"Ground cushion"

On Feb 26, 7:28*am, Bertie the Bunyip wrote:
WingFlaps wrote :



On Feb 26, 7:04*am, Bertie the Bunyip wrote:
"gatt" wrote

ews.com:

In the book "Takeoffs and Landings" by Leighton Collins--who is
referenced in "Stick and Rudder"--there is no mention of the term
"ground effect."

Rather, Collins talks briefly about "ground cushion" and how air
"piles up" beneath the wings.

Do people still teach this?

Wel, it's a rose by any other name sort of thing.. Basicaly what's
happening is the air around the wing's pressure is influenced by the
ground. You have a high below the wing in flight and it gets higher
and influences the way the air flows around the wing keeping it
laminar longer
.
( slower)

I thought it was purely due to a reduction in induced drag. This
causes the lift vector to be larger.

??I'm not with you here..


OK, I'll explain myself but I'm sure you know this stuff -you're not
trolling me I hope ...
The wing produces a force vector that is broken into 2 components lift
and drag. The wing tip vortex is a major contributer to the rotation
of the wing force vector. Within ~1.5 wingspans the wing tip vortex is
suppressed by friction with the ground. This has two effects : a big
reduction in drag (so you float a long way in landing as you do not
shed energy so fast) and the rotation of the wing force vector towards
vertical increases lift. In landing/takeoff configs the high AOA leads
to a very backward wing force vector so ground effect increases lift a
lot in those configs. This then easily explains a take off stall -as
you rotate and climb out of ground effect drag increases and lift
decreases. The loss of lift requireds you to further increases AOA
which further increases drag. If drag becomes greater than thrust you
stall.

Cheers