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What about this idea... dual centerboards


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#1 DaveK

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Posted 07 June 2013 - 04:13 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

 



#2 MisterMoon

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Posted 07 June 2013 - 04:16 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. 

 

Angle of attack is where the lift comes from. It doesn't take all that much AOA in a dense fluid like water to make a lot of lift. To prove this you your self next time you are on the boat, observe your rudder! A little AOA goes a long way, eh? 



#3 ssi

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Posted 07 June 2013 - 04:19 PM

OMG!!!!!  You are a genius, better run in Doug's Patent Office and get your intellectual rights protected.

 

It's the best thing that's ever happened to dinghy sailing.  Wow, are you a rocket scientist by night, by chance?



#4 DaveK

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Posted 07 June 2013 - 04:28 PM

No but I act like one on TV



#5 BalticBandit

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Posted 07 June 2013 - 04:33 PM

David, this is what scows and other "bilge boarders" do.   Its also what most "canters" do as well (look at their photos) since the "keel" doesn't offer much lateral resistance when "canted" all the way to weather.   So they drop in "daggerboards".

 

Now these daggerboards are angled, but the reason for angling them is not the one you think.    even a perfectly flat DB will generate lift if you give it an angle of attack (try it with a piece of cardboard out of a car window).  So what causes a symmetric blade to generate lift, just as in an airplane with a symmetric wing (stunt planes for example like the Chipmunk), is the Angle Of Attack caused by the hull "sliding" to leeward somewhat.

 

all hulls "slide" - but planing hulls like skiffs slide very little, but make up for it by much much much higher flow rates which then generate the same amount of lift but at a lower angle of attack.  In skiffs its not unusual to see a beginner pointing way above the rest of the fleet, but tracking the same as them, only way slower.   You need to put the bow down, accelerate and then the foil starts to generate the same amount of lift but at a lower AoA.

 

Boats like the 5o5  allow for "gybing" centerboards.  By carving the top of the CB into a diamond shape, with the peak of the diamond behind the Center of Lift, the CB"gybes" (twists) to windward just like the piece of cardboard in the car experiment above.     BUT since for a given foil the optimal angle of attack remains the same this does not generate more "lift"  What it does is allow the hull to be sailed that many degrees lower.  Which means the hull is sliding through the water at a lower AoA than before and hence generating less drag and allowing the boat to go just a bit faster.

 

But once it is planing upwind, the 5ohs lock their boards on centerline



#6 Disambiguated

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Posted 07 June 2013 - 04:40 PM

The Melges 17 uses asymmetric bilge boards, not sure about the other scows.



#7 Gouvernail

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Posted 08 June 2013 - 07:40 AM

Mine looks a lot like this and I'm sorry Dave but I don't think it would work for shit and putting it in the water would ruin it for playing my old music collection..

 

 

490541-dual_604_direct_drive_vintage_tur



#8 barney

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Posted 08 June 2013 - 08:57 AM

ixylon yolle:

ixylon-segeljolle-segelboot-foto-bild-69



#9 Cavandish

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Posted 08 June 2013 - 09:21 AM

187676651.jpg



#10 Amati

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Posted 09 June 2013 - 09:06 AM

This might amuse...


http://www.airfoildb...eria=flatbottom

For example,


http://www.airfoildb...]=382&id[]=1152

#11 Lake Shark

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Posted 09 June 2013 - 01:44 PM

The Melges 17 uses asymmetric bilge boards, not sure about the other scows.

 

most of them just use unshaped slabs of stainless or aluminium, maybe there has been some development in E's and A's but most likely not



#12 mercruiser

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Posted 09 June 2013 - 03:40 PM

This was a graceful design- not very popular but the right idea:

 

http://www.capecodsh...php?boat=gemini



#13 Amati

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Posted 11 June 2013 - 01:16 AM

If this wrong I don't want to be right:


http://www.acontaine...1/crossbow2.jpg

And on so many levels, digitally......

#14 Amati

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Posted 25 June 2013 - 03:49 PM

Anyway, some interesting towing studies on the advantages of bilge keels

http://www.brayyacht..._twinkeels.html

#15 Doug Lord

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Posted 25 June 2013 - 03:57 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.
 
I'm not on much drugs today..... just a thought

-------------------
Speaking of two boards, check out the C Class thread in Multihulls: the Canadians(Blunted and Fred) are using one curved foil and one straight board in each hull of their new C Class Cat......

http://forums.sailin...howtopic=144228      pictures, post 96
 



#16 LakeBoy

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Posted 25 June 2013 - 04:49 PM

Try to look up a "Lone Star 13" dinghy from the 1960s or 70s.  They used two bilgeboards mounted in the inside edge of the seats so it was easy to cross the cockpit sole given the absence of a centerboard trunk.

 

 

Everything old is new again!

 

Of course, I think they used flat plate aluminum, not asymetrical designs.



#17 Mr. Swordfish

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Posted 28 June 2013 - 09:05 PM

The idea that in order to generate lift a foil must be flat on one side and curved on the other may be one of the biggest misconceptions in popular fluid dynamics.

 

See http://www.grc.nasa....ane/wrong1.html

 

wrong1.gif



#18 Amati

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Posted 28 June 2013 - 11:01 PM

It just occurred to me, Dave, that you might mean two daggerboards in the same daggerboard well as one symmetrical daggerboard would be?

And you would effectively cut the existing single symmetrical daggerboard in half, so that you'd wind up with two assymetrical boards with flat surfaces, and you'd lower one and raise on each tack? So each daggerboard would have 1/2 the thickness of the symmetrical board?

I can think of some advantages and some disadvantages. I don't know anyone has tried it. ?

The two assymetrical boards would be thinner, but if you shaped the leading edges to roundish ellipses, and didn't get too much separation, you be generating some lift at lower aoa's, but maybe with lower lift, so lower drag, and skinnier can mean less drag, so.. Skinnier is better at lower Reynolds numbers..

If this is what you're thinking of, why not try it, and check the airfoil data site I suggested above and see if there any shapes in the same vein that might work differently but maybe better in some ways and try it out. Find a typical symmetrical airfoil, like NACA 0015, and see what would happen with 7% thickness solutions that have flat or flattish bottoms.

Couldnt hurt?

Flat bottom airfoils do develop lift, after all. It's what else is going on while the lift is developed is the rub.

#19 DaveK

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Posted 29 June 2013 - 12:07 AM

The idea that in order to generate lift a foil must be flat on one side and curved on the other may be one of the biggest misconceptions in popular fluid dynamics.

 

No..... not at all that it has to be flat on one side. But it is obvious in your picture that lift is generated towards the longer side of the foil. Lots of RC planes have what I call symetric foil wings with the ailerons making the difference in how they perform and somewhat the stabilizer. Obviously this doesn't really apply here since water isn't blown at 100mph over a boats surfaces and these planes can do stupid tricks of anti gravity at very low speeds. But I just have a hard time grasping lift on sym foil at 5 knts. Maybe it produces more of gyroscope, I wanna go straight in this direction attitude, but lift isn't in my head.

 

And Amati, yes I was thinking about just splitting an existing board down the middle of a trunk. It would have to be stronger and reinforced. So the faster you go, the more lift u get until maybe your going so freaking fast the apparent wind just slams you straight into it.....lol. But that was the idea!



#20 BalticBandit

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Posted 30 June 2013 - 05:55 PM

No Dave, in RC planes Aoa makes the diff not ailerons.    And lift on those planes is the same as it is on a foil, just different Reynolds numbers and drag profiles.  Have you ever seen a 3D stunt plane fly really really slowly?  It flies purely on AoA.. in fact my 3D stunt plane has ZERO foil curvature.  Its just flat bits of foam.  So it flies purely on AoA.

 

Think about it. 

  • Take a very thick symmetric foil (cut it out of cardboard). draw a line through it right down the center  and then draw a heavy line on a piece of graph paper and line them up.  This configuration  does not generate any lift - as you realize since the "distance travelled" is identical.

     

  • Now keeping the "line of flow" the same   incline the foil by 2degees to the flow line.
  • Now find the "forwardmost" edge of the foil where it touches the horizontal lines of the graph paper.  This will be ever so slightly to one side of the centerline of the foil you drew in Step 1. 

     

  • Pin  the foil to the graph paper and go fine some thread or yarn in your wife's/GF's dresser.  pin and mark one end on the new "front point" of the foil.  drape the thread tightly along the edge of the foil to the TE on one side.  Mark that distance on the thread.

     

  • Now without unpinning the front point, drape the thread along the other side.

you will find that with a slightly inclined foil, there is a slight difference in the two "surfaces" because the "centerline" has shifted.  And thus it is the AoA that creates the "longer distance" that the Bernoulli Effect requires



#21 Mr. Swordfish

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Posted 01 July 2013 - 01:36 PM

Aaack.

 

And thus it is the AoA that creates the "longer distance" that the Bernoulli Effect requires

 

 

While it is correct that angle of attack is essential to the creation of lift, you do not need a longer distance on one side vs the other to generate lift.

 

Again, this is one of the most common misconceptions about aerodynamics.  Path length difference is a red herring that has nothing to do with creating lift.  Look at your sails - they are a thin membrane with a negligible difference in path length from one side to the other, yet they do a fine job of generating lift.

 

You might want to read this article in Physics Today to better understand lift and how aerodynamic lift comes about.  Bernoulli's principle has little to do with it.

 

"How do Wings Work" by Holger Babinsky



#22 BalticBandit

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Posted 01 July 2013 - 01:52 PM

Holger's explanation is circular.  it also does not correspond to what pressure and velocity sensors on the surface of a sail tell us.  Dave - please ignore that article, it will only confuse you and the discussion.



#23 Mr. Swordfish

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Posted 01 July 2013 - 02:59 PM

Circular?  How so? It starts with basic physics (newton's laws on motion) applies a bit of calculus, and reaches a conclusion.  How is that circular?

 

 

Anyway, there are peer-reviewed papers written by PhD physicists and engineers that are published in scientific journals, eg Babinsky's paper.

 

And then there are old wives' tales repeated  by people who don't know what they are talking about, e.g. nonsense about path length differences and the equal-transit-time fallacy.

 

You can choose which ones to believe. 

 

Wikipedia has a good article on Lift.  Follow the citations for many articles discussing lift.  The discredited "longer path hypothesis" is now rejected by nearly all academic papers.



#24 FORCE

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Posted 01 July 2013 - 05:02 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

 

You are assuming that a centerboard must produce lift in order for the boat to sail to windward.  (OK, I can hear the howls now, but listen). More than lift, windward work requires sufficient lateral plane to vector the sail's force to the forward direction, rather than making leeway.   Granted, a foil shaped lateral plane is a bit more efficient than a non-foil shaped plane as a keel shape to windward, however, you cannot fully substitute the QUANTITY of lateral plane merely with lift.  A flat metal plate has no lift, yet does a fine job providing lateral plane allowing a boat to sail to windward.  Try it on a saling dinghy!   Additionally, the theory that windward ability is ONLY a function of foil lift is a misconception further diminished when the vessel is deeply heeled..... when the foil is actually providing even more heeling moment to the boat and proportionately less lift to windward.   Against popular opinion, I have found that that you cannot replace large amounts of lateral plane area with very small and efficient foils and expect to sail to weather without significant leeway.  Surprisingly, lower aspect plate shaped metal centerboards that are generous in area can make the vessel sail better to windward than smaller foils, because the centerboarder can add much more surface area to it's lateral plane (at least 20% more than the fin keeled foil) and the extra wetted surface can be retracted as it is not being sailed hard on the wind.  The extra wetted surface of this additional lateral plane is negligible when considered in the overall scheme of drag when sailing to windward, be it mononhull or multihull.  So yes, a boat with a proportionately oversized centerboard can outpoint and outfoot a comparable one with a much smaller foil (all other things being equal) with the  of extra wetted surface that can be lifted out when off the wind, where lower wetted surface really counts (as opposed to windward work, where it does not).  Granted, the large area plate centerboard should have a fine taped trailing edge and parabolic entry.   

 

Another factor to centerboard efficiency is the slot through which it drops.  If there is any significant opening to the trunk aft of the lowered centerboard, you will receive a very large drag component caused by the sucking and water circulation as the water passes by.   Such a trunk must have lips on it, to this off.  To avoid this, I have built 2 large cats and one waterballaested monohull all which use a quadrant-board, rather than a drop down centerboard, which never leaves the bottom of the trunk open to water circulation.   The cats had floaters, and the monohull quadrant board was a sinker, beling made of shaped solid aluminum.    On the first cat I built a traditional centerboard trunk, and could hear this incredible sucking noise from the pendant line exit hole, and saw the vacuum turbulence in the wake at speeds over 15 knots.   So out came the old cases and boards, replaced with the quadrant board designs, and I never looked back since.  As evidence of the reduced drag of the quad boards, the vessel powered a full knot and a half faster at the same RPM.   

 

In the case of the monohull, the oversized quadrant board was used in conjunction with an 18" NACA airfoil shaped minikeel, which was built around the board trunk.  The volume of the NACA minikeel was calculated exactly to all of the lead ballast, and work well together to get the ballast low and provide huge lateral plane when wanted.  The OVNI's would sail much better and increase their stability using such a design.  'Nuff rambling on about centerboards for today.

 

 



#25 BalticBandit

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Posted 01 July 2013 - 05:02 PM

Its circular because he uses the logic of the "stream flows" to argue that there must be centripetal accel.  But he offers no explanation for such acell.

 

The Wikipedia article actually is not all that good in that it does not explain why if you have faster flowing air you don't get massive sheer turbulence in the wake (it should be seen as massive downwash)  when what you actually get is tip rotor turbulance.

 

Furthermore the Wiki article shows a "longer travel path" just as the "debunking" video does (although it cheats slightly by having partially stalled flow)

 

Yes lift - particularly ultra low speed lift - which is what we sailors deal with, is a complex phenomenon  with non-linear acceleration and pressure profiles along the whole of the section of the foil.   

 

But you have a person who is struggling to understand lift in the first place.  You introducing things like Holger's paper just makes it harder to understand



#26 BalticBandit

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Posted 01 July 2013 - 05:04 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

 

You are assuming that a centerboard must produce lift in order for the boat to sail to windward.  (OK, I can hear the howls now, but listen). More than lift, windward work requires sufficient lateral plane to vector the sail's force to the forward direction, rather than making leeway.   Granted, a foil shaped lateral plane is a bit more efficient than a non-foil shaped plane as a keel shape to windward, however, you cannot fully substitute the QUANTITY of lateral plane merely with lift.  A flat metal plate has no lift, yet does a fine job providing lateral plane allowing a boat to sail to windward. 

Wrong.  A metal plate at an AoA has lift because of the AoA.  Why can get rather complex but it has measureable lift

 

 

Against popular opinion, I have found that that you cannot replace large amounts of lateral plane area with very small and efficient foils and expect to sail to weather without significant leeway.

Then what you have found is contrary to what most aerodynmicists and skiff sailors know to be the case.

 

the best example would be comparing a Shields (20'DWL) vs an Ultimate 20 and an 18' skiff (just under 20' DWL)
 

Each has a progressively narrower and higher lift keel and each progressively makes less leeway.



#27 Mambo Kings

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Posted 01 July 2013 - 05:24 PM

Angle of attack? Ive tried attacking Ultra from many different angles but I never seem to get the lift that I need .

#28 FORCE

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Posted 01 July 2013 - 05:56 PM

BalticBandit stated:

"Wrong.  A metal plate at an AoA has lift because of the AoA.  Why can get rather complex but it has measureable lift"

 

 

Yes, or course there is a "measurable" lift.  Everything is relative.  That was not the point.  The point was that amount of lateral plane is hs more effect than a smaller foil with more lift when sailing to windward, all factors considered.  My statements stand as they were written and qualified.

 

 

BalticBandit stated:

"Then what you have found is contrary to what most aerodynmicists and skiff sailors....... Each has a progressively narrower and higher lift keel and each progressively makes less leeway."

 

 

Yes, narrower, but not the same depth.  Their keels are deeper, giving more area.   And the higher aspect with give a certain amount of additional lift, which will also compensate for some of the reduced lateral plane. That was covered in the article, and still stands as written.



#29 FORCE

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Posted 01 July 2013 - 06:09 PM

I  did not realize that I was in a dinghy forum. 

This centerboard treatise was intended for bluewater boats.



#30 Mr. Swordfish

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Posted 01 July 2013 - 06:44 PM

> Its circular because he uses the logic of the "stream flows" to argue that there must be centripetal accel.

 

It is not a circular argument.  The logic goes something like this:

 

1) Any time an airstream follows a curved path there is a pressure gradient normal to the directon of flow.  This follows form a simple application of Newton's 2nd law to the flow kinematics, and was derived by Euler hundreds of years ago.  .

 

2) The air follows a path that is curved because of the shape of the airfoil and the angle of attack.

 

Put 1 and 2 together, and we obtain lift: since the air follows a path that is curved, there is a pressure differential, with lower pressure on the inside of the curve and a higher pressure on the outside.  See the article for more details.

 

 

> Furthermore the Wiki article shows a "longer travel path" just as the "debunking" video does

 

Agreed that some airfoils have a longer path.  And some don't.  And if you invert one with a longer path and apply a high enough angle of attack you'll get lift with the "wrong" side up.  Longer path is neither a necessary nor sufficient condition for lift, and explanations that use it are not based on physics.

 

>But you have a person who is struggling to understand lift in the first place.

 

A fair comment, as Babinsky's article assumes freshman college level physics and calculus and not everyone has that.  For those that do, it's probably the simplest, most easily understandable presentation of the physics underlying aerodynamic lift.  For an explanation aimed at High School level students, NASA's is pretty good:  Lift comes from flow turning.

 

If you want me to make it simpler than that, I'll just say "air goes down, plane goes up" or "air coming in over the beam is deflected towards the stern - air goes backwards, boat goes forward."  Anyone who can understand that much  will be miles ahead of someone trying to apply a non-physical, non-sensical model based on path length differences.



#31 BalticBandit

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Posted 02 July 2013 - 12:02 AM

> Its circular because he uses the logic of the "stream flows" to argue that there must be centripetal accel.

 

It is not a circular argument.  The logic goes something like this:

 

1) Any time an airstream follows a curved path there is a pressure gradient normal to the directon of flow.  This follows form a simple application of Newton's 2nd law to the flow kinematics, and was derived by Euler hundreds of years ago.  .

 

2) The air follows a path that is curved because of the shape of the airfoil and the angle of attack.

 

Put 1 and 2 together, and we obtain lift: since the air follows a path that is curved, there is a pressure differential, with lower pressure on the inside of the curve and a higher pressure on the outside.  See the article for more details.

And for this to work one flow path has to be more curved than the other.  And if you have a flowpath that has less curvature, simple integral calculus will tell that the path with more curvature is longer.  So for this to be a refutation of the "flow path" theory, you have to have some magic causing there to be a curved flow path that is not based in length

 

> Furthermore the Wiki article shows a "longer travel path" just as the "debunking" video does

 

Agreed that some airfoils have a longer path.  And some don't.  And if you invert one with a longer path and apply a high enough angle of attack you'll get lift with the "wrong" side up.


Except that's not true.  Because the flow path in that inverted case is still "longer" on the lift side.  What you are not taking into account is the viscosity of air (or water) which is what in fact makes the path effectively longer in the same way that a flat plate orthogonal to the wind direction will have lateral flows around the edges proportional to the viscosity of the material and the energy in the flow - but there will be a "wind shadow" somewhere in the middle

 

So even in the case of a tilted 'flat plate' which according to you would not have a "longer path" actually a longer path does exist - one generated by the viscosity of air and its inability to make "instantaneous" turns absent a hard surface to bounce the molecules off of.

 

you can see this effect on the opensided cargo containers that have dropcloths on their sides - you see the effects of those static air pockets in the deflection of the dropcloths.  Were you to attach a gossamer light sheet of elastic plastic to the lee side of a flat plate, you would see the same static bubble form around the leading edge.  This is what essentially causes the "longer distance"  whereas on the "windward side" the air molecules bounce off the flat plate and are immediately deflected laterally.

 

 

And the reason a sail works is due to the same effect.  there is essentially an "air pocket" formed on the inside of the sail where the viscosity of the air overcomes the sheering flow.  Thus the lower energy path is a straight line from the LE edge to the TE  and voila! Bob's Your Uncle and you have a differentiated path length.



#32 RobG

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Posted 02 July 2013 - 06:02 AM

Great thread hijack. The OP wants dual asymmetric boards and probably doesn't give a rats arse whether the longer path on the low pressure side is a cause or consequence of lift (it's a consequence).

 

The largest impediments to dual, side–by–side asymmetric boards are practical. The lifted board needs to stay below the boom and not foul any control lines such as the vang or mainsheet. It must also be easy to raise and lower, perhaps swing boards would do the job, but they need to slide past each other without fouling. I can imagine some collisions between leading and trailing edges in the lower parts of the boards.

 

Probably not too hard to get working, but I can't see that it would beat a gybing board where the amount of "gybe" is easily tuned and requires no extra work when tacking.



#33 BalticBandit

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Posted 02 July 2013 - 08:28 AM

Rob, Dave was asking why NOT dual bilge boards in place of a daggerboard because he cannot understand how a symmetric DB works.  So the first part was explaining that boats like the A Scow do exactly this but mainly because they are designed to be sailed extremely heeled so a swing bilge board is feasible (they also are fairly big so the extra weight of two DB trunks is not an issue).

 

The second part has been to try and explain to Dave how symmetric foils work to generate lift



#34 Mr. Swordfish

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Posted 02 July 2013 - 02:16 PM

Great thread hijack.

 

Ok, fair enough.  One more and I'm done, at least in this thread.

 

 

Q: Why does a 5O5 go faster than an Optimist?

A: Because it traverses a longer path in the same amount of time.

 

This is a correct answer.  It is also a useless answer, because it just restates the definition of speed.  You might as well say it goes faster because it does.

 

Q: Why does the air on the top of a wing go faster than the air on the bottom?

A: Because it traverses a longer path in the same amount of time.

 

This is equally correct and equally useless.  Using longer path to  explain increased speed is simply circular reasoning.

 

Now, often people will try to bolster the longer path idea with some intuitive explanation of why the air has to speed up, and this usually results in a non-physical and nonsensical hypothesis like the condition of equal-transit-time, or Baltic Bandit's erroneous idea that the air flowing on the windward side of a sail travels in a straight line from the leading edge to the trailing edge.  There's all sorts of nonsense as people try to back fill the reason for the increased speed, and the basic problem is that the now-discredited "popular" explanation gets the cause and effect backwards:

 

The old "popular explanation" says that increased speed causes a lower pressure, and then flounders around trying to explain why the air speeds up, offering either incorrect reasons, or circular non-explanations.

 

The modern scientific explanation  first explains the pressure distribution (as a consequence of Newton's laws or by using Euler's equation for curved fluid flow: dp/dR =rho * v^2/R) and then uses Bernoulli's formula to see that increased speed is a consequence, not the cause of the pressure distribution.  See Klaus Weltner's  articles for example.  Here is one that is available free.

 

You can choose which one to believe.  Many people have faith in the popular explanation because it's what they learned when they were just starting out, and nothing I'm going to say is going to sway them away from those beliefs if their faith is strong enough.



#35 BalticBandit

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Posted 02 July 2013 - 04:20 PM

Except that the pressure distribution over a sail does not match Euler's equation except at the same macro level as Bernoulli. 

 

Again we are trying to explain a very complex interaction concept to someone I don't think is well versed in Diff Alg. 

 

and in fact the velocity measurements show that the air on the "inside" curvature of the jib is relatively stagnant compared to the flow down the "center line" so much so that on a deeply drafted cat rigged sail mid--sail tells will sometimes stream forward!!!  even though LE and TE tells are streaming aft.

 

Yes, the flow acelleration has to do with lateral acceleration (both positive and negative) vectors associated essentially incompressible flow around an object.  But if in fact TE velocities were different in any signifcant way, TE tells would not stream aft as they do with an unstalled sail - they instead would sheer towards the lower velocity flow just as the edges of puffs do.  But they don't.  Which indicates TE velocities are largely symmetric.

 

so if LE and TE velocities are largely symmetric, then the net sum of the velocity changes between those points has to add up to zero as well because otherwise there would either be a gain or loss of air molecules on one side or the other of the sail - and we know that's not so.

 

It gets very complicated very fast.  More than we can probably discuss in this forum.  But I'm trying to explain how AoA - which is the key here - makes a symmetric foil generate lift in a way that is cognatable to someone not doing piece wise integrated meshing of Euler's equations.  I invite you to help, but the blog you presented and the Wikipedia article do not help at all



#36 Amati

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Posted 07 July 2013 - 02:10 AM

The molecules, once split,do not meet again at the trailing edge!

I think the Kutta condition is a lot more interesting than you are giving it credit for. Or maybe you're just assuming it's existenc

To me the kewl thing about asssymetric foils is that they develop lift at -0 aoa. But that might lead to the bow turning in a way that would be, um, inconvenient?

Either that, or as I have spewed before, lift is a quantum event. And I haven't seen any mathematical arguments here for that. Yet. :)

#37 RobG

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Posted 07 July 2013 - 12:21 PM

he molecules, once split,do not meet again at the trailing edge!

Yes, and there is no law of physics to say they must. It's one of those fallacies that persists. BB can even tell us about air travelling forward on the high pressure side of a sail—those mollecules must really get their skates on to meet their partners at the trailing edge. And then magically reduce speed to match the low pressure side.
 

To me the kewl thing about asssymetric foils is that they develop lift at -0 aoa. But that might lead to the bow turning in a way that would be, um, inconvenient?

Aircraft don't fly with the main wing at zero AoA as it reduces the margin for tripping into negative AoA. This might be relevant with respect to sailing boats when sailing dead down wind, or very fast on a broad reach as apparent wind boats tend to do. I expect gybing boards are locked on the centreline when not going to windward for just that reason (e.g. so they don't "gybe" as the result of a sudden bear away in a puff).

#38 Ron D

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Posted 07 July 2013 - 01:17 PM

With regards to twin boards, this is already done on boats with leeboards. Each board is shaped for its side and is raised and lower according to the tack. Leeboards unclutter the interior but the extra effort complicates your ability to do quick tacks.

#39 BalticBandit

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Posted 07 July 2013 - 05:28 PM

he molecules, once split,do not meet again at the trailing edge!

Yes, and there is no law of physics to say they must. It's one of those fallacies that persists. BB can even tell us about air travelling forward on the high pressure side of a sail—those mollecules must really get their skates on to meet their partners at the trailing edge. And then magically reduce speed to match the low pressure side.
 

>>To me the kewl thing about asssymetric foils is that they develop lift at -0 aoa. But that might lead to the bow turning in a way that would be, um, inconvenient?

Aircraft don't fly with the main wing at zero AoA as it reduces the margin for tripping into negative AoA. This might be relevant with respect to sailing boats when sailing dead down wind, or very fast on a broad reach as apparent wind boats tend to do. I expect gybing boards are locked on the centreline when not going to windward for just that reason (e.g. so they don't "gybe" as the result of a sudden bear away in a puff).

 

Actually  There is a law of physics that says the two adjacent molecules have to meet - though not directly

 

 

  • A sailboat going to weather is arguably in a "steady state" condition (yes wind and wave variance alters that but for simplifying the model we remove that variance to see if we have a correct model).

     

     

  • That means that at no point along the surface of the sail can there be any changes in pressure, mass accumulation/loss or changes in the balance of forces

     

     

  • A sailboat travelling in wind is a frame of reference from the sailboat being static and the wind moving. - We can just as easily change the frame of reference to have the air static and the sailboat moving through static air...BTW this does happen in "Real Life" - the best example being a race in which the wind died completely but the tide was running at about 3 knots. We set the kite and reached across the sound in zero wind by literally moving the sailboat through the air

     

     

  • So from the frame of reference of static air and a moving sailboat if we tag two molecules of air at the leading edge of the sail, and track them to the TE, since the air is static, then they can ONLY be displaced/accelerated/decell.  as long as they are interacting with the sails.  The moment they cease to interact with the sails, since there is no other force operative, they EITHER

     

    Have to return to their presail contact static equilibrium (adjacency)
    OR

     

  • There has to be some other energy input into the system to compensate for the change in energy/mass density in the part of the air after the sailboat has passed

 

 

 

Now we know there is SOME turbulence in the air after a sailboat has passed so perhaps there is some relaxation of the "molecules remeet" condition.  So what is the cause of that turbulence?  Well it basically is the combination of two factors[list=1]the change in the Potential Energy of the part of the air that has energy extracted from it to drive the boat, and any sheer forces caused by the "pre" and "post" conditions at the LE and TE not matching.

 

Now if the amount of energy involved in the sheer forces is equal to the PE lost to drive the boat, then the net energy transfer into the boat and the net energy transfer out of the boat into turbulence sums to zero and there is no energy left over to drive the boat.  And we know this not to be true.

 

In fact we know that the TE turbulence is fairly limited since if we run multiple fore-aft sails (like in a set of turbine blades) in series, while the amount of energy we can extract out of each sail is reduce, and the AWA also progressively decreases, we also know that the turbulence on the TE is limited.  thus the Kinetic Energy of the two airstreams at the TE is relatively the same.

 

If the KE is relatively the same, then if any mass is lost on one airstream (if one molecule arrives ahead of the other it is increasing mass of its airstream relative to the other) the only way the KE can be the same is if that airstream is flowing faster.

 

So the perverse conclusion of one molecule flowing faster than the other and arriving first is that the other molecule has to suddenly acclerate at the TE to make up for the difference in KE.  So suddenly the slower moving airstream heats up and moves faster!!!!!

 

and we know that's not the case.   So that means that the assumption that the molecules do not meet violates the conservation of energy.  A basic law of physics.

 

 

Now as to 0 AoA in airplanes - yes one reason is to stay in a meta-stable condition, another is to stay in the optimum part of the Lift/drag bucket which invariably is slightly more than 0 AoA.

 

Now as to gybing boards - no that's not why you lock them out downwind.  The reason you lock them out downwind is that downwind you want to MINIMIZE LIFT  in fact if you could somehow generate negative lift with your board, that might very well be ideal (although if you have ever raced in light air  in a way that one gybe runs with the current and the other runs across the current (with the current carrying you to leeward) you will have experienced the sensation that there is "less wind" on the cross current gybe and the sails feel "softer".  So with a negative AoA on the board you may well make less VMG than with a 0 AoA.

 

 

 

 

As to leeboards - they actually CLUTTER the interior, complicate the maneuvers and increase weight.  Why?  because you now have two DBs instead of one, TWO DB trunks, and each gybe/tack you have to drop one and raise the other...  Works OK on a boat like an M scow where you have extra hands and the boat sails heavily heeled, but not at all in a skiff that is sailed flat.



#40 fastyacht

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Posted 08 July 2013 - 03:44 AM

Oh-My-God.

 

Circulation.

Viscocity.

Ideal Versus Real fluids.

Ideal fluid, as well as real fluid at instantaneous start-up, has zero circulation, zero net downwash and zero lift, and the stagnation points are symmetrical. Once that starting vortex moves off the trailing edge, the trailing stagnation point goes to the trailing edge, downwash begins and so does lift. Without viscocity there is no lift. Without drag there is no lift.

 

Now, for the big shocker--the one that really matters but I didn't see anyone say, is the *definition* of lift!

 

Lift is merely the force acting normal to the incident airflow direction--in other words 90 degrees to the flow. That's all it means. Period, If whate ver you are doing is creating any net force at 90 degrees--net--not gross--then you have lift. Baseball, rotating cylinder, flat plate. None of it matters. If there is *any* force acting perpendicular to the flow, you have lift.

 

Hand out the window = lift.

 

Not complicated. Making less drag for a given lift gets more complicated. But of course a symmetrical section develops lift. It creates a redirection of the flow, and whether you are doing circulation or momentum or etc you get to the same place. What won't get you there is Bernoulli. That's the wrong explanation I still find in every middle school science textbook along with a wrong understanding of speed of sound. Great. STEM at work. (Don't get me started on that !)



#41 RobG

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Posted 08 July 2013 - 03:52 AM

Your ideal wold doesn't exist. The condition that "there [can't] be any changes in pressure, mass accumulation/loss or changes in the balance of forces" means the sail can't be interacting with the air at all.

 

Turn the sail to be square to the breeze, clearly the turbulence on both sides of the sail means that the chance of any two molecules that started adjacent being reacquainted with each other later is zero. Absolutely zero (or at least some vanishingly small number).

 

So from that position until the sail is aligned perfectly with the wind and causes zero disruption (at which point the hypothetical molecules might just get to the trailing edge and still be within hailing distance) there are an infinite number of states where the molecules will not end up anywhere near each other.

 

An alternative view of aerodynamic lift is to consider the exchange of kinetic energy between the air and the foil. The foil deflects the air, causing it to accelerate at some different velocity. The result is a force on the wing (action, equal and opposite reaction) to produce "lift". Behind the foil is an area of low pressure caused by the air that wasn't deflected continuing with its initial velocity (momentum). But that causes a low pressure zone, so the air near the foil is pushed into the low pressure area by surrounding air.
 
If only a small deflection is caused, the air flows reasonably steadily around and into the low pressure zone. At some point, the deflection causes a larger low pressure zone that the air can't fill without becoming turbulent (the angle depends on the density of the air and speed relative to the foil). So the low pressure side is curved to fill in the void and allow a bigger deflection before the flow becomes turbulent.
 
The problem with the "longer distance" theory is that on a sail, the distance on both sides is almost identical. Consider an asymmetric spinnaker on a skiff where the leading edge is perhaps 4mm thick and the distance across the surface might be 4 or 5 metres. Now think if the massive power produced by the sail—that can't be explained purely in terms of different distances for each side. Where a traditional pole and reaching spinnaker is used, the difference is even closer to identical as there is only a luff tape on the leading and trailing edges to create the longer path.
 
Lastly, there are modern square riggers that are extremely efficient using a DynaRig, of which the Maltese Falcon is probably the best known (http://en.wikipedia...._Falcon_(yacht)).
 
The longer path theory is wrong and should be consigned to the dust bin, it's a consequence, not a cause, of lift.
 
Now to bebunk the coriolis effect… ;-)


#42 BalticBandit

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Posted 08 July 2013 - 08:02 AM

Your ideal wold doesn't exist. The condition that "there [can't] be any changes in pressure, mass accumulation/loss or changes in the balance of forces" means the sail can't be interacting with the air at all.

not quite.  the point I am making is that there cannot be any NET changes in Mass/Energy in the system.  Since we take some PE and translate it into lift aka KE, there will be some localized loss of PE equal to the KE extracted.   But a loss of PE does not mean that the mass from which the PE was extracted now has  more KE... that violates fundamental laws of mass/energy conservation.

Turn the sail to be square to the breeze, clearly the turbulence on both sides of the sail means that the chance of any two molecules that started adjacent being reacquainted with each other later is zero. Absolutely zero (or at least some vanishingly small number).

Simply wrong - the two molecules starting in the middle front of that sail's stagnation zone will slowly flow to each edge.  there they will spill around the edge, accelerate through the turbulence zone, decelerate near the end of the turbulence zone and meet again in steadystate some distance down wind.

 

An alternative view of aerodynamic lift is to consider the exchange of kinetic energy between the air and the foil. The foil deflects the air, causing it to accelerate at some different velocity.

and if we measure the relative velocities (and imputed accelerations) of the particles along various parts of the foil, it turns out that this isn't the case.  That instead we get both acceleration and decelleration on both sides of the sail.



#43 BalticBandit

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Posted 08 July 2013 - 08:06 AM

If only a small deflection is caused, the air flows reasonably steadily around and into the low pressure zone. At some point, the deflection causes a larger low pressure zone that the air can't fill without becoming turbulent (the angle depends on the density of the air and speed relative to the foil). So the low pressure side is curved to fill in the void and allow a bigger deflection before the flow becomes turbulent.

you are confusing two effects here.

 

The problem with the "longer distance" theory is that on a sail, the distance on both sides is almost identical.

its not if you take into account the stagnation zone on the interior of the curvature that fills the area between the LE and TE.  So your Asso example is the wrong analysis

 

 

Lastly, there are modern square riggers that are extremely efficient using a DynaRig, of which the Maltese Falcon is probably the best known (http://en.wikipedia...._Falcon_(yacht)).

 

Have you ever been on a square rigger?  I have.  the sails get "shaped" to generate a powerful deep sail shape WRT the wind with draft forward.   So its standard airfoil analysis, nothing special.

 

Note that in fully square running, the sails run in a stalled mode.  That's why if you want to be fast, you don't sail DDW with the kite strapped in max projection and the main with outhaul at Max and halyard at max,   but instead you "heat up" about 5-15 degrees even on an IOR slug, and you rotate the kite so that you get flow over it  and you ease the outhaul and halyard and vang so that your main can generate lift and its leech matches the leach curvature of the kite to some extent



#44 Mr. Swordfish

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Posted 08 July 2013 - 01:43 PM

What won't get you there is Bernoulli. That's the wrong explanation I still find in every middle school science textbook along with a wrong understanding of speed of sound. Great. STEM at work. (Don't get me started on that !)

 

 

Well, it's getting better.  Just about everybody in this thread seems to get it, and there is only one lone holdout for the incorrect and useless 'popular explanation', whereas ten years ago the ratio would have been more or less the opposite.  Once NASA's Glenn Research Center came out with their website about aerodynamic lift a few years ago, the popular explanation has been dying a slow (too slowly for my tastes) inexorable death.

 

Widespread misconceptions do not die easily, but we're making progress.  This YouTube video is pretty good - it went viral a short time ago and instructors who try to present the old wrong explanation will run into a lot of resistance if anyone in the class has seen it.



#45 RobG

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Posted 08 July 2013 - 01:49 PM

Have you ever been on a square rigger?  I have.  the sails get "shaped" to generate a powerful deep sail shape WRT the wind with draft forward.   So its standard airfoil analysis, nothing special.

I don't need to sail on a square rigger to know the principles of how they work any more than I need to sit in a rocket to know how a rocket works. The point is that the distance across both the high and low pressure sides is the same. Having an invisible pocket of stagnant air fill the belly to provide a short cut is a novel idea (and means you've invented some air since the molecules are supposed to meet their partners at the trailing edge, or are they too stalled on the low pressure side?). While stagnant pockets occur in some conditions, it's slow and not assisting lift so it's far better to trim it out if you can.

Forward thrust is provided by deflecting the air, not by a different path length. The sail is shaped to efficiently deflect the air while keeping a smooth(ish) flow.

You can keep repeating yourself if you want, I'll stop now.

#46 Mr. Swordfish

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Posted 08 July 2013 - 01:53 PM

 
Now to bebunk the coriolis effect… ;-)

 

Does this help?



#47 BalticBandit

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Posted 08 July 2013 - 02:57 PM

No the distance is not the same, and if you measure the flow velocities on a sail's both surfaces you find that the flow velocity of the air at "max chord" is significantly slower than if the sail were simply "flagging out"   in fact if you measure the flow velocities in the whole cambered section ( Using ultrasound doppler) you find that its an area of relatively stagnant air. - Next time you are out for a sail in light wind with no race, try this experiment.  Light a cigarette (or a punk which these days is cheaper) and let it generate smoke.

 

Now standing abeam of the max draft point hold the smoke source centerline on the boat.  slowly move it towards the sail.  you will find a boundary point where the air suddenly is moving much slower than ambient flow.  That's the stagnation pocket.  and essentially the air is flowing around the stagnation pocket in a straighter line than the inner curvature of the sail would suggest.

 

this stagnation is a consequence of the viscosity of the air.

 

 

As to swordfish's youtube video - its actually playing fast and loose.  if you run it very slowly and look at the video at roughly 1.01 seconds, you can see the two smoke trails reconverging.  It happens about 10% of chordlength aft of the foil, but it happens.  And if it did not you would be violating one of the basic laws of physics.  why it happens slightly aft of the TE has again to do with viscosity an the fact that the molecules have no "intelligence" and are operating in each case on localized forces that tend not to fully backpropogate.

 

But the video plays so fast and loose with facts that its basically kindness to call it junk science.  In fact the whole bit about "how airplanes fly upside down" and the "flat airfoil of the Wright bros" is so wrong as to be laughable.



#48 RobG

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Posted 09 July 2013 - 03:20 AM

As to swordfish's youtube video - its actually playing fast and loose.  if you run it very slowly and look at the video at roughly 1.01 seconds, you can see the two smoke trails reconverging.

Perhaps we watched different videos.

Attached Files



#49 Amati

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Posted 09 July 2013 - 03:35 AM

so is the 'Kutta Condition' the phrase that shall not be uttered?

 

popcorn please, lite buttery topping...



#50 Amati

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Posted 09 July 2013 - 04:42 AM

Maybe a diet Coke...

 

 

http://astropt.org/b...-automobilismo/

 

:)



#51 BalticBandit

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Posted 09 July 2013 - 06:51 AM

so is the 'Kutta Condition' the phrase that shall not be uttered?

 

popcorn please, lite buttery topping...

Well that's kinda my point in responding to Swordfish and RobG.   That to really explain how lift works you have to go to esoteric stuff like Kutta and Kelvin Circulation.   And that leaves someone like DaveK  really struggling.

 

And the problem with the "One Minute Physics" explanation is that it ignores these necessary components while adding in other aspects - for example the notion that the trace paths don't meet at the TE... that is to some extent true - but that has to do with the Kutta TE vortex.  and the two tracepaths DO meet at the end of that vortex.  And you can argue that the vortex itself, because it is turbulent flow, acts as an extension to the foil itself (you can see this in how water vortices resist lateral external flows)

 

The problem with "lift equals forces pushing up and back" is that the amount of lift generated actually exceeds the amount of molecular deflection:  ie if we take a flat board of surface area X and hold it a say a 30deg AoA  we will get Xflat lift out of it

 

and if we take an assymmetric (or symmetric - but I'm choosing Assymetric to vary only one aspect at a time)  foil that is of surface area X and hold it at a 30deg AoA we will get Xcurved amount of lift  And LiftXflat < LiftXcurved  So the "vertical lift + aft drag"  does not work as a lift equation



#52 BalticBandit

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Posted 09 July 2013 - 06:55 AM

As to swordfish's youtube video - its actually playing fast and loose.  if you run it very slowly and look at the video at roughly 1.01 seconds, you can see the two smoke trails reconverging.

Perhaps we watched different videos.

No we didn't... notice how between the 1st and the 6th frame the bottom flow is catching up to the top flow.  and if you were to allow that to go on for a few more frames and some more distance - which they don't because then the do meet and they would have to talk about Kutta vortices and vortices as extensions to the foil - they do meet up  they have to.  Its the basic physics law of the conservation of mass and energy



#53 Mr. Swordfish

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Posted 09 July 2013 - 02:19 PM

 


From Stick and Rudder by Wolfgang Langewiesche, page 9, published 1944:

 

 

The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down.

 

It shoves the air down with its bottom surface, and it pulls the air down with its top surface; the latter action is the more important. But the really important thing to understand is that the wing, in whatever fashion, makes the air go down. In exerting a downward force upon the air, the wing receives an upward counterforce--by the same principle, known as Newton's law of action and reaction, which makes a gun recoil as it shoves the bullet out forward; and which makes the nozzle of a fire hose press backward heavily against the fireman as it shoots out a stream of water forward. Air is heavy; sea-level air weights about 2 pounds per cubic yard; thus, as your wings give a downward push to a cubic yard after cubic yard of that heavy stuff, they get upward reactions that are equally hefty.

 

That's what keeps an airplane up. Newton's law says that, if the wing pushes the air down, the air must push the wing up. It also puts the same thing the other way 'round: if the wing is to hold the airplane up in the fluid, ever-yielding air, it can do so only by pushing the air down. All the fancy physics of Bernoulli's Theorem, all the highbrow math of the circulation theory, all the diagrams showing the airflow on a wing--all that is only an elaboration and more detailed description of just how Newton's law fulfills itself...

 

Thus, if you will forget some of this excessive erudition, a wing becomes much easier to understand; it is in the last analysis nothing but an air deflector. It is an inclined plane, cleverly curved, to be sure, and elaborately streamlined, but still essentially an inclined plane. That's, after all, why that whole fascinating contraption of ours is called an air-plane.


 

Replace "wing" with "sail" and "down" with "aft" and you've got a simple cogent explanation of what makes a sailboat go.  Yes, there are more complicated ways to explain it (and sometimes these more complicated explanations are even correct), but this is the basis.

 

As for Baltic Bandit's 'theories", they're not even wrong, so there's no point in arguing with him.



#54 BalticBandit

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Posted 09 July 2013 - 03:24 PM

And again, if that were the case, then a curved foil would not generate more lift than a flat plate... and yet it does.  In fact if it was just Newton's law, then what would matter is purely the surface area of the foil and not the foil itself.  So gliders and fighter jets and long haul commercial jets would all have mostly the same wing loadings (per surface area)

 

And yet they don't.  Not even close.

 

The notion of "not even wrong" is reserved for describing claims that do not comport with observable measurements, which is essentially what you are claiming  is that a bird and a jet should have similar wing loadings and they don't

 

Furthermore that a flat foil vs an assymetric foil should have substantially the same lift characteristics  And yet we see from NASA windtunnel data this is nonsense http://www.hq.nasa.g...P-468/ch5-2.htm   In particular Fig 5.3 shows that wings with similar surface areas will have as much as a 3x change in the amount of lift they generate simply with changes in the airfoil shape.

 

A FACTOR OF THREE.   Newton's law says that's impossible. since for a given surface area travelling at a given speed, the number of air molecules deflected will be identical.

 

  So do tell where that difference magically comes from hmm?



#55 BalticBandit

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Posted 09 July 2013 - 03:34 PM

See the thing is to make it really work you need fairly complex math http://my.fit.edu/~d...ctures/TAFT.ppt  But that's not what Dave needs  Because he's trying to understand why a symmetric foil generates lift at a nonzero AoA  and substantially more lift than the equivalent flat plate surface area.

 

And Newton's law actually says it doesn't - which we all know experimentally is bull.



#56 BalticBandit

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Posted 09 July 2013 - 03:40 PM

Note BTW that "Stick and Rudder" up front says  that

 

"What's wrong is that ....it is the theory of building an airplane rather than flying"  (IOW their explanation is not really the way it works)  pp5

 

they then go on to say

 

"The only way to understand unknown things is in comparison to known things"  pp7

 

 

So the source up front is saying that the explanation it is going to offer and which Swordfish cites.... Is NOT actually how a wing works, but an explanation that is "good enough" for a pilot.  But of course it doesn't explain the phenomenology DaveK was asking about.

 

And which your "model" of how lift works also fails to explain.



#57 fastyacht

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Posted 09 July 2013 - 09:23 PM

 

As to swordfish's youtube video - its actually playing fast and loose.  if you run it very slowly and look at the video at roughly 1.01 seconds, you can see the two smoke trails reconverging.

Perhaps we watched different videos.

No we didn't... notice how between the 1st and the 6th frame the bottom flow is catching up to the top flow.  and if you were to allow that to go on for a few more frames and some more distance - which they don't because then the do meet and they would have to talk about Kutta vortices and vortices as extensions to the foil - they do meet up  they have to.  Its the basic physics law of the conservation of mass and energy

 

If what you mean by catching up is vortices then yes, you are correct, the moecules "catch up" in the same conservation sense that volume rate of flow at any cross section of a pipe is the same.  But this kind of catching up--embedded in the vortex--is completely different from the "catching up" hypothesis of the old oversimplified wrong description of lift so long perpetuated along with the paradoxical bernoulli theory of lift.

As far as section shape and lift, well, yes, of course a flat plate is worse than a symmmetric foil, which is worse than a cambered foil. That is totally compatible with Newton, by the way :-)  The flat foil, for a given angle of attack, area and aspect ratio, deflects less air than a more efficient foil. This is because the better foil can carry a greater adverse pressure gradient than the flat foil without separation. Greater gradient means more mass accelerate favorably downward rather than unfavorably forward. This applies to both face and back but as we all know, the back is the one where the greatest screw-ups and greatest gains are to be had. Completely compatible with Newton.



#58 BalticBandit

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Posted 09 July 2013 - 09:59 PM

No vortices tend to be static, that's why tip vorticies on landing jumbos are such a problem for the next plane in.  What you have is the vortex essentially acting like an extension of the foil.

 

As for a cambered foil being more efficient than a symmetric foil, hmm how is that compatible with Newton when the surface areas are the same?

 

If it is surface area, for an identical AoA, Area, and aspect ratio a flat foil deflects the IDENTICAL amount of air as one that has curvature.  Furthermore if it was deflecttion as the primary force then it would not matter where Max Thickness was. It could be at 75% and Newtonian equations would give you an identical result

 

Except we know that Max Thickness aft of 50% kills lift and generates drag compared to even a flat foil... Hmmmm

 

And given that a cambered foil generats THREE TIMES the lift of a flat foil  do explain how this is compatiible with newton?  No extra molecules are deflected, no extra deflection force is involved, yet lift is 3x.... IOW what you are telling me is that Fflat = MairAdeflection due to AoA = 1/3 Fcambered  Huh??

 

 

- your "greater Gradient" doesn't exist in any theory nor does it exist in the Newtonian explantation that Swordfish and RobG are touting. 



#59 fastyacht

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Posted 10 July 2013 - 12:00 AM

No vortices tend to be static, that's why tip vorticies on landing jumbos are such a problem for the next plane in.  What you have is the vortex essentially acting like an extension of the foil.

 

As for a cambered foil being more efficient than a symmetric foil, hmm how is that compatible with Newton when the surface areas are the same?

 

If it is surface area, for an identical AoA, Area, and aspect ratio a flat foil deflects the IDENTICAL amount of air as one that has curvature.  Furthermore if it was deflecttion as the primary force then it would not matter where Max Thickness was. It could be at 75% and Newtonian equations would give you an identical result

 

Except we know that Max Thickness aft of 50% kills lift and generates drag compared to even a flat foil... Hmmmm

 

And given that a cambered foil generats THREE TIMES the lift of a flat foil  do explain how this is compatiible with newton?  No extra molecules are deflected, no extra deflection force is involved, yet lift is 3x.... IOW what you are telling me is that Fflat = MairAdeflection due to AoA = 1/3 Fcambered  Huh??

 

 

- your "greater Gradient" doesn't exist in any theory nor does it exist in the Newtonian explantation that Swordfish and RobG are touting. 

 

Separation is caused by too steep a pressure gradient for attached flow. The reason max thick isn't 75% aft is due to separation issues. Indeed, you get more momentum of air normal to free stream, using a cambered foil, for a given angle of attack. Yes, you can increase the angle of attack of the flat plate to the point where you have identical lift, but you will then have significantly greater drag. That's why we use shaped airfoils. At small angles of attack, the benefits of shape become less and less. Indeed, a stubby airfoil can create negative lift if you get premature separation from it some distance down the chord.

 

Note that landing configuration on a jetliner is for high lift, and high drag; cruising design is for minimized drag at design speed. You can design a foil many different ways.

 

I'm not sure why you are trying to disprove momentum. And your vortex thing is goofy. The starting vortex spins around and around but the rest of the 2-diml circulation simply becomes downwash--the vortex is in fact around the foil but free of it, other than at the starting point, you won't see a swirl. Wingtip vortices are spanwise flow phenomena and have nothing to do with circulation theory per se--except of course that the total circulation circuit of a 3Dml foil will include them (but they are of no consequence to the fundamental idea of 2-diml lift)



#60 BalticBandit

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Posted 10 July 2013 - 09:21 AM

No its not separation issues.  Anyone who has built a model airplane wing and accidently dropped it knows that lift will generate more on the thicker side than the thinner side.  in fact if "separation" issues where the issue, then a lower curvature would generate much slower stall characteristics which are what you would like.  In fact high lift foils with max curvature further forward have nastier stall characteristics than ones where the max curvature is further aft.

 

As for your explanation that you can get the same amount of lift by increasing the AoA and that's why we use curved foils ignores the core point.  The core point is that

 

For an IDENTICAL UNDERSIDE Surface area, and an IDENTICAL FLOW RATE (velocity)  and an IDENTICAL AoA - the curved foil can generate THREE TIMES as much lift as a flat plate. 

 

Newton's equations tell us that F=MA.. 

  • Since the flow rate is the same, ADeflection is the same (from a Newtonian perspective)

  • Since the surface area is the same and the AoA is the same  Mdeflection is the same

  • From #1 and #2 we therefore conclude that under Newton, FFlat Plate == FCambered upper surface  foil

And yet we absolutely know that this is not true.   So Newton's laws are insufficient in how swordfish and his video presented them to explain lift.

 

 The starting vortex spins around and around but the rest of the 2-diml circulation simply becomes downwash--

That would require the "downwash" (or on a sailboat "upwash" or "leebow header" ) to be equal to the full velocity difference that you measure between the upper and lower surfaces.  And yet .. when you actually instrument the surfaces - THIS DOES NOT HAPPEN.

 

 

Now I'll admit I was worried that you would not get the point about wingtip vorticies (which are  a result IN PART of spanwise flow, (parenthetically if the "deflected flow" theory was right, then the winglets that are most effective at stopping such vorticies would be ones that droop rather than curve upwards, and yet it is upwards curving ones that kill such vortexes  odd that).  The point I was making with Tip Vortices is precisely the "momentum" argument.  Namely that the air within the vortex has momentum (including gyroscopic) and thus it acts as a barrier to flows incident upon it.

 

Here's another way to think about, a vortex is a bit like a spinning ping pong ball.  to displace the spinning ping pong ball with a lateral force vector, you have to

  • apply enough force to move the Mplastic mass of plastic of the ball

  • apply enough force to move the Mcontained air mass inside the plastic ball

  • apply enough force to overcome the gyroscopic momentum of Mplastic gyro+Mcontained air Gyro

This means that vortex flow that extends behind the foil for some distance,  acts as a lightweight extension of the foil.  (as though it was trailing a lfroth of soap bubbles - just like you see in the hull wake of a ship)

 

So to actually assert that the air molecules at LE don't meet at same velocity at TE, you have to extend the actual TE to the end of this TE vortex.  And in fact if you look at the video Swordfish and RobG cite - at the end of that vortex tail, the two are in fact back together. Thereby CONSERVING MOMENTUM.

 

 

 

So called "circulation theory" - which is in fact how aerodynamicists calculate lift - has inherent in it the notion of the flow velocity on the upper surface being faster than on the lower surface - (as the Diff Eqs in the PPT I linked show).  And yet that is what you claim to be refuting.

 

And it is the claim that somehow "top" air has gained more momentum magically that is a direct consequence of Swordfish and RobG's claims and why I state that they violate a fundamental law of physics - namely conservation of momentum



#61 RobG

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Posted 10 July 2013 - 01:55 PM

so is the 'Kutta Condition' the phrase that shall not be uttered?
 
popcorn please, lite buttery topping...

Well that's kinda my point in responding to Swordfish and RobG.   That to really explain how lift works you have to go to esoteric stuff like Kutta and Kelvin Circulation.

No, you don't. They are artifacts, they are consequences, they are incidental, they have very little to do with creating lift. You might as well say you need to understand the general theory of relativity to understand planetary motion. You don't. You only need to understand gravity.

If you want to do precise calculations of planetary motion of the accuracy required to get a rocket from Earth to Mars, Einstein's your man. But it isn't necessary for an understanding of the basics.

Another problem with your "the particles meet up again" theory is that it is contrary to the second law of thermodynamics. You know, the one that says entropy never decreases, yet here you are claiming the restoration of the initial order (i.e. a decrease in entropy) is fundamental to your theory.

The smoke in the video is gradually diffusing, becoming less and less ordered (see, it obeys the law). So some of the smoke from the upper surface eventually mixes with some of the smoke from the lower surface.

And how can a stationary vortex have anything to do with the lower smoke catching up with the upper smoke?

The problem with "lift equals forces pushing up and back" is that the amount of lift generated actually exceeds the amount of molecular deflection:  ie if we take a flat board of surface area X and hold it a say a 30deg AoA  we will get Xflat lift out of it

A flat board at 30° AoA will stall, you'll get little lift and much drag.
 

and if we take an assymmetric (or symmetric - but I'm choosing Assymetric to vary only one aspect at a time)  foil that is of surface area X and hold it at a 30deg AoA we will get Xcurved amount of lift  And LiftXflat < LiftXcurved  So the "vertical lift + aft drag"  does not work as a lift equation

So where is the proof that the lift "exceeds the amount of molecular deflection"? You haven't used any actual mathematics, you've just restated your hypothesis using algebra (i.e. replaced one set of symbols with another).

 

Oh sorry, wasn't going to post again…



#62 Amati

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Posted 10 July 2013 - 02:18 PM

This might amuse-


http://web.mit.edu/1...e_Notes/f18.pdf

Although I still think that quantum entanglement has some promise- all those electrons pining for each other's company would have to be loving the one they're with in molecules entangled & coupled. (he said coupled!) to a massive and continuous Heisenberg uncertainty leap. And back! Damn 2D foils. Getting in the way and all. Torturing innocent fluids.

The mystery of sailing.......

::):

#63 BalticBandit

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Posted 10 July 2013 - 02:45 PM

Amati - circulation and vortex sheets are a more precise and finer grained way of modeling higher speed flow over the top surface and lower speed flow over the bottom surface.

 

The reason the  "longer distance" mental model is useful is because we know that as you increase the curvature (and hence the "length") of the top surface, you get more and more lift (along with more drag)  with your max amount of lift-drag occurring with a half circle of sufficient chord to allow attached flow to exist on the whole surface.

 

 

Rob - if you "understand gravity" then you are way ahead of the teams of physicists at CERN, who are just starting to sort through how gravity works (and yes they do use both Special and General relativity)

 

 

 

As to thermo -  sorry entropy can be maintained in any number of ways ranging from a change in temperature of the flow to a change in linearity of the flow.  The two molecules meeting at TE has nothing to do with Entropy but purely of mass energy and momentum conservation.

 

 

And while the smoke in the video is "gradually diffusing"  your problem is that the video frame is too small and cuts off too soon.  I've seen a larger version of that sort of trail and the molecules do meet up... they have to, otherwise you are digging a permanent hole in the air... which is called a vacuum.

 

A flat board at 30° AoA will stall


Not necessarily.  Think Snipes... but that doesn't really matter, change that to 3deg AoA... and make it a very high aspect ratio plank.  It won't stall.  And the cambered version of the same surface will generate up to 3 times as much lift WITH NO INCREASED DEFLECTION POSSIBLE

 

what part of the inability to increase deflection by adding a curve to the backside of the foil do you not get? 

 

  • How does adding a curve to the top surface increase the number of molecules bouncing off the bottom surface?
  • do the molecules on the bottom come with xray vision goggles so that when they see a curved top surface they bounce harder?
  • do the molecules on the bottom have precognition so more of them bunch up together when they previsage hitting a curved top surface?
  •  

Or is it some other effect other than Newtonian F=MA deflected displacement?

 

 

So where is the proof that the lift "exceeds the amount of molecular deflection"?

In the NASA report I linked where they showed the amount of lift that various types of airfoils

 

  • of equal surface area
  • of equal AoA
  • of equal velocity streamflows

     

     

If the surface area, AoA and streamflow velocityis the same then the bottom side Newtonian "deflection" is identical.  It has to be since there is no other way for the molecules to be deflected other than by bouncing off the bottom surface (at least according to the video explanation Swordfish and you are touting).

 

and since the law of Conservation of Momentum is one of Newton's laws, you cannot say that more momentum is imparted to one type of foil over another if the net impact force of all of the molecules of air <b>is identical</b>  To say that violates the basic law of "conservation of momentum"



#64 Mr. Swordfish

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Posted 10 July 2013 - 03:15 PM

For the record, those are not my videos, and it is not my theory. 
 
Here's what the American Association of Physics Teachers recommends:
 

"At least for an introductory course, lift on an airfoil should be explained simply in terms of Newton’s Third Law, with the thrust up being equal to the time rate of change of momentum of the air downwards."

 
So, I'm not making this stuff up out of whole cloth, it's what the serious educators recommend as a correct yet easily understandable explanation.  Yes, there are more detailed treatments that are appropriate for those going beyond an introductory course; if you are interested in them pretty much any college level aerodynamics text will suffice.  You won't find 'equal-transit-time' or 'longer path' arguments in any of them, except when they point out that these 'popular' notions are incorrect.
 
I'd recommend the recently published Understanding Aerodynamics: Arguing from the Real Physics by Doug McLean since it focuses more on the physical conceptual ideas; other treatments are usually computationally-oriented and the physics tends to get lost in the math. Chapter 7.3 is a must read for anyone seriously interested in the subject.

#65 fastyacht

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Posted 10 July 2013 - 05:30 PM

It is a vortex street, not a sheet. Haha.

Baltic:

You keep arguing that there is no conservation of momentum--that the lift is somehow not related to m*a.  What, then, are you proposing???



#66 BalticBandit

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Posted 10 July 2013 - 06:29 PM

For the record, those are not my videos, and it is not my theory. 
 
Here's what the American Association of Physics Teachers recommends:
 


"At least for an introductory course, lift on an airfoil should be explained simply in terms of Newton’s Third Law, with the thrust up being equal to the time rate of change of momentum of the air downwards."

 
So, I'm not making this stuff up out of whole cloth, it's what the serious educators recommend as a correct yet easily understandable explanation.  Yes, there are more detailed treatments that are appropriate for those going beyond an introductory course; if you are interested in them pretty much any college level aerodynamics text will suffice.  You won't find 'equal-transit-time' or 'longer path' arguments in any of them, except when they point out that these 'popular' notions are incorrect.
 
I'd recommend the recently published Understanding Aerodynamics: Arguing from the Real Physics by Doug McLean since it focuses more on the physical conceptual ideas; other treatments are usually computationally-oriented and the physics tends to get lost in the math. Chapter 7.3 is a must read for anyone seriously interested in the subject.

notice the emphasis.   since intro courses are extremely simplistic and don't even come close to being able to deal with Bernoulli's principles,  this isn't very helpful for someone who is asking about gybing centerboards and why the work.

 

FY... if you put a straw in a bottle of soda and blow across the top of it - what Newtonian equations make the soda spray out of the straw???

 

Furthermore - I'm the one INSISTING THERE IS CONSERVATION OF MOMENTUM... so how do you get to my supposedly arguing that there is no conservation of momentum is beyond me.



#67 Mr. Swordfish

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Posted 10 July 2013 - 07:26 PM

OK, Goldilocks.

 

I give you  Babinsky and you complain that it's too complicated

so I give you Langewische, and you complain that it's too simple

so i give you Weltner, and you complain that it's too complicated

and I give you the American Association of Physics Teachers and you complain that it's too simple

 

I could point you at another couple of dozen resources, but as none of them comport with your, .... ahem .... novel   theories I doubt it will help.



#68 BalticBandit

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Posted 11 July 2013 - 07:32 AM

For me its all fine.  I've got the Diff Eq math to deal with circulation theory and its informed my trimming (quite successfully) for nigh unto 3 decades.

 

The point is that Langewische and AAPT are too simple for what DaveK wants as an explanation.   Why?  well because anyone with a fairly simple understanding of foils can figure out that those are wrong.  (flat plate vs. Curved upper surface with all else identical and one gens more lift means "deflection" is simply wrong)

 

Babinsky and Weltner are in fact too complicated for what DaveK is asking for - namely a fairly quick "Rule of Thumb" way of understanding why a symmetric foil with a positive AoA, generates lift on one side and not both sides.

 

You've not offered anything that works better than the "longer distance travelled" (which in fact IS going on, but has only a partial effect).



#69 Mr. Swordfish

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Posted 11 July 2013 - 04:18 PM

The problem with this "just right" longer-path answer is that it's not based on physics.

 

So while it sometimes gives the right answer, it fails more often than it succeeds.  It's like computing 16/64 by cancelling the 6's i.e.

 

16/64 = 16/64 = 1/4

 

Granted, it gives the right answer in this particular case, but it's not based on mathematics, and if you're doing arithmetic this way you're going to arrive at the wrong answer more often than not.

 

Likewise, if you try to understand lift without a clear notion of the definition of lift, you wind up making nonsense statements like having a foil that "generates lift on one side and not both sides".   This is impossible, unless you are working with a different definition of the  word "lift" than that used by scientists and engineers.

 

I think I'm done here. For those lurking who've gotten this far in the thread, read the links I've posted for more info, and don't believe the non-physical misconceptions stemming from the longer-path and equal-transit-time fallacies.  They're simply wrong.

 

The (simplified, and good enough for most people) reason why a foil generates lift is that the fluid is deflected by the foil.  Fluid has mass, so if the fluid goes in one direction, the foil is pushed in the other direction in accordance with the law of conservation of momentum. The more fluid you deflect ( either by increasing the angle of attack, increasing the size of the foil or changing its shape, or by finding faster moving fluid to deflect) the more force you get from the foil.

 

The rest is just details. 

 

Granted, this idea does not explain why some foils are more efficient than others -  that requires a more detailed analysis, and for that you need real physics, not the nonsense expoused by the discredited 'popular explanation' based on longer paths and equal transit time.



#70 Doug Lord

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Posted 11 July 2013 - 05:51 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

Why not consider a circular, sliding asymmetric foil-flatish side outboard.Gives some "foil assist" vertical lift to reduce wetted surface and promote earlier planing. Slide side to side as you tack or center to mostly retract. Mount can allow a fore and aft angle of the board to change the angle of incidence of the lifting portion of the foil independently of the lateral resistance portion-sorta like a curved foil on an A Class. One board works like two boards:

Attached Files



#71 BalticBandit

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Posted 11 July 2013 - 06:19 PM

The problem with this "just right" longer-path answer is that it's not based on physics.

 

So while it sometimes gives the right answer, it fails more often than it succeeds.  It's like computing 16/64 by cancelling the 6's i.e.

 

16/64 = 16/64 = 1/4

 

Granted, it gives the right answer in this particular case, but it's not based on mathematics, and if you're doing arithmetic this way you're going to arrive at the wrong answer more often than not.

 

Likewise, if you try to understand lift without a clear notion of the definition of lift, you wind up making nonsense statements like having a foil that "generates lift on one side and not both sides".   This is impossible, unless you are working with a different definition of the  word "lift" than that used by scientists and engineers.

 

I think I'm done here. For those lurking who've gotten this far in the thread, read the links I've posted for more info, and don't believe the non-physical misconceptions stemming from the longer-path and equal-transit-time fallacies.  They're simply wrong.

 

The (simplified, and good enough for most people) reason why a foil generates lift is that the fluid is deflected by the foil.  Fluid has mass, so if the fluid goes in one direction, the foil is pushed in the other direction in accordance with the law of conservation of momentum. The more fluid you deflect ( either by increasing the angle of attack, increasing the size of the foil or changing its shape, or by finding faster moving fluid to deflect) the more force you get from the foil.

 

The rest is just details. 

 

Granted, this idea does not explain why some foils are more efficient than others -  that requires a more detailed analysis, and for that you need real physics, not the nonsense expoused by the discredited 'popular explanation' based on longer paths and equal transit time.

Well its not altogether wrong the way the Newtonian model can trivially be shown to be.  BTW the Babinsky explanation is not exactly misleading but it is so incomplete that it really might as well be considered wrong.  If you want we can go into  what it is that he assumes and tries to say and ends up being misleading on - but Id rather do that in another post.

 

Where the "longer path" model is not altogether wrong (and Babinsky does get this part wrong)  is that in the case of sails, even a sail with no mast (ie a lufftape only jib like on many a modern skiff ) has a stagnant air pocket on the windward side of the jib,  and this pocket serves to create a "shorter path" for the "bottom" stream vs the lifting stream

 

Secondly, what is really doing the work - as you allude to but don't quite get to the root of - is the curvature.  Yes the curvature induces centripetal accel - but that's not really the bit that causes the flow and pressure changes we observe when instrumenting the leeward side of a sail.   And this is where the real explanation gets very hard to explain to folks -  What's causing the accelerated flow (and it does accelerate as even your video shows)  is that you essentially are trying to force more air molecules through a smaller space.

 

HUH?  But there is essentially infinite space to leeward of the sail you say.

Sure but that also means there is essentially infinte mass resisiting displacement and compression. in the lateral direction.

 

So what you really have are the same flow dynamics you would have if you took a foil and pulled it through the center of a pipe.   the walls of the pipe act as constraints on the lateral displacement of the fluid.  now the fluid has to go somewhere (imagine a tube that is closed with a track down the middle and the foil on the track - and it is all filled with water.  I can clearly move the foil through the water so the water has to go somewhere)   and in the case of the upper/lifting surface, the ONLY way it can move ffrom the LE to the TE in sufficient volume to (hey FS this is for you - To comply with the law of Conservation of Momentum)  is to move more incompressible molecules through the same space faster.  That means that as a molecule moves across the curved surface, since it is doing so faster, it "vibrates" (brownian vibration is what causes pressure of any sort) against the surface fewer times.  Thus there is lower pressure there.

 

Now in the Real World of open air,  the essentially infinite incompressible mass of air acts as the walls of the pipe.  And yes there is some lateral compression of air, but that has to do with viscosity efffects.  So it is the laterall displacement of some of the sttreamflows into the spatial location of the other stream flows that forces them to accel.  

 

And as they accel. they cause a drop in pressure.

 

Since the lateral accel of a curved surface is greater than of a flat surface (where the lateral displacement is limited to the AoA, the flow velocity over a curved surface is greater and hence there is even lower pressure than for a flat surface.  Thus you do get some push from the newtonian deflection, but you also get "suck" from the reduced pressure on top.  and that is instrumetably measureable.

 

So in essence, because the air molecule is being moved further (travelling a longer distance) in the curved surface case, it generates more lift than the flat plate case.  So the "longer path" model, while inaccurate  in its completeness - is less wrong than the hash that Babinsky puts forth (which while not actually wrong, is so incomplete it might as well be wrong).

 

The "vortex sheet" is one way of modeling this because it allows you to take into account the differing accel and decel patterns we actually measure on the surface of the foil.  so it is a more amenable calculation model for DiffEqs  of the real world.



#72 BalticBandit

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Posted 11 July 2013 - 06:21 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

Why not consider a circular, sliding asymmetric foil-flatish side outboard.Gives some "foil assist" vertical lift to reduce wetted surface and promote earlier planing. Slide side to side as you tack or center to mostly retract. Mount can allow a fore and aft angle of the board to change the angle of incidence of the lifting portion of the foil independently of the lateral resistance portion-sorta like a curved foil on an A Class. One board works like two boards:

Doug - Fuck Off.  Go pollute another thread.  You are not even remotely on topic here as DaveK was trying to understand how foils work, not invent some new rube-goldberg piece of crap that will never work in the real world. 



#73 mikeys clone no1

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Posted 21 July 2013 - 02:47 AM

As to swordfish's youtube video - its actually playing fast and loose.  if you run it very slowly and look at the video at roughly 1.01 seconds, you can see the two smoke trails reconverging.

Perhaps we watched different videos.

any videos where aoa is 0?



#74 BalticBandit

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Posted 22 July 2013 - 10:53 AM

And therein lies the rub.  This is what you get if it is symmetrcal

 



#75 BalticBandit

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Posted 22 July 2013 - 10:55 AM

And here is what you get with assym foil at very close to 0 AoA

 



#76 BalticBandit

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Posted 22 July 2013 - 10:59 AM

Then there is this one

 



#77 BalticBandit

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Posted 22 July 2013 - 11:03 AM

Now from this CFD analysis from Doyle Sails, you can see that it rapidly gets pretty complex  and in fact

  • the path on the leeward side DOES accelerate and then decelerate before meeting at the TE
  • and that the path taken is "longer" (though less so that would at first be obvious)
  • and that there is a "stagnation zone" in the "pocket" of the sail on the windward side



#78 RobG

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Posted 22 July 2013 - 01:47 PM

 

As to swordfish's youtube video - its actually playing fast and loose.  if you run it very slowly and look at the video at roughly 1.01 seconds, you can see the two smoke trails reconverging.

Perhaps we watched different videos.

any videos where aoa is 0?

Watch this video made in 1963 from 05:30 onward. It shows both positive and negative angle of attack, and utterly disproves the "equal transit" theory without even mentioning it: 

http://youtu.be/DOUfyDHxkYQ?t=5m27s



#79 DaveK

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Posted 22 July 2013 - 02:11 PM

 

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

Why not consider a circular, sliding asymmetric foil-flatish side outboard.Gives some "foil assist" vertical lift to reduce wetted surface and promote earlier planing. Slide side to side as you tack or center to mostly retract. Mount can allow a fore and aft angle of the board to change the angle of incidence of the lifting portion of the foil independently of the lateral resistance portion-sorta like a curved foil on an A Class. One board works like two boards:

Doug - Fuck Off.  Go pollute another thread.  You are not even remotely on topic here as DaveK was trying to understand how foils work, not invent some new rube-goldberg piece of crap that will never work in the real world. 

 

Ya know, you're really a dickhead yourself. Doug isn't polluting anything compared to your bullshit. This was a very simple idea of weather it would work or not and apparently it does on scows. Enough said. I don't need any long winded self appraising diatribe from you about how lift works. It doesn't mater and my question was answered by post 6. You are seriously much more obnoxious then Doug!!



#80 Mr. Swordfish

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Posted 22 July 2013 - 03:26 PM

Sorry about the thread hijack, Dave.

 

To return to your original question, yes in principle dual asymmetrical centerboards should work to increase lift, but the complications involved in adapting a conventional centerboard boat are probably not worth the effort when much of the benefit could be obtained via a single symmetrical jibing centerboard.  This has been done before.  For boats that already have 2 daggerboards such as scows and cats, it's not much of a complication to use asymmetrical foils, and many do.

 

As for your implied question about how a symmetrical centerboard really produces much lift, the lift comes about for the same reason as any symmetrical foil - when the foil  meets the water at a non-zero angle of attack there is lift.  For this to happen, the boat must have some non-zero leeway, and this is where it gets a little complicated. 

 

For a conventional centerboard dinghy, the leeway is so small as to be unmeasurable - or at least Frank Bethwaithe was unable to measure it  when he tried, there's a long discussion of it in High Performance Sailing.  So what's happening is that as soon as the boat starts slipping to leeward, the centerboard develops lift from the non-zero angle of attack and this counteracts the leeway.  The net result is a very small amount  of leeway.  Since water is very dense, a small angle of attack generates a lot of lift. 

 

How much lift do you get from this vanishingly small amount of leeway? My understanding is that most of the force from the centerboard is due to it's resistance to sliding sideways - i.e. it's an example of a Normal force  rather than a result of hydrodynamic lift.  Since the boat resists sliding sideways through the water even when there is no forward motion, this lateral resistance force can't be the result of lift as you can't have lift until there is motion. In theory, once the boat is moving forward through the water it will develop additional force from lift, but I'm uncertain of the magnitude of the increase - I'd speculate that it is a small increase over the stationary lateral resistance force, but I'm not sure.  Any pointers to articles addressing this (as opposed to random people on the intertubes pulling theories out of their posterior) cheerfully accepted.



#81 BalticBandit

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Posted 22 July 2013 - 04:01 PM

And I would challenge your understanding Mr Swordfish.  The best example I can offer is skiff like the 49er or the Musto with is kite in really  light air.  Here's how you run the "experiment".... Bear off untll you are at about 100 deg TWD (this would be your normal course with a kite in 4-6 knots. now let the boat coast to a stop after easing sails.   Hoist the kite, and sheet on the main and then the kite.  The boat will now slide sideways - completely overcoming "normal force" , and because of the imbalance in the sail plan (huge kite, relatively small high lift main) the bow will slide sideways and you will be unable to counter act it with any helm control.


Now do the same, but ease your sails and use the rudder to try and bear the boat off just a bit faster than your sliding will cause.  And a soon as you have some steerage, bring the nose up to accelerate the airflow around your airfoils.  pretty soon, if you do it right, you are now moving at 2x-3x the Speed Through Water, and you are able to track at about 100deg to the TWD.  And yet your lateral force loads - due to dramatiically increased lift out of the sails - are much higher. 

 

If in fact it was just normal force generatiing most of the lateral resistance,then this would not be possible.  And yet it is.  IOW your explanation is iinsufficient.



#82 fastyacht

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Posted 22 July 2013 - 04:02 PM

"normal force"?

 

???

 

It's lift. I don't know what dinghies you sail in, but I can easily measure leeway...regardless of what you may think FB said about it.

 

Asymmetrical foils can achieve a higher lift/drag ratio. That's why you use them.

 

A traditional double-ended Polish fishing boat (forget the name) is sailed (always was) with asymmetrical daggerboards. -- Actually they are ogival and symmetrical around the y axis. You simply pull up the board after (durng?) tacking and flip it nose for tail.



#83 BalticBandit

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Posted 22 July 2013 - 04:04 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

 

 

 

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

Why not consider a circular, sliding asymmetric foil-flatish side outboard.Gives some "foil assist" vertical lift to reduce wetted surface and promote earlier planing. Slide side to side as you tack or center to mostly retract. Mount can allow a fore and aft angle of the board to change the angle of incidence of the lifting portion of the foil independently of the lateral resistance portion-sorta like a curved foil on an A Class. One board works like two boards:

Doug - Fuck Off.  Go pollute another thread.  You are not even remotely on topic here as DaveK was trying to understand how foils work, not invent some new rube-goldberg piece of crap that will never work in the real world. 

 

Ya know, you're really a dickhead yourself. Doug isn't polluting anything compared to your bullshit. This was a very simple idea of weather it would work or not and apparently it does on scows. Enough said. I don't need any long winded self appraising diatribe from you about how lift works. It doesn't mater and my question was answered by post 6. You are seriously much more obnoxious then Doug!!

Well then fuck you too Dave for not having a clear idea of what you are asking.  Within 20 minutes of you asking that question I'm the one who explained to you that

1) Bilge boarders did that. 

 

2) Then of course I made the mistake of trying to explain to you the question you had implicitly raised in the first sentence of your post.  Only to have folks like RobG and Mr Swordfish deploy bogus criticisms.

 

 

If you don't want explanations, don't bother asking the questions.  As FY points out, there are boats that deploy Assym foils.  and "gybing" boards and "trim tab" boards also work to generate asymetrical lifting surfaces. BUT two boards

A) add a lot of unnecssary weight

B) don't give you that much benefit over symmetric boards

C) only really give you an edge in boats that are sailed with a heel - which most dinghies are not since heeling reduces power generated by the saiils



#84 Steam Flyer

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Posted 22 July 2013 - 05:16 PM

So I never really get how a centerboard really produces much lift since it's symetrically balance in a foil shape. I do get that it reduces lateral forces to leeward and only wants to move in the direction it's pointed. Now if I really wanted a centerboard that did that and lifted me to weather, I'd think that a flat sided foil on one side would really produce lift and work. But would need 2 centerboards. One for each tack. Or one could swing up while the other was down.

 

I'm not on much drugs today..... just a thought

 

"Only wants to move in the direction it's pointed" = lift.

 

Kicking the can past all the pissing & moaning ...

 

http://forums.sailin...howtopic=129928

 

FB- Doug



#85 Mr. Swordfish

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Posted 22 July 2013 - 06:47 PM

"normal force"?

 

???

 

 

Fair enough - I understand that what I wrote might be a bit confusing, and the concept of "normal force" is not really quite applicable.  Here's what I'm trying to say:

 

Imagine standing in calm waist deep water next to your dinghy with its centerboard down.   As long as you are just standing next to it, the centerboard is exerting no force on the boat.  Now, try pushing the boat sideways - it resists moving sideways as the centerboard exerts a force counter-acting your pushing force.  The harder you push, the harder it pushes back, and if you stop pushing it stops pushing back.  This is somewhat similar to the classic example of the 10 pound brick sitting on a table - gravity pulls the brick down with a force of ten pounds and the table pushes the brick up with a "normal force" of ten pounds in response.  Turn off gravity and the table stops pushing, double the weight of the brick and the table pushes twice as hard.

 

Since we're dealing with a fluid rather than a solid object like a table, the use of the term "normal force" is not quite applicable, but the idea is the same.  Even when motionless the centerboard will exert a resistance force when you try to push the boat sideways.  This isn't lift, at least not according to the accepted definition of lift.

 

Now, imagine the boat moving forward through the water with the centerboard down and the sails trimmed.  The sails are pushing the boat sideways, the centerboard is exerting a force in the opposite direction, and since this force is roughly perpendicular to the fluid flow it is correct to call it lift.  The question is: how much stronger is this force  than what we got in the static case? I don't know the answer to that.  I'd be interested in reading more about it.



#86 Mr. Swordfish

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Posted 22 July 2013 - 07:05 PM

I don't know what dinghies you sail in, but I can easily measure leeway...regardless of what you may think FB said about it.

 

 

Ok, I just re-read the section in Bethwaithe's High Performance Sailing, and as you imply I remembered it wrong:

 

FB was trying to measure the leeway on a dinghy sailing close hauled with a gybing centerboard set to 3 degrees angle of attack.  In this particular scenario, the leeway was too small for him to measure.  For symmetric keelboats and the like, he indicates that their typical  leeway is about 3 degrees, and that dinghys with high-aspect centerboards seem to have less leeway in practice than they should in theory.  I'll try to transcribe the relevant passages so you can read his words rather than my recolletions of them.

 

FWIW, I sail a dinghy with a high aspect deep centerboard and feel like the leeway is barely perceptible when I'm sailing it right.  Of course, I can always pull up the centerboard or let the boat heel to generate quite perceptible (actually, impossible to ignore) leeway.  And when the centerboard stalls the boat slips to leeward and is slow to boot.

 

So, bottom line is that boats with non-gybing symmetrical centerboards make leeway, which gives the centerboard an angle of attack, which generates lift opposing the leeward motion, which limits the leeway.  The leeway is small, on the order of a couple of degrees depending on the boat and how it's sailed.



#87 fastyacht

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Posted 22 July 2013 - 08:42 PM

"normal force"?

 

???

 

 

Fair enough - I understand that what I wrote might be a bit confusing, and the concept of "normal force" is not really quite applicable.  Here's what I'm trying to say:

 

Imagine standing in calm waist deep water next to your dinghy with its centerboard down.   As long as you are just standing next to it, the centerboard is exerting no force on the boat.  Now, try pushing the boat sideways - it resists moving sideways as the centerboard exerts a force counter-acting your pushing force.  The harder you push, the harder it pushes back, and if you stop pushing it stops pushing back.  This is somewhat similar to the classic example of the 10 pound brick sitting on a table - gravity pulls the brick down with a force of ten pounds and the table pushes the brick up with a "normal force" of ten pounds in response.  Turn off gravity and the table stops pushing, double the weight of the brick and the table pushes twice as hard.

 

Since we're dealing with a fluid rather than a solid object like a table, the use of the term "normal force" is not quite applicable, but the idea is the same.  Even when motionless the centerboard will exert a resistance force when you try to push the boat sideways.  This isn't lift, at least not according to the accepted definition of lift.

 

Now, imagine the boat moving forward through the water with the centerboard down and the sails trimmed.  The sails are pushing the boat sideways, the centerboard is exerting a force in the opposite direction, and since this force is roughly perpendicular to the fluid flow it is correct to call it lift.  The question is: how much stronger is this force  than what we got in the static case? I don't know the answer to that.  I'd be interested in reading more about it.

 

the force generated is identical. otherwise your free-body diagram would fly off the sheet of paper never to return. The difference is the stedy-state velocity, but even that isn't important. If what you are wondering is whether you drift sideways faster without fwd motion, versus proper speed, all other (aero) equal, well, you can figure that out reasonably easily.



#88 Steam Flyer

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Posted 23 July 2013 - 12:22 AM

 

... ...

 

Since we're dealing with a fluid rather than a solid object like a table, the use of the term "normal force" is not quite applicable, but the idea is the same.  Even when motionless the centerboard will exert a resistance force when you try to push the boat sideways.  This isn't lift, at least not according to the accepted definition of lift.

 

Now, imagine the boat moving forward through the water with the centerboard down and the sails trimmed.  The sails are pushing the boat sideways, the centerboard is exerting a force in the opposite direction, and since this force is roughly perpendicular to the fluid flow it is correct to call it lift.  The question is: how much stronger is this force  than what we got in the static case? I don't know the answer to that.  I'd be interested in reading more about it.

 

the force generated is identical. otherwise your free-body diagram would fly off the sheet of paper never to return. The difference is the stedy-state velocity, but even that isn't important. If what you are wondering is whether you drift sideways faster without fwd motion, versus proper speed, all other (aero) equal, well, you can figure that out reasonably easily.

 

Yah I think so... considering that pretty much any boat including the Niña, the Pinta, and the Santa Maria, will make less leeway the faster it's going, should be easy to figure out.

 

Whether or not the resistance to side force when the boat is moving straight sideways should be called "lift" well that's another story. My understanding is that "lift" is always perpendicular to fluid flow. But no point in quibbling over semantics.

 

FB- Doug



#89 fastyacht

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Posted 23 July 2013 - 12:34 AM

. "My understanding is that "lift" is always perpendicular to fluid flow. But no point in quibbling over semantics."

 

You are correct, because that is exactly what it is defined to be. You ain't quibbling.



#90 RobG

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Posted 23 July 2013 - 03:16 AM

. "My understanding is that "lift" is always perpendicular to fluid flow. But no point in quibbling over semantics."

 

You are correct, because that is exactly what it is defined to be. You ain't quibbling.

Sure, but I think most people think of lift as a force resulting entirely from flow. The force resulting from pushing a board sideways through the water is mostly drag, i.e. the force opposite to thrust.



#91 BalticBandit

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Posted 23 July 2013 - 05:58 AM

And now Rob you start to see where the "Newtonian Deflection" approach to what generates lift is so fatally flawed.  Swordfish's assertion about how it is the "lateral push" through the water is demonstrably wrong as my example of how skiffs behave indicates (note that if you go look at the threads in which JB talks about the benefits of a self-tacking jib, the same point is made albeit differently - lift from a moving board is dramatically higher than lift from a stalled board)

 

Now in reality both are a form of "lift" though the "lift" from a lateral "drag" is occurring only on a small part of the surface (LE and TEs and a little bit "in" from each)  but separating them into "draggy" vs. Lifted behaviours is right.

 

Now if the "deflected flow" model was accurate then there would be no sudden transition where the blades "catch" (ie move from stalled, separated flow, to attached turbulent flow) - and yet there is.  Why?  because if "deflected flow" is what is generating the primary component of lift, then whether or not there is attached or unattached flow on the upper side of the foil has no bearing on the linear increase in the NET Kinetic Energy transferred by the elastic collisions on the lower side.

 

So Swordfish's notion that somehow it is the net energy transfer from these collisions simply fails to explain all the known observable phenomenon.   QED it is the WRONG MODEL



#92 Mr. Swordfish

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Posted 23 July 2013 - 01:49 PM

. "My understanding is that "lift" is always perpendicular to fluid flow. But no point in quibbling over semantics."

 

You are correct, because that is exactly what it is defined to be. You ain't quibbling.

Sure, but I think most people think of lift as a force resulting entirely from flow. The force resulting from pushing a board sideways through the water is mostly drag, i.e. the force opposite to thrust.

 

 

Well, I think a common misunderstanding is that lift and drag are distinct physical phenomena.  They're not - they're just components of the same thing.  Here's how it works:

 

When a fluid flows past a solid object it exerts a net force on it.  This force is a vector, so it can be resolved into components depending on how one chooses the co-ordinate system.  Engineers designing airplane wings usually choose the co-ordinate system with an axis parallel to the flow and another perpendicular to the flow, and call the components of the aerodynamic force "lift" and "drag".  But there's no reason why on has to choose that particular coordinate system, so lift and drag are just artifacts of the choice of coordinate system.

 

When it comes to sailboats, it's more convenient to choose the coordinate system to be parallel/perpendicular to the centerline of the boat.  Some treatments use this convention, and then there's no need to talk of lift and drag at all.



#93 fastyacht

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Posted 23 July 2013 - 02:20 PM

 

. "My understanding is that "lift" is always perpendicular to fluid flow. But no point in quibbling over semantics."

 

You are correct, because that is exactly what it is defined to be. You ain't quibbling.

Sure, but I think most people think of lift as a force resulting entirely from flow. The force resulting from pushing a board sideways through the water is mostly drag, i.e. the force opposite to thrust.

 

 

Well, I think a common misunderstanding is that lift and drag are distinct physical phenomena.  They're not - they're just components of the same thing.  Here's how it works:

 

When a fluid flows past a solid object it exerts a net force on it.  This force is a vector, so it can be resolved into components depending on how one chooses the co-ordinate system.  Engineers designing airplane wings usually choose the co-ordinate system with an axis parallel to the flow and another perpendicular to the flow, and call the components of the aerodynamic force "lift" and "drag".  But there's no reason why on has to choose that particular coordinate system, so lift and drag are just artifacts of the choice of coordinate system.

 

When it comes to sailboats, it's more convenient to choose the coordinate system to be parallel/perpendicular to the centerline of the boat.  Some treatments use this convention, and then there's no need to talk of lift and drag at all.

 

+1

 

Look at Marchaj's old books. If I remember correctly he shows resolution on both coordinate systems in at least one sketch.

Indeed, there is only one true force--and as you say, lift is the normal to the flow component of it.

In Rob's example, you have zero lift and 100% drag.



#94 Mr. Swordfish

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Posted 23 July 2013 - 02:24 PM

So Swordfish's notion that somehow it is the net energy transfer from these collisions simply fails to explain all the known observable phenomenon.   QED it is the WRONG MODEL

 

 

I'm not going to spend much energy arguing with you,  but it appears that you are under the mistaken impression that I am advocating what NASA calls "Incorrect theory of lift #2"

 

This is sometimes mistakenly referred to as the "Newtonian" theory of lift, but it's easily shown to be false and serves as the basis for many  strawman arguments.  See http://www.grc.nasa....ane/wrong2.html for more detail.

 

wrong2.gif

   NASA also presents a  correct Newtonian theory of flow turning..

 

 

 

newton3.gif

 

Perhaps you'd like to contact NASA and tell them that their theories of aerodynamics are wrong.  I'm sure that they would appreciate hearing from you.



#95 BalticBandit

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Posted 23 July 2013 - 03:57 PM

you keep mixing and matching explanations.   Your claim about how there wasn't going to be much difference between simply pushing a hull laterally and the same amount of normal force generated once flow began is EXACTLY the "incorrect theory #2".... and now you are saying that's not what you were talking about.  aiyayyai,,,

 

As for drag vs lift.  FY that's not altogether accurate either, because you have more than one form of drag and the vector directions are different.  yes all the force vectors sum to a simple single vector, but you can't get the direction of that vector correct unless you separate out the surface flow drag, the induced drag from the lift as well as the lift "direction"



#96 fastyacht

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Posted 23 July 2013 - 04:08 PM

you keep mixing and matching explanations.   Your claim about how there wasn't going to be much difference between simply pushing a hull laterally and the same amount of normal force generated once flow began is EXACTLY the "incorrect theory #2".... and now you are saying that's not what you were talking about.  aiyayyai,,,

 

As for drag vs lift.  FY that's not altogether accurate either, because you have more than one form of drag and the vector directions are different.  yes all the force vectors sum to a simple single vector, but you can't get the direction of that vector correct unless you separate out the surface flow drag, the induced drag from the lift as well as the lift "direction"

No, wrong.

 

You measure the total force vector. From that, you simply resolve to two axes. This is the "flying kite" method.  Or, on a fixed stand, you read the load cell ouput and once again, simply solve for the total force vector and any axes you choose.

 

"Surface flow drag" versus "induced drag" is a scaling issue somewhat analogous to separating out frictional and residuary resistance in model tests. This has absolutely nothing to do with the topic of this thread, nor with how lift works per se.



#97 BalticBandit

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Posted 23 July 2013 - 07:33 PM

Sure, but to understand how that changes with shape (chord, thickness, max thickness point) you have to understand all of the components.  The "Flying Kite" is fine if you don't need to change the parameters of the kite

 

and while surface drag is not related to lift parasitic or induced drag does.



#98 fastyacht

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Posted 24 July 2013 - 01:17 PM

Sure, but to understand how that changes with shape (chord, thickness, max thickness point) you have to understand all of the components.  The "Flying Kite" is fine if you don't need to change the parameters of the kite

 

and while surface drag is not related to lift parasitic or induced drag does.

 

Nope. Induced drag is a result of lift. Parasitic is not. We can't measure either of these drag cvomponents individually--but the *definition* of induced drag is drag due to lift, whereas parasitic drag is drag due to interactions.  As with the model test situation, we have to be clever, we have to calculate an estimate of one or the other, if possible based on an additional experiment, and subtract that from the total drag, to get the other.  The skin friction portion of parasitic drag is calculated from surface area and reynolds number and form factor--and subtracted. Parasitic interaction is calculated from strut angle and size etc from other experiments, and subtracted. That leaves you with induced drag.

 

Why are we discussing this again?



#99 Doug Lord

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Posted 24 July 2013 - 02:27 PM

Sure, but to understand how that changes with shape (chord, thickness, max thickness point) you have to understand all of the components.  The "Flying Kite" is fine if you don't need to change the parameters of the kite

 

and while surface drag is not related to lift parasitic or induced drag does.

 

Nope. Induced drag is a result of lift. Parasitic is not. We can't measure either of these drag cvomponents individually--but the *definition* of induced drag is drag due to lift, whereas parasitic drag is drag due to interactions.  As with the model test situation, we have to be clever, we have to calculate an estimate of one or the other, if possible based on an additional experiment, and subtract that from the total drag, to get the other.  The skin friction portion of parasitic drag is calculated from surface area and reynolds number and form factor--and subtracted. Parasitic interaction is calculated from strut angle and size etc from other experiments, and subtracted. That leaves you with induced drag.

 

Why are we discussing this again?

 

Because beebie doesn't have a clue what he's talking about or even what this thread was about........



#100 Amati

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Posted 25 July 2013 - 02:44 AM

At this point, I feel it is necessary to, for the umpteenth time


http://www.modelrese...er_airfoils.htm

Enjoy-




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