John Mohr
I would explain ground effect as the high pressure that is underneath the wing, when you are low to the ground. I can envision it like a pillow, like it starts packing up between the wings and the ground. The closer you get the wings to the ground, the more the pressure builds and the more cushion you get from this pillow.
I can only speak from my experience, because there are not a lot of other people that use ground effect in air shows. I don’t know anybody, in modern times that uses it to the extent that I do in my Stearman. Probably back when they were flying underpowered airplanes in the early days of air shows, in the barnstorming days, in the 20s and 30s, I’m sure they used it. I am sure they did. Because, when you are flying an airplane that is right on the ragged edge, and you need, say, 120 mph for a loop to ensure you come out of the backside without mushing into the ground and enough energy for the next maneuver, 110 versus 120 is huge. A little less than 10%, or it could be 10% in a Stearman. Well, that is a huge number. When you are flying an airplane that is 200 knots or higher, that might not be such a huge number.
I don’t know if it increases proportionally. In other words, I don’t know if you would still have 10%. So, if you were in ground effect with an Extra 300 or something, a Pitts, would you gain 20, so it would be 220? I don’t know that.
All I know is my experience with the Stearman and what it does for me. Then 10 to 15% is huge. It would mean the difference between enough energy to do my next maneuver, and continue to do maneuvers, or to stop and climb.
Some shows I’ve done, like over in the Middle East, where they don’t understand ground effect, and they had people from the UK with rules that said you can’t go below 50 feet and you can’t do a loop below 300 feet and all those sort of things that they tried to impose on us…well, to me, that meant that I had to start out higher and to continue to lose altitude throughout my maneuvers. So the show that I do, the magic of it, I attribute a lot of that to ground effect. I can see that it really does make the difference between flying the whole series of maneuvers and staying at the same altitude versus starting high and continuing on down as you lose energy.
Ground effect is most pronounced when your wings are level to the ground and you are definitely within half a wingspan or less. In some cases, I am in a shallow turn. I don’t want to be in too steep a turn when I am that close to the ground. Obviously, I’d drag a wingtip. I know that when I get it down to where my wheels are just inches off the ground and moving along in that manner, it is more pronounced than when I am even half a wingspan above the ground. It is definitely noticeable. I can pick up that 10 mph, 15 mph by staying right down there.
The other thing that happens with my airplane because it is not constant speed, it is fixed pitch, as I increase airspeed, my RPM increases also and with a round engine which is limited to about 2100 RPM, again, 10%. Another 100 RPM in a round engine, in a 220 Continental, is a huge thing. I not only get the airspeed gain, but I start to get an RPM gain. And, when I start to get that, I develop a little more power. It pulls me a little faster and the whole thing compounds itself for the good. If you stay down there long enough with a nice straight run of, I don’t know, several hundred feet or longer, you can tell things happen. You can tell the increase in speed, the increase in the RPM. Even if it is only 50 RPM, it still makes a difference.
I also know from seaplane flying that when you break clear of the water, in many cases, you’ve reached the maximum planing speed of the hull in a float plane or a flying boat. It gets to a point, sometimes on a high density altitude day or a high altitude lake, with a heavy load where you’ve reached the maximum planing speed of the hull before you can get it in the air. In other words, you are just below stall speed. So, if you force it into the air, suck back on the yoke really quick and pop it into the air, it’s probably not ready to fly. And if you climb away from the ground any amount you would probably end up stalling and settling back into the ground, or the water, in this case.
By keeping it just inches off the water, you are taking advantage of ground effect and you can actually feel this cushion. You gradually reduce your pitch and the airspeed increases until you finally get your best rate or best angle of climb, then you can start climbing away.
The same thing happens in soft field takeoffs, even in a tricycle gear aircraft, any type of airplane…high wing airplane, it doesn’t matter. You can get the airplane off the ground, or out of a muddy field, or what have you, probably at just below stall speed, or what is written in the book as stall speed. So you use this cushion of air to your advantage to get it airborne, reduce the drag, see if you can accelerate to your best climb speed and climb away from there.
There are accounts during World War II of bombers that lost two engines, maybe even three, and they were back over the English Channel heading home and they thought they were going to have to ditch. As they got lower and lower over the water until they got down right above the water, all the sudden the ground effect helped them out and they gained enough lift to continue on and stay in ground effect to continue to where they were going, or at least to get close enough to land, ditch and be rescued.
