"FF" wrote in message ...
"B Lacovara" wrote in message
...
I've been under the impression that the biggest gain in ground
effect is
cutting off the tip vortices thus significantly reducing drag.

Could be dangerous for me to try to interpret Hoerner (in "Fluid
Dynamic Lift"), but the effect on the vortices is indirect, as it's
more relevantly the downwash behind the wing which is being altered
and therefore induced angle of attack, and hence drag. He thus
measures and graphs the height above the ground as the height from the
trailing edge. There's really no "cushion" of air involved, as the
pressure of the air under the wing can be zero or negative with
respect to free-stream air, per normal except at high lift
coefficients, and wing can still exhibit ground effect.

Fred F.

The helicopter manual I have here says the same thing: the
presence of the surface reduces AOA. However, this is for a hovering
machine, and the ground prevents the generation of the descending
column of air that can occur at altitude, necessitating the higher AOA
(more collective) to maintain altitude in the hover. In a fixed-wing
airplane, the ground's effect on AOA will be somewhat less than that.
Kerschner says that tip vortices (or rather, the ground's interference
with them) are definitely a part of the reduced drag, as well as the
reduced AOA caused by reduced downwash angle.
An airliner in the piston days left Honolulu for the Mainland, and
before the halfway mark it had lost two of the four engines. The pilot
turned back to Hawaii, prepared the pax for ditching as the airplane
settled toward the ocean, and found that once he was within a few feet
of the waves it stopped sinking. He made it all the way back to
Honolulu that way, forcing it a bit lower to gain a little speed to
pull up high enough to get to the runway. Can't remember where I read
that, and the details might be off some, but it's true. I think.
Anyone else recall it?

Dan