UNDERSTANDING GROUND EFFECTWhat is ground effect (GE) and how does it effect RC helicopter controls? Depending on what kind of helicopter you have, you might not even notice it. First off GE is not just limited to helicopters; anything that is flying close the ground will experience it. GE for any aircraft is caused by the increase in air pressure as the aircraft is flying close to the ground. This increased air pressure is caused by the air being pushed downwards by wings or rotors. Since the aircraft is close to the ground, the downward rush of air has no place to go, so air pressure builds under the aircraft. This high pressure area causes interesting changes to induced drag and total lift from the wing or rotor, but to keep things simple we will just look at GE as a bubble of high air pressure under our helicopter. Helicopters will of course experience GE when they are hovering close to the ground. That is why we refer to it as the Ground Effect Hover. So how close to the ground do we have to be? It depends on the model, but half the rotor diameter or one rotor length is generally where the effect begins and it gets stronger as you get closer to the ground. When your helicopter is in the ground effect hover, the helicopter will be more sensitive and will seem a bit harder to control. The down wash of air off the rotor blades (called rotor wash) is also causing many air flow instabilities that add to the ground effect sensitivity. As I said earlier, you may not even notice GE or rotor wash with your RC helicopter while you hover. There are several circumstances that can make the effects more or less noticeable. First if the wind is blowing, the effect won’t be as noticeable. The wind is basically blowing that high pressure air bubble under your helicopter out of the way. Second, if you are hovering over thick grass or brush, the high pressure bubble won’t build up as strong as if you are hovering over pavement or water. In my own experience, larger and heavier RC helicopters don’t experience GE as much as small light RC helis. I should probably rephrase that. They do experience it, but because of their mass the helicopter controls don’t seem to be overly sensitive.
On the other hand when my small Blade 400 or micro electrics are a few inches off the ground, the controls do become more sensitive and require more skill to keep them hovering perfectly still. Micro sizes are very difficult, GE and rotor wash throw them all over the place - another reason to stay away from anything smaller than a 400 size electric when learning. Remember, you learn to fly RC helicopters from the ground up. This is not easy if your bird is being pushed all over the place by rotor wash and GE is making the controls overly sensitive. Ground Effect is something you shouldn't concern your self too much about. However I thought it import to touch on so you know why your RC helicopter might seem more sensitive and difficult to control while you are hovering close to the ground.
|
