Lift Survey
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Lift Survey
I'm doing a survey about pilot perceptions on various aviation concepts. First on the list is lift....
Please answer the questions to the best of your knowledge, based on what you have read and/or you have learned from your flight instructor(s).
Select only one answer for each question, unless otherwise noted.
1. Lift is best explained by:
a. Bernoulli’s equation (pressure differential between top and bottom of wing)
b. Newton’s laws (equal and opposite reaction)
c. Some lift is created via Bernoulli’s equation; some is created via Newton’s laws
d. Either Bernoulli’s or Newton’s can be used to explain total lift production
Please answer the questions to the best of your knowledge, based on what you have read and/or you have learned from your flight instructor(s).
Select only one answer for each question, unless otherwise noted.
1. Lift is best explained by:
a. Bernoulli’s equation (pressure differential between top and bottom of wing)
b. Newton’s laws (equal and opposite reaction)
c. Some lift is created via Bernoulli’s equation; some is created via Newton’s laws
d. Either Bernoulli’s or Newton’s can be used to explain total lift production
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As I know it, the lift force for a given aerofoil is directly proportional to the dynamic pressure, which is conserved in a laminar flow over a wing. this conservation is a form of bernoullis eqn, so i would answer a).
regards
Jon
regards
Jon
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Originally Posted by Henry Hallam
b.
You cannot apply Bernoulli's law between the upper and lower surface of the wing since there is no streamline connecting them.
You cannot apply Bernoulli's law between the upper and lower surface of the wing since there is no streamline connecting them.
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Jon
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There is an argument for Newtons laws being used, in addition to bernoulli, as one Newtonian law states that an object will accelerate in the direction of a resultant force, but that was not the one refered to in the original question.
Regards
Jon
Regards
Jon
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There are lots of different ways of looking at and explaining lift generation, so the answer has to be d. The momentum change resulting from downwash can explain lift in terms of Newton's third law, the pressure difference over the wing surfaces can be used to explain lift in terms of Bernoulli's equation and vorticity is another way of accounting for lift. There are, I am sure, other techniques, but it's a long time since my Aero Eng degree. All different techniques are actually just looking at the same thing in different ways.
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Excellent! It's been at least six months since this knackered old thread has been given a repeat airing!! Posters please make sure we include a few of the standard jokes about lift being created by money, gremlins, etc, etc, etc.
But, if you're conducting a survey and want serious answers rather than an argument bordering on religious faith, I'd have to vote (d), though it's not as black and white as all that.
Pitts2112
But, if you're conducting a survey and want serious answers rather than an argument bordering on religious faith, I'd have to vote (d), though it's not as black and white as all that.
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Not that one again 
Bernoulli can be derived from Newton; the two are different ways of looking at the same thing. Bernoulli just found a convenient way of working stuff out for the special case of fluids. If you did a simulation on every molecule around the wing, using Newtonian mechanics, you would (eventually) end up with Bernoulli.
So the answer is a) and b) and d). c) is meaningless.
Bernoulli can be derived from Newton; the two are different ways of looking at the same thing. Bernoulli just found a convenient way of working stuff out for the special case of fluids. If you did a simulation on every molecule around the wing, using Newtonian mechanics, you would (eventually) end up with Bernoulli.
So the answer is a) and b) and d). c) is meaningless.
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Not sure who your reply is aimed to, HH.
The explanation about equal travel times on top and bottom (i.e. particles separated at the leading edge meeting up again at the trailing edge) is false, yes.
To generate lift, you need to change the direction of the airflow. You can't change the direction of airflow without creating a pressure difference. Pressure difference gives you lift... The two are tied together. They are the same thing.
The explanation about equal travel times on top and bottom (i.e. particles separated at the leading edge meeting up again at the trailing edge) is false, yes.
To generate lift, you need to change the direction of the airflow. You can't change the direction of airflow without creating a pressure difference. Pressure difference gives you lift... The two are tied together. They are the same thing.
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And for a proper explanation of lift rather than the superficial knowledge implied by the four choices of answer you offer, an understanding of viscosity and circulation will be required.
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Here's a hamster wheel I made earlier: http://www.pprune.org/forums/showthread.php?t=207289
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I am not a physicist but I think viscosity doesn't come into this (it plays a part but is not necessary to create lift through redirection of the fluid flow - all you need is for the fluid to have nonzero MASS), and circulation theory is just a handy way to look at the problem; it doesn't represent reality any more than anything else.
I doubt that X-Plane, when simulating the airflow over the airframe and the prop blades, is using circulation theory.
I doubt that X-Plane, when simulating the airflow over the airframe and the prop blades, is using circulation theory.
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Originally Posted by IO540
I am not a physicist but I think viscosity doesn't come into this (it plays a part but is not necessary to create lift through redirection of the fluid flow - all you need is for the fluid to have nonzero MASS), and circulation theory is just a handy way to look at the problem; it doesn't represent reality any more than anything else.
Circulation is central to the production of lift and has nothing whatsoever to do with handy concepts. No circulation ... no lift, regardless of aerofoil shape or angle of attack.
And viscosity is central to the production of circulation. No viscosity ... no circulation and therefore no lift (or drag).
This is covered by all the main aerodynamics text books, but the mathematics can be rather daunting. The best non-mathematical treatise of lift generation that I've found is Volume 1 of Chris Carpenter's 'Flightwise - Principles of Aircraft Flight'. He is a former head of aerodynamics at RAF Cranwell. Some quotes from that book:
"Lift is generated by the production of circulation around the wings, by which some velocity is added to the stream-wise velocity over the top of the wing, and some is subtracted from the velocity beneath the lower wing surface."
"Viscosity is essential for the production of any aerodynamic force whatsoever."
Or, if that book is not your flavour, try 'The Design of the Aeroplane' by Darrol Stinton, a truly remarkable treatise that should be on the shelf of anyone with a deep interest in these matters:
"Viscosity is the property that makes a fluid 'real', as against mathematically 'ideal', and it only becomes important when fluids are set in motion. We could solve many problems if it could be disposed of, but we would banish lift as well."
"When a circulatory (bound vortex) motion is induced in a straight steady flow by an aerofoil, both motions can be added vectorially to produce the flow pattern seen around a lifting surface ....... the circulatory motion results in an aerodynamic cross force which can be resolved into lift, normal to, and drag, parallel with the flight path"
Last edited by Islander2; 29th Mar 2006 at 08:23.
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An interesting thought about Newton's law
Every action has it's equal and opposite reaction right? Think about this--when your wing produces upward lift (due to Bernoulli's principle), it has to push down on the air. Your wing produces lift by continuously pushing (or sucking) air down!! If your wing produces 20,000 lbs of lift in the upward direction, then a mass of air must be accelerated in the down direction so that (mass of air) * (average acceleration of air) = 20,000 lbs. It's interesting to realize that as you go faster, the more air mass you run into, and the less you need to accelerate it downward to get 20,000 lbs. Thus the decrease in angle of attack required, etc.
I personally think that downwash is not what causes lift, just the natural by-product of a wing sucking air down. That's right, I said it--the wing sucks air down. I think I might be full of crap though, cause this stuff just came to me while falling asleep one night...
Same principles work for drag--you have to "drag" air along with you to get drag. Of course, you have to "thrust" air behind you to get thrust. These two cancel out if thrust = drag, so if you looked at the average movement of a body of air your airplane just flew through, it would just be moving down, not forward or back. Again, I could be full it, so don't flame me.
I personally think that downwash is not what causes lift, just the natural by-product of a wing sucking air down. That's right, I said it--the wing sucks air down. I think I might be full of crap though, cause this stuff just came to me while falling asleep one night...
Same principles work for drag--you have to "drag" air along with you to get drag. Of course, you have to "thrust" air behind you to get thrust. These two cancel out if thrust = drag, so if you looked at the average movement of a body of air your airplane just flew through, it would just be moving down, not forward or back. Again, I could be full it, so don't flame me.
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Islander - I maintain that all this is derived from Newtonian mechanics.
All the other stuff is just a way of looking at the problem on a bigger scale - no matter how many books contain the stuff.
All the other stuff is just a way of looking at the problem on a bigger scale - no matter how many books contain the stuff.
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Originally Posted by IO540
Islander - I maintain that all this is derived from Newtonian mechanics.
I have two problems with that:
a) Saying that lift is governed by Newtonian mechanics, and that's that, is about as helpful in understanding the subject as .............!!
b) Lift as a Newtonian reaction requires the airfoil to deflect the streamlines downwards. Unfortunately, without circulation - which requires viscosity - the streamlines aren't deflected downwards! Look in any text at a representation of inviscid flow around an aerofoil and tell me how you get a Newtonian reaction from that. So a knowledge of Newtonian mechanics alone ain't sufficient.
Genghis said:
"Just butting in, Flyandteach was asking about pilot's perception, not about what actually happens."
Edited to add one last observation before getting my hat and coat, never to be seen on this thread again! From Chris Carpenter's book referenced earlier:
"Interestingly, therefore, Newton's momentum theory of lift, although useless for the aerodynamics of conventional flight, actually comes into its own in modern hypersonic flight analysis." (my italics)
Last edited by Islander2; 30th Mar 2006 at 10:47.