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A friend of mine says he鈥檚 seen a helicopter loop the loop at an air show. Is it really possible? And if it is, how?

鈥� In theory, most helicopters can perform a loop. With enough speed and distance from the ground they can use their momentum to do a loop and overcome the downward force produced by the rotors. The main problem is that when the helicopter is upside down, the rotors tend to bump around and flex too much. To overcome this you can use a rigid rotor 鈥� although rigid rotors then cause other problems.

Remote-control helicopters can actually fly upside down. They do this by adjusting the angle of attack of their rotor blades (this is called collective pitch) so that they are in the opposite position to normal flight.

They can get away with it because the forces acting on a small helicopter are lower and the joints and mechanisms are normally less complicated.

Johneng

鈥� During a properly flown loop centrifugal force is greater than gravity, even when the aircraft is inverted. The pilot is pushed into the seat throughout and experiences positive .

However, if the loop is too large or it is flown at too low a speed, the pilot falls away from the seat, restrained only by a harness, and experiences negative g-forces. In older helicopters the rotor blades are flexible and hinged, flapping up and down, but under positive g-forces they always flap upwards. Under negative g-forces the blades may bend downwards far enough to hit the tail of the helicopter, with fatal results. This is why loops are discouraged.

Modern military helicopters have stiffer, hingeless, rigid rotors, giving much greater agility. Even under negative g-forces the rotor blades remain a safe distance from the tail, allowing loops to be flown safely.

Richard Whybray, Omagh, Tyrone, UK

You can see helicopters performing rolls and loops for yourself by searching for 鈥渉elicopter loop the loop鈥� at 鈥� Ed