At any rate, why is such ultra-low-level flight aerodynamically impossible? Because the reactive force of the hugely powerful downwash sheet,
coupled with the compressibility effects of the tip vortices, simply will not allow the aircraft to get any lower to the ground than approximately one
half the distance of its wingspan—until speed is drastically reduced, which, of course, is what happens during normal landings.
And that is complete BS. While it is true that in normal flight attitude at speed you may not be abled to decend while half your wingspan in height,
that hardly has anything to do with 9/11 or anything really.
Let's go into the basics. Lift is created by a pressure diferance between under the wing and over the wing. As aircrafts centre of gravity is infront
of the centre of lift, therefore, to stop the aircraft nosediving towards the ground, you will need to have a tailplane that pushes the tail down.
That means that in level flight the pressure diferance between the top and bottom of the wings should create a force equal to the weight of the
aircraft plus the aerodynamic weight of the tailplane pushing it down. Understand?
The amout of lift can be calculated by using a formula.
One of the variables in the sum is
'dynamic pressure', basically the pressure under the wing. Obviously if you increase angle of attack, dynamic pressure will dramatically go up and
the plane should rise. If you decrease angle of attack dynamic pressure will go down and the plane will sink.
Down low, however, in ground effect, the ground interferes with the downwash increasing the pressure underneath the wing - increasing lift - so to
stay at that altitude you will have to have a lower angle of attack than if you were not in ground effect. To decend all you would need to do would be
lower the nose slightly - reducing lift and allowing you to fly into the ground.
At high speeds a low angle of incedence will still create a pressure diferance capable of lifting the plane easily. Any change in pitch will create a
large diferance in pressure leading to a change in direction almost instantaneously. Increase in lift caused by ground effect will easily be remedied
by lowering the nose ever so slightly, you will NOT be pitching the nose down five degrees to go any lower at half a wingspan as that would cause the
wing to have a negative angle of incedence which would smoosh the plane into the ground rather quickly.
The 757 was going over 450mph at 3500 feet per minute, that's five times the decent rate an airliner is doing on final while going over twice as
fast. That's a heck of a load of momentum. The 757 has a wingspan of 124 feet, ground effect therefore should only be 'dramatic' when 64 feet high,
at 3500 feet per minute it takes only a second to do that.
Somehow I don't think ground effect is going to make it magically defy physics and be unable to get bellow half its wingspan in less than a second.
It may be hard for a pilot to do what they say it did on 9/11, but the 757, sure was, easily capable of doing it.
[edit on 16/1/2008 by C0bzz]