Ground effect effectiveness

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 effectiveness of ground effect is more at lower speeds, where
induced drag dominates. At high speed, parasitic drag dominates
and the effect doesn't reduce that.

(Induced drag is from energy lost to downwards-thrown air, which
has to receive enough momentum per unit time to support the weight
of the airplane. Since energy goes as the square of this downward
velocity, you're better off throwing twice as much air half as fast,
which has the same momentum but half the energy, which still supports
your weight. Hence long wings.

Parasitic drag is from skin friction and turbulence produced and
pressure drag, that does not help in keeping you aloft; this is the
chief drag at high speed, where you're throwing vast quantities of
air downwards per unit time and so at very small downward velocity.
Ground effect doesn't help this.

If you want to capture a live bird in a closed garage, keep him
flying poking him when he lands with a long pole ; flying at low
speed for long is not possible, just a few minutes, and the bird
will exhaust himself. The same bird can fly fast hundreds of miles.)
--
Ron Hardin


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

"Tony" wrote in message oups.com...
:I haven't found a decent reference for this -- can anyone help?
:
:
: References would be helpful: I hate having stories I write wrong for
: technical reasons.
:

Try:
http://www.australianhovercraft.com/...aft_photos.htm

You all provided a wealth of good references, thank you all.



On Jan 7, 12:11 pm, "Tony" wrote:
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?

Would we be talking about a few percent less drag, or is it a big
number, like 30%? Sea gulls and other long winged birds tend to fly
just above the water, ducks and geese like to reduce drag by flying in
vees, but don't often cruise just above the water.

References would be helpful: I hate having stories I write wrong for
technical reasons.

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...

Excellent writeup - thanks.

Jose
--
He who laughs, lasts.
for Email, make the obvious change in the address.

("Tony" wrote)
another great lead -- thanks.


WIG info:
http://www.se-technology.com/wig/index.php
Wing in Ground-effect page. Good stuff


MontBlack-Sea

Tony wrote:
I haven't found a decent reference for this -- can anyone help?

How about...

http://www.aerospaceweb.org/question...cs/q0130.shtml

http://en.wikipedia.org/wiki/Ground_effect

Kev

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.