Helicopter Lift Theory - Equal Transit, Skipping Stone & 1/2 a Venturi - all wrong...
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Crab.
Look at Jymil's post #15. The streamlines ARE deflected down. They are at least 1/2cm lower on the right than the left.
Why would accelerating air downwards create a downward force on the wing ? Is that what you observe on a pool table or in a rocket engine ?
Your point about the underside of the wing is absolutely correct, but the point about Newtonian mechanics is that they do not attempt to deal with why or how the wing does what it does, which part of the wing does what, and less still the best shape to achieve it. They simply describe the aggregate consequences of it.
You can still have long arguments about how the wing actually achieves the acceleration without calling into question Newton laws.
Look at Jymil's post #15. The streamlines ARE deflected down. They are at least 1/2cm lower on the right than the left.
Why would accelerating air downwards create a downward force on the wing ? Is that what you observe on a pool table or in a rocket engine ?
Your point about the underside of the wing is absolutely correct, but the point about Newtonian mechanics is that they do not attempt to deal with why or how the wing does what it does, which part of the wing does what, and less still the best shape to achieve it. They simply describe the aggregate consequences of it.
You can still have long arguments about how the wing actually achieves the acceleration without calling into question Newton laws.
Punto - I am just playing devil's advocate. In order to deflect the air to follow the curved path along the wing an acceleration downwards (relative to the wing) is required - if the air is in contact with the wing, it must push down on it (as the wing will push up against it)
However, jymil's post with the turning of fluids is a graphic which does show downward displacement - the cambridge video is actual streamlines which show negligible downwards displacement but a significant upwards displacement.
Which of these is the bad science?
If all the clever scientists in the world can't agree, what hope have us mere pilots got?
It would be nice to have a definitive explanation - probably a combination of all the current theories.
However, jymil's post with the turning of fluids is a graphic which does show downward displacement - the cambridge video is actual streamlines which show negligible downwards displacement but a significant upwards displacement.
Which of these is the bad science?
If all the clever scientists in the world can't agree, what hope have us mere pilots got?
It would be nice to have a definitive explanation - probably a combination of all the current theories.
if the air is in contact with the wing, it must push down on it (as the wing will push up against it)
However, jymil's post with the turning of fluids is a graphic which does show downward displacement - the cambridge video is actual streamlines which show negligible downwards displacement but a significant upwards displacement.
You cannot beat physical law, and Newtons second law is the only possibility here, unless you wish to claim that it all works by magic or other supernatural forces.
RVDT: you don't really know who you believe do you? You've stuck with your preferred theory for some time and refuse to budge when the truth comes along
"KISS": stick to cambridge and you won't ever get disproven.
What is a travesty is that for so long, so many mil pilots were taught bo**ocks and went along with it. Shame on you all.
How many others believe the low pressure upper surface definition?
"KISS": stick to cambridge and you won't ever get disproven.
What is a travesty is that for so long, so many mil pilots were taught bo**ocks and went along with it. Shame on you all.
How many others believe the low pressure upper surface definition?
Last edited by Thomas coupling; 19th Nov 2012 at 18:01.
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Peter Garrison had a recent article in Flying Magazine which discussed the subject (and its history). His final conclusion certainly has some truth in it - "we do not understand lift; we merely talk about it".
You Will Never Understand Lift | Flying Magazine
You Will Never Understand Lift | Flying Magazine
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@[email protected]: watch this video, it shows actual streamlines with varying angle of attack. This should make it clear air is indeed deflected downwards.
If this doesn't convince you, I'll hover 10ft above you to prove some significant amount of air is indeed deflected downwards :-)
If this doesn't convince you, I'll hover 10ft above you to prove some significant amount of air is indeed deflected downwards :-)
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Downwash
The video from RVDT's post has also an interesting point at around 21 min:
The speed and direction of the downwash from an observer on the ground would be more or less straight down. Wind tunnels are actually showing reality in an inverted way: it's not the air flowing around a static wing, but the wing plowing through the air. So if you picture the wing going from right to left and the air being still, you see the air is mainly deflected downwards.
The speed and direction of the downwash from an observer on the ground would be more or less straight down. Wind tunnels are actually showing reality in an inverted way: it's not the air flowing around a static wing, but the wing plowing through the air. So if you picture the wing going from right to left and the air being still, you see the air is mainly deflected downwards.
I see no reason for claiming a fluid in contact must exert a force in any particular direction.
The wings in the tunnel are deflecting air downwards, I don't dispute that but the amount of downwash is very small - even at high AoA the streamlines even a short way below the wing get closer together but no more so than those a short way above it. How can you emphasise the downwash and ignore the upwash?
Strange how everyone is such an instant expert in this field.
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rehash
This link, from post #316, by RDVT in Vortex ring / settling with power says it all How Airplanes Fly: A Physical Description of Lift
How Airplanes Fly: A Physical Description of Lift
TC,
Oh really? Or don't you believe that any more?
[What Newtonians and Bernoulians dont say is that the path that the air has to travel is longer on the upper surface and hence the air has to speed up to join its counterparts flowing under the aerofoil]
Punto: If you look at BOTH the bernoulli and the newton explanations, they both discriminate in favour of reduced air pressure on the upper surface - low pressure - to produce lift?
Have you not recognised this yet?
Have you not recognised this yet?
So the only thing wrong with the Bernoulli explanation is the flawed concept of equal transit times to explain why the air should accelerate over the top of the wing.
But the Newton explanation doesn't actually explain this either.
If you want something to travel in anything other than a straight line you must apply a force to it - what is the force that makes the air follow the wing?
Coanda is an effect, not a force and was first noticed during WW1 when observing the smoke from the guns and engines of the old fighters as it tracked round the fuselage so it isn't new.
I don't have the right answer - I'm neither a physicist nor a mathematician but Bernoulli (apart from equal transit time which I don't think was part of his theorem anyway) seems to be the closest to explaining all the observable phenomena.
The pretty picture of the downwash over the fog bank is fine but the vertical penetration of that downwash is limited - is there really enough air moving downwards at enough of a rate to hold up a multi-ton jet??
At least with a helicopter downwash you can see a column of air being displaced downwards (and feel its effects if you are underneath it)to balance the mass of the aircraft
But the Newton explanation doesn't actually explain this either.
If you want something to travel in anything other than a straight line you must apply a force to it - what is the force that makes the air follow the wing?
Coanda is an effect, not a force and was first noticed during WW1 when observing the smoke from the guns and engines of the old fighters as it tracked round the fuselage so it isn't new.
I don't have the right answer - I'm neither a physicist nor a mathematician but Bernoulli (apart from equal transit time which I don't think was part of his theorem anyway) seems to be the closest to explaining all the observable phenomena.
The pretty picture of the downwash over the fog bank is fine but the vertical penetration of that downwash is limited - is there really enough air moving downwards at enough of a rate to hold up a multi-ton jet??
At least with a helicopter downwash you can see a column of air being displaced downwards (and feel its effects if you are underneath it)to balance the mass of the aircraft
From the same NASA pages that JB77UK used at the beginning of this thread
Bernoulli and Newton
Summary - it is complicated and only really clever people can understand it
The last line
well that counts me out
Bernoulli and Newton
Summary - it is complicated and only really clever people can understand it
The last line
To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.
Punto: Please find time to read 407too's post where he offers a link about Newton's theory. At the very end, where the 'experts' actually describe the effect they specifically describe the formation of a low pressure area above the wing. 
Crab: The Bernoulli theorem " equal transit times" aspect has well and truly been disproved, many years ago. IF one was to adopt the BT solution (as opposed to the other camp belonging to the NT solution), then it is a fact that the air over the surface with the greater cadence (in most cases this refers to the upper surface - but it could be the lower surface if the a/c was inverted!!)...does speed up and accelerate past its counter molecule transitting the other side of the wing. This increase in speed causes the reduction in air pressure and voila. Job done.
BT and NT come to the same conclusions:
(a) Low pressure is formed on the upper surface.
(b) Lift is formed as a reaction.
Next........................................................ .
Crab: The Bernoulli theorem " equal transit times" aspect has well and truly been disproved, many years ago. IF one was to adopt the BT solution (as opposed to the other camp belonging to the NT solution), then it is a fact that the air over the surface with the greater cadence (in most cases this refers to the upper surface - but it could be the lower surface if the a/c was inverted!!)...does speed up and accelerate past its counter molecule transitting the other side of the wing. This increase in speed causes the reduction in air pressure and voila. Job done.
BT and NT come to the same conclusions:
(a) Low pressure is formed on the upper surface.
(b) Lift is formed as a reaction.
Next........................................................ .
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The wing occupies a certain amount of space. Fact.
When the wing moves from that space something must replace it. There cannot be nothing. Fact.
Solution = air.
That air is moved into place by the ambient airpressure. It flows smoothly down following the surface of the wing that is constantly moving out of the way. This movement of air that was stationery now has kinetic energy that has come from the plane moving. The plane keeps moving because of thrust provided by the powerplant. The wing doesn't pull it down, it is pushed down by surrounding air. No powerplant, potential energy from height.
Please note this is only addressing the top surface. The bottom surface is so obvious it shouldn't need explanation. Also it's my theory and not proven but I'm sticking with it until someone comes up with something simpler.
When the wing moves from that space something must replace it. There cannot be nothing. Fact.
Solution = air.
That air is moved into place by the ambient airpressure. It flows smoothly down following the surface of the wing that is constantly moving out of the way. This movement of air that was stationery now has kinetic energy that has come from the plane moving. The plane keeps moving because of thrust provided by the powerplant. The wing doesn't pull it down, it is pushed down by surrounding air. No powerplant, potential energy from height.
Please note this is only addressing the top surface. The bottom surface is so obvious it shouldn't need explanation. Also it's my theory and not proven but I'm sticking with it until someone comes up with something simpler.
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TC.
Newton may have had a theory at one point in time but now he has a law, in fact three of them. When I quote Newton, I'm not doing it through anyone's intermediation (eg 407too's post), nor am I supporting a Newtonian theory of lift (whatever that might be). I'm simply setting out what his insights offer by explaining lift as a reaction to air being accelerated downwards.
From a pilot teaching perspective it's perfect, because it builds on existing universal knowledge (rockets, pool tables, school physics), and avoids the pitfalls of other approaches by leaving the mechanism out of it.
If other people have chosen to bend Newton to their aims as many have tried with Bernoulli, then good luck to them, but they're nothing to do with me other than standing on the shoulders of the same giants.
Newton may have had a theory at one point in time but now he has a law, in fact three of them. When I quote Newton, I'm not doing it through anyone's intermediation (eg 407too's post), nor am I supporting a Newtonian theory of lift (whatever that might be). I'm simply setting out what his insights offer by explaining lift as a reaction to air being accelerated downwards.
From a pilot teaching perspective it's perfect, because it builds on existing universal knowledge (rockets, pool tables, school physics), and avoids the pitfalls of other approaches by leaving the mechanism out of it.
If other people have chosen to bend Newton to their aims as many have tried with Bernoulli, then good luck to them, but they're nothing to do with me other than standing on the shoulders of the same giants.
Last edited by puntosaurus; 20th Nov 2012 at 18:22.