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14 hours ago, Zonker said:

Did you notice the cow velocity - the flow is hypersonic. Which would cook the cow. Surely there are easier ways to bbq a cow that making it go that fast?

 

Also the cow lacks a keel, thus it would suffer massive loss of stability as it bears off. 

But what about the udders?  You know, the udder things that got left out of the equation.

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Did you ‘go wrong’ when you calculated that the wind causes a pressure on the sails rather than creates lift? Foils create lift forces many times greater than the drag. 20...40 times? Also the volume

Did you just prove that you scientifically can’t sail to your rating?

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On 1/18/2021 at 7:02 PM, Sailor Al. said:

 

As we all know, with no motor, and no energy from the water or crew a sailboat gets all its moving energy from the wind.

 

 I thought a boat gets its forward motion(moving energy?) and lift from sails and the low pressure exerted on the windward side of the keel and rudder face which counteract the lateral motion of the air pressure exerted on the sails. Low pressure on the windward side of the sails allow the boat to point up to the angle that the jib or Genoa luffs due to equalizing pressure on both sides of the shaped cloth. 
 

As far as crew energy-moveable ballast that exert pressure on the sails and keel to increase speed as in a roll tack...plus1 there. The keel is subject to an increase in low pressure on the windward side forcing the boat’s keel to be pushed laterally into the low pressure area. That lateral distance is due to the energy exerted against the sail initially but is compounded by angle of heel, attack and the distribution of the crews weight at critical times.


You also haven’t taken into account the density of the air due to temperature in your quest to figure out the frictional forces of the atmosphere. Your powered aircraft vs glider example falls short of my expectation for the same reason. You can’t fly a helicopter out of the Grand Canyon on a 120degree day no matter how hard the rotors spin...

I’m not an engineer, just a sailor, but you aren’t an engineer(I hope) and need to figure out how a boat works before punching in data points and slinging your personal opinion(Dogma) into a room full of idiots who know a hell of a lot more about things I can’t wrap my head around, just like you. 

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This is insane: Cows never have keels.

I’m quite chuffed about blowing up the thread overnight with my glider poke. The glider example is useful for demonstrating the error when @Sailor Al. wrote that keels provide no thrust, only righting moment. The fundamental characteristic of gliding flight is that gravity pulls the craft down thru the air which causes the foils to squirt it forward. Switch around the labels and that is exactly what a keel is doing. Simple. 

Glider descent slope angle is comparable to sailboat leeway angle.

And, duh, a glider flight is extended if the air is rising, or the terrain is descending. Not unlike a powered aircraft flight being extended by aerial refueling.

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3 hours ago, Sailor Al. said:

Without gravity to hold the atmosphere down there would be no air for them to fly in, so it's a ridiculous proposition.

Dude. Listen carefully. Your answer will be telling. First case you are in a glider at 10000ft flying 90 kts ias in still air and Palomar peak 20 mi away is exactly at your telltail tip.

Now you are at the same location but updraft of 200 fpm. You are at 90 ias. Same temp altitude exactly.

Where is Palomar peak in your field of view?

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18 hours ago, Sailor Al. said:

Ah, yes, keel lift, that's delving into hydrodynamics.

The keel is moving through the water at Boat Speed with an AoA of the angle of the leeway angle, typically around 5°.

I know it produces lift, effectively increasing the righting moment, but that comes at the expense of drag.

I don't think it provides thrust, quite the opposite, it increases the drag. Do you have any thoughts?
 

The lift from a keel does NOT increase righting moment, if it did it would greatly increase leeway. It is the weight of the keel that provides the righting moment, that is why they tend to be made of very heavy materials. 

A keel does produce thrust, that is how a sailboat is able to sail upwind, by using the keel to react against the water - just like a glider using gravity to react against the air to produce thrust. It does not matter if the water current is moving in one particular direction or not, just like the glider does not care if the airmass is moving up or down or is stationary. It is the relative motion of the different forces and the foils exploiting that difference that create the forward movement.

Yes of course the keel increases drag. You cannot have lift without drag.

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20 minutes ago, Thread Killer said:

The lift from a keel does NOT increase righting moment

A keel does produce thrust, that is how a sailboat is able to sail upwind, by using the keel to react against the water - just like a glider using gravity to react against the air to produce thrust. It does not matter if the water current is moving in one particular direction or not, just like the glider does not care if the airmass is moving up or down or is stationary. It is the relative motion of the different forces and the foils exploiting that difference that create the forward movement.

Yes of course the keel increases drag. You cannot have lift without drag.

I chuckled when this guy wrote thst. It decreases rm. Duh.(or--reference frame issue--increases HA)

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I don’t think that this guy realizes that a heeling boat is falling back to windward onto its keel the entire time upwind instead of the keel swinging back to leeward  under the CE of the sailplan. 

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2 hours ago, Sail4beer said:

I don’t think that this guy realizes that a heeling boat is falling back to windward onto its keel the entire time upwind instead of the keel swinging back to leeward  under the CE of the sailplan. 

I don't think this guy has ever been sailing.  Or wasn't paying attention when the crew on the Windjammer cruise gave her 5 minute explanation just before rum swizzle time.

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On 1/18/2021 at 8:30 PM, Sailor Al. said:

Trying to spark a serious discussion on the subject. Maybe the wrong forum.:unsure:

HAHAHAHHAAH ya think?

 

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historically seen a sail boat on the top of the river Nile / ägypt did not know whether the energy comes from the air or the raining clouds  far away to Nubien, and the crocodile says that the boats lay under the side of the air... (a ship is a hole in the air in which we throw our money) for the croc a sailboat is only a construct which must be pressed into the air, did the energie to sail comes from the moving air? ask one little croco, no it comes from movin NIL... during 5000 years every croc learnt  this in scool, and it is important  to have boats  heavier than air, nothing else is able to do a sailtrip under the air one said,  and eight crocodiles grilled the cow they had fished in the air

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5 hours ago, Thread Killer said:

The lift from a keel does NOT increase righting moment, if it did it would greatly increase leeway. It is the weight of the keel that provides the righting moment, that is why they tend to be made of very heavy materials. 

Yes, you are correct: by providing the lift to counteract the aerodynamic side force, a keel, by virtue of its projection below the CLR, does provide a moment arm to increase the heeling moment. 

5 hours ago, Thread Killer said:

A keel does produce thrust, that is how a sailboat is able to sail upwind, by using the keel to react against the water - just like a glider using gravity to react against the air to produce thrust. It does not matter if the water current is moving in one particular direction or not, just like the glider does not care if the airmass is moving up or down or is stationary. It is the relative motion of the different forces and the foils exploiting that difference that create the forward movement.

However here you are incorrect here: a keel does not produce thrust. The attached diagram, from Fossati's "Aero-hydrodynamics of Sailing Yachts" shows the total hydrodynamic force Fl can be resolved into Lift Pl (from which the keel increases the heeling moment) and Resistance Rl which acts in the opposite direction to thrust, as drag.

image.png.994d8aa13b44ce57743a891137f4b92a.png

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6 hours ago, fastyacht said:

Dude. Listen carefully. Your answer will be telling. First case you are in a glider at 10000ft flying 90 kts ias in still air and Palomar peak 20 mi away is exactly at your telltail tip.

Now you are at the same location but updraft of 200 fpm. You are at 90 ias. Same temp altitude exactly.

Where is Palomar peak in your field of view?

I'm not sure what type you're flying but if you're doing a steady 90 kts in still air, I would suggest your VS indicator is pointing seriously in the down direction and you'll be looking for a landing site pretty soon.

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3 hours ago, Sailor Al. said:

I'm not sure what type you're flying but if you're doing a steady 90 kts in still air, I would suggest your VS indicator is pointing seriously in the down direction and you'll be looking for a landing site pretty soon.

Non-answer. OK. So you can't answer. Or won't answer. What are you afraid of? Getting the wrong answer?

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4 hours ago, haribo said:

historically seen a sail boat on the top of the river Nile / ägypt did not know whether the energy comes from the air or the raining clouds  far away to Nubien, and the crocodile says that the boats lay under the side of the air... (a ship is a hole in the air in which we throw our money) for the croc a sailboat is only a construct which must be pressed into the air, did the energie to sail comes from the moving air? ask one little croco, no it comes from movin NIL... during 5000 years every croc learnt  this in scool, and it is important  to have boats  heavier than air, nothing else is able to do a sailtrip under the air one said,  and eight crocodiles grilled the cow they had fished in the air

Brilliant! Some of my best days sailing were when I set a course for a nice place in the atmosphere. The rushing seas pressing my keel and ship forward against the reluctantly yielding atmosphere.

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I am rather puzzled by the original posting.  It seems to imply that the potential maximum force on a sail can be estimated as the product of the dynamic pressure and the area of the sail.  This seems to be fundamentally flawed.  The dynamic pressure represents the kinetic energy of the air per unit volume.  It is therefore very relevant to the potential maximum force on a sail but does not tell us a lot more than that for a given system stronger winds can produce greater loads. 

My understanding is that for inviscid, incompressible flow at a constant elevation the dynamic pressure is equal to the stagnation pressure minus the static pressure. 

The computation (area times dynamic pressure) of force presented in the original posting seems to me to imply that the windward side of the sail is subject to stagnation pressure whilst the leeward side of the sail is subject to static pressure.  This implies that the air at all locations on the windward side of the sail is stationary whilst the air at all locations on the leeward side is moving at the remote free stream speed.  I think this is neither the case nor even approximately the case - for a real world or an idealized sail. 

My understanding is that the air flow around the sail in question can be characterized as having speed higher than remote free stream speed on the leeward side and lower than remote free stream speed on the windward side.  The air on the leeward side having greater speed has lower static pressure, whilst the air on windward side having lower speed has higher static pressure.  Ignoring friction the force on the sail at any local area of the sail is the difference in static pressure at that location times the increment of area considered.  This varies continuously over the sail.

So I think that multiplying sail area by free stream dynamic pressure is not helpful when trying to estimate the force on the sail under discussion. 

The post goes on to say “Whilst this is in the right ballpark as we're looking for 2000 Newtons, it's still far short of the answer as it would require 100% conversion of wind energy to force,”. As far as I can see there is no discussion of the distance this force moves.  My understanding is that the concept of converting energy to force is invalid.  I think that energy is the ability to do work and the amount of energy is the amount of work that can be done.  Energy can be used to create a force – or if you like can be converted to a force.  However any magnitude of force can be produced by any amount of energy… but if the system is designed to produce a large force the distance that force can move is small.  The energy is converted to work and the amount of work is the force time distance moved by the force.  So an amount of energy that could be used to create a force of 2,000 newtons which moves through 10 metres could instead be used to produce a force of 20,000 which moves through 1 metre. 

The original post finishes by asking “So where have I gone wrong?”.  I think the answer is that the underlying conceptual analysis is completely muddled and wrong. 

 

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7 hours ago, Sailor Al. said:

Yes, you are correct: by providing the lift to counteract the aerodynamic side force, a keel, by virtue of its projection below the CLR, does provide a moment arm to increase the heeling moment. 

However here you are incorrect here: a keel does not produce thrust. The attached diagram, from Fossati's "Aero-hydrodynamics of Sailing Yachts" shows the total hydrodynamic force Fl can be resolved into Lift Pl (from which the keel increases the heeling moment) and Resistance Rl which acts in the opposite direction to thrust, as drag.

image.png.994d8aa13b44ce57743a891137f4b92a.png

That illustration does not depict what you think it does. Fossati is not wrong, but that drawing is not right either...or lacks detail...or completeness...in the context of your claims...

The glider examples, and the topsy-turvy text from @haribo are actually quite helpful. Thank you @mountsbay for your apt and scholarly contribution.

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The whole concept of 'thrust' in either gliders or sailboats is misguided.

Trying to determine how the 'thrust' is produced in a sailboat is the fundamental problem here, because there is no thrust produced, and trying to shoehorn 'thrust' into the system will result in conceptual errors in the way the system operates.

Where the energy comes from and goes to in the sailboat system is entirely dependent on the frame of reference. Unlike a system with thrust, where the energy source is typically a chemical reaction.

 

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12 hours ago, Sailor Al. said:

Yes, you are correct: by providing the lift to counteract the aerodynamic side force, a keel, by virtue of its projection below the CLR, does provide a moment arm to increase the heeling moment. 

However here you are incorrect here: a keel does not produce thrust. The attached diagram, from Fossati's "Aero-hydrodynamics of Sailing Yachts" shows the total hydrodynamic force Fl can be resolved into Lift Pl (from which the keel increases the heeling moment) and Resistance Rl which acts in the opposite direction to thrust, as drag.

image.png.994d8aa13b44ce57743a891137f4b92a.png

That's exactly the diagram the girl on the Windjammer cruise used!  Except she then said...If you sign on for the full two week cruise next year, I'll describe how to lee bow the current.  Now...Rum swizzle time!

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6 hours ago, mountsbay said:

The original post finishes by asking “So where have I gone wrong?”.  I think the answer is that the underlying conceptual analysis is completely muddled and wrong. 

He ignored the spherical cow in his basic assumptions. OK, I agree a cow with a keel is crazy - but maybe spoilers to get more lift or keep flow attached because the cow is not a very aerodynamic animal. Possibly fairings on the legs

image.png.3d11f5c80cc9dffb1ec5560a9f823466.png

Now a greyhound - that's a well optimized aerodynamic beastie. Surely man must look to nature for inspiration when designing sailboats and trying to go upwind. The greyhound can also easily go DDWFTTW. The cow, not so much.

image.png.a136fbdf79a931173192bcd4e2778fcb.png

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5 hours ago, El Borracho said:

Fossati is not wrong, but that drawing is not right either...or lacks detail...or completeness...in the context of your claims...

If his drawing is not right, how can he not be wrong?

Please point out to me and the professor of applied mechanics, where it is wrong.

image.png.a4cb3573765240f78d5795310e2a2951.png

 

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This is how the vectors of lift on sails and leeway from a keel or centerboard propelling a sailboat forward was explained to me when I was about 8 years old. Except it was a watermelon seed not a grape as in this Google search image. Now that I think about it the old Plastrend Flying Dutchman that I learned to sail in is probably about as stiff as a grape these days.

Fingers pinching a grape - Buy this stock photo and explore similar images  at Adobe Stock | Adobe Stock

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4 hours ago, JohnMB said:

The whole concept of 'thrust' in either gliders or sailboats is misguided.

Trying to determine how the 'thrust' is produced in a sailboat is the fundamental problem here, because there is no thrust produced, and trying to shoehorn 'thrust' into the system will result in conceptual errors in the way the system operates.

 

Okay, what is the proper word to call the forward force in a glider? I think it is thrust but I am open to being educated.

In a Cessna thrust comes from the rotating wings of the propeller acting against the air, their movement through the air being powered via an engine.

In a glider the the thrust comes from the wings reacting against the air, powered by gravity.

The main difference (other than the obvious) between the two being that the bulk of the lift in the Cessna's case is being used to move the aircraft forward while the bulk of the lift in the glider's case is being used to hold the glider up.

Another analogy to support the glider being powered by gravity;

A bike rolling down a hill is powered by gravity in the same way that a glider is. The slope of the hill will determine how fast the bike goes.

If you were to raise the hill while the bike was rolling down it, the bike would still roll down the hill but it would roll slightly faster- depending upon how fast the hill was rising.

To maintain the same rolling speed while the hill is rising you need to tilt the hill to reduce the steepness of the slope. The tilting of the hill correlates with the slight change in attitude that the glider would take on while maintaining the same airspeed in lifting air. I believe this is what fastyacht was trying to illustrate in post #204.

Without gravity the rising hill (or lifting air) only produces thrust during the short time that there is a change in acceleration. In steady rising air (without gravity) the glider would initially get a bit of forward movement from the change in acceleration then would slow down to a stop, then rise with the air mass like a balloon. You need gravity to keep the glider moving forward, reacting against the air; whether it be still, rising or sinking.

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24 minutes ago, Thread Killer said:

Okay, what is the proper word to call the forward force in a glider? I think it is thrust but I am open to being educated.

The "forward" force on a glider is either its weight or its lift, but NEITHER of these is a thrust. Thrust is a mechanical force reacting with the air, and you need an engine or rocket or something of that kind to produce thrust. For gliders you only get weight, lift and drag.

The question of which is these acts in the forward direction depends heavily on how you define forward on a glider. And I have no idea what is standard for this. I can get my head around forward being a direction normal to it the weight vector, or normal to the lift vector, or fixed to the fuselage, but I don't know which makes most sense in a glider. Any thoughts?

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10 hours ago, fastyacht said:

Non-answer. OK. So you can't answer. Or won't answer. What are you afraid of? Getting the wrong answer?

Your post was in response to my comment:

"Without gravity to hold the atmosphere down there would be no air for them to fly in, so it's a ridiculous proposition."

I couldn't see how any answer to your question would add to the discussion, so I chose not to answer it.

However, while looking at your question, I thought it worthwhile to make some side observations about your parameters. A standard club glider will have a best L/D speed of around 50 kts, with a glide slope of around 40:1. If your 10,000 ft altitude was also AGL, then in still air at 50 kts, you would be descending at around 125 ft/min and would be on the ground in  80 minutes.

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10 minutes ago, JohnMB said:

The "forward" force on a glider is either its weight or its lift, but NEITHER of these is a thrust. Thrust is a mechanical force reacting with the air, and you need an engine or rocket or something of that kind to produce thrust. For gliders you only get weight, lift and drag.

The question of which is these acts in the forward direction depends heavily on how you define forward on a glider. And I have no idea what is standard for this. I can get my head around forward being a direction normal to it the weight vector, or normal to the lift vector, or fixed to the fuselage, but I don't know which makes most sense in a glider. Any thoughts?

If gliders only had weight (gravity), lift and drag then they would never move forward.

I can agree that thrust is most commonly defined as the forward force produced by some form of engine. If you look up the definition of "thrust" you won't easily find it associated with engineless vehicles.

However, in engineless vehicles the forward force is still called thrust. I don't have my glider books at hand right now but I am 99.99% confident that "thrust" is the term used to describe the forward force in gliders.

In a glider "thrust" is the forward portion of the lift vector.

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19 hours ago, shaggybaxter said:

I don't think this guy realises much at all aside from spreadsheet=i must be smart.

Wait till he finds the graphs tab in Excel. Uber smart then.

 

Hey, let's not lower the tone to insults.

My 30+ years implementing terabyte relational databases has taught me that databases are brilliant for storing and manipulating large quantities of data.

Spreadsheets are brilliant for presentation of computational work on small data, and yes, you can't beat a graph to assist in the presentation.

 

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1 minute ago, Thread Killer said:

If gliders only had weight (gravity), lift and drag then they would never move forward.

I can agree that thrust is most commonly defined as the forward force produced by some form of engine. If you look up the definition of "thrust" you won't easily find it associated with engineless vehicles.

However, in engineless vehicles the forward force is still called thrust. I don't have my glider books at hand right now but I am 99% confident that "thrust" is the term used to describe the forward force in gliders.

In a glider "thrust" is the forward portion of the lift vector.

I don't agree.

In your last statement you make clear that gliders can move forward with only weight, lift and drag.  Adding 'thrust' whether defined as the forward component of lift or the forward component of gravity doesn't add value. NASA seems to be quite happy with out it. https://www.grc.nasa.gov/www/k-12/airplane/glider.html

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3 minutes ago, JohnMB said:

I don't agree.

In your last statement you make clear that gliders can move forward with only weight, lift and drag.  Adding 'thrust' whether defined as the forward component of lift or the forward component of gravity doesn't add value. NASA seems to be quite happy with out it. https://www.grc.nasa.gov/www/k-12/airplane/glider.html

You guys have hijacked my sailing thread. Can you take your discussion to a gliding forum please? 

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5 minutes ago, JohnMB said:

I don't agree.

In your last statement you make clear that gliders can move forward with only weight, lift and drag.  Adding 'thrust' whether defined as the forward component of lift or the forward component of gravity doesn't add value. NASA seems to be quite happy with out it. https://www.grc.nasa.gov/www/k-12/airplane/glider.html

That's okay. I think that NASA explanation is quite lacking.

Whether you want to separate out the forward component of lift and call it thrust, or just forget about it and only use lift, it really doesn't matter. The physics are the same, the power source is the same. It is just semantics.

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5 minutes ago, Sailor Al. said:

You guys have hijacked my sailing thread. Can you take your discussion to a gliding forum please? 

Thanks, but no. If you can't understand how a glider moves forward through the air, there is no chance you will ever understand how a sailboat sails upwind.

I suggest you take your "discussion" to https://www.boatdesign.net/forums/hydrodynamics-aerodynamics/

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5 minutes ago, Sailor Al. said:

Maybe I should have done, I wasn't aware of its existence, thanks I'll. have a look. But I did start the thread and hijacking is discourteous.

Welcome to Sailing Anarchy.

I feel it fair to point out that you participated in the glider discussion so to claim a hijacking is not quite reasonable in my opinion.

an·ar·chy
/ˈanərkē/
noun
  1. a state of disorder due to absence or nonrecognition of authority.
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3 minutes ago, Sailor Al. said:

Maybe I should have done, I wasn't aware of its existence, thanks I'll. have a look. But I did start the thread and hijacking is discourteous.

oops i seem to have strayed into sailing discourtesy by accident. can someone direct me back to sailing anarchy.

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@Sailor AI.  I don't know if you read my post but it seems to me to address the issues you raised in your first post.  In a nutshell it seems to me that the conceptual analysis underlying your post is completely muddled and wrong.  Maybe I am wrong - if so I would like to know why

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34 minutes ago, JohnMB said:

I don't agree.

In your last statement you make clear that gliders can move forward with only weight, lift and drag.  Adding 'thrust' whether defined as the forward component of lift or the forward component of gravity doesn't add value. NASA seems to be quite happy with out it. https://www.grc.nasa.gov/www/k-12/airplane/glider.html

Okay, I must concede.

I did check my glider books and indeed, they do not mention thrust.

Conceptually, I think that it is useful to call the forward component of lift "thrust" because in reality, that is what moves the glider forward. I think it is a bit harder to visualize where the forward movement comes from when it is described as a small component of the lift - but that is the standard so... I was wrong. I guess.

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3 minutes ago, Thread Killer said:

Okay, I must concede.

I did check my glider books and indeed, they do not mention thrust.

Conceptually, I think that it is useful to call the forward component of lift "thrust" because in reality, that is what moves the glider forward. I think it is a bit harder to visualize where the forward movement comes from when it is described as a small component of the lift - but that is the standard so... I was wrong. I guess.

https://science.howstuffworks.com/transport/flight/modern/glider3.htm

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1 hour ago, Sailor Al. said:

Your post was in response to my comment:

"Without gravity to hold the atmosphere down there would be no air for them to fly in, so it's a ridiculous proposition."

I couldn't see how any answer to your question would add to the discussion, so I chose not to answer it.

However, while looking at your question, I thought it worthwhile to make some side observations about your parameters. A standard club glider will have a best L/D speed of around 50 kts, with a glide slope of around 40:1. If your 10,000 ft altitude was also AGL, then in still air at 50 kts, you would be descending at around 125 ft/min and would be on the ground in  80 minutes.

Answer the question i asked. Stop prevaricating.

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9 hours ago, mountsbay said:

I am rather puzzled by the original posting.  It seems to imply that the potential maximum force on a sail can be estimated as the product of the dynamic pressure and the area of the sail.  This seems to be fundamentally flawed.  The dynamic pressure represents the kinetic energy of the air per unit volume. 

Not quite. Dynamic pressure, 1/2ρV^2  is measured in Newtons/square metre. "kinetic energy of the air per unit volume" would be measures in joules/cubic metre. As with all pressure calculations, when you multiply the pressure by the area its acting on, you get a force , in Newtons. So multiplying the Dynamic pressure by the sail area provided the total force on the sail.  

It is therefore very relevant to the potential maximum force on a sail but does not tell us a lot more than that for a given system stronger winds can produce greater loads. 

Precisely,  that's what I'm working to calculate

My understanding is that for inviscid, incompressible flow at a constant elevation the dynamic pressure is equal to the stagnation pressure minus the static pressure. 

You may be right, but the classical computation for Dynamic pressure (above) works for me.

The computation (area times dynamic pressure) of force presented in the original posting seems to me to imply that the windward side of the sail is subject to stagnation pressure whilst the leeward side of the sail is subject to static pressure.  This implies that the air at all locations on the windward side of the sail is stationary whilst the air at all locations on the leeward side is moving at the remote free stream speed.  I think this is neither the case nor even approximately the case - for a real world or an idealized sail. 

And that's the nub of the Betz factor. Since in practice when the air encounters the sail, it is only slowed down, not completely stopped , so only a proportion of the wind energy is extracted by the sail. That is one of the factors that limit the efficiency of the overall system (air/water/ sails/hull). 

My understanding is that the air flow around the sail in question can be characterized as having speed higher than remote free stream speed on the leeward side and lower than remote free stream speed on the windward side.  The air on the leeward side having greater speed has lower static pressure, whilst the air on windward side having lower speed has higher static pressure.  Ignoring friction the force on the sail at any local area of the sail is the difference in static pressure at that location times the increment of area considered.  This varies continuously over the sail.

So I think that multiplying sail area by free stream dynamic pressure is not helpful when trying to estimate the force on the sail under discussion. 

The post goes on to say “Whilst this is in the right ballpark as we're looking for 2000 Newtons, it's still far short of the answer as it would require 100% conversion of wind energy to force,”. As far as I can see there is no discussion of the distance this force moves.  My understanding is that the concept of converting energy to force is invalid.  I think that energy is the ability to do work and the amount of energy is the amount of work that can be done.  Energy can be used to create a force – or if you like can be converted to a force.  However any magnitude of force can be produced by any amount of energy… but if the system is designed to produce a large force the distance that force can move is small.  The energy is converted to work and the amount of work is the force time distance moved by the force.  So an amount of energy that could be used to create a force of 2,000 newtons which moves through 10 metres could instead be used to produce a force of 20,000 which moves through 1 metre. 

The original post finishes by asking “So where have I gone wrong?”.  I think the answer is that the underlying conceptual analysis is completely muddled and wrong. 

That's absolutely right, my original algorithm attempted to resolve the question by resolving the forces involved. That's why, when I realised I had left distance out of the equation, I re-worked the algorithm using power, and have arrived at the quite satisfying answer of 35% efficiency.  

 

 

Edited by Sailor Al.
Pressure is newtons/sq m, not joules/sq m
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22 minutes ago, mountsbay said:

@Sailor AI.  I don't know if you read my post but it seems to me to address the issues you raised in your first post.  In a nutshell it seems to me that the conceptual analysis underlying your post is completely muddled and wrong.  Maybe I am wrong - if so I would like to know why

Yes, sorry, I have taken a while to digest your post. Thanks, I hope my comments are relevant.

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2 hours ago, Thread Killer said:

If gliders only had weight (gravity), lift and drag then they would never move forward.

I can agree that thrust is most commonly defined as the forward force produced by some form of engine. If you look up the definition of "thrust" you won't easily find it associated with engineless vehicles.

However, in engineless vehicles the forward force is still called thrust. I don't have my glider books at hand right now but I am 99.99% confident that "thrust" is the term used to describe the forward force in gliders.

In a glider "thrust" is the forward portion of the lift vector.

I prefer to call it "Ooomph".  Or is that too technical?

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@Sailor AI.  Most of the issues I discuss in my first posting require only a grasp of high school physics to understand.  Stagnation, static and dynamic pressures and their significance were not covered when I was at high school but they are simple concepts easily understood once you have a firm grasp of high school physics.  The following reference may help you understand these terms and their significance https://www.aft.com/learning-center/video-tutorials/video/static-vs-stagnation-pressure

You also need to be able to use logic. You say  As with all pressure calculations, when you multiply the pressure by the area its acting on, you get a force , in Newtons. So multiplying the Dynamic pressure by the sail area provided the total force on the sail.   This clearly implies that your assertion that dynamic pressure multiplied by sail area gives the total force on the sail somehow logically follows from the fact that when you multiple pressure pressure times area you get a force.  This does not follow logically and is in fact nonsense.

So... learn a bit more physics... think a bit more clearly

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You cannot figure out horsepower from projected area and stagnation pressure. Can't be done. This is getting silly.

Horsepower developed by a sailboat is real. But it isn't correlated to the stagnation pressure and the sail area. The amount of lift  generated isn't even enough. You need the L/D too. If you are talking about downwind in a lead mine it is no longer a problem so much of L/D but upwind or higher performance (or lead mine in tiny zephyrs) it is everything. L/D of the whole system is what the difference is between a hydrofoiling moth and a lowrider moth and a classical moth. All with 70 kg sailor/pilot, but very different results on the same area and weight.

AI you are not going to invent your way into first prinicples consideration of power developed by a sailboat. But you can learn how it works in the empirical sense. Like I said Waaaaay upthread go read everything 12 metre and you will find it.

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1 hour ago, Sailor Al. said:

You guys have hijacked my sailing thread. Can you take your discussion to a gliding forum please? 

What the about my cows?  Cow aerodynamics forum anywhere?

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11 minutes ago, Left Shift said:

I prefer to call it "Ooomph".  Or is that to technical?

 

The important question then becomes how much oomph does a spherical cow have? (Note we are ignoring farting cows, and especially bulls to avoid having to deal with thrust).

Secondary question does the size and shape of the cows keel affect its oomph?

 

Damn cross post with zonker... though it was time to get us back on rack again.

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3 minutes ago, JohnMB said:

 

The important question then becomes how much oomph does a spherical cow have? (Note we are ignoring farting cows, and especially bulls to avoid having to deal with thrust).

Secondary question does the size and shape of the cows keel affect its oomph?

 

Damn cross post with zonker... though it was time to get us back on rack again.

Now you are making me hungry. A rack of ribs sounds just right right now right?

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1 hour ago, Sailor Al. said:

And that's the nub of the Betz factor. Since in practice when the air encounters the sail, it is only slowed down, not completely stopped , so only a proportion of the wind energy is extracted by the sail. That is one of the factors that limit the efficiency of the overall system (air/water/ sails/hull). 

You are treating the sail as a turbine.  It’s not, it’s a foil.  The air velocity differential that produces the pressure field is between the windward and leeward sides not the upstream and downstream ends of the sail.  You should not think of a sail as “slowing down” the air. An ideal sail would not have zero wind speed at the leach. 

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Just FYI - an "Ooomph" (3 ooo's) = 12 "Good Shoves".  Or maybe it's the other way around - hard to keep track with those damn imperial units of measure.

And if you are worried about thread hijacking you're in the wrong place.

Using a spreadsheet to try to figure out the amount of energy extracted by a sailboat seems... well, futile but knock yourself out dude.

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14 minutes ago, Zonker said:

What the about my cows?  Cow aerodynamics forum anywhere?

I want to know the if the lift changes dependent upon how much cow has eaten. If only the forrard stomach is full, does that hinder the windward performance? In light airs, our cow seems to track better if we all shimmy forrard on the lee side. But she does have a fat arse....so horses for courses?

 

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14 minutes ago, mountsbay said:

This clearly implies that your assertion that dynamic pressure multiplied by sail area gives the total force on the sail somehow logically follows from the fact that when you multiple pressure pressure times area you get a force.  This does not follow logically and is in fact nonsense.

Since the very definition of pressure is force per unit area, why is it nonsense to assert that Dynamic pressure (of the wind) times the sail area equals the force on the sail?

I learned that in high school before completing my degree in mathematical physics, so please ease back on the derision.

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Just now, Sailor Al. said:

Since the very definition of pressure is force per unit area, why is it nonsense to assert that Dynamic pressure (of the wind) times the sail area equals the force on the sail?

I learned that in high school before completing my degree in mathematical physics, so please ease back on the derision.

Oh contraire Dr Al, I think you'll find the derision needle has yet to move. It trends as logarithmic, so its always a slow starter. 

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7 minutes ago, shaggybaxter said:

I want to know the if the lift changes dependent upon how much cow has eaten. If only the forrard stomach is full, does that hinder the windward performance? In light airs, our cow seems to track better if we all shimmy forrard on the lee side. But she does have a fat arse....so horses for courses?

 

I think I answered my own question.

Filling only the rear stomach definitely adds a bow up attitude. Needs some work on the CE, I might break open a spreadsheet. 

1836921560_FunnyPic(83)_jpg.jpg.ee685431116113b4ff5b7134302829d3.jpg

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14 minutes ago, Zonker said:

Just FYI - an "Ooomph" (3 ooo's) = 12 "Good Shoves".  Or maybe it's the other way around - hard to keep track with those damn imperial units of measure.

And if you are worried about thread hijacking you're in the wrong place.

Using a spreadsheet to try to figure out the amount of energy extracted by a sailboat seems... well, futile but knock yourself out dude.

Here I though an "Ooomph" would equate to a kilo-shove.   Or is that an "Oh, Oh, Oh"?  The big brother to the "Oh La La"!

And just where is that spreadsheet?

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11 minutes ago, Sailor Al. said:

Since the very definition of pressure is force per unit area, why is it nonsense to assert that Dynamic pressure (of the wind) times the sail area equals the force on the sail?

I learned that in high school before completing my degree in mathematical physics, so please ease back on the derision.

Fucking hell. You have to integrate the pressure at all points, including vacuum, all normal to the cloth---oh and you cannot forrget the mast too. And the eddies will fuck you up. Also the boundary layer will majorly fuck youu up. But have at it.

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6 minutes ago, shaggybaxter said:

I think I answered my own question.

Filling only the rear stomach definitely adds a bow up attitude. Needs some work on the CE, I might break open a spreadsheet. 

1836921560_FunnyPic(83)_jpg.jpg.ee685431116113b4ff5b7134302829d3.jpg

This is fucking brillian.t A real cowmaran porpoising across florida baty!

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2 minutes ago, Sailor Al. said:

I learned that in high school before completing my degree in mathematical physics, so please ease back on the derision.

Slept in during your early morning aerodynamics classes huh?

I had that problem too, with a FEA course on a Thursday morning. If I slept through it, I didn't have any other courses that morning. It was a miracle I passed the course. Thankfully the Prof was a nice fellow with a very thick Israeli accent teaching the course for the first time. He did it so badly the entire class failed the final exam and then we all got shoved up the curve.


Never mind, while I may have digressed - the internet has things like that solved. MIT has very nicely put a ton of their courses online. For free. So smart kids in India can see the same lectures as the rich kids in the USA. Well the course notes anyway.

This one might help you:

https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-100-aerodynamics-fall-2005/

This course extends fluid mechanic concepts from Unified Engineering to the aerodynamic performance of wings and bodies in sub/supersonic regimes. 16.100 generally has four components: subsonic potential flows, including source/vortex panel methods; viscous flows, including laminar and turbulent boundary layers; aerodynamics of airfoils and wings, including thin airfoil theory, lifting line theory, and panel method/interacting boundary layer methods; and supersonic and hypersonic airfoil theory. Course material varies each year depending upon the focus of the design problem

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34 minutes ago, Zonker said:

What the about my cows?  Cow aerodynamics forum anywhere?

Here is fine, but I'm still trying to work out why there weren't any vortices around the cow's ears? Maybe cows have inadvertently developed the answer to control surfaces for super sonic flight?

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Just now, Mark Morwood said:

Here is fine, but I'm still trying to work out why their weren't any vortices around the cow's ears? Maybe cow's have inadvertently developed the answer to control surfaces for super sonic flight?

They use sonic cloaking. It is as if they arent there. Thats why cows area able to run as fast as roadrunners.

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20 minutes ago, Sailor Al. said:

Since the very definition of pressure is force per unit area, why is it nonsense to assert that Dynamic pressure (of the wind) times the sail area equals the force on the sail?

I learned that in high school before completing my degree in mathematical physics, so please ease back on the derision.

@ Sailor AI.  I explained that in my first post.

Sorry about what you consider to be the derision.  I did not realise you had studied high school physics.  In light of that I should not have suggested you "learn a bit more physics" but rather I should have suggested that you "re-learn some basic physics".

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5 minutes ago, Mark Morwood said:

Here is fine, but I'm still trying to work out why there weren't any vortices around the cow's ears? Maybe cows have inadvertently developed the answer to control surfaces for super sonic flight?

Great! Was worried about hijacking a serious discussion.

I think the CFD results are horizontal and vertical slices along the middle and center of yon cow, thus you're missing the flow around the ears.

We really need to see a 3D streamline presentation to truly judge. Like this. You can see stagnation on the front of the ears and beginnings of vortices behidn, but at really high wind speeds perhaps the cows ears fold back to reduce wind resistance. (it is amazing how many cows have been CFD'd)

image.png.c0858b78b6d9e0627768a75e9f5f1093.png

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4 minutes ago, Zonker said:

Great! Was worried about hijacking a serious discussion.

I think the CFD results are horizontal and vertical slices along the middle and center of yon cow, thus you're missing the flow around the ears.

We really need to see a 3D streamline presentation to truly judge. Like this. You can see stagnation on the front of the ears and beginnings of vortices behidn, but at really high wind speeds perhaps the cows ears fold back to reduce wind resistance. (it is amazing how many cows have been CFD'd)

image.png.c0858b78b6d9e0627768a75e9f5f1093.png

This is udder nonsense,  my kindergarten geometry class notes indicate that this is not a spherical cow.

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2 minutes ago, JohnMB said:

This is udder nonsense,  my kindergarten geometry class notes indicate that this is not a spherical cow.

Of course if you'd done any recent formal physics you'd understand that topologically speaking the cow depicted is indistinguishable from a sphere - QED it is a spherical cow.

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@John MB.  I think the spherical cow concept was introduced as a simplification to facilitate analysis.  Zonker has applied advanced modelling techniques which obviate the need for such measures and allow a more rigorous analysis.

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Does that mean a coffee cup is just a lumpy donut? I must admit I still have some more study to do in the area of topology and cows.

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It's a little known fact that cows have contributed greatly to the fields of aerodynamics.

On February 18, 1930, the first cow to ever take flight ascended into the sky in a Ford Tri-Motor. As part of the celebration of the International Air Exposition in St. Louis, Elm Farm Ollie was flown from Bismarck, Missouri to St. Louis, a distance of 72 miles. Elm Farm Ollie was the first cow to take flight and the first cow to be milked on a plane 

elm-farm-ollie-first-flying-cow.jpg.a14eea42d5a0c374a8100f746f7b60cc.jpg

It's an even littler know fast that on this flight Elm Farm Ollie earned membership to the mile high club. Her sires would continue her love of exploring the arcane world of fluid dynamics, but to avoid public scorn and derision they changed their name by deed poll from Elm Farm to....Yeager.     

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31 minutes ago, Mark Morwood said:

Of course if you'd done any recent formal physics you'd understand that topologically speaking the cow depicted is indistinguishable from a sphere - QED it is a spherical cow.

Awesome we have invented a new field, (for the cow to stand in presumably).

We can now formally set up the first topological aerodynamics department in the world. The fundamental basis of the study is that there are physical laws of aerodynamics which are invariant under topological transformation. Our job is to find them, I'm think the DoD would be the place to go for an initial grant. We will also need a wind tunnel suitable for the exploration of the dynamics of all shapes of spherical cow.

 

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6 minutes ago, JohnMB said:

The fundamental basis of the study is that there are physical laws of aerodynamics which are invariant under topological transformation.

I've already proven that  aerodynamical drag and lift of all topologically equivalent shapes are equal at 0 speed.

Generalizing to speeds != 0 should be easy.

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14 minutes ago, mathystuff said:

I've already proven that  aerodynamical drag and lift of all topologically equivalent shapes are equal at 0 speed.

Generalizing to speeds != 0 should be easy.

Sure, but by inclination I am an experimentalist so I would want to do some wind tunnel test to verify any theory you propose. The question is will PETA get upset is we install suitable equipment to modify the topology of the cows for wind tunnel testing?

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5 minutes ago, JohnMB said:

Sure, but by inclination I am an experimentalist so I would want to do some wind tunnel test to verify any theory you propose. The question is will PETA get upset is we install suitable equipment to modify the topology of the cows for wind tunnel testing?

They really shouldn't as homeomorphisms are reversible on cowspheres.

Obviously I would want my theoretical findings to be tested by experiment.

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On 1/18/2021 at 7:02 PM, Sailor Al. said:

As we all know

There are traditions to uphold, so...

show us some tits and get the fuck out!

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I'm thinking of setting up a spreadsheet to design a nuclear power station.  If the hive-mind of SA is the right place to resolve all the forces applying at the intersection of two different fluids with a complex solid object, then surely designing a radioactive boiler will be a doddle.

Should I start a new thread, or can we do it here?

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8 minutes ago, TwoLegged said:

I'm thinking of setting up a spreadsheet to design a nuclear power station.  If the hive-mind of SA is the right place to resolve all the forces applying at the intersection of two different fluids with a complex solid object, then surely designing a radioactive boiler will be a doddle.

Should I start a new thread, or can we do it here?

I vote for here.

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Nuclear reactors are kinda like sails You move a fluid and that fluid transfers energy across sails or blades or whathave you and someboy calls it thrust and somebody esle says, "not its not/."

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6 minutes ago, fastyacht said:
15 minutes ago, TwoLegged said:

I'm thinking of setting up a spreadsheet to design a nuclear power station.  If the hive-mind of SA is the right place to resolve all the forces applying at the intersection of two different fluids with a complex solid object, then surely designing a radioactive boiler will be a doddle.

Should I start a new thread, or can we do it here?

I vote for here.

Great!

OK, I got a whistling kettle, the dynamo off grand-dad's bicycle, and a top-grade garden hose for the cooling.  Is there anything else I need?

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