Wing in Ground Effect?

[BllFs6]

Hi all...

I know this is somewhat off topic perhaps...but maybe somebody here has the
tidbit of info I need....

As Im sure you all know....the Lift to drag ratio of a wing goes way up once it
gets close to ground.....and roughly speaking the effect becomes pronounced
when the wing has an altitude on the same order as the span of the wing or
less.....

What I was looking for was:

1..... a graph that shows the L/D ratio as the wing gets closer and closer to
the ground......anything rough will do...I found such a rough graph once a year
or 2 ago on the internet....but I cant find it again......

2.....question.....as you approach the ground VERY closely (lets assume your
landing gear are up AND you somehow maintain a constant forward speed AND you
maintain the same angle of attack) does the drag GO down or does the lift go
up?

3.....lets assume a wingspan on the order of 6 feet and a chord of 2 to 3
feet.....flying say 6 inches to a foot about the ground at 40-60mph....(yeah,
Im gonna be building the worlds fastest flying lawnmower :).....what kind of
lifting force could I reasonably expect? .....if you dont have the values for
the ground effect itself......I could still use the numbers for "high" altitude
values and fudge them from there to get back of the envelope ground effect
numbers....

take care all

Blll

[James R. Freeman]

on # 1 look at JavaFoil on the WWW enter your airfoil and if you check the
box called "Include Ground Eff" it will include it on the CL/CD graph

"BllFs6" > wrote in message
...
> Hi all...
>
> I know this is somewhat off topic perhaps...but maybe somebody here has
the
> tidbit of info I need....
>
> As Im sure you all know....the Lift to drag ratio of a wing goes way up
once it
> gets close to ground.....and roughly speaking the effect becomes
pronounced
> when the wing has an altitude on the same order as the span of the wing or
> less.....
>
> What I was looking for was:
>
> 1..... a graph that shows the L/D ratio as the wing gets closer and closer
to
> the ground......anything rough will do...I found such a rough graph once a
year
> or 2 ago on the internet....but I cant find it again......
>
> 2.....question.....as you approach the ground VERY closely (lets assume
your
> landing gear are up AND you somehow maintain a constant forward speed AND
you
> maintain the same angle of attack) does the drag GO down or does the lift
go
> up?
>
> 3.....lets assume a wingspan on the order of 6 feet and a chord of 2 to 3
> feet.....flying say 6 inches to a foot about the ground at
40-60mph....(yeah,
> Im gonna be building the worlds fastest flying lawnmower :).....what kind
of
> lifting force could I reasonably expect? .....if you dont have the values
for
> the ground effect itself......I could still use the numbers for "high"
altitude
> values and fudge them from there to get back of the envelope ground effect
> numbers....
>
> take care all
>
> Blll

[B Lacovara]

Check the archives from Soaring Magazine (the SSA Publication). Maybe 10 years
ago??, the Air Force test pilots school did a study of ground effect using a
glider (I think I remember it being a Blanik). It involved flying at best L/D
compared to diving for the ground and riding out the ground effect. It was
interesting data.

Bob

[Robert Bonomi]

In article >,
BllFs6 > wrote:
>
[[.. munch ..]]

>2.....question.....as you approach the ground VERY closely (lets assume your
>landing gear are up AND you somehow maintain a constant forward speed AND you
>maintain the same angle of attack) does the drag GO down or does the lift go
>up?

Both, "more-or-less". You get a 'cushion' of higher-pressure air that is
'trapped' under the wing. Lift is due to the pressure difference between
the top and bottom of the wing. _Either_ a decrease in upper-surface
air pressure, or an increase in lower-surface pressure will result in
increased lift.

The trapped air-cushion also acts like a nearly 'frictionless bearing',
reducing drag.

I've seen a Cherokee 6 use up more than 12,000 ft of runway, due to ground-
effect. Full flaps, the stall warning sounding, and aimed right at the
'numbers', get to about 2' above touch-down, and the PIC *cuts* the engine.
Not to idle, but _off_. We follow the stationary prop for more than two
miles *without* touching down -- then he kicks the engine, throttles up, and
goes around for another attempt. He -had- been cleared by the tower for
touch-and-go practice. This one was logged as a "missed touch-and-go".

It was amazing how -little- speed we lost during that two mile 'float' down
the runawy. Only shed about 5 kts.

[B Lacovara]

I've been under the impression that the biggest gain in ground effect is
cutting off the tip vortices thus significantly reducing drag.

Bob

[nafod40]

BllFs6 wrote:
> Hi all...
>
> I know this is somewhat off topic perhaps...but maybe somebody here has the
> tidbit of info I need....
>
> As Im sure you all know....the Lift to drag ratio of a wing goes way up once it
> gets close to ground.....and roughly speaking the effect becomes pronounced
> when the wing has an altitude on the same order as the span of the wing or
> less.....
>
> What I was looking for was:
>
> 1..... a graph that shows the L/D ratio as the wing gets closer and closer to
> the ground......anything rough will do...I found such a rough graph once a year
> or 2 ago on the internet....but I cant find it again......

I'll give you the equation for drag in terms of altitude above ground.

D = drag
D0 = zero lift drag (parasitic)
DL = drag due to lift, proportional to lift^2, equl to K x L^2, where K
is some constant.

D = D0 + K x L^2

Now, K gets adjusted as you get closer to the ground by...

K' = K x (33 (h/b)^1.5 )/(1+33(h/b)^1.5)

Where h is height, b is wingspan.

As h/b goes to zero, K' goes to zero too, leaving only parasitic drag D0

[FF]

"B Lacovara" > wrote in message
...
> I've been under the impression that the biggest gain in ground
effect is
> cutting off the tip vortices thus significantly reducing drag.

Could be dangerous for me to try to interpret Hoerner (in "Fluid
Dynamic Lift"), but the effect on the vortices is indirect, as it's
more relevantly the downwash behind the wing which is being altered
and therefore induced angle of attack, and hence drag. He thus
measures and graphs the height above the ground as the height from the
trailing edge. There's really no "cushion" of air involved, as the
pressure of the air under the wing can be zero or negative with
respect to free-stream air, per normal except at high lift
coefficients, and wing can still exhibit ground effect.

Fred F.

[Kyle Boatright]

"Robert Bonomi" > wrote in message
ervers.com...
> In article >,
> BllFs6 > wrote:
> >
> [[.. munch ..]]
>
> >2.....question.....as you approach the ground VERY closely (lets assume
your
> >landing gear are up AND you somehow maintain a constant forward speed AND
you
> >maintain the same angle of attack) does the drag GO down or does the lift
go
> >up?
>
> Both, "more-or-less". You get a 'cushion' of higher-pressure air that is
> 'trapped' under the wing. Lift is due to the pressure difference between
> the top and bottom of the wing. _Either_ a decrease in upper-surface
> air pressure, or an increase in lower-surface pressure will result in
> increased lift.
>
> The trapped air-cushion also acts like a nearly 'frictionless bearing',
> reducing drag.
>
> I've seen a Cherokee 6 use up more than 12,000 ft of runway, due to
ground-
> effect. Full flaps, the stall warning sounding, and aimed right at the
> 'numbers', get to about 2' above touch-down, and the PIC *cuts* the
engine.
> Not to idle, but _off_. We follow the stationary prop for more than two
> miles *without* touching down -- then he kicks the engine, throttles up,
and
> goes around for another attempt. He -had- been cleared by the tower for
> touch-and-go practice. This one was logged as a "missed touch-and-go".
>
> It was amazing how -little- speed we lost during that two mile 'float'
down
> the runawy. Only shed about 5 kts.

Obviously, this Cherokee 6 had the "eternal motion" upgrade.

A Cherokee 6 is a fairly draggy airframe. Assuming no odd circumstances (
i.e. moving from a large tailwind to a large headwind or a severely downhill
runway), I'd pay $50 to anyone who could float a Cherokee 6 over 1000' down
the runway if the excercise was started with the flaps down, the engine
stopped, and the aircraft a 2' of altitude and within 10 knots of stall
speed.

KB

[Dan Thomas]

"FF" > wrote in message >...
> "B Lacovara" > wrote in message
> ...
> > I've been under the impression that the biggest gain in ground
> effect is
> > cutting off the tip vortices thus significantly reducing drag.
>
> Could be dangerous for me to try to interpret Hoerner (in "Fluid
> Dynamic Lift"), but the effect on the vortices is indirect, as it's
> more relevantly the downwash behind the wing which is being altered
> and therefore induced angle of attack, and hence drag. He thus
> measures and graphs the height above the ground as the height from the
> trailing edge. There's really no "cushion" of air involved, as the
> pressure of the air under the wing can be zero or negative with
> respect to free-stream air, per normal except at high lift
> coefficients, and wing can still exhibit ground effect.
>
> Fred F.

The helicopter manual I have here says the same thing: the
presence of the surface reduces AOA. However, this is for a hovering
machine, and the ground prevents the generation of the descending
column of air that can occur at altitude, necessitating the higher AOA
(more collective) to maintain altitude in the hover. In a fixed-wing
airplane, the ground's effect on AOA will be somewhat less than that.
Kerschner says that tip vortices (or rather, the ground's interference
with them) are definitely a part of the reduced drag, as well as the
reduced AOA caused by reduced downwash angle.
An airliner in the piston days left Honolulu for the Mainland, and
before the halfway mark it had lost two of the four engines. The pilot
turned back to Hawaii, prepared the pax for ditching as the airplane
settled toward the ocean, and found that once he was within a few feet
of the waves it stopped sinking. He made it all the way back to
Honolulu that way, forcing it a bit lower to gain a little speed to
pull up high enough to get to the runway. Can't remember where I read
that, and the details might be off some, but it's true. I think.
Anyone else recall it?

Dan

[nafod40]

BllFs6 wrote:
> 1..... a graph that shows the L/D ratio as the wing gets closer and closer to
> the ground......anything rough will do...I found such a rough graph once a year
> or 2 ago on the internet....but I cant find it again......

This site has reams of gouge (Navy term).
http://www.se-technology.com/wig/index.php

> 2.....question.....as you approach the ground VERY closely (lets assume your
> landing gear are up AND you somehow maintain a constant forward speed AND you
> maintain the same angle of attack) does the drag GO down or does the lift go
> up?

Both. Drag goes down due to span-related ground effect. Lift goes up due
to chord-related ground effect.

> 3.....lets assume a wingspan on the order of 6 feet and a chord of 2 to 3
> feet.....flying say 6 inches to a foot about the ground at 40-60mph....(yeah,
> Im gonna be building the worlds fastest flying lawnmower :).....what kind of
> lifting force could I reasonably expect? .....if you dont have the values for
> the ground effect itself......I could still use the numbers for "high" altitude
> values and fudge them from there to get back of the envelope ground effect
> numbers....

Check that web site. Might have the answers.

[Robert Bonomi]

In article >,
Kyle Boatright > wrote:
>
>"Robert Bonomi" > wrote in message
ervers.com...
>> In article >,
>> BllFs6 > wrote:
>> >
>> [[.. munch ..]]
>>
>> >2.....question.....as you approach the ground VERY closely (lets assume
>your
>> >landing gear are up AND you somehow maintain a constant forward speed AND
>you
>> >maintain the same angle of attack) does the drag GO down or does the lift
>go
>> >up?
>>
>> Both, "more-or-less". You get a 'cushion' of higher-pressure air that is
>> 'trapped' under the wing. Lift is due to the pressure difference between
>> the top and bottom of the wing. _Either_ a decrease in upper-surface
>> air pressure, or an increase in lower-surface pressure will result in
>> increased lift.
>>
>> The trapped air-cushion also acts like a nearly 'frictionless bearing',
>> reducing drag.
>>
>> I've seen a Cherokee 6 use up more than 12,000 ft of runway, due to
>ground-
>> effect. Full flaps, the stall warning sounding, and aimed right at the
>> 'numbers', get to about 2' above touch-down, and the PIC *cuts* the
>engine.
>> Not to idle, but _off_. We follow the stationary prop for more than two
>> miles *without* touching down -- then he kicks the engine, throttles up,
>and
>> goes around for another attempt. He -had- been cleared by the tower for
>> touch-and-go practice. This one was logged as a "missed touch-and-go".
>>
>> It was amazing how -little- speed we lost during that two mile 'float'
>down
>> the runawy. Only shed about 5 kts.
>
>Obviously, this Cherokee 6 had the "eternal motion" upgrade.
>
>A Cherokee 6 is a fairly draggy airframe. Assuming no odd circumstances (
>i.e. moving from a large tailwind to a large headwind or a severely downhill
>runway), I'd pay $50 to anyone who could float a Cherokee 6 over 1000' down
>the runway if the excercise was started with the flaps down, the engine
>stopped, and the aircraft a 2' of altitude and within 10 knots of stall
>speed.

Agreed, about the airframe, but that big slab of a low wing makes for an
impressive air-cushion. *No* odd circumstances, per your list. Main
runway at DSM, summer 1975. PIC was check-pilot from the FBO there -- I'm
drawing blank on the FBO name, "{mumble} Aviation" I think. Check-flight
prior to an XC rental.

His entire point of the exercise was the ground-effect issue. 2nd time
around, he shows the 'effective' landing technique. Leaves the engine on,
settles into ground effect, whereupon he reaches down for the flaps lever,
_up_ flaps, and 'thump' onto the wheels.