Wednesday, March 14th, 2012 07:45 am GMT -6 Wednesday, March 14th, 2012 07:45 am GMT -6Wednesday, March 14th, 2012 07:45 am GMT -6
 
Aquaglide 2. Photo by Stefan Richter.

It’s a boat…no. It’s a plane…maybe. It’s a ground effect vehicle…yes. Shouldn’t be too hard to turn it into a true airplane, though.

 

 

 

Ground Effect

When an airplane is less than a wing span above the ground, a reduction in drag and an increase in lift is experienced. These beneficial effects grow stronger as the wing gets closer to the ground.

The reduction in drag is due to a reduction in force of the wing tip vortices. The drag created directly and indirectly by these vortices is what we call induced drag. The effect is proportional to the height above the ground and the wing span. The wing acts as if it effectively had a longer span. When a wing is 10% of its span above the ground, the induced drag is cut in half. That is a huge gain in efficiency!

The increase in lift is due to an increase in air pressure on the bottom of the wing. The effect works best when the bottom of the wing is flat. This increase in lift is proportional to the height above ground and the wing chord size. When the wing is 10% of its chord above the ground, the lift is increased by 30%. Note that for typical airplane wings 10% of the chord is a much smaller distance than 10% of the wing span. The difference is proportional to the wing’s aspect ratio, which for a typical airplane is about seven.

I have flown full-size airplanes in ground effect many times. It is like you stop slowing down. I have flown half the length of a runway at three feet (1 m) off the ground with barely any loss in airspeed. It helps that I was flying a high performance sailplane at the time, but still. You feel like you can go on forever like that.

Ground Effect Vehicles

Because of the dramatic increase in efficiency possible by flying very close to the ground, a whole class of air vehicles have been designed and built to take advantage of it. They go by various names, but they are generically called ground effect vehicles (GEVs).

Don’t confuse a ground effect vehicle with a hovercraft. Hovercraft use a cushion of air to stay off the ground. They cannot lift off away from the ground. GEVs are really more like airplanes that fly close to the ground for higher efficiency. Some GEVs can fly away like a true airplane.

GEVs are most efficient when they are very close to the ground. Since it is very hard to guarantee that land is flat and without obstructions for long stretches, GEVs normally operate at cruising speeds over water. They travel over land only when they are starting up or stopping.

Caspian Sea Monster

Russia has a long history of developing ground effect vehicles. They call them “ekranoplan” vehicles. This literally means “ground effect”.

They developed what I believe to be the largest heavier-than-air flying machine ever built. Not sure what to make of what their spy pictures were showing them, Americans dubbed the aircraft “The Caspian Sea Monster”. Weighing about three times as much as a Boeing 747, it was absolutely massive. It could cruise around at 350 mph (560 kph), skimming right above the surface of the water.

Aquaglide 2. Photo by Stefan Richter.

Aquaglide

The Aquaglide is a modern GEV made in Russia. It can carry no more than about a dozen passengers at reasonably fast speeds.

What I find most interesting about the Aquaglide is that the air from its propellers flows right over its wings. When it is getting going, it can also direct the air from the propellers under the wings.

With the air flowing over the wings, a very short take-off and landing model airplane could be built. Because of the very short wing span, some means for having sufficient roll stability would have to be devised. Using a helicopter gyro would be an easy solution.

This would be a very unique model airplane, being a far more interesting project than almost anything else I can think of. I guarantee you that if you build and fly one of these, every eyeball on the field will be on it.

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