Nick sez,
"Note that real GEM's do create a pressure below the aircraft, and they have a skirt to allow the pressure to build. A manometer that reads the pressure under the skirt would show a significant pressure rise. Helos in ground effect are not the same beast at all."
The previously mentioned web page on
Ground Effect sez,
" The principle of Wing-in-ground-effect or (W.I.G) has been known for a long time. Ground effect is a natural phenomenon, as an aerofoil approaches the ground its lifting ability increases and the drag reduces"
It never uses the word
'skirt'
Nick, you appear to be basing the cause of ground effect totally on tip vortices. This is contrary to what I have been able to discover.
Perhaps the tip vortices contribute to ground effect, by acting as a kind of skirt, and this helps retain the pressure.
~or~
Perhaps the increased pressure results in the blades operating in a slightly denser medium and they are therefor slightly more efficient.
Dave J.
_____________________________
.... and furthermore
Having no
(good) reputation to protect, I'll go out on a limb and make the following totally unheard of and totally unsupported claim.
[list=1][*]Pressure is an increase in air density.[*]Higher air density results in greater rotor efficiency.[/list=1] Therefor the higher pressure caused by ground effect results in a greater thrust for a given collective pitch settling.
Tear that to pieces, ~ kindly.
Dave J.