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RE: Cambridge scientist debunks flying myth - 2/16/2012 11:32 AM

Tim Green

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quote:

ORIGINAL: Shoe

quote:

ORIGINAL: Tim Green
Air cannot support anything that's heavier than it.

A vehicle that has the same density as air still has weight. In order for that vehicle to be held airborne by the air, the air must provide an upward force equal to the vehicle's weight. I think everyone would agree that there is no momentum transfer when the air provides an upward force of this kind (you don't feel upwash or downwash around a hot-air balloon... yes RMH, flatulent passengers could provide such a thing). At first, this might appear to be a violation of Newton's Laws. The air pushing up on the balloon means that the balloon must be pushing down on the air (Newton's 3rd law). If the balloon is pushing down on the air, then doesn't Newton's 2nd law require the air's momentum to change? The answer is obviously ''no''. The reason the air's momentum doesn't change is because the downward force the balloon exerts on the air is balanced by an upward force that the ground exerts on the air. Newton's 2nd law only requires an object to experience a rate of momentum change when there is an UNBALANCED force on it. What does this have to do with airplanes? Although the mechanism is different (dynamic lift vs. buoyancy), the same fundamental principle still applies. If the air doesn't experience an unbalanced force, then it won't experience a rate of momentum change. Anyone suggesting the air MUST experience a change in its vertical momentum in order to generate a lift force is misapplying Newton's Laws. Is it POSSIBLE for air to experience a change in momentum associated with an aerodynamic force? Yes, but it is certainly not REQUIRED by Newton’s Laws.

quote:

ORIGINAL: Tim Green
Airplanes lift in response to the large amounts of air they accelerate in a downward direction.

Possibly. In addition to the large amounts of air that airplanes accelerate downward, airplanes also accelerate large amounts of air upward (there are plenty of photos showing smoke going downward behind a wing, and going upward outboard of the wing tips at the same time). As far as Newton is concerned, the air going upward is JUST as important as the air going downward. In order to show that airplanes lift in response to the large amounts of air they accelerate in a downward direction, you have to show that the difference between the rate of downward momentum transfer and the rate of upward momentum transfer is equal to the lift. Tim, you have not shown this, you have simply stated it to be the case. Those who have done a disciplined accounting (Drela, Lissaman, Betz, Kroo among others) have shown that the above statement is misleading at best. Under almost all conditions, the downward force exerted by a wing on the air is balanced by the upward force exerted by the ground on the air, and there is no net momentum transfer. If your statement is true, then it must be possible to provide an accounting for the total momentum (other than ''FEEEAAL the Physics'' ) that shows a direct balance between lift and the rate of net vertical momentum transfer. You say no math is required, and I would welcome an approach that doesn’t include math, but simply ignoring all the air that is being deflected upward isn’t a valid approach. If you can't provide such an accounting, then you're simply stating a belief that makes sense to you. I will point out that this belief isn’t consistent with the reality of the situation.

I used to believe in a balance between wing lift and downward momentum transfer. It wasn’t until I added up the momentum of all the downward going air and subtracted the momentum of all the upward going air that I realized this balance doesn’t exist. Still skeptical, I calculated the pressure footprint of a wing on the ground. Sure enough, under most conditions, the ground pushes up on the air with the same net force that the wing pushes down. Newton’s 2nd Law tells you this mean’s the wing isn’t changing the vertical momentum of the air. I suggest you take another look at Lissaman’s paper, it might change the way you look at airplanes.

Why don't you drop the nonsense about upwash being equal to downwash. It simply doesn't make any practical (or academic) sense. Props blow backward - not forward - no matter how many words you publish in an attempt to make them change direction. For wings, they blow downward, not upward. Same for rotors on choppers. And lets throw fans in as well - they all blow in one direction - that's how they work. If they blew two directions at the same time, they simply wouln't work. Please stop this nonsense.

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RE: Cambridge scientist debunks flying myth - 2/16/2012 11:49 AM

Tim Green

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quote:

ORIGINAL: rmh

quote:

ORIGINAL: Tim Green

quote:

ORIGINAL: rmh

quote:

ORIGINAL: Tim Green

quote:

ORIGINAL: rmh

quote:

ORIGINAL: Tim Green

[

. I think Einstein said, that if you cannot make a 5 year old understand your theory, then you don't understand it.

Lawyers told me the same thing -and it is true-juries need simple ,plain meaningful explanation
ALL the varied explanations of lift /drag etc., go to one simple fact
Lift and propulsion -by whatever means- have lowest pressure in direction of intended movement- highest in opposite direction
parachutes have opposite pressure difference.
nature simply works in this manner.
How this is all done efficiently, and calculated for power needed etc., is another thing alltogether
(only two patents? -I' ll see your two and raise you two more)

I've got to ask - why do you believe that wings, rotors, etc., don't react to the air they shove down?

Because Newton does believe that - so why don't you?

Air pressure (you believe in that ?) seeks a steady pressure state.
ANY unbalance in pressure tries to rebalance - so the ''shove'' is nothing more than pressure trying to rebalance
It's just that simple

You didn't answer the question - why do you believe that wings don't react to the air they accelerate and cause to move in a downward direction?

IOW - is it true that you don't believe anything physical, like a wing, reacts to the change in momentum (downward and faster) produced in the air by a wing passes through that air?

I believe in the Easter Bunny but I am not influenced by descriptions re momentum exchange.
They simply describe movements which occur in some cases
really - differences in pressure is all lift is about.

I'm sorry, but the Easter bunny's not going to let this slide.

Ok - So it's plain that you don't believe in Newton's principles of action and reaction when it comes to air. Although texts are not in sync regarding causes of lift, I believe that all physics texts are in sync regarding action and reaction when it comes to moving volumes of air in regards to f=ma. That a body will react with a given force when it accelerates a given mass of air according to the equation f=ma. No physics texts hold this point in dispute. So I'm confused as to where you derived your statements from? The bunny, perhaps?

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RE: Cambridge scientist debunks flying myth - 2/16/2012 12:25 PM

Yak 52

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You clearly do not understand how a prop or rotor works. The downwash field from the blade or wing can be measured and mathmatically calculated. The upwash is beneath the blade, as is the downwash. What you feel beneath the rotor or behind the prop is NOT the downwash it's a slipstream that has been accelerated by the lift of the 'wing' in the system.

You can 'feeeel the physics' but it's also helpful if you can correctly identify which physics you are feeling. It appears that you are able to do with out instrumentation and can accurately conjure the correct understanding in your mind. Sounds like mystics not physics to me.

And no one is arguing 'against' Newtonian physics... it's just more complex than that.

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RE: Cambridge scientist debunks flying myth - 2/16/2012 1:18 PM

Shoe

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I can't stop the "nonsense" you refer to because it just happens to be reality. You are basing your description of how wings work on the physics of props/rotors/fans/leaf blowers. There are subtle but very important differences between wings and spinning things that have compact wake structures. Unfortunately, you have to open your mind to be able to appreciate this. If you can't see that wings also deflect air upward (and there's a WHOLE lot more air outboard of a wing than between the wingtips), then you clearly haven't spent enough time studying the airflow around a wing. Please stop your nonsense about transfer of net downward momentum to the air until you can SHOW that it exists. Just saying it exists doesn't make it so. Several posters have provided a rigorous accounting that shows your position is flawed. You have provided essentially nothing beyond "this appeals to my intuition". Sorry, but that doesn't cut it, your intuition has led you astray with respect to wings.

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RE: Cambridge scientist debunks flying myth - 2/16/2012 4:54 PM

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Tim

I guess that your simplistic view of lift is derived from your simplistic view of Newton's second law. F=MA is a special case of Newton's second law.

Newton's second law says that the Force is equal to the product of the mass times the time rate of change of the momentum. Or at least that's what my physic book says. Perhaps it does dispute others that you have read.

I do agree that it is only necessary to develop an intuitive understanding of Newton's laws to fly our planes (feel the physics), but to proscribe that a thorough and detailed analysis of lift is "nonsense" is just absurd.

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RE: Cambridge scientist debunks flying myth - 2/16/2012 7:32 PM

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quote:

ORIGINAL: Yak 52

What you feel beneath the rotor or behind the prop is NOT the downwash it's a slipstream that has been accelerated by the lift of the 'wing' in the system.

But,.........that is the momentum transfer which existence Shoe denies and claims that he has demonstrated here.

Then,.........is the atmosphere heavier when an airplane flies in it?
Maybe this time somebody chooses not to ignore my question and explains why.

quote:

ORIGINAL: Shoe

Please stop your nonsense about transfer of net downward momentum to the air until you can SHOW that it exists. Just saying it exists doesn't make it so. Several posters have provided a rigorous accounting that shows your position is flawed. You have provided essentially nothing beyond ''this appeals to my intuition''.

With all due respect, Shoe, nobody has proven anything in this discussion yet.
Regarding scientific demonstration, Tim's points are as valid as anyone's else, IMHO.

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RE: Cambridge scientist debunks flying myth - 2/16/2012 8:48 PM

Shoe

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ORIGINAL: Lnewqban
Then,.........is the atmosphere heavier when an airplane flies in it?

The weight of the atmosphere isn't changed, but the pressure it exerts on the ground is increased (assuming the density of the airplane is greater than that of the air).

This is similar to when a boat is set afloat on the ocean. Because a boat displaces water, it increases the depth of the ocean VERY slightly. This extra depth means that the ocean pushes down on the earth with slightly more pressure. All the additional pressure times area adds up to the weight of the boat.

For an airplane in ground effect, the pressure "footprint" is fairly compact. For an airplane hundreds of spans above the ground, the pressure footprint is enormous (and the amount of pressure increase correspondingly VERY small).

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RE: Cambridge scientist debunks flying myth - 2/16/2012 9:27 PM

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Others have argued that this was incorrect. I never completly bought this explaination as the case of the two streams meeting at the same point only occured on some airfoils.

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RE: Cambridge scientist debunks flying myth - 2/16/2012 9:30 PM

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Please don't tell the communists and liberals or they will claim the earth is threatened by global over pressurization.

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RE: Cambridge scientist debunks flying myth - 2/17/2012 2:39 PM

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quote:

ORIGINAL: Shoe

The weight of the atmosphere isn't changed, but the pressure it exerts on the ground is increased (assuming the density of the airplane is greater than that of the air).

Shoe,

From my point of view, the atmosphere and airplane as a whole system, weights more than the atmosphere alone when the airplane is sitting on the tarmac.

Hence, the atmosphere becomes the means to connect the airplane and the Earth and its gravitational field.

A crane lifting that same airplane would be accomplishing the same effect.

The forces involved in the case of the crane are all static (no need to use energy since no work or heat exchange happens there).

The case of an airplane is different, the lift and drag forces are dynamic, they need to be constantly fed, and because of that they are associated to movement and work, which is transferred energy.

If the airplane is not able to develop any interaction with magnetic or gravitational fields, then its only option is to resort to the mechanical trick of trading momentum by supporting force.

The concept of flying happening without a net exchange of momentum with air is not logical to me.
We cannot fly on the wing above the mass of air that we call atmosphere, because we need to impart movement to that mass and obtain a reactive lifting force.

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RE: Cambridge scientist debunks flying myth - 2/17/2012 4:14 PM

Shoe

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The concept of a wing creating lift without momentum exchange doesn't really appeal to me either. The idea that lift is the result of a wing pushing down and imparting downward momentum to the air makes for a very tidy and intuitive explanation. It just makes sense from a Newtonian point of view. There's only one itty bitty problem with this explanation. If it's accurate, then you should be able to add up all the momentum being added to the air and show that the rate of change is equal and opposite the lift force. This addition requires some care, but it's certainly something you can do. When you do this, you are faced with the rather inconvenient result that, for the case of a wing in steady flight, there isn't any momentum exchange going on. Sure the air is deflected downward behind the wing (rather vigorously), but the air is also deflected upward outboard of the wingtips (not as vigorously, but there's a lot more air set in motion outboard of the wingtips than between them). I'm not aware of any way to determine the relative magnitudes of the upward and downward contributions short of actually adding them up and comparing them. Again, when you do this you find that they are the same... no net momentum transfer. Is this result in some way inconsistent with Newton's Laws? No. Again, as long as there is no unbalanced force being exerted on the air, then there is no need for its momentum to change. The same analysis that allows you to add up the momentum allows you to find a wing's pressure footprint on the ground. The upward force from this pressure footprint sums up to equal the lift. Not a very satisfying or particularly intuitive result, but that's how a wing in steady flight does its business.

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RE: Cambridge scientist debunks flying myth - 2/17/2012 4:28 PM

Tim Green

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quote:

ORIGINAL: Yak 52

You clearly do not understand how a prop or rotor works. The downwash field from the blade or wing can be measured and mathmatically calculated. The upwash is beneath the blade, as is the downwash. What you feel beneath the rotor or behind the prop is NOT the downwash it's a slipstream that has been accelerated by the lift of the 'wing' in the system.

You can 'feeeel the physics' but it's also helpful if you can correctly identify which physics you are feeling. It appears that you are able to do with out instrumentation and can accurately conjure the correct understanding in your mind. Sounds like mystics not physics to me.

And no one is arguing 'against' Newtonian physics... it's just more complex than that.

The terminology is out of my hands. I'll continue to use downwash, as it's more common than slipstream.

Downwash: The air a wing accelerates in a downward direction.
Lift: An opposing reaction by a wing to the downwash (f=ma) generated by that wing.

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RE: Cambridge scientist debunks flying myth - 2/17/2012 4:44 PM

Tim Green

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quote:

ORIGINAL: Shoe

The concept of a wing creating lift without momentum exchange doesn't really appeal to me either. The idea that lift is the result of a wing pushing down and imparting downward momentum to the air makes for a very tidy and intuitive explanation. It just makes sense from a Newtonian point of view. There's only one itty bitty problem with this explanation. If it's accurate, then you should be able to add up all the momentum being added to the air and show that the rate of change is equal and opposite the lift force. This addition requires some care, but it's certainly something you can do. When you do this, you are faced with the rather inconvenient result that, for the case of a wing in steady flight, there isn't any momentum exchange going on. Sure the air is deflected downward behind the wing (rather vigorously), but the air is also deflected upward outboard of the wingtips (not as vigorously, but there's a lot more air set in motion outboard of the wingtips than between them). I'm not aware of any way to determine the relative magnitudes of the upward and downward contributions short of actually adding them up and comparing them. Again, when you do this you find that they are the same... no net momentum transfer. Is this result in some way inconsistent with Newton's Laws? No. Again, as long as there is no unbalanced force being exerted on the air, then there is no need for its momentum to change. The same analysis that allows you to add up the momentum allows you to find a wing's pressure footprint on the ground. The upward force from this pressure footprint sums up to equal the lift. Not a very satisfying or particularly intuitive result, but that's how a wing in steady flight does its business.

Shoe - a rotor is a rotating wing. It shoves air down, not up. Also, some have hovered their rc choppers over scales, and measured the force of the downwash - it all pretty much added up as it should have. The reading on the scales are pretty much equal to a choppers static weight - depending on how accurately the experiment is carried out.

The wing vortices you speak of do not cancel out the downwash - if they did - we wouldn't have lift. Yeah, there's some upwash in front of the wing, but it's captured by the wing (air crests top of wing, and Coanda effect "glues" air to top back surface of wing), accelerated and sent back downward ultimately (as it follows the top of the wing (thanks to Coanda effect) down to the trailing edge). Note that the energy in the upwash isn't even close to canceling all the energy in the downwash. That's why fans, etc., don't blow both directions at once. Even though they experience upwash, as well as downwash.

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RE: Cambridge scientist debunks flying myth - 2/17/2012 5:19 PM

Shoe

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Interesting to see the conclusion you draw from the RC helicopter hovering over a scale. I think we'd agree the rotor is pushing down on the air with a force equal to the weight of the helicopter. The reading on the scale shows it is pushing up on the air with a force almost exactly equal to the weight of the helicopter. If the rotor and the scale are the only two things pushing up or down on the air, then the air is not experiencing an unbalanced force. If the air is not experiencing an unbalanced force then Newton's 2nd Law requires it's rate of change of momentum to be zero. Counterintuitive, but the rotor in ground effect is adding as much upward as downward momentum to the air.

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RE: Cambridge scientist debunks flying myth - 2/17/2012 6:16 PM

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quote:

ORIGINAL: Shoe

There's only one itty bitty problem with this explanation. If it's accurate, then you should be able to add up all the momentum being added to the air and show that the rate of change is equal and opposite the lift force. This addition requires some care, but it's certainly something you can do. When you do this, you are faced with the rather inconvenient result that, for the case of a wing in steady flight, there isn't any momentum exchange going on.

Thanks, Shoe.
I have no reason to doubt about the accuracy and result of those calculations.

However, I insist, keeping an airplane flying requires work, which is energy.
That energy is transferred from the fuel (chemical energy) into the air (mechanical energy).

Mechanical energy is either heat or work.
If there isn't any momentum exchange, then all that energy becomes just heated air?

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RE: Cambridge scientist debunks flying myth - 2/17/2012 6:43 PM

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Maybe the effects of all these updrafts and downdrafts around/above/below wings are actually significantly reduced by the large amounts of air that is compressed to inflate billions of car, truck and aircraft tyres. Now all this air that is compressed is causing air pressure out there to drop, causing global warming and make aircraft more difficult to fly..........

Cheerz

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RE: Cambridge scientist debunks flying myth - 2/17/2012 6:50 PM

Shoe

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Eventually, when the flight is over and the hangar doors are closed, all the disturbances made by the airplane's flight will have been dissipated by viscosity. All you're then left with is the heating.

It's important to note that a wing's lift force is defined as being perpendicular to the wing's direction of travel. This means that the lift force, by definition, does no work (work results from displacement in the direction of a force). If there's any drag acting on the wing (and there usually is), then to maintain steady flight, you must provide power equal to: drag x speed. All the energy you are providing eventually ends up as heat, but in the near term, some of it goes into the kinetic energy of the trailing vortex system (causing induced drag), some of it goes almost directly to heat (through viscous shearing), and some of it goes into the turbulent motion of the air (viscosity dissipates this pretty quickly). There are other places the energy can go in the near term once you get transonic.

A wing generating lift without vertical momentum transfer will still require work/ energy/ power to maintain steady flight as long as there is drag acting on the wing.

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RE: Cambridge scientist debunks flying myth - 2/17/2012 9:18 PM

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Does not the vertical component of a wing (lift) overcome gravity which is accelerating the wing (and aircraft) downward at 32'/sec/sec not constitute work?
This discussion is getting good. I have an analogy: since we are discussing fluid mechanics really and we are trying to define how lift is created and it's resultant force on the earth and the atmosphere here is what I submit. Take an undersea submersible which is ballasted to slightly heavier than its displacement of water and put it in a swimming pool. we now have a heavier than water flying machine. This machine has dive plates (wings) and we start it up. First of all the volume of water in the pool can be measured and its weight determined. The sub also has weight which must be added to the weight of the water. We now have a closed system which is analagious to the atmosphere with an airplane in it. If we start the submersible, tilt the wings up the sub should move forward and rise off the bottom of the pool and manouver about (fly). The pool and sub still weigh as much as they ever did and the dive planes are lifting the sub off the bottom of the pool. Without a motor the sub would not move and without dive planes the sub would not lift off the bottom. This would be a closed system which allows us see that nothing is either added or subtracted from the experiment.
So what we have is a requirement to have enough power to move the sub fast enough to give the planes enough "bite" to fly through the water.
In a aircraft the choice of airfoil is determined by the job the aircraft has to do and the choice of engine has to do with moving the aircraft fast enough so the plane will fly.
The airfoil generates a tremendous vaccuum on the upper surface as well as pressure from below which I think is due to angle of attack. I have had poorly secured hatches "explode" off the top of a wing on a heavy camera plane.
The vortexs and downwash seen in the agricultural planes earlier are not contributing to the lift; they are the result of lift. Notice the tip vortex curling around to go to the low pressure area on the top and the downwash is a result of the aircraft deflecting the air downward just as the wake of a boat spreads out behind.
Peter

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RE: Cambridge scientist debunks flying myth - 2/18/2012 1:15 PM

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Yes- you can measure how much work a horse does by the weight,speed etc., involved
However , the size of the hoofprints in the road or amount of horse droppings resulting from the work are just that -

< Message edited by rmh -- 2/18/2012 8:33 PM >

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RE: Cambridge scientist debunks flying myth - 2/19/2012 2:41 PM

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quote:

ORIGINAL: Bundubasher

Maybe the effects of all these updrafts and downdrafts around/above/below wings are actually significantly reduced by the large amounts of air that is compressed to inflate billions of car, truck and aircraft tyres. Now all this air that is compressed is causing air pressure out there to drop, causing global warming and make aircraft more difficult to fly..........

...................Not far from reality

< Message edited by Lnewqban -- 2/19/2012 5:03 PM >

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RE: Cambridge scientist debunks flying myth - 2/19/2012 5:00 PM

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Two beautiful conclusions:

quote:

ORIGINAL: Shoe

A wing generating lift without vertical momentum transfer will still require work/ energy/ power to maintain steady flight as long as there is drag acting on the wing.

quote:

ORIGINAL: PeterC

Does not the vertical component of a wing (lift) overcome gravity which is accelerating the wing (and aircraft) downward at 32'/sec/sec not constitute work?

Lift and drag are just two conventions (one perfectly vertical vector and one perfectly horizontal vector) that we make out of a single (reactive) natural force.

That single reactive force consumes energy or work (from the fuel or from the gravitational acceleration), which manifests itself in a transfer of a combination of momentum and heat.

In other words, anything that flies (or floats) on its wings (or fins) MUST accelerate and heat the fluid around it.

Check this video out:

http://www.youtube.com/watch?v=UqBmdZ-BNig

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RE: Cambridge scientist debunks flying myth - 2/19/2012 6:05 PM

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A bit of a nit pick, but lift is traditionally defined as the force component acting perpendicular to a wing's direction of motion. Drag is traditionally defined as the force component acting opposite a wing's direction of motion. Only in the case where a wing's motion is a straight horizontal line do these definitions match up with the vertical and horizontal components you suggest. With the traditional definitions, the lift can't consume any work or energy because there is no displacement of the wing in the direction of the lift. As an analogy, if you were to grab a 10kg object from a 1m high shelf, carry it across a room and place it on another 1m high shelf, the upward force you exerted on the object while carrying it would not have done any work (the object's total energy is unchanged). It's the drag force that consumes all the energy.

I'm OK with the statement that a wing must accelerate the air around it to fly. You can accelerate the air without adding net momentum as long as you don't accelerate all the air in the same direction. Yes, you ultimately heat the air, but I don't think that's a feature of the process that really contributes to an explanation of how lift is generated.

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RE: Cambridge scientist debunks flying myth - 2/28/2012 10:11 AM

Shoe

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From: Stuttgart, GERMANY
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Apologies for the lengthy post, but I have a (fairly) simple “thought experiment” that might help convey my concerns about the wing momentum transfer discussion.

Imagine you are standing next to a long, straight, level stretch of railroad track (frictionless of course), with a string of boxcars on it extending for a mile in either direction. If you were to exert a force of magnitude F on one of the boxcars, and the direction of the force was to the right as you faced the track, then Newton’s Second Law says the string of boxcars would accelerate to your right at a rate of F/m (where m is the total mass of the string of boxcars). Newton’s Third Law says you would experience a “reaction” force to your left of magnitude F.

Now imagine that you took the track directly underneath the string of boxcars and bent it into a circle with a circumference of 2 miles (so you’d be able to connect the first and last boxcars). If you were to again exert a force of magnitude F (to your right) on a boxcar, the result would appear very much the same. The boxcars in front of you would accelerate to the right at a rate of F/m, and you would experience a reaction force to your left of magnitude F. There's a difference though...

In the case of the straight track, the center of mass of the string of boxcars accelerates to your right at a rate of F/m (the string experiences net momentum change). In the case of the circular track, the boxcars in front of you accelerate to your right at a rate of F/m, but their counterparts on the opposite side of the circle accelerate to your left at a rate of F/m. The center of mass of the circular string of boxcars remains at the center of the circle (no net momentum change). The harder you push to the right, the harder the track pushes the string of boxcars to the left to keep them on the track. This example shows it’s possible to achieve a reaction force by pushing against an object without transferring net momentum to it (even though you have put the object in motion). All that is required is that you cause something else push on that object with equal magnitude in the opposite direction.

If you make a mental leap from boxcars to “bits” of air, which of the above setups better represents a lifting wing? Before going there, I think it's easiest to start out considering a RC helicopter hovering in ground effect over a scale. As pointed out a few posts back, the scale will show a reading equal to the helicopter's weight when the helicopter is sitting on it, and a reading very nearly equal to the helicopter's weight when the helicopter is hovering over it. This shows that the force exerted by the scale on the air is (very nearly) equal and opposite the force exerted by the helicopter rotor on the air. In theory, if you made the scale's "footprint" big enough and the hover low enough, the forces would balance exactly. For a rotor in ground effect, the air experiences no unbalanced force, and therefore its net momentum doesn't change. I think the circular train track setup is an excellent analogy to the helicopter in ground effect (where the force you exert on the boxcar is analogous to the force exerted by the helicopter rotor on the air, and the force exerted by the circular track on the boxcars is analogous to the force exerted by the scale on the air). A rotor in ground effect imparts no NET momentum to the air. Any downward momentum created in one location is offset instantaneously by upward momentum created elsewhere (if Newton is to be believed).

What happens as the helicopter moves out of ground effect? If it moves completely out of ground effect, it makes sense that the straight track setup becomes the better analogy. Far away from the ground, if the downward force exerted by the rotor on the air is no longer balanced by an upward force exerted by the ground (or scale) on the air, then the air can experience a net rate of vertical momentum change.

How about a wing in steady, level flight? In ground effect, I think it again makes sense that the circular track is the more accurate analogy (the ground inhibits any net momentum transfer). How about as you move the wing out of ground effect? It's tempting to conclude that the straight track becomes the better analogy, just as it did for the rotor, but this is not necessarily the case. If you account for all of the air set in motion, it turns out that a lifting wing (unlike a rotor out of ground effect) adds just as much upward as downward momentum to the air (if the aspect ratio of your control volume is constrained by the ground). It has been suggested that a balance between upward and downward momentum transfer would mean the wing couldn’t produce lift. The circular track example above illustrates how it is possible have a reaction force without net momentum transfer.

The bottom line is that both cases (the straight and the circular track) represent POSSIBLE ways to model the underlying physics of a wing flying out of ground effect (both are entirely consistent with Newton’s Laws). To know which one better represents what’s really going on, you have to actually sum up all the momentum transfer. Those who have done the summation have concluded that the circular track analogy better represents how a wing pushes against the air to generate lift. Those who suggest that a wing works by transferring downward momentum to the air need to qualify that statement by noting that, in a global sense, the air’s net downward momentum doesn’t change. Upward momentum transfer is just as much a feature of wing-generated lift as downward transfer. Wings do not generate force the same way rockets do (rockets use pure, unadulterated NET momentum exchange), and they are different from rotors and props in subtle, but important ways.

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(in reply to Tim Green)

Post #: 98

RE: Cambridge scientist debunks flying myth - 2/29/2012 8:18 AM

beepee

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For me, the train analogy does not work. You are not transferring linear momentum to the train cars in the circle, you are transferring rotational momentum. It is still momentum.

Bedford

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Post #: 99

RE: Cambridge scientist debunks flying myth - 2/29/2012 9:52 AM

Shoe

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From: Stuttgart, GERMANY
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quote:

ORIGINAL: beepee
For me, the train analogy does not work. You are not transferring linear momentum to the train cars in the circle, you are transferring rotational momentum. It is still momentum.

Bedford

Fair enough, but several here have invoked Newton's Second Law to suggest a required balance between the rate of change of the air's linear vertical momentum and the lift force acting on the wing. The point I am trying to make is that balance doesn't exist (it's definitely not required by Newton's Laws). Newton would certainly make a distinction between linear and angular momentum.

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