Helicopter Lift Theory - Equal Transit, Skipping Stone & 1/2 a Venturi - all wrong...
Punto: for the sake of the industry take time out to read the links. You genuinely might learn something rather than re-inventing the wheel.
You've only started Instructing since 2006 - how many studes have you contaminated with your ad lib theories?
You've only started Instructing since 2006 - how many studes have you contaminated with your ad lib theories?
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Whoa Buddy. Wind it in. This is not about me.
I've read How Airplanes Fly: A Physical Description of Lift, and like these guys I start with Newton. Since I'm not qualified to work at the National Accelerator Laboratory Batavia IL, or the Dept. of Aeronautics and Astronautics University of Washington, that's where I stop. And when I stop I make it clear that what I've told them is the beginning of the story, not the end. I then point them at google if they're interested in finding out more.
You might like to note that the article in question is entitled A Physical Description of Lift, not A Newtonian Theory of Lift. And BTW I think it's an excellent article.
I've read How Airplanes Fly: A Physical Description of Lift, and like these guys I start with Newton. Since I'm not qualified to work at the National Accelerator Laboratory Batavia IL, or the Dept. of Aeronautics and Astronautics University of Washington, that's where I stop. And when I stop I make it clear that what I've told them is the beginning of the story, not the end. I then point them at google if they're interested in finding out more.
You might like to note that the article in question is entitled A Physical Description of Lift, not A Newtonian Theory of Lift. And BTW I think it's an excellent article.
Punto - please explain how the acceleration of air upwards - ie UP and over the top of the wing is conveniently ignored when you talk about the air being accelerated downwards to produce so much lift.
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All this was discussed in series 1, programme 1 of the BBC radio 4 comedy "Cabin Pressure" with Arthur the cabin attendant asking Martin the pilot all the right questions ! The pilot didn't know then either 
Avoid imitations
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Air molecules are very small, very sensitive, herding creatures. But they are surprisingly strong for their size.
If you upset them they run away.
Clonking them with an aerofoil gets them very upset.
In their rush to get away they push the aerofoil.
The aerofoil is connected to the aircraft so up it goes.
Simple, really.
(Best told in the theme of Peter Cook and Dudley Moore).
If you upset them they run away.
Clonking them with an aerofoil gets them very upset.
In their rush to get away they push the aerofoil.
The aerofoil is connected to the aircraft so up it goes.
Simple, really.
(Best told in the theme of Peter Cook and Dudley Moore).
Last edited by ShyTorque; 21st Nov 2012 at 10:40.
Shy: love it
Punto: I would have thought that leaving your stude with half a story is worse than not telling them in the first place. Either you as an individual would want to know what is really happening, or you as a professional MUST know what is happening. Believe either Bernoulli or Newton but take your pick and follow thru!
Punto: I would have thought that leaving your stude with half a story is worse than not telling them in the first place. Either you as an individual would want to know what is really happening, or you as a professional MUST know what is happening. Believe either Bernoulli or Newton but take your pick and follow thru!
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I can't believe nobody has mentioned the role of the bound spanwise vortex, circulation and the role of the shed vortices ..... 
While I think of it .... regardless of how that flowfield near to the airfoil is displaced in actuality ... the nett result (think of a fixed wing here) is a spanwise vortex shed at rotation and left on the runway (flow at top towards flight path) , two trailing vortices from the tips of the wings (of opposite sign, flow "leaks" from bottom to top), and a bound vortex that travels with the wing creating the "circulation" (greek symbol Kappa from memory) that can be thought of as a superposition on the free air velocity and kind of fitting the Bernouilli theory (faster on top).
So you have a "box" of vortex threads with a nett vorticity of zero that make a pump that forces air down through the middle of the "box", exchange of momentum equals a force that is the lift etc. Bigger lift = bigger vortex, hence the wake behind "heavies".
Confused yet? I am and I only just opened the bottle.
By the way, when a fixed wing lands and the lift is reduced to zero, a big-ass vortex rolls off the front of the wing (because it left one behind at takeoff and they have to cancel to zero). Don't believe me? Accelerate your spoon through the crema on a long black or espresso at an angle of attack, then abruptly stop it. You will see the vortex shed at the start (takeoff) and another one of opposite sign roll off when you stop the spoon.
Now ... lets take that fixed wing, make it 3 of them, rotate them around an axis, and then ..... wait ...
While I think of it .... regardless of how that flowfield near to the airfoil is displaced in actuality ... the nett result (think of a fixed wing here) is a spanwise vortex shed at rotation and left on the runway (flow at top towards flight path) , two trailing vortices from the tips of the wings (of opposite sign, flow "leaks" from bottom to top), and a bound vortex that travels with the wing creating the "circulation" (greek symbol Kappa from memory) that can be thought of as a superposition on the free air velocity and kind of fitting the Bernouilli theory (faster on top).
So you have a "box" of vortex threads with a nett vorticity of zero that make a pump that forces air down through the middle of the "box", exchange of momentum equals a force that is the lift etc. Bigger lift = bigger vortex, hence the wake behind "heavies".
Confused yet? I am and I only just opened the bottle.
By the way, when a fixed wing lands and the lift is reduced to zero, a big-ass vortex rolls off the front of the wing (because it left one behind at takeoff and they have to cancel to zero). Don't believe me? Accelerate your spoon through the crema on a long black or espresso at an angle of attack, then abruptly stop it. You will see the vortex shed at the start (takeoff) and another one of opposite sign roll off when you stop the spoon.
Now ... lets take that fixed wing, make it 3 of them, rotate them around an axis, and then ..... wait ...
Avoid imitations
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and the role of the shed vortices
Come on guys, lighten up a little.
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Now that is funny TC. Do you know what is happening ? Really ?
I can't do better than to extend the quote from the NASA article that crab referred to in Post 37:
I notice that there's $1m on offer from the Clay Mathematics Institute for solving the existence and smoothness problems in the Navier-Stokes equations, so maybe you should take some time out of aviation management and put us straight. For the benefit of the industry, you understand.
Best,
Punto
I can't do better than to extend the quote from the NASA article that crab referred to in Post 37:
The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel's father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.
Best,
Punto
Last edited by puntosaurus; 21st Nov 2012 at 13:45.
Punto - but the wind tunnel streamlines seem to show pretty much an equal and opposite effect with little evidence of a net downwash (certainly not enough to hold up the aircraft).
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Well since I made that assertion, I thought I'd better check, and if you follow each individual streamline, by my reckoning the average deflection down is just over 1/2cm. Only the top three actually go up. I don't think the displacement in a bounded system like a wind tunnel tells you much anyway, and besides, the Newtonian issue is not the displacement but the acceleration imparted to the air.
The article in post 32 does a good job of both explaining exactly how much (a lot !) of air is involved, and presenting a picture which illustrates it.
The article in post 32 does a good job of both explaining exactly how much (a lot !) of air is involved, and presenting a picture which illustrates it.
Last edited by puntosaurus; 21st Nov 2012 at 18:21.
Wikipedia - Lift (force)
The explanation seems reasonable and the "Reference and Notes" also has
most of the players quoted.
Of note -
The explanation seems reasonable and the "Reference and Notes" also has
most of the players quoted.
Of note -
In short, Bernoulli’s equation is a straight-forward application of Newton’s laws, often
misinterpreted. There are other details that must be included in a full treatment of
aerodynamic/hydrodynamic flow, including viscous drag and turbulence, but there should be no
surprises in Bernoulli’s equation.
The difficulties are removed by recognizing that Bernoulli’s equation tell us that a pressure
difference causes a change in speed, and pressure differences are caused by curvature of flow,
interpreted locally as producing a centrifugal force.
For those who wish to avoid the details, it is only necessary to point out that where there is
curved fluid flow, there is a pressure difference (i.e., lift), and from Bernoulli’s equation (simply
and properly interpreted) the existence of the pressure difference tells us there must be a speed
difference.
Robert P. Bauman
Professor of Physics Emeritus
University of Alabama at Birmingham
misinterpreted. There are other details that must be included in a full treatment of
aerodynamic/hydrodynamic flow, including viscous drag and turbulence, but there should be no
surprises in Bernoulli’s equation.
The difficulties are removed by recognizing that Bernoulli’s equation tell us that a pressure
difference causes a change in speed, and pressure differences are caused by curvature of flow,
interpreted locally as producing a centrifugal force.
For those who wish to avoid the details, it is only necessary to point out that where there is
curved fluid flow, there is a pressure difference (i.e., lift), and from Bernoulli’s equation (simply
and properly interpreted) the existence of the pressure difference tells us there must be a speed
difference.
Robert P. Bauman
Professor of Physics Emeritus
University of Alabama at Birmingham
So does the change in pressure cause the speed to increase or does the increase in speed cause the pressure to drop?
Either way the lift is a combination of the suck at the top of the wing and the curving of the airflow - possibly
Either way the lift is a combination of the suck at the top of the wing and the curving of the airflow - possibly
The latter crab. Speed change first. Henceforth the reduced pressure allows the object to backfill this space which is offering less resistance than was previously there. The "wing" in this instance moves upwards.
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TC, you are moving into the vacuum (suck theory) (Bernoulli).
The jist of the article in post #32 is that the air molecules have had work (force) applied to them, drawing them down, an equal and opposite force is applied to the wing (the item applying the force), lifting it up. It is not moving to fill a void.
The jist of the article in post #32 is that the air molecules have had work (force) applied to them, drawing them down, an equal and opposite force is applied to the wing (the item applying the force), lifting it up. It is not moving to fill a void.