Ground effect effectiveness

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.

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.

There was an article in Soaring magazine years ago about some tests
done at Edwards AFB by USAF test pilot students on ground effects -
using a Blanik and a Grob-103, I think.

That might be available somewhere - there is a Soaring directory
somewhere, try SSA.org.

In gliding, especially with state of the art gliders (L/D in the 40 to
60 range), failure of your landing drag devices (dive brakes, 90 degree
flaps, even tail chutes) can be a real emergency - you can float in
ground effect for miles without slowing down, unable to land! And with
wingspans of 50 to 80+ feet, slipping at ground effect altitude is a
dangerous proposition! In the pattern, I would much rather have my
gear fail to extend than my spoilers fail!

A classic glider landing mishap is watching a pilot in a new-to-him
glider float the whole length of the runway raising and lowering the
gear, until he does a "tree-stop" off the far end - the result of
confusing the manual gear handle for the spoiler handle!

Kirk
Ls6-b "66"

Aerodynamics for Naval Aviators, page 380, figure 6.9 is a graph of percent
reduction in induced drag coefficient versus ratio of wing height to wing
span.

Bob Gardner

"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?

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.

"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.

Danny Deger

wrote in message
s.com...

Tony wrote:
I haven't found a decent reference for this -- can anyone help?
...
In gliding, especially with state of the art gliders (L/D in the 40 to
60 range), failure of your landing drag devices (dive brakes, 90 degree
flaps, even tail chutes) can be a real emergency - you can float in
ground effect for miles without slowing down, unable to land! And with
wingspans of 50 to 80+ feet, slipping at ground effect altitude is a
dangerous proposition! In the pattern, I would much rather have my
gear fail to extend than my spoilers fail!


http://www.soaridaho.com/Schreder/St...und_Effect.htm

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

"Tony" wrote:

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

Here's an online reference, including some graphs:

http://www.se-technology.com/wig/htm...en=aero&code=0

There are also a number of real-life examples where ground effect was
beneficial.

One was the Ekranoplan ground effect vehicles that operated in the
Caspian Sea. You can search for that name or the "Caspian Sea Monster"
to learn more about them.

Fighter pilots during WWII would often take advantage of ground effect to
extend their endurance when returning from sorties.

There was also the story of the MATS C97 (military version of the
Stratocruiser) that had its #1 prop separate from the aircraft, and lost
both port engines just past the point-of-no-return on a flight to Hawaii.
Initial calculations by the flight engineer suggested that they would
have to ditch 30 minutes from their destination. The captain jettisoned
all excess weight, and flew in ground effect for six hours, with full
right trim, and having to stand on the right rudder pedal with both feet
for the duration of the flight. They eventually landed safely after a
missed approach with 30 minutes of fuel remaining.

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

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:
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