Ground effect effectiveness

On Sun, 7 Jan 2007 10:52:24 -0800, Danny Deger wrote
(in article ):


"Tony" wrote in message
oups.com...
I haven't found a decent reference for this -- can anyone help?

Consider a clean low speed airplane -- maybe one of the kit built ones.
Does anyone have some quantitative measure of how much drag is reduced
if the airplane is flown say half or quarter of a wingspan above the
ocean?


If I recall correctly it is about 20%. It is enough that the Russians built
an seaplane with small wings that cruised in ground effect to reduce drag.
It is not just a couple of percent for sure. Your technothriller will be
valid to assume a substantial reduction in drag by flying in ground effect.

I remember there was some discussion a couple years back of building a giant
ground effect container ship/plane. It would cross the Pacific in ground
effect, then fly the short distance to a coastal airport. Probably not
economically feasible, but it could be done. For one thing, why fly it to an
airport? All the cranes to unload it are at ports. Seems to me that skipping
the flying step would greatly simplify things.

C J Campbell wrote:
On Sun, 7 Jan 2007 10:52:24 -0800, Danny Deger wrote
(in article ):


"Tony" wrote in message
oups.com...
I haven't found a decent reference for this -- can anyone help?

Consider a clean low speed airplane -- maybe one of the kit built ones.
Does anyone have some quantitative measure of how much drag is reduced
if the airplane is flown say half or quarter of a wingspan above the
ocean?


If I recall correctly it is about 20%. It is enough that the Russians built
an seaplane with small wings that cruised in ground effect to reduce drag.
It is not just a couple of percent for sure. Your technothriller will be
valid to assume a substantial reduction in drag by flying in ground effect.

I remember there was some discussion a couple years back of building a giant
ground effect container ship/plane. It would cross the Pacific in ground
effect, then fly the short distance to a coastal airport. Probably not
economically feasible, but it could be done. For one thing, why fly it to an
airport? All the cranes to unload it are at ports. Seems to me that skipping
the flying step would greatly simplify things.

Two problems that killed the Russian's ideas (besides money):
(1) The huge waves encounted at sea means the thing has to rise out of
ground effect, and (2) the span and power needed to fly to the airport
ruin the economics of the thing.

Dan

Ron Hardin wrote:
The efficiency of ground effect comes from replacing having to throw
air downwards, which costs energy (less energy the more air you throw
at a lesser speed, ie. long wings), with just hovering over a high
pressure area that you only have to set up once, instead of
continuously creating it.

You get the reduced induced drag of a longer-winged craft without
the parasitic drag of longer wings (the point of long wings being
to reduce the downward speed of thrown air).

The proximity of the ground does two things: It interferes
with wingtip vortex formation, the source of a major part of induced
drag and which destroys lift over the outer part of the wing at low
speeds, and it decreases angle of attack by reducing the upflow ahead
of the wing and reducing the downwash. Longer wings lose less area to
vortices, making them more efficient at low speeds.
The pressure under the wing is not significantly higher in
ground effect.

Dan

wrote:
The pressure under the wing is not significantly higher in
ground effect.

It's exactly the same. It holds up the airplane. Little work has been
expended to do it, though.

--
Ron Hardin


On the internet, nobody knows you're a jerk.

Ron Hardin wrote:
wrote:

The pressure under the wing is not significantly higher in
ground effect.


It's exactly the same. It holds up the airplane. Little work has been
expended to do it, though.


It doesn't have to be the same. It is the difference in pressure that
provides the force to hold up the airplane, not just the pressure under
the wing.


Matt