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The truth is that there is no requirement for nature to understand our definitions, our formulae, or even basic mathematics. We can fly because of how nature behaves, not because of how we describe it. We make up a bunch of words to describe what we have observed, then realize it is a bit wrong and fix it. Then repeat.
The descriptions that start with single molecules of air and Newtonian Law are the most accurate place to start. When the system is real-sized, this description becomes far too complex to solve and offers no intuition. The descriptions that deal with airflow shapes, multiple engineering terms, pressure fields, etc. are attempts to simplify the Newtonian calculations using assumptions and bounds with the caveat that the accuracy may reduce. and may in fact be completely wrong (especially true in earlier models). The descriptions that are given to pilots typically are lacking scientific accuracy, contain very few formulae, and don't yield useful results. However, they are the ones that are most intuitive and tend to be the easiest way to generate sufficient understanding to fly the airplane.
In the end, every explanation is inaccurate to some extent. The most accurate explanation is the Newtonian one. The complex explanation is just a derivation from Newtonian physics. Talking only about pressure gradients lies between these other two methods, and I think is just confusing.
This is how I like to think of it:
- First, think of just one molecule coming towards a wing. There is only one thing the wing can do to help the wing counter gravity, that is to deflect the molecule downwards.
- If you continue to deflect more molecules downwards, you will have more molecules below the level of the wing than you have above, so you will have greater pressure below.
- The molecules will move from highest pressure to lowest pressure. We've seen this, it seems intuitive and we've even added words to the physicist's vocabulary to explain why. Entropy, Brownian motion, etc.
- The move from high to low manifests itself in a number of ways, most of which are important for aerodynamicists to understand. Flow behind a wing can be upwards. Wing tip vortices will develop. There is a lot of flow disruption behind the wing, but in the production of lift, the wing only causes the initial effect. Everything else is a result of the disturbances to the air that have already been caused.
- The pressure gradient (created by moving air molecules downwards) will also change the flow in front of the wing. I think this is a key point to acknowledge, because this one seems to conflict with the starting point of considering just one molecule. It doesn't conflict, but does require that we now think of one molecule in front of the wing that already has some vertical velocity.
- If the single molecule starting point is still causing some concern, then consider a group of molecules, bumping into each other with a net vertical velocity. What can the wing do to that group to help the wing counter gravity? Would just compressing them help at all? Or would you have to change the net vertical velocity as well?
Sorry, no formulae to punch into your calculators. Just a description of what I believe is going on.
Capt Pit Bull, according to your understanding of physics, falling bodies don't accelerate, because otherwise momentum wouldn't be conserved. Include gravity into your consideration of downwash, and you will find that momentum and energy are conserved.
Ground effect reduces drag, and hence increases L/D, giving a gain in performance. Its not a "lift increaser", its a "drag decreaser".
Sorry, have to disagree with that.
There's a definite shift in the lift-curve slope, with increased CL at a given AoA (at least at low AoA), in ground effect. It's not just a drag effect.
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Originally Posted by Jetstream Rider View Post
Ground effect reduces drag, and hence increases L/D, giving a gain in performance. Its not a "lift increaser", its a "drag decreaser".
Sorry, have to disagree with that.
There's a definite shift in the lift-curve slope, with increased CL at a given AoA (at least at low AoA), in ground effect. It's not just a drag effect.
I stand corrected.
The particles bouncing off the lower surface theory does not account for the fact that around 4/5 of the lift comes from the top surface (see Kermode, early pages). Have a look here:
Forwards and backwards on the arrows at the bottom give more good info.
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To sum up, the wing throws air at the ground. No downwash, no lift.
I disagree. Look at the flow sim and lift over a spinning cylinder. Up = down, especially on the cylinder, yet you get a large amount of lift. We only don't use that for aeroplanes as the drag is very high.
The maths of flow is by nature an approximation, or a "trick" to get the world to work. Bernoulli's equation is a description, not the whole text.
Jetstream Rider, that link you provided only discusses one idea on how the downwash is created. The "4/5 of lift [that] comes from the top surface" is also generated by deflecting air molecules against gravity, its just that its not doing it through a collision with the lower surface of the wing.
Interestingly, areas of lower pressure don't pull. Instead, areas of higher pressure push. Granted, these are definitions we have made, but intuitively, what is attaching the area of low pressure to the surface it is supposedly pulling on? With this in mind, I'd find it interesting to see how Kermode has decided that the top surface is producing more lift than the bottom surface.
The spinning cylinder redirects airflow. Creates downwash in the production of lift. This is consistent with momentum theory (and inconsistent with "skipping stone" theory). Not sure why you brought this up.
Indeed there is no "pull" from sucking, pressure can only push.
Most of the lift is due to the top surface geometry, hence 4/5 coming form the top of the wing. Take away that geometry and it just doesn't work the same way.
The spinning cylinder directs as much air up as it does down - we can't choose to ignore the upwash and only look at the downwash, so yes the air is re-directed, but as much up as down. Resolving the entire flow, as much momentum goes up and goes down, so the skipping stone theory doesn't add up. There are further links on the NASA page I linked to above.
There is no need to "attach" low pressure to the wing - the wing is pushed in to the air by the high pressure below, but its the top surface that creates the low pressure that makes the pressure below the wing the higher of the two.
Certainly the skipping stone theory is incorrect, although along with the equal time fallacy, there may be truth mixed in with the error.
Fallacy Number 1. The "Longer Path" theory, or the "Equal Transit Time" theory. This theory is one of the most widely circulated, incorrect explanations. The theory states that airfoils are shaped with the upper surface longer than the bottom. The air molecules have farther to travel over the top of the airfoil than along the bottom. In order to meet up at the trailing edge, the molecules going over the top of the wing must travel faster than the molecules moving under the wing. Because the upper flow is faster, then, from Bernoulli's equation, the pressure is lower. The difference in pressure across the airfoil produces the lift.
Fallacy Number 2. The "Skipping Stone" theory is often seen on web sites and in popular literature. The theory is based on the idea that lift is the reaction force to air molecules striking the bottom of the airfoil as it moves through the air. Because this is similar to the way in which a flat rock thrown at a shallow angle skips across a body of water, it is called the "Skipping Stone" theory of lift. It is sometimes called a Newtonian theory of lift, since it involves Newton's third law.
Fallacy Number 3. The “Venturi” theory is based on the idea that the airfoil upper surface is shaped to act as a nozzle which accelerates the flow. Such a nozzle configuration is called a Venturi nozzle and it can be analyzed classically. Considering the conservation of mass, the mass flowing past any point in the nozzle is a constant; the mass flow rate of a Venturi nozzle is a constant. The mass flow rate m dot is equal to the density r times the velocity V times the flow area A:
m dot = r * V * A = constant
For a constant density, decreasing the area increases the velocity.
Turning to the incorrect airfoil theory, the top of the airfoil is curved, which constricts the flow. Since the area is decreased, the velocity over the top of the foil is increased. Then from Bernoulli's equation, higher velocity produces a lower pressure on the upper surface. The low pressure over the upper surface of the airfoil produces the lift.
The Real Story. There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments.
Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning
From a Newtonian perspective, lift is generated by turning a flow of air. The flow turning creates a downwash from the wing.
Ground Effect. The proximity of the ground reduces the downwash. At a height equal to the span the induced drag is reduced by 2%. At a height equal to 1/10th the span induced drag is reduced by 50%. The reduction in drag will only occur where the induced drag is predominant. At high speeds where parasite drag is predominant ground effect will not result in a drag reduction. Ground effect will only be significant during take off and landing. As the ground is approached during landing the induced drag progressively decreases and a constant angle of attack will produce an increasing CL, and the reduced downwash produces a change in longitudinal stability and a change in trim. The reduction in induced drag and increasing CL will cause the aircraft to “float”.
Am I naïve to think that Dr. Dick Whitcomb, inventor of area rule, the supercritical wing and winglets, and his colleagues may know what they are talking about?
if you can do both differential and integral calculus--and have some background in mechanics---I highly recommend the following book listed below---trust me it's a light book in terms of the mathematics of aerodynamics--but it give a lot of the true basis of section lift-though geometric 2D airfoil analysis---as well several--{hundered} or so pages of data on various NACA airfoils---very good for beginning aero--injunirs and such
Ira H. Abbott and Albert Von Doenhoff ---Theory of Wing Sections [1959]
Capt Pit Bull, according to your understanding of physics, falling bodies don't accelerate, because otherwise momentum wouldn't be conserved.
Well, that might be your understanding of my understanding!
Falling body acceleration conserves momentum quite happily, you just need to consider the other mass involved in the gravitational attraction. You don't need any air around to explain it either.
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Include gravity into your consideration of downwash, and you will find that momentum and energy are conserved.
Like I said, expand the view to a 3 dimensional model and conserving momentum become easy. Two vortices with equal and opposite angular momentum (for unaccelerated flight of course), job done.
pb, you are correct, that was my understanding of your thread. However, you did say that momentum was not conserved when the air goes down. However, the free falling mass increases its momentum and you contend that momentum is conserved in that case because the earth moves.
What if the free falling mass started hurtling small bundles of mass to the ground with sufficient effort that the large mass reduced its momentum to zero? Get my point?
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Like I said, expand the view to a 3 dimensional model and conserving momentum become easy. Two vortices with equal and opposite angular momentum (for unaccelerated flight of course), job done.
This sounds like a gyro-levitation device (those don't work). If creating two equal and opposite vortices is the action, then the reaction is going to be two equal and opposite moments. That would sum to zero as well.
Lift and drag are mechanical forces generated on the surface of an object as it interacts with a fluid. The net fluid force is generated by the pressure acting over the entire surface of a closed body. The pressure varies around a body in a moving fluid because it is related to the fluid momentum (mass times velocity). The velocity varies around the body because of the flow deflection described above.
I'm happy that mathematical descriptions for lift are far from exact, are necessarily approximations and sometimes are quite wacky (yet still yield good results). I'm also happy that we don't have an analysis toolkit that is good enough to describe flow and lift adequately (especially not turbulence), regardless of computer size or complexity of model. What I was after by starting this thread was to improve my conceptual understanding.
Anderson says "...the aerodynamic forces and moment on the body are due to only two basic sources: 1. Pressure distribution over the body surface 2. Shear stress distribution over the body surface".
My current understanding therefore is that in a flow, energy, mass, and momentum are conserved. In doing so and by passing flow around a closed surface the pressures and therefore forces within that fluid change. This causes lift, by a number of mechanisms including viscosity and we can roughly
describe it. The force on the closed body is caused by pressure changes and shear stress distribution, these pressure changes are caused by "turning" the flow and by the fact it has momentum. By turning it carefully and with experience we can approximate what nature will give us.
In short, lift is complex and hard to understand and we've not got that near describing it exactly by using mathematics. We do know however that it is caused by pressure changes within a fluid, caused by flow turning and the fact it has momentum.
You seem to be extrapolating my posts beyond what I've stated.
My position is simply that lift can not be adequately explained by a simple 2d model, and that the 'air deflected downwards' is not a complete model.
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What if the free falling mass started hurtling small bundles of mass to the ground with sufficient effort that the large mass reduced its momentum to zero? Get my point?
Sure. I don't dispute it.
But extend your model. Where are these lumps of mass coming from, and where do they go after they've impacted the other body?
Weather systems aside, if you took a large fleet of aircraft and just flew them around all day, would the atmosphere get gradually compressed in the lower levels as the wings deflected air downwards?
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This sounds like a gyro-levitation device (those don't work).
No mention of Gyro theory from me. I'm talking about a wing, and wake vortices.
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If creating two equal and opposite vortices is the action, then the reaction is going to be two equal and opposite moments. That would sum to zero as well.
The moments sum to zero, (which is why our aircraft isn't undergoing any angular acceleration about its longitudinal axis.) However, given that the two wings are on opposite sides of the aircraft, this is consistent with an upwards force on both wings.
Bottom line is most of this comes into the 'picture paints a thousand words' category, so I'll leave it there for tonight.
Interesting to explain wing lift scientifically/mathematically/physically or whatever but the bird flying around in the pressurised cabin of an aircraft nevertheless transfers its weight to the aircraft by air pressure waves and the aircraft then transfers its weight, which includes that of the bird, to the earth's surface by more pressure waves.
Boats/ships do the same thing. Space things don't.
...aircraft then transfers its weight, which includes that of the bird, to the earth's surface by more pressure waves.
Really?
I can deal with the aircraft exerting the same force on the air as the air exerts on the aircraft, but is the force of the aircraft on the air not taken up by viscosity and eventually heating?
I'm not saying you're wrong, but could you explain it a bit more please?
The hydrofoil at rest is a displacement vessel (Archimedes and all that).
But as it gathers speed, its weight is taken up by the foils, and the water reacts to the dynamic load of the downwash of the foils. If it were a model hydrofoil in a long tank aboard an aircraft, its wing still carries the same load.
Just as I suspected, with one or two illustrious exceptions -
I've been putting my life in the hands of pilots for years, and you still haven't agreed how the wings work !
I've always gone for a mixof the ' airflow deflected down ' idea- I've been under low flying fast jets, and that's how it seemed to me ! Mind you they were at relatively high alpha, flaps & power on, for chase work.
Also the ' baby sucking on a straw ' pressure theories, but have only studied aeronatics slightly compared to most posting here.
I once attended a lecture by Bill Bedford, when he was asked by an idiot ( who naturally qualified later as a BAe 'manager' for about 5 minutes ) " what new developments are in the pipeline, and will we see the P-1154 ?"
This was 1978 ! Bill gave him a withering look, and replied " well we haven't got anti-gravity paint worked out yet ! "
Last edited by Double Zero : 30th June 2008 at 19:58.
So what I wrote above would be correct - forces are transferred to the wing by pressure forces, caused by turning the air. These pressure forces are as a result of conserving momentum, energy and mass. The body feeling the reaction of the force imparted to the flow.
I'm quite happy with that, with two exceptions.
How does one explain lift without downwash (spinning cylinder and reflex trailing edge) and lift without power for infinite spans?
I don't think "lift is produced by throwing air at the ground" is quite as accurate as that. Surely "lift is produced by turning air and feeling the reaction" would be better - albeit with the ground (or other solid objects) eventually taking the momentum away from the air.
As I said I am no expert ( did play with a wind tunnel a few times ) but surely what you're describing is ground effect, not true wing lift - velocity, chord & all that ?
--but it give a lot of the true basis of section lift-though geometric 2D airfoil analysis---as well several--{hundered} or so pages of data on various NACA airfoils---very good for beginning aero--injunirs and such
Linear Mode
