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#1 Captain Bob

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Posted 24 April 2006 - 09:21 PM

Square top mains seems to be the hottest stuff at the moment. But what are the main advantages apart from more sailarea. I looked around at the internet but I couldnīt find anything. This place is usually a good place to get more informed or more confused.

#2 Great Red Shark

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Posted 24 April 2006 - 09:25 PM

A tri-angular sail comes to a point on top - not a very effective shape for lots of reasons. Square-tops can produce a higher aspect-ratio (think airplane wings).

The new sqare-tops on the I-14s have luff & leaches that are parallel for 80%+ of the hoist ! Amazing.

Why was this not done earlier ?

Material advances.

Just like somebody could have made a boat like the Melges24 back when the J-24 was designed, but it would have wieghed 2,500 lbs.

Actually, they did have square-tops way back when, they were called gaff-rigs.

#3 Large Thomas

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Posted 24 April 2006 - 09:30 PM

Square top mains seems to be the hottest stuff at the moment. But what are the main advantages apart from more sailarea. I looked around at the internet but I couldnīt find anything. This place is usually a good place to get more informed or more confused.

The twist. Better control. Changed sail's center of gravity.
I'll have a look to find something more precise...

#4 Tyler Durden

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Posted 24 April 2006 - 09:37 PM

Compared to a 'pinhead' main of the same area, the fathead has a lower CoE, and the top part of the sail is more efficient. The fathead also can be given more twist for depowering and general sail shaping.

I think the real reason these sails are popular on older boats is that they can be retrofitted much more cheaply than a taller mast. So now you have the advantages in trimming and depowering, as well as more sail area up and down wind, without replacing the rig.

#5 MoMP

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Posted 24 April 2006 - 09:53 PM

Agree with what's posted above. Let me go further with the question.

What's more efficient, a squarehead main or a (for lack of a better term) fathead main.

I went with an elliptical fathead over a sharp squarehead on my tri for the same reasons that an elliptical daggerboard is more efficient than a square daggerboard.

Can anyone speak to this?

#6 Great Red Shark

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Posted 24 April 2006 - 10:06 PM

an elliptical daggerboard is more efficient than a square daggerboard



I don't know that this is an absolute.

Perhaps it's much easier to produce a low-drag dagger on an elipse, what are the latest foils on the 14s ?

( I really don't know, that's not a rhetorical question)

I see the foils on Rutan's birds are still squared off.

At least in my expereince the benefits of the fat-heads ( more SA, more effective, better twist dymanic) were enjoyed further in square-top designs, but they are not without complications ( backstays are a big problem, for example)

#7 Captain Jack Sparrow

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Posted 24 April 2006 - 10:13 PM

Its more effective sail area. On the 12 Metres it was found that they could cut off the top 15% of the triangular mainsail and there was nearly zero loss in performance. The bendy masts on the British challenger (Lionheart?) and Australia I in 1980 followed this discovery.

#8 DDW

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Posted 24 April 2006 - 10:55 PM

Compared to a 'pinhead' main of the same area, the fathead has a lower CoE, and the top part of the sail is more efficient. The fathead also can be given more twist for depowering and general sail shaping.

I think the real reason these sails are popular on older boats is that they can be retrofitted much more cheaply than a taller mast. So now you have the advantages in trimming and depowering, as well as more sail area up and down wind, without replacing the rig.

Huh???

Compared to a conventional main, a squarehead will most likely have a HIGHER center of effort if both are properly trimmed. Furthermore, you will typically want LESS twist in the squarehead, because a more favorable downwash distribution makes tip stall less of a problem.

The answer to the question is that a squarehead, fathead, or elliptical sail are all attempting to do the same thing, and that is minimize the drag penalty paid for developing lift. Long ago, it was shown that if the downwash from an airfoil was distributed (spanwise) elliptically, then drag was minimized (also the airfoil will stall all at the same time over its span, rather than tip first). The easiest way to do this is with an elliptical planform, if the airfoil has no twist. A tapered, square headed planform comes very close to the ideal, while a triangular planform departs a long way from the ideal. Of course, like everything else on a sailboat, it is made far more complex by the presence of wind gradient, unavoidable sail twist, hull stability limitations, heel angle, the jib, the boom-deck gap, to name just a few of the problems.

#9 Tyler Durden

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Posted 24 April 2006 - 10:59 PM

Squarehead or pinhead on same mast, squarehead has higher CoE.

Squarehead of pinhead OF SAME AREA, ie. pinhead on taller mast, the squarehead has lower CoE. at the very least it is no higher.

#10 amolitor

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Posted 24 April 2006 - 11:09 PM

Squarehead or pinhead on same mast, squarehead has higher CoE.

Squarehead of pinhead OF SAME AREA, ie. pinhead on taller mast, the squarehead has lower CoE. at the very least it is no higher.


Pretty sure you're STILL wrong, bucky. Keeping the area the same, chop the top 10% of the triangle off and paste it on the upper leech of what's left to make a fathead. CoE goes down, not up.

EDIT: My bad, My 'correction' was simply repeating with TD said. Oops. Got so caught up in thinking he was wrong I reversed it myself. TD's right, and I am wrong.

#11 kschultz419

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Posted 24 April 2006 - 11:22 PM

amolitor I don't see the sense in that at all. If you have the same SA and are moving some of it further up the lever arm, how can the center of effort be CLOSER to the pivot point than when you started? I see that when you take the two types on the same mast, the COE is higher on the fatty, but if we're talking SA constant and shape variable, with the fathead you are compressing the height so the COE has to move down.

#12 hoom

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Posted 24 April 2006 - 11:33 PM

Not if its a short boom square top compared to a long boom triangle.

Why was this not done earlier ?

Materials is only partly true.
Rules played the larger part.
In classes where fatheads have been legal ie skiffs they were initially tried in the 80s or possibly earlier.

#13 BIAM

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Posted 25 April 2006 - 12:14 AM

another big benefit is in light air....

in the really light stuff there is usually more pressure at the top of the sail than down on the water.....the additional foil higher up really helps out.....

#14 in_TO

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Posted 25 April 2006 - 12:44 AM

This may be helpful

http://onedesign.com...article6-1.html

Fathead mains are alluded to briefly on page two.

#15 hdglightning

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Posted 25 April 2006 - 03:12 AM

Fat head sails are simply going for more sail area. In simple terms, you create lift (force) based on the area of the wing. The airfoil itself only "reduces" lift or increases drag (from 0). So in an ideal world with no mast restriction we would have an elliptical (kind of) planform with a nice wing mast (rotating of course) with the amount of SA that perfectly balances out the righting moment of the specific wind and situation.

Of course that does not happen.

So...for most classes there is a restriction on something, normally mast height (14's for example). By adding sail area they generate more driving force. But this also raises the total drag as generating more lift generates more drag. Offsetting this drag is that fact that they can reduce camber, which loses some lift but greatly reduces drag. Add in better twist profile at the upper section (wider chord length up high makes for a better twist profile) and the overall greater sail area...and you get more lift with very little increased drag. Win win in my book.

As to square top vs pinhead. Generally the ideal is a elliptical planform. Check out the 18's to see what that looks like. They are very efficient vs even square tops, though that is ignoring the mast's influence. A square top actually generates more drag due to tip vortex issues, the lack of taper creates that. But the gains again are greater than the losses...making it better overall.

Downwind...when barndooring it, SA wins. When reaching/tacking downwind, again the ability to minimize drag is ideal, and the twist capability of the square heads win again (less drag and more driving force).

Bad thing about square tops is that while they are better overall...it also can be much worse if done wrong. The drag goes way up, and offsets all of the increased lift. So the boat heels like mad, but goes slower.

As to Rutan and airplanes with square wings. Even Burt's high tech stuff has "limits" on wingspan and structural issues. The goal is to make the stuff as close to perfect as possible, again balancing the lift vs the drag penality for the tip losses. It is not an easy calculation!

#16 DDW

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Posted 25 April 2006 - 05:20 AM

A square top actually generates more drag due to tip vortex issues, the lack of taper creates that.

Huh??? again.

Even the most radical square tops have a taper ratio (tip chord/foot chord) of 1/3 or so. This is close to the ideal taper ratio for a tapered (untwisted) wing. Compared to a pointy tip, you are reducing drag due to tip vortex issues, not increasing it. Efficient wings don't have pointed tips. I am ignoring near sonic and supersonic wings, which are an entirely different animal. High performance glider wings, for example, have squared off tips. On the other hand a nice extreme elliptical tip as seen on some dinghies and a square top are aerodynamically very similar and between these it will be the smaller details, not the difference in planform, that differentiate their performance.

#17 Alaris

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Posted 25 April 2006 - 05:27 AM

How similar are the foil dynamics of sails vs. airplane wings and keels? Because you don't see many elliptical or triangular keels and you will not see either on any modern airplane.

#18 Christian

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Posted 25 April 2006 - 05:35 AM

another big benefit is in light air....

in the really light stuff there is usually more pressure at the top of the sail than down on the water.....the additional foil higher up really helps out.....





Ding-ding-ding - we have a winner! (in addition to a more efficient wing shape)

#19 DDW

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Posted 25 April 2006 - 05:40 AM

How similar are the foil dynamics of sails vs. airplane wings and keels? Because you don't see many elliptical or triangular keels and you will not see either on any modern airplane.

On a modern airplane: triangular - no; elliptical - yes, on anything efficient.

#20 Alaris

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Posted 25 April 2006 - 06:18 AM


How similar are the foil dynamics of sails vs. airplane wings and keels? Because you don't see many elliptical or triangular keels and you will not see either on any modern airplane.

On a modern airplane: triangular - no; elliptical - yes, on anything efficient.


The most popular general aviation airplane in the world:
Posted Image

Arguably the most efficient type of winged aircraft, by order of necessity, a Schweizer sailplane:
Posted Image

Where are the elliptical wings? These are modern and successful designs. See also any Mooney, Beech Bonanza, Cirrus, Lear, Cessna, Piper, etc. The elliptical wing tip went out of favor with the biplane.

#21 kiwi4shore

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Posted 25 April 2006 - 06:20 AM

On my boat,Dale7.6,one off design for light air,so large sail area,I have a problem with a really heavy helm that occurs quickly when the wind increases only a small amount.I know I can slap a reef in but with lake sailing I would be constantly reefing,unreefing,a real pain.
So back on topic,not wanting to reduce sail area would a shorter boom and a square top sail,Lost area at boom added to top of sail cure the helm problem or will it just open a pandoras box of other problems?

#22 Per

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Posted 25 April 2006 - 07:23 AM

The key in the elliptical wing debate is the fact that it the wing loading and not the planeform that should be elliptical. A square ended wing can still have a (close to) elliptical wing loading.

#23 macca

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Posted 25 April 2006 - 07:30 AM

Square top mains seems to be the hottest stuff at the moment. But what are the main advantages apart from more sailarea. I looked around at the internet but I couldnīt find anything. This place is usually a good place to get more informed or more confused.



We have had Square tops on cats for 20 years, so it took you monohull punters a long time to catch on!!

#24 Sumner

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Posted 25 April 2006 - 08:58 AM


A square top actually generates more drag due to tip vortex issues, the lack of taper creates that.

Huh??? again.

Even the most radical square tops have a taper ratio (tip chord/foot chord) of 1/3 or so. This is close to the ideal taper ratio for a tapered (untwisted) wing. Compared to a pointy tip, you are reducing drag due to tip vortex issues, not increasing it. Efficient wings don't have pointed tips. I am ignoring near sonic and supersonic wings, which are an entirely different animal. High performance glider wings, for example, have squared off tips. On the other hand a nice extreme elliptical tip as seen on some dinghies and a square top are aerodynamically very similar and between these it will be the smaller details, not the difference in planform, that differentiate their performance.


What DDW said. At the top of a mainsail, air moves from the high-pressure side to the low-pressure side, creating a vortex that causes drag. The same thing happens with keels. Wings and bulbs on the bottom of keels help to reduce this effect. This is the same reason there are winglets on the wing tips of many aircraft. For sails, the shape that best minimizes the formation of a vortex at the top is tall and narrow ending in an ellipse-like shape (such as those on AC boats).

EDIT: Here's an example (nod to Valencia Sailing):

#25 BH Junky

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Posted 25 April 2006 - 09:13 AM

Why no elliptical wings? As mentioned wing loading, and ease of build. Elliptical wings are a nightmare to build (Spitfire) compared to trapezoid wings (P-52). The pay off is rarely worth it, especially in powered a/c. Plenty of gliders (sailplanes for the lot across the pond) from the 50's had (near) elliptical wings. If you actually look at the trailing edge of the Cessna pictured you'll notice the trailing edge kinks in, modelling an elliptical plan form.

By sticking winglets on wings and wings on keels the aerodynamics / hydrodynamics start working towards the theoretical ideal elliptical shape again. Tip vortices are extremely complex and have been keeping alot of people very busy.

On of the prctical issues with any fat head main is the enormous compression loads on the battens, something that only in the last few years has been effectively tackled wth new materials and design techniques.

Some of the reasons square tops have not come through thick and fast are simply practical:
- backstays. Even a whip on a fractional rig can't clear a proper fathead. One would have to set up a dual backstay system.
- Storage. Look at the pics of the VO70's. When parked the top (square) section of the main is still upright. Not very practical for yacht owners.
- Cost. The type of battens and baten cars used are top end and the mast will need a track rather than the more common luff cord. Retrofitting is prohibtive for Joe Six Pack.

#26 Presuming Ed

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Posted 25 April 2006 - 09:26 AM

One successful aircraft built with an elipitical wing planform...

Posted Image

#27 BH Junky

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Posted 25 April 2006 - 09:40 AM

Sex in the sky!! We see one flying around Northumberland every now and then; the SOUND is just the best thing ever.

#28 Sumner

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Posted 25 April 2006 - 09:47 AM

Sex in the sky!! We see one flying around Northumberland every now and then; the SOUND is just the best thing ever.


Yeah, very cool. Saw one flying over Lymington last summer. Makes you wonder what an entire squadron of them must have sounded like.

#29 mustang__1

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Posted 25 April 2006 - 12:32 PM


Sex in the sky!! We see one flying around Northumberland every now and then; the SOUND is just the best thing ever.


Yeah, very cool. Saw one flying over Lymington last summer. Makes you wonder what an entire squadron of them must have sounded like.


come to oshkosh and see the next best thing. they usualy throw up a few mustangs. i need to go back, its been a long time since i was there...

#30 MoMP

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Posted 25 April 2006 - 01:12 PM

Square top main? is this supposed to new? I think it called a gaffer? Ok the new version is minus the gaff, but you get my point. Stick a carbon rod in the top add a peak halliard and bingo, its a gaffer.

It all goes arround and arround. Like prods, can any one explain to me the difference between a prod and a bowsprit. Kind of the same from where I am standing. Both thick out bow or should I call it something new like "wave cleaver".

Any way you get my point.

Chow


You're right about technologies coming around again and again. Sailing is a very mature sport. The differences with today's squareheads and the gaffs of years past is the building materials, design power and year's of experiences tweaking out an additional few knots per hour.

Last generation's building material: Timber
Today's: Carbon Fiber, Nomax, resins, etc.

Last generation's sails: Sailmaker's eye, hemp, cotton.
Today's: Kevlar, Cuban fiber, CAD design, etc.

#31 Alaris

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Posted 25 April 2006 - 01:51 PM

One successful aircraft built with an elipitical wing planform...

Posted Image


That is 65 year old technology. Just because it was done doesn't mean it was the best option.

#32 DDW

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Posted 25 April 2006 - 02:57 PM



How similar are the foil dynamics of sails vs. airplane wings and keels? Because you don't see many elliptical or triangular keels and you will not see either on any modern airplane.

On a modern airplane: triangular - no; elliptical - yes, on anything efficient.


The most popular general aviation airplane in the world:
Posted Image

Arguably the most efficient type of winged aircraft, by order of necessity, a Schweizer sailplane:
Posted Image

Where are the elliptical wings? These are modern and successful designs. See also any Mooney, Beech Bonanza, Cirrus, Lear, Cessna, Piper, etc. The elliptical wing tip went out of favor with the biplane.

I did say "anything efficient". You have picked designs that are 50 years old. Cessna doesn't care how efficient their planes are (because they cruise at low lift coefficients, the induced drag is secondary), and Schweizer is hardly "the most efficient type of winged aircraft". Schweizer hasn't made sailplanes in decades, and was uncompetitive decades prior to that. These aircraft were built that way because sheet metal breaks are straight.

Every modern composite racing sailplane has an approximately elliptical planform. Some use a double taper wing, some a symitar wing (curved leading edge). Look at a modern manufacturer like Schleicher.
Attached File  ASH26toplow.jpg   103.06K   46 downloads
Note the double tapered planform.

#33 DDW

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Posted 25 April 2006 - 04:14 PM

And here is a modern glider wingtip:
Attached File  Wingtip.jpg   100.71K   36 downloads
Note the obvious break in the leading edge where the second taper starts. The winglets are very difficult to design such that they add performance, many actually cost performance. You can see many other sailplanes in the background with similar wing planforms. The performance of these is almost double what that old metal Schweizer will do.

#34 Breitling

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Posted 25 April 2006 - 04:38 PM

(Cessna & Schweizer images)

Where are the elliptical wings? These are modern and successful designs. See also any Mooney, Beech Bonanza, Cirrus, Lear, Cessna, Piper, etc. The elliptical wing tip went out of favor with the biplane.

No, not at all. Neither Cessna nor Schweizer are modern designs. Cessna don't use elliptical wings cos wing efficiency is not their goal. Modern glider designs are nearest to an elliptical wing than a trapezoidal one.

Attached File  nimbus_4.jpg   10.15K   30 downloads

Wingtip vortex is a great induced drag source. It is caused by the air flowing around the wingtip from the high pressure area below the wing to the low pressure area above it. If you reduce wing chord to the minimum at the wingtip, you're managing to reduce induced drag as well. Elliptical wings has a near zero wing chord at the wingtip, that's why they don't produce induced drag. Winglets has the same purpose: to minimize airflow around the wingtip.

The problem with elliptical wings is their building complexity. It is easier and cheaper to build a multi-tappered wing, which obtains similar performance.

#35 DDW

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Posted 25 April 2006 - 06:37 PM

Wingtip vortex is a great induced drag source. It is caused by the air flowing around the wingtip from the high pressure area below the wing to the low pressure area above it. If you reduce wing chord to the minimum at the wingtip, you're managing to reduce induced drag as well. Elliptical wings has a near zero wing chord at the wingtip, that's why they don't produce induced drag. Winglets has the same purpose: to minimize airflow around the wingtip.

The wingtip vortex is the ONLY source of induced drag. A wing with no tips (infinite span) has no induced drag. Elliptical wings still produce induced drag, but the minimum possible with a given aspect ratio. It was shown by Monk in the '30s that to produce minimum induced drag, you needed an elliptical distribution of downwash velocity, and the easiest way to insure that was with an elliptical planform. Note that a triangular planform also reduces the chord to zero at the tip, but has a nasty downwash distribution and lots of induced drag. The Ventus C in your picture has triple taper and also polyhedral, the latter is partly for induced drag control, but mostly to improve handling at low speeds.

#36 P_Wop

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Posted 25 April 2006 - 10:10 PM

Interesting that the Spitfire elliptical wing was eventually cropped at the tip to produce the truncated elliptical shape as shown here in the XIV-E. Lots of reasons for this. But the elliptical trailing edge was still judged best.

I agree with others above - the noise of the RR Merlin is unique and wonderful - I remember overflights of Spitfires over our house in the Isle of Wight during the D-Day 50th anniversary. My late father had been a young RAAF Spitfire squadron medic, and on late nights in '43 with 3 men missing after a sortie, he'd walk to the end of the airfield in the dusk and pray for that sound - it was the noise that he most wanted to hear.

Working together in the garden one day in that summer of '94, his head went up. "Spitfire,' he whispered. I could hear absolutely nothing. Then at last I heard it, and it grew, and a Mark VI with D-Day stripes flew 50 feet over our trees, waggling wings as he did. Magical moment.

Attached Files



#37 PNWGuy

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Posted 25 April 2006 - 10:33 PM

Spitfire!

http://www.alexispar...m/oh_my_god.htm

#38 BH Junky

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Posted 26 April 2006 - 07:18 AM

And here is a modern glider wingtip:
Attached File  Wingtip.jpg   100.71K   36 downloads
Note the obvious break in the leading edge where the second taper starts. The winglets are very difficult to design such that they add performance, many actually cost performance. You can see many other sailplanes in the background with similar wing planforms. The performance of these is almost double what that old metal Schweizer will do.


DDW, yours? Nice toy! Used to fly a Janus C and a DG500 fully aero at our club, long time ago. It was frowned upon, but the best ride in the Janus when there were no thermasl, or last flight, was: flaps up, dive down from 300m, fly by next to the motorway at about 15 - 30 ft doing 270 km/hr (avoiding lamp posts), the guy in the back waving at the motorists. Pull up, regain at least 200 m and go to circuit.

#39 atwinda

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Posted 26 April 2006 - 03:31 PM

The challange for aquear heads is to have the sail keep it's leech profile until the wind builds enough that you want the top of the sail to twist off, at which point the huge square head does a mighty fine job of it. Molded sails have the advantage of designed shape into the cloth and will hold their leech profiles into much higher wind ranges. (see that new i-14 main).

#40 B30

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Posted 26 April 2006 - 04:49 PM

[/quote]
The wingtip vortex is the ONLY source of induced drag. A wing with no tips (infinite span) has no induced drag. [/quote]
DDW, I never studied fluid dynamics, but what you said above seems very important even if I don't understand it. Could you explain this a bit more?

#41 DDW

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Posted 26 April 2006 - 05:01 PM

And here is a modern glider wingtip:
Attached File  Wingtip.jpg   100.71K   36 downloads
Note the obvious break in the leading edge where the second taper starts. The winglets are very difficult to design such that they add performance, many actually cost performance. You can see many other sailplanes in the background with similar wing planforms. The performance of these is almost double what that old metal Schweizer will do.


DDW, yours? Nice toy! Used to fly a Janus C and a DG500 fully aero at our club, long time ago. It was frowned upon, but the best ride in the Janus when there were no thermasl, or last flight, was: flaps up, dive down from 300m, fly by next to the motorway at about 15 - 30 ft doing 270 km/hr (avoiding lamp posts), the guy in the back waving at the motorists. Pull up, regain at least 200 m and go to circuit.

Yes, the ASH26E in the foreground is mine, as was the Duo Discus 5H (with partners) in the background, when the picture was taken. I sold the Duo since, having already made all of my friends sick. The ASH is amazing, 50:1 glide ratio, still better than 30:1 at over 100 knots. High speed passes are frowned upon at one gliderport I frequent, but encouraged at another...

#42 DDW

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Posted 26 April 2006 - 05:42 PM


The wingtip vortex is the ONLY source of induced drag. A wing with no tips (infinite span) has no induced drag.

DDW, I never studied fluid dynamics, but what you said above seems very important even if I don't understand it. Could you explain this a bit more?

This is all explained in any elementary book on aerodynamics. Drag from wing sections is divided for convenience of description, calculation, and measurement into two forms: profile drag and induced drag. Profile drag is produced by air pressure on the wing and is dependent only on section shape and airspeed (at least in a simple view), and is independent of the planform and the lift produced. Induced drag is the drag caused by the production of lift and is entirely due to the pressure loss at the tip and the resulting vortex. In a wind tunnel, if you make the planform span from wall-to-wall, you eliminate the pressure loss around the tip (it is sealed by the wall) and the airfoil behaves as though it had infinite span, so there is no induced drag component (again ignoring secondary effects). This is how profile drag is measured. For an airfoil with a finite span, the drag penalty paid for lift is proportional to the lift coefficient squared divided by the aspect ratio, for an elliptical wing. For a non-elliptical wind, you must add factors to account for the non-ideal downwash distribution. Thus my comment about Cessnas and in fact most power planes - in cruising flight the wing operates at low lift coefficients, therefore induced drag tends to be low no matter what the planform is. Contrast that to gliders, which often operate at high lift coefficients (in thermalling and slow cruising flight). That is why gliders are always designed with the highest aspect ratio possible under structural and rule constraints. For example my 18M glider has a span of 59.2 ft, and a root chord of about 2.5 ft. with a double tapered planform (tip chord is about 8 in).

How important is induced drag in sailing? I think very important, particularly for pointing ability. It can be mathematically shown (Lancaster's theorem) that the most efficient AWA for a boat on the wind is the numerical sum of the hydrodynamic drag angle and the aerodynamic drag angle. These angles are the arctan of the drag to lift ratio. Probably 1/2 the drag is induced drag (aerodynamically), hydrodynamically it is greatly dependent on boat speed: at higher speeds, hull drag predominates, at lower speeds induced drag becomes a greater percentage.

Interesting stuff, at least to me, and not particularly well studied on a sailboat.

#43 B30

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Posted 26 April 2006 - 05:56 PM



The wingtip vortex is the ONLY source of induced drag. A wing with no tips (infinite span) has no induced drag.

DDW, I never studied fluid dynamics, but what you said above seems very important even if I don't understand it. Could you explain this a bit more?

This is all explained in any elementary book on aerodynamics. Drag from wing sections is divided for convenience of description, calculation, and measurement into two forms: profile drag and induced drag. Profile drag is produced by air pressure on the wing and is dependent only on section shape and airspeed (at least in a simple view), and is independent of the planform and the lift produced. Induced drag is the drag caused by the production of lift and is entirely due to the pressure loss at the tip and the resulting vortex. In a wind tunnel, if you make the planform span from wall-to-wall, you eliminate the pressure loss around the tip (it is sealed by the wall) and the airfoil behaves as though it had infinite span, so there is no induced drag component (again ignoring secondary effects). This is how profile drag is measured. For an airfoil with a finite span, the drag penalty paid for lift is proportional to the lift coefficient squared divided by the aspect ratio, for an elliptical wing. For a non-elliptical wind, you must add factors to account for the non-ideal downwash distribution. Thus my comment about Cessnas and in fact most power planes - in cruising flight the wing operates at low lift coefficients, therefore induced drag tends to be low no matter what the planform is. Contrast that to gliders, which often operate at high lift coefficients (in thermalling and slow cruising flight). That is why gliders are always designed with the highest aspect ratio possible under structural and rule constraints. For example my 18M glider has a span of 59.2 ft, and a root chord of about 2.5 ft. with a double tapered planform (tip chord is about 8 in).

How important is induced drag in sailing? I think very important, particularly for pointing ability. It can be mathematically shown (Lancaster's theorem) that the most efficient AWA for a boat on the wind is the numerical sum of the hydrodynamic drag angle and the aerodynamic drag angle. These angles are the arctan of the drag to lift ratio. Probably 1/2 the drag is induced drag (aerodynamically), hydrodynamically it is greatly dependent on boat speed: at higher speeds, hull drag predominates, at lower speeds induced drag becomes a greater percentage.

Interesting stuff, at least to me, and not particularly well studied on a sailboat.

Thanks. I am not going to pretend I understand all of what you wrote, but it does have my attention. For someone like myself with little (no) background in fluids, what books would you recomend to begin to learn about this more?

#44 Gone To Plaid

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Posted 26 April 2006 - 08:35 PM

Thanks. I am not going to pretend I understand all of what you wrote, but it does have my attention. For someone like myself with little (no) background in fluids, what books would you recomend to begin to learn about this more?


For a pure investigation of fluid dynamics as it pertains to sailing:
Aero-Hydrodynamics of Sailing, C A Marchaj, 1979
I think he may have just re-written & released a new edition quite recently, however, so much of the book is pure theory that very little changed.

For a more practical look at application in the real world:
Sail Performance: Techniques to Maximise Sail Power, C A Marchaj, 1990
In this book he speaks more as (and to) a layman, and looks at practical examples of sails, and applies theory to them, as opposed to the other way around, as from the previous book.

For a look at the point of view of a sailboat experimenter on the cuting edge:
High Performance Sailing, Frank Bethwaite, 1993
He delevs into much more than just fluid dynamics here, talking about wind, current, and mostly, his lifelong study/experiments with skiff type boats.

Other references:
Theory of Wing Sections, Abbott & Doenhoff, 1959 (1st ed. 1949)
Study/Textbook, includes tested data for many NACA foil sections.

Foundations of Aerodynamics: Bases of Aerodynamic Design, Kuethe & Chow, 1998 (1st ed. 1905)
Theoretical textbook

#45 Lease

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Posted 26 April 2006 - 10:51 PM

Begs the question, since knowledge of aerodynamics appears to be in attendance, would then a squaretop benefit from an endplate?

For that matter, would tip losses be mittigated by a wing fence at some point along the span, if such a thing could be moulded into the sail/batten?

Likely, the answer is that velocities are too low to extract any real benefit, but it would be interesting to find out.

#46 DDW

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Posted 27 April 2006 - 02:05 AM

Begs the question, since knowledge of aerodynamics appears to be in attendance, would then a squaretop benefit from an endplate?

For that matter, would tip losses be mittigated by a wing fence at some point along the span, if such a thing could be moulded into the sail/batten?

Likely, the answer is that velocities are too low to extract any real benefit, but it would be interesting to find out.

Very unlikely. It is quite difficult even for well funded German universities with full sized wind tunnels to get winglets right, and when they do it is about a 2-3% improvement on a sailplane, and then only in a limited speed range. A boat sail has a much lower aspect ratio (and hence more induced drag) so there is more opportunity. However one of the things that make sailboats so intractable to the airplane aerodynamicist is that it must operate over such a wide range of conditions - say 1 to 25 knots. There are no fixed wing aircraft that operate (in incompressible flow) over that large a range. It is relatively easy to get winglets on an airliner where they must perform well only in a very narrow range of cruise speeds.

#47 TP30

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Posted 27 April 2006 - 08:48 AM

First off, I must agree - the noise from a Spitfire is THE BEST noise in all the world. Simply beautiful.

Secondly, I second Marchaj and Bethwaite as (relatively) simple to understand books re sail aerodynamics. These are probably the two most thumbed books I own.

Thirdly I'm going to back up what others have said above, and then add a little twist. Lifting Line theory does show that an elliptical distribution offers maximum efficiency, hence the Spitfire wing. Tapered wings with a root:tip ratio approx 2.5:1 offer a much simpler to build solution with only a small loss in performance, hence the popularity of this planform (but they are NOT superior). Having said that, lifting line theory is only an approximation. It does not deal with the finer details. It is no mistake that dorsal fins and albatross wings are not pure ellipses (or tapered). Tank tests and wind tunnel results show that the ellipse is outperformed by these natural shapes. This is because the dorsal fin etc recovers some of the energy lost via tip vortices (induced drag).

Finally, it must be remembered that sails are not wings. Not only do sails operate at different Reynolds numbers, and across a much wider range of conditions (speeds, angles of attack etc), but boats capsize. This introduces constraints on sail shapes that are not relevent to birds/planes/gliders. For example, imagine a glider wing. It generates lift and drag perpendicular and parallel to the angle of attack respectievely. Thus the direction of drag is mostly backward (slowing the plane down), but also slightly upward (keeping it in the air). The direction of lift is mostly upward (keeping it up) and slightly forward (speeding it up).
Now imagine a boat sailing upwind, by turning the wing upwards. Drag is mostly backward, and slightly sideways (heeling). Lift is slightly forward but MOSTLY SIDEWAYS, therefore heeling the boat. Once the heeling moment exceeds the available righting moment any extra lift cannot be utilised.
For both the glider and the boat, drag should be minimised. For the glider lift should be maximised. For the boat, thrust (the forward component of lift) should be maximised for a limiting heel force (the sideways component of lift).

Of course, this only applies when close hauled. When the apparent wind is across the boat on a reach, lift IS thrust, so should be maximised.

Sorry if thats a little unclear, I'll try to post a picture later if I get the time.

#48 TP30

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Posted 27 April 2006 - 11:00 AM

As promised, some diagrams.

Essentially, for gliders look to maximise a, b and c, whilst minimising d. For boats maximise b whilst minimising the rest.

Attached Files



#49 Grant

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Posted 27 April 2006 - 12:08 PM

Agree with what's posted above. Let me go further with the question.

What's more efficient, a squarehead main or a (for lack of a better term) fathead main.

I went with an elliptical fathead over a sharp squarehead on my tri for the same reasons that an elliptical daggerboard is more efficient than a square daggerboard.

Can anyone speak to this?



I sail a I14 with a fat head, not quite square, but we basically went for these recently since T foils have evolved. In the past you could not bear away in 20-25 kts plus with a fat head...you would nosedive straight away.
They give more sail area downwind too. We are currently working on an even bigger more rectangular version. Some of the problem is getting the twist right, with the sail maker cutting the panels at different angles to help this happen.

Our centreboard looks like those glider wings, very long 1750mm, square tip 120 mm wide tapering to a parrallel section for the top 600, and only 230 wide overall

#50 Grant

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Posted 27 April 2006 - 12:12 PM

Agree with what's posted above. Let me go further with the question.

What's more efficient, a squarehead main or a (for lack of a better term) fathead main.

I went with an elliptical fathead over a sharp squarehead on my tri for the same reasons that an elliptical daggerboard is more efficient than a square daggerboard.

Can anyone speak to this?



I sail a I14 with a fat head, not quite square, but we basically went for these recently since T foils have evolved. In the past you could not bear away in 20-25 kts plus with a fat head,,,you would nosedive straight away.
They give more sail area downwind too. We are currently working on an even bigger more rectangular version. Some of the problem is getting the twist right, with the sail maker cutting the panels at different angles to help this happen.

Our centreboard looks like those glider wings, very long 1750mm, square tip 120 mm wide tapering to a parrallel section for the top 600, and only 230 wide overall

#51 Damp Freddie

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Posted 27 April 2006 - 03:11 PM

I'm with more elipticals, and the Melges 24 is about as much roach as I like the look of in practice on a keel boat - on the water-

the new VO70s - have you noticed one thing
? the square top is almost always way twisted off. Waste of time.

Then when you reef you have loads of sail area where you don't want it , exacerbated with the loss of tune you get with reefing.

IMHO this long chord doesn't fly in as broad a wind range, boat speed or wind shear- on a yacht. True in optimum puffs it gives bags more power and pointing lever, but it will stall in many winds and some sheers, swell or chop and also,k importantly in regatta sailing, during manoevres. The flow will detach and then re-attach in a heavy handed way.

Otherwise it is ego jewelry danlging periously on the broachy haired chest of big boy sailors....change for the sake of change. Imagine all that pain in the arse batten flicking if it ever takes on with general racing boats?

But On a high performance dinghy the amount of apparent wind and actual relative wind is a narrower range than in yacht sailing


.....and not only have you say twin traps but your trimmer can play the damn thing every second of the way....

for my mediocre dinghy skills I like eliptical a la bethwaites

#52 DDW

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Posted 28 April 2006 - 01:47 AM

For both the glider and the boat, drag should be minimised. For the glider lift should be maximised. For the boat, thrust (the forward component of lift) should be maximised for a limiting heel force (the sideways component of lift).

You had me agreeing up to this point. Glider lift should not always be maximized. In steady state flight the lift is fixed, and equal to the weight of the glider. In that sense it is similar to a fully powered up boat that begins to depower the rig and feather it. On gliders, we can simply reduce the angle of attack. While there is no chance of the rigid wing luffing, the profile drag does increase, this is why gliders have full span flaps when legal in class. It allows reducing camber, therefore reducing profile drag, similar to flattening the main. On boats on the other hand, you generally want to maximize lift until you are fully powered up, then minimize drag while keeping lift constant. Gliders are always flying faster than the fully powered up case, while boats are very frequently sailing below this equivalent (when the glider would stall).

It is true that gliders do not have a heeling moment problem, and structural limitations have been pretty much eliminated by all carbon structures. However the figure of merit is still L/D, with the tradeoff in boats that there is finite stability, which limits aspect ratio. When these limits aren't strong (like C class cats in light conditions) very high aspect ratios are common.

Racing gliders also must fly efficiently over a large speed range compared to airplanes (though not compared to boats, as I pointed out previously). On a strong day, a glider will thermal at 48 knots, cruise at 135 or more, and must be efficient at both speeds.

#53 Jay_Madrid

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Posted 29 April 2006 - 11:43 PM

Frank Bethwaite's book "High Performance Sailing" is a interesting read regarding hull and sail design. It's oriented to skiffs but a lot of what it covers is applicable to other sorts of sailboats.

#54 JimC

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Posted 30 April 2006 - 10:56 PM

Interesting that the Spitfire elliptical wing was eventually cropped at the tip to produce the truncated elliptical shape as shown here in the XIV-E.


More complicated than that: there were clipped wing variants right through the production series. But the prime reason for the Spitfire wing wasn't induced drag - Mithchell knew he would get the same performance from a conventional taper wing. What it was all about was that the elliptical planform permitted him to use a thin wing section and still fit guns in it, whereas with a conventional taper wing it would have had to have been a thicker section to get enough width where the guns needed to be.




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