Ground effect effectiveness

[Tony]

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.

[Larry Dighera]

On 7 Jan 2007 09:11:36 -0800, "Tony" > wrote in
. com>:

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

What you seek is probably contained within this great resource:
http://aerodyn.org/summary.html

[[email protected]]

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"

[Tony]

What a reference!

Thanks for a great lead -- it will help me avoid making at least some
stupid mistakes. I'm sure to make some others in the story, but it
won't be the fault of this reference.

I doubt that anyone buy into the idea that the because the airplane was
painted sky blue it would be lighter -- but a long wingspan powered
glider 5 feet above the ocean surface: that would work, so long as it
avoids boat masts.

If you see that in a technothriller next year, you can smile knowing
you played a part.





On Jan 7, 12:27 pm, Larry Dighera > wrote:
> On 7 Jan 2007 09:11:36 -0800, "Tony" > wrote in
> . com>:
>
> >I haven't found a decent reference for this -- can anyone help?What you seek is probably contained within this great resource:http://aerodyn.org/summary.html

[Bob Gardner]

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]

another great lead -- thanks.

On Jan 7, 1:31 pm, "Bob Gardner" > wrote:
> 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 ooglegroups.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.- Hide quoted text -- Show quoted text -

[Danny Deger]

"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

[Capt. Geoffrey Thorpe]

> 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/Stories/Ground_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.

[James Robinson]

"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/html/main.php?open=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.

[Larry Dighera]

>
>
>On Jan 7, 12:27 pm, Larry Dighera > wrote:
>> On 7 Jan 2007 09:11:36 -0800, "Tony" > wrote in
>> . com>:
>>
>> >I haven't found a decent reference for this -- can anyone help?

>What you seek is probably contained within this great resource:http://aerodyn.org/summary.html

On 7 Jan 2007 10:30:52 -0800, "Tony" > wrote in
. com>:

>What a reference!

Yes. I thought so. That's what makes the Internet so marvelous.

>Thanks for a great lead -- it will help me avoid making at least some
>stupid mistakes.

You're welcome. We can both thank the thoughtful users who are among
the readership of this newsgroup for making us aware of it.

>I'm sure to make some others in the story, but it
>won't be the fault of this reference.
>
>I doubt that anyone buy into the idea that the because the airplane was
>painted sky blue it would be lighter -- but a long wingspan powered
>glider 5 feet above the ocean surface: that would work, so long as it
>avoids boat masts.
>
>If you see that in a technothriller next year, you can smile knowing
>you played a part.

Best of luck with your literary work.

If you'd like to contribute something to rec.aviation.stories I'd be
happy to accept your submissions.

[Ron Hardin]

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.

[Blueskies]

"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/hovercraft_photos.htm

[Tony]

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.

[Jose[_1_]]

> 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.

[Montblack]

("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

[Kev]

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

How about...

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

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

Kev

[C J Campbell[_1_]]

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.

[[email protected]]

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

[[email protected]]

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

[Ron Hardin]

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.

[Matt Whiting]

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