How do high-altitude helicopters work? --- Kurt, Long Valley, N.J.
The highest altitude helicopter currently in existence is the AS350. A pilot named Didier Delsalle of France landed it on the summit of Mount Everest (8,850 meters) in 2005:
There are two problems with high altitude flight that the helicopter design needs to overcome. The first is a loss of engine power at high altitudes. The second is the loss of rotor lift as the air thins out. So as you go higher, you have less power and less lift from the engine/rotor.
To solve this, you need to have plenty of power. The first step is to use a gas turbine engine, which is lightweight to begin with and performs better at high altitudes. Then you use a very large engine relative to a "normal" helicopter so that there is plenty of power at high altitude. The rotor design is also modified to optimize performance in thin air.
The AS350 uses an 850 horsepower turbine. The helicopter weighs 2,771 pounds empty and carries a maximum of 143 gallons (about 900 pounds) of fuel. The maximum weight at sea level is between 5,000 and 6,000 pounds. It is a large helicopter that can hold up to 6 people. At high altitude, the maximum weight would be significantly reduced, so you would cut way back on the fuel in the tank and fly with just one or two passengers and no cargo.
In some flight simulator programs you can fly helicopters. Try these aircraft at high altitudes and you will see the problem. They simply run out of lift at a certain height.