Can someone explain the reduction of lift-induced drag while in the ground effect. I have read that while in the ground effect region that drag can be reduced by 75% I do not fly my aircraft at any AMA functions or do I know anything of the AMA other than some basics but if what I am reading is accurate then I can see that staying in the ground effect region would make for one fast aircraft. I have experienced this phenomenon first hand with a few of my all wings
When I bring them in for a belly landing they do not want to land.
I've noticed when making the transition from tailed aircraft to tailless the landings are much more soft with the tailless aircraft & the landing speed is much less
The distance that is needed to land is so short that it is almost like hovering down to the landing spot. I have landed some of my 36" span electrics in less than a couple feet..

Well... when in ground effect, you can reduce your operating coefficient of lift, which will definitely reduce lift induced drag.

Ground effect is due to the air's inability to move downward as the wing passes, so the air underneath the wing is compressed.
High velocities over the upper surface aren't needed. The lower surface experiences laminar flow in the boundary layer. This results in a lower Cl for the same lift, and a lower Cd.
It's as if the wing's AR is increased.
The Shuttle and SR-71 land softly because of the compression of the air.
Your wings see the same thing.
The problem with exploiting the condition is turning. Any bank changes the distribution of the air pressure in an exponential manner, the lower wing seeing the most, the rising wing experiencing a large change.
This is why those planes built to use the condition have short spans. Ekranoplan, Lippisch, etc..

It's more an interferance effect that can extend the lift coefficient of the plane to higher values than it would otherwise. It's not that the ground effect makes the plane able to fly faster or even allows it to. As such it's not really usable other than at takeoff and landing times.

I've got a wonderfully underpowered bipe that lifts off and then needs to have a few moments to accelerate just off the ground before it will trim to a hands off climb. That's also ground effect in action.

HO-229,

I don't think you could get much of a speed advantage from ground effect. Ground effect does indeed reduced induced drag, but induced drag is negligible at high speed. I think that if a plane is build for speed, and flown fast, trying to take advantage of ground effect would have a bigger effect on the life span of the plane than the speed.

banktoturn

"I think that if a plane is build for speed, and flown fast, trying to take advantage of ground effect would have a bigger effect on the life span of the plane than the speed."
.
At Mudd Lake NV, in 1977, when Darryl Greenamyer was flying his F-104RB for the low-altitude absolute speed record, Bob Reichardt brought his restored/replica Mr. Mulligan to try for speed records in its classes. After watching the first day's practices ( I was filming the event for "scientific purposes"), I was alarmed at the altitude he was flying at..
At dinner that night, I mentioned this to him, suggesting he might fly a little higher.. He told me he was taking advantage of the ground effect.
He did set a class record for the plane that weekend... but....
Almost everyone went home after Darryl's successful flights on Saturday and Sunday... Bob decided to try on Monday for the record for the next class. He took his wife along....
There's a Goodyear commercial that shows the result...
With no cars and motorhomes on the lakebed to provide real-time altitude information, he flew into the lakebed at a shallow angle. The tires disintegrated, broke the wheel pants, the plane tumbled nose-first into the lake bed at something over 300 mph.
This image is from one of the passes he made on Sat. or Sun. The red bottle is at the top of a 15 foot pole...

Paul,

I agree in principle but disagree in the details of your analysis.

By definition in subsonic aerodynamics, air is considered to be an incompressible fluid, and compression is a factor only as a wing approaches Mach 1 (the "compressibility factor" first experienced at the end of WWII as airplanes were approaching the speed of sound in a dive).

At subsonic speeds, the reduction in drag (and increase in lift) in ground effect is caused by the reduction of the downwash angle (the angle the air departs the trailing edge relative to the chordline).

In free-stream flight (out of ground effect), the downwash angle develops based on a number of structural and environmental factors.

As the wing is brought closer to the ground, the downwash flow cannot compress since the air is incompressible at these speeds, so it interacts with the ground by reducing its departure angle from the trailing edge of the wing.

The "mathematical" effect of this downwash angle reduction is a tilting forward of the total aerodamic force vector, while the practical effect is a reduction in apparent drag and increasing apparent lift components...

Cheers!

Jim

That's a pretty dramatic effect on the life span of the plane.

banktoturn

.
Actually, that was the 2nd time that plane had crashed there! When Ben Howard had it in the '30s, it had force-landed.
Riechardt had picked up the wreckage after 40 years, and rebuilt/replaced most of it.
.
Here's a similar frame of one of Darryl's passes.. not much higher, but well out of ground effect. and 3 times faster, or 1000 mph! The sonic boom was extemely uncomfortable at 600 distant.. note there are people directly -under- the plane! That must have been really bad!

The effects in ground effect aren't "apparent", they're real. But difficult to exploit. There's a complicated explanation using a "virtual wing" and Prandtl circulation in my old aero book. I never did get too much into Prandtl.
I have used the bow-wave off semis and buses while cruising on my motorcycle. In just the right position, I could back off on the throttle, and not lose any speed, the bow-wave supplying a push.

Absolutely! Since the air is incompressible at highway speeds, it pushes itself (and you) ahead of the vehicle, reducing your power requirements. Good thing, too, since if it were not so, you could have become a Peterbilt hood ornament in the experiment!

Cheers!

Jim

One chooses to place oneself off to the -side- and front of the semi....
Actually even riding alongside there's the reduced throttle for the same speed, close to the vehicle, a lot like migrating ducks and geese exploit with their v-formations..
And a bicyclist has pedaled well beyond 150 mph drafting a specially setup racecar that pulled him along.

Here's how Anderson explains ground effect in his book "Introduction to Flight":

"When an airplane is flying close to the ground, the strength of the wing tip vortices is somewhat diminished because of the interaction with the ground. Since these tip vortices induce downwash at the wing, which in turn generates induced drag, the downwash and hence induced drag are reduced when the airplane is flying close to the ground. "

Picture a plane flying towards you. You'll "see" a vortex on each wing tip causing air to flow down and away from the bottom surface, around the tip, and back down towards the top surface. This flow causes a net downwards flow on the wing. This is the downwash that Anderson is talking about. It deflects the relative flow over the wing in a downwards direction. Now picture a side view of the airplane. The downwash caused by the vortices causes the wing to fly along at a lower angle of attack. To compensate for this, you need to increase the angle of attack of your wing to make up for the AOA lost to downwash. Increased AOA means more lift, but also means more drag... induced drag. When you're near the ground the vortices can't create this downwash, and the effect disappears.

Anderson also includes a handy formula for calculating the effect of ground effect on induced drag.

I=((16*h/b)^2 ) / (1+(16*h/b)^2)
where h is your height above ground, and b is your wingspan. This formula should give you a number between 0 and 1. For example, if your 6in. off the ground with a 4 ft wingspan, I=0.8. In other words, the induced drag created by the aircraft will only be 80% of what it is when you're at altitude. You can see how this effect depends on how many "wingspans" above the ground you are. Which is why aircraft with really long wingspans are affected by ground effect much more..

I hope I haven't confused you more

Tom

(BTW... I highly recommend "Introduction to Flight" by John Anderson to anyone interested in a better understanding of airplane aerodynamics. It's very easy to read... not much math, and is full of historical tid bits.)

Some of this is getting too complicated.

In ground effect the wing's collecting air (incompresible as noted) between it and the ground. We all know that this is going to result in added lift as the air tries to escape, but the wing keeps collecting new air between it and the ground.

If you're finding that you need formulas or exotic theory to figure out ground effect, get a paper bag, fill it with air, tie off the open end, then sit on it.

If ground affect had some other magic to it, then near the ground the wing could be flown at negative AOA and there'd still be lift. But there isn't. AOA is needed to trap air between the wing and the ground.

I think the paper bag experiment falls short of capturing what is really going on. It's a pretty big stretch to compare the bounded compressible paper bag example to the unbounded (on three sides anyway) incompressible wing in ground effect.

I don't think anybody has suggested that this is the case. In the case where ground effect is strongest (as your height approaches zero), the induced drag goes to zero (per Tom's formula), and the lift curve slope (lift per degree of AOA) increases to a finite value. Even at zero height, the image vortex "magic" says that a wing needs some positive angle of attack to generate lift