Leaving Ground Effect

[Dazzler]

According to the PHAK, when an aircraft leaves ground effect, it will experience a reduction in stability and a nose-up pitching moment. Why?

I would have thought that the nose would pitch down because less lift is generated out of ground effect than in it (for the same power setting and angle of attack)

[turbojet28]

I'll be honest, I'm a probably a little rusty on my aero, but I always was told (in layman's terms) that the wing will create more downwash out of ground effect because the ground isn't there to "block" the downwash. This results in an increase in induced drag and this downwash now will push down on the horizontal stabilizer from the top (at least in a high wing), making the nose go up. This was always the "simple" version of my understanding.

[seagull]

Downwash is enhanced a lot by the air circulation at the wingtips, which is blocked by the ground, effectively giving the wing the performance it would have with more aspect ratio. This downward air acts on pitching moment as described, pretty much.

One thing to point out here, to those that think that the downwash is where we get lift due to Newton's 3rd law, here is a good way to point out the fallicy of that. The downwash is a RESULT of the lift force, not the cause of it.

[PhilosopherPilot]

[ QUOTE ]
One thing to point out here, to those that think that the downwash is where we get lift due to Newton's 3rd law, here is a good way to point out the fallicy of that. The downwash is a RESULT of the lift force, not the cause of it.

[/ QUOTE ]

That is the whole point, the lift force creates a downwash, and that displacement of air is part of the 3rd law... You cannot separate Bernoulli and Newton! They are part and parcel to each other. Bernoulli is not a completely independant system of physics, it is a description of forces that fit into the Newtonian system.

The upwash and corresponding downwash created by the Bernoulli effects alter the average relative wind. Remember that lift acts perpendicularly to the relative wind, which is not perfectly horizontal due to the downwash. Therefore lift acts up, and a little aft when the downwash is not interfered with. That aft portion of lift is induced drag.

Get close to the ground, the wingip vortices are interupted, the downwash is made more horizontal, and the average relative wind shifts to a more horizontal direction. Now, imagine that the lift acts perpendicularly to that new average relative wind, and it is MORE vertical, LESS aft, and you therefore get more lift and less drag in ground effect. (I know a diagram would help.)

Anyway, that is how ground effect works. The previous post was correct, the increased downwash interacts with the horizontal stabilizer, and creates the nose-up moment as you leave ground effect.

Grayson

[seagull]

Bernoulli is an adaption of Newton's laws, but not the 3rd one, rather the 1st and 2nd.

[PhilosopherPilot]

[ QUOTE ]
Bernoulli is an adaption of Newton's laws, but not the 3rd one, rather the 1st and 2nd.

[/ QUOTE ]

Right, but that doesn't mean that the 3rd law of motion goes out the window, which is what I meant when I said that Bernoulli still works within Newtonian Physics. It's not completely separate system as most people seem to want to make it. It is absurd to ask the question, "Which is correct Bernoulli or Newton?" because they are interconnected.

G

[seagull]

No argument there, I think we're both saying the same thing in different terms!

[ QUOTE ]
It's not completely separate system as most people seem to want to make it. It is absurd to ask the question, "Which is correct Bernoulli or Newton?" because they are interconnected.

G

[/ QUOTE ]

[PhilosopherPilot]

I agree.

G

[MrCole]

While I don't have much to add to the wonderful posts by Seagull and Philospher, I'm attaching something I posted recently on the subject.

Dave

Regarding the question of downwash and its role in lift, I also agree with the previous poster in that it isn't really one or the other. Actually lift produces downwash, but downwash impacts the character of the lift and drag. If I'm not mistaken the downwash is caused by two related effects. The first is the wingtip vortices, which impart a downward velocity component to the free airstream passing the wing. If we had an infinitely long wing we could eliminate the wingtip vortices. But there is also another type of vortex, that created by the air trying to move from the higher pressure area to the lower pressure area not around the tips, but over the leading edge. When the sum of the vortices are added to the free airstream, the result is a relative wind of the wing sections that is inclined to the relative wind of the free airstream.

The lift vector of each section is perpendicular to the local relative wind, but the local relative wind of each section is inclined to the relative wind of the total wing, as measured some distance ahead of the wing. Since we measure lift and induced drag not in reference to the local wing sections but the entire wing, the induced angle between wing relative wind and section relative wind causes the section lift to be resolved into a wing lift component parallel to the overall relative wind, but also a induced drag component perpendicular to it.

If we could create a wing that was infinitely large and eliminate the vortices, then there would be no vertical velocity components added to the free stream, the relative wind of the sections and the overall wing would be the same, and induced drag would be zero. In a way this is what ground effect represents. Because of the ground plane the vertical velocity components due to the vortices are reduced, the wing section relative wind approaches that of the overall wing, reducing the angle between the two, and the lift vector of the section is closer to perpendicular as measured by the wing AOA, and induced drag is reduced.