2010

The X2 and X3 Helicopters

Traditional helicopters have a single main rotor and a tail rotor. And they have a problem: top speed is much slower than a turboprop or jet airplane. In civilian situations that means longer flight times. In military situations it means that helicopters are easier to shoot down. Example: the Apache helicopter (AH-64) has a maximum speed of 182 mph (292 kph). If people are shooting at you, you would prefer to escape at a much higher speed.

This speed limitation is a well-known engineering problem. Consider the main rotor blades as they spin. The tip of each blade slices through the air at 500 mph (800 kph) in hover. Now the helicopter accelerates to 100 mph (160 kph). As a blade moves forward, in the direction of flight, the tip is actually traveling at 600 mph (960 kph). As a tip moves backwards (retreating), it is going 400 mph (640 mph). Two things happen as the forward speed increases: the advancing tip approaches supersonic speeds, which damage the rotor, and the retreating blade stops creating lift—it’s called retreating blade stall.

The engineers of the X2 (Sikorsky) helicopter overcame this problem in 2008 in a creative way. First, they use two counter-rotating rotors. Lift remains balanced on both sides of the rotors as the speed increases. Second, they slow the rotational speed of the rotors as forward speed increases. Third, they add a large pusher prop to move the helicopter through the air faster.

The engineers of the X3 (Eurocopter) helicopter used a completely different approach in 2010: 1) the main rotor follows the tradition of a regular helicopter, 2) there is no tail rotor, 3) the helicopter has short wings, and 4) there are propellers on both wings. One of the wing propellers spins faster than the other to replace the action of the tail rotor. The main rotor has five blades and slows during high-speed flight, with the short wings picking up part of the load.

The X2 and X3 are able to go nearly 300 mph (480 kph) with these engineering innovations. Two engineering teams, two radically different solutions that both yield better performance.

SEE ALSO Helicopter (1944), Apache Helicopter (1986).

Pictured: An X2 helicopter in flight in Germany.