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Old Jan 01, 2013, 12:59 PM
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OK, so the downwash does affect the airplane once it's off the surfaces of the plane.

My point being that in a steady subsonic flow, a change in the boundary conditions (a change in a wing's height above the ground, or the deflection of a flap for example) will, in general, change the flow conditions everywhere else in the flow.
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Old Jan 01, 2013, 01:09 PM
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But not when it's away any distance from the plane.. wingspan is the usual distance beyond which the air doesn't/can't affect the plane.
The rotor/airflow downwash is "trapped" by the ground. Other than blowing your hair around if you're near, it has no effect on the plane. Listening to and seeing the downwash hit the ground behind a large airplane landing is an interesting diversion at those locations where one can get close to the approach path, and not be "noticed" in an official manner..
The swirling noise and dust movement is pronounced behind a 747, for instance. At the middle marker, where the plane is 250' up, the plane will be touching down a mile away when the turbulence gets to ground level.
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Old Jan 01, 2013, 02:53 PM
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Originally Posted by Sparky Paul View Post
But not when it's away any distance from the plane.. wingspan is the usual distance beyond which the air doesn't/can't affect the plane.
The rotor/airflow downwash is "trapped" by the ground. Other than blowing your hair around if you're near, it has no effect on the plane. Listening to and seeing the downwash hit the ground behind a large airplane landing is an interesting diversion at those locations where one can get close to the approach path, and not be "noticed" in an official manner..
The swirling noise and dust movement is pronounced behind a 747, for instance. At the middle marker, where the plane is 250' up, the plane will be touching down a mile away when the turbulence gets to ground level.
Here's another take - note that last paragraph is a disclaimer of sorts. Also, these two authors modified their theory since their last publication. Ground effect is not completely known yet, it seems.

From http://www.allstar.fiu.edu/AERO/Flightrevisited.pdf ...

Ground Effect

The concept of ground effect is well known to pilots. This effect is the increase in efficiency of a wing as it comes to within about a wingís length of the ground. The effect increases with the reduction in the distance to the ground. A low-wing airplane will experience a reduction in the induced drag of as much as 50 percent just before touchdown. This reduction in drag just above a surface is used by large birds, which can often be seen flying just above the surface of the water. Pilots taking off from deep-grass or soft runways also use ground effect. The pilot is able to lift the airplane off the soft surface at a speed too slow to maintain flight out of ground effect. This reduces the resistance on the wheels and allows the airplane to accelerate to a higher speed before climbing out of ground effect.

What is the cause of this reduction in drag? There are two contributions that can be credited with the reduction in drag. The ground influences the flow field around the wing which, for a given angle of attack, increases the lift. But, at the same time, there is a reduction in downwash. It can be surmised that this additional lift must come from an increase in pressure between the wing and the ground. In addition, since lift is increased for a given angle of attack, the angle of attack can be reduced for the same lift, resulting in less downwash and less induced drag.

Ground effect introduces a fundamental change from the discussion of flight at altitude. When no ground is present, the relationship between lift, drag and downwash is straight forward. But, near the ground, there is an action-reaction between the wing, the air and the ground. At altitude the ground is so distant that this effect does not exist. Near the ground this interaction helps produce lift and reduce downwash due to an increase in pressure below the wing.

The details of ground effect are extremely complex. Most aerospace texts devote a paragraph or two and donít attempt to describe it in depth. The truth is that so much is changing in ground effect that it is difficult to describe by pointing to a single change in the air flow or a term in an equation. There is no simple way to describe how the airflow adjusts to satisfy the change in conditions.
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Old Jan 01, 2013, 02:57 PM
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You always have to remember that pressure waves propagate at the speed of sound. I imagine that ground effect does not affect a plane flying faster than sound, but that would be a bit hard to test. Incidentally, some VTOL configurations suffer from a "suck down" effect when near the ground. Particularly where thejet efflux is right under the wing, like in the case of the harrier. The fast flows coming from the jet engine create a lower pressure area under the wing, that causes the plane to stick to the ground. This can be mitigated by increasing the ground clearance and by doing a rolling takeoff.
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Old Jan 02, 2013, 05:46 AM
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Originally Posted by ShoeDLG View Post
OK, so the downwash does affect the airplane once it's off the surfaces of the plane.

My point being that in a steady subsonic flow, a change in the boundary conditions (a change in a wing's height above the ground, or the deflection of a flap for example) will, in general, change the flow conditions everywhere else in the flow.
Is it possible that treating the air as an incompressible fluid is incorrect? Could it be that, by introducing a second boundary (the ground), the density of the local parcel of air is increased and by operating in a denser medium the efficiency of lifting surfaces is improved?
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Old Jan 02, 2013, 05:51 AM
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Originally Posted by ShoeDLG View Post
can you say that the air experiences a downward acceleration equal to the force exerted on it by the rotor and an upward acceleration equal to the force exerted on it by the ground. This analysis considers the air to be a collection of particles that are free to move relative to each other, not a rigid monolithic mass.
bill,
thanks, i appreciate the thoughtful response

but all i really read is that instead if representing the air between the rotor and scale with a single piece of foam, its now represented by many layers, each transferring the upward force on its bottom surface to its top surface, and not of which experiencing any displacement, hence v = a = dm/dt = 0.


i don't understand why the air can't be considered parcels of air, the way it is considered when flowing over an airfoil surface, where a force is exerted on each surface of the parcel by surrounding parcels, causing it to accelerate and change both increase and decrease velocity.

it other words, the air between rotor and scale experiences a relatively constant (de) acceleration such that its velocity decreases from the maximum just beneath the rotor to zero (or more sideways) above the scale.


perhaps it's just plain wrong to apply f=ma to a fluid like air in this manner. Maybe "potential flow"(?) must be used, or whatever approach is used to determine the surface velocity of air as it moves across an airfoil from leading edge to trailing edge.

why is "net" momentum important?
why not local momentum in a small region below the rotor, above the scale and everywhere in between?
why not velocity at different points between the rotor and scale?
or why not displacement (to whatever extent it can be described)?

how would you describe the air in a wind tunnel?
aren't there walls in a wind tunnel that cause the air's velocity perpendicular to the fan to become zero?

really just looking for understanding
greg
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Old Jan 02, 2013, 07:01 AM
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bill,


perhaps it's just plain wrong to apply f=ma to a fluid like air in this manner.

why is "net" momentum important?
why not local momentum in a small region below the rotor, above the scale and everywhere in between?

really just looking for understanding
greg
I was taught, many years ago, that f=ma only applies to rigid bodies with constant mass or, if the body is not rigid, one must be able to define the centre of mass.
A moving column of air is not rigid and the centre of mass is indefinable.

I think, in this discussion, that "net" momentum could be useful in the understanding of the mechanics of ground effect.

Also looking for understanding.
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Old Jan 02, 2013, 09:33 AM
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well, not only that, but the action/reaction principle only applies to solid bodies in contact with each other. In order to then extend it to fluids you must derive the behaviour of individual particles, and then extrapolate statistical averages that consider large amounts of particles as a whole. The entire principle of conservation of energy didn't work for gases until Mr Boyle started to also look at pressure and temperature. Incidentally, "Daniel Bernoulli in 1737-1738 derived Boyle's law using Newton's laws of motion with application on a molecular level." (http://en.wikipedia.org/wiki/Boyle%27s_law)
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Old Jan 02, 2013, 10:56 AM
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Originally Posted by TangoKilo View Post
I was taught, many years ago, that f=ma only applies to rigid bodies with constant mass or, if the body is not rigid, one must be able to define the centre of mass.
A moving column of air is not rigid and the centre of mass is indefinable.

I think, in this discussion, that "net" momentum could be useful in the understanding of the mechanics of ground effect.

Also looking for understanding.
NASA clearly believes that fluids, including air, fall under Newton's 3 laws.

You can try to understand every cubic inch of space around the wing - hard.
Or you can look at the macro ouputs - copter's rotors are wings - copters throw air down, and in measurable quantities (put a scale under a hovering copter). Therefore, in the macro world - we see that the copter is hovering, that air is being moved downward and accelerated by the copter. Therefore, the copter will react with a lifting force, to all the air it accelerates downward.

Newton says for every action, there's an equal and opposing reaction. The air is being accelerated downward. By what? Well, there's nothing else ther but a copter - so it's the copter that reacts with an opposing force. And, since the air is being shoved downward, the copter will react with an opposing lifting force.

That's physics - plain and simple (Shoe doesn't like the word simple - just ignore him when he rants about that)

But it is simple - and model copters allow you to see, feel and understnad lift right in front of you. The more air they blow down, the higher they go. Action and reaction. Simple as that.

Working at understanding the flow around the rotors or wings - that's clearly difficult, requires great resources - wing tunnels, tufts of string tied to the wings, pressure sensors all over the place. We can only read what others have done, for the analysis of every cubic inch of space around a wing while it's working.

But, the macro level clearly shows the cause of lift is a reaction to the action of accelerating air downwards. And just about anyone, can look at a copter hovering over a scale, and come to an understanding of action and reaction. But not everyone, apparently.
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Old Jan 02, 2013, 11:07 AM
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You should be wary of "simple" explanations. Often they turn out to be horribly wrong. I am all in favour of Occam's Razor, but it must be applied sparingly and carefully.
Also, when analysing an action/reaction system, you must be very careful not to confuse the two. It's a common beginner mistake. The reaction is static, not dynamic.
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Old Jan 02, 2013, 01:39 PM
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You should be wary of "simple" explanations. Often they turn out to be horribly wrong. I am all in favour of Occam's Razor, but it must be applied sparingly and carefully.
Also, when analysing an action/reaction system, you must be very careful not to confuse the two. It's a common beginner mistake. The reaction is static, not dynamic.
Action and reaction - one body acts on another body or fluid, and reacts with an opposite and opposing force. (Air is a fluid.)

The wing acts on the air (accelerates it downward), and therefore will react with an opposite and opposing force - lift.

If something's simple, you don't dress it up. Well, I don't, at any rate.

You know, there's only two objects in play - the wing and the fluid (air). If one moves, then the other caused that motion, and the other will react with an opposite and opposing force.
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Old Jan 02, 2013, 02:25 PM
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action only concerns the force, not the resultant acceleration, which is a measure of the force. Reaction is the force that pushes back. that's why it's only "that simple" when working with solids. That's also why time does not get into it. It's an equilibrium equation. Motion is a measure of the effect, but it's not part of the equation either
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Old Jan 02, 2013, 02:53 PM
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action only concerns the force, not the resultant acceleration, which is a measure of the force. Reaction is the force that pushes back. that's why it's only "that simple" when working with solids. That's also why time does not get into it. It's an equilibrium equation. Motion is a measure of the effect, but it's not part of the equation either
Then what's going to react, to having accelerated all that air downward, if not the wing? What other object up there, with the plane and the air, could have accelerated the air downward?

I'm curious - cause the answer to that question, will have a lifting force applied to it, from having accelerated that air down.

Here's something to chew on
http://www.allstar.fiu.edu/aero/Flightrevisited.pdf

Actually, I answered, without really getting your point. Still don't get your point.
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Old Jan 02, 2013, 03:51 PM
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That's why you should again read a primer on newtonian phisics. You are mixing the 2nd and 3rd laws of motion in an ugly mess. the third law states "Third law: When a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction". It does not concern itself with motion. and the bodies only exert forces on each other when they are in contact. It helps to solve collisions on a billiard table (ignoring elasticity, though), but does not work for fluids
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Old Jan 02, 2013, 04:40 PM
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That's why you should again read a primer on newtonian phisics. You are mixing the 2nd and 3rd laws of motion in an ugly mess. the third law states "Third law: When a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction". It does not concern itself with motion. and the bodies only exert forces on each other when they are in contact. It helps to solve collisions on a billiard table (ignoring elasticity, though), but does not work for fluids
I'm pretty sure you're wrong about this. Fluids have mass, and can exert a force on a body.

NASA says this about it ...

For a body immersed in a moving fluid, the fluid remains in contact with the surface of the body. If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body.
http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

You are of course, free to disagree with NASA.
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