To paraphrase helicopter pioneer Igor Sikorsky: If you're in trouble, an airplane can fly over and drop flowers, but a helicopter can save your life. It can deftly maneuver through tight spots and alight in remote places. It can float next to a mountain to search for the lost. And the best sound a wounded soldier can hear is that telltale rotor beat, just minutes before being evacuated to a hospital. When roads are impassable, bridges have been destroyed, and the electricity has been knocked out, helicopters can still deliver supplies and rescue people.
What they can't do is fly fast. The world speed record for a helicopter, claimed by a modified Westland Lynx in 1986, is 400 kilometers per hour. A Curtiss biplane bested that speed in 1923. The Westland Lynx was a good start, but it was more like a race car than a family sedan—impressive but not quite practical for routine missions. Today's working helicopters tend to top out at around 270 km/h.
At Sikorsky Aircraft Corp., in Stratford, Conn., we decided in the 1970s that we wanted to build a really fast helicopter. The goal was to reach 480 km/h without sacrificing the vehicle's other strengths. Almost 40 years later, Sikorsky is now close to meeting that goal. In August 2010, a technology demonstrator, known as the X2, reached 435 km/h, unofficially breaking the helicopter speed record. A few hurdles remain before Sikorsky can claim the official record, which is maintained by the Fédération Aéronautique Internationale (FAI), the world's air sports organization. Having gotten this far, we anticipate that Sikorsky will soon begin producing commercial helicopters using X2 technologies.
To take a crack at the speed record, we had to make some fundamental changes to conventional helicopter design. The reason why becomes clear when you consider the difference between how helicopters and airplanes fly.
When an airplane barrels down a runway, air flows over the wings to produce lift. At a certain speed, the pilot pitches the nose up slightly, increasing the angle of the wing to the air. That creates enough extra lift for the airplane to take off. Once a plane is airborne, its speed is limited only by the amount of thrust its engines can provide.
A helicopter generates lift quite differently. It manipulates the air flowing over its spinning rotor blades, allowing the body of the aircraft to hover. The lift generated by the rotor blades can be angled using the helicopter's flight controls, allowing it to fly sideways, pivot, or even backward.
Compared with the fixed wings of an airplane, a helicopter's rotating blades make for a much more complicated design. Each blade must withstand the forces of rotation, which can amount to many times the weight of the aircraft on each blade. A helicopter also needs a powerful engine and a large transmission to reduce the engine's rotation rate to something appropriate for the large rotors. For example, a U.S. Army UH-60 Black Hawk engine's output of 20 900 revolutions per minute turns the main rotor only 258 times per minute, a ratio of 81 to 1.
But here's the catch. When a helicopter flies forward, the rotor blades experience a dramatic variation in airspeed. That's easy to see if you imagine a miniature version of yourself perched on the tip of a helicopter rotor blade. If the helicopter were hovering, you'd feel a constant 800-km/h wind in your face as the rotor spun around. If the helicopter were to fly forward, you would note that the wind was stronger on what's called the advancing side, when the rotor was moving in the same direction as the helicopter, but that it would be noticeably weaker when the rotor was on the retreating side. By the time the helicopter reached 150 km/h, you would feel a wind speed of 950 km/h on the advancing side, versus 650 km/h on the retreating side. The relative speed of the wind on the retreating side gets lower and lower the faster the aircraft flies. At 300 km/h, the wind on the advancing side would reach 1100 km/h, while the wind on the opposite side would be 500 km/h.
Eventually, the helicopter would reach a point at which the difference between the lift on the advancing and retreating sides of the rotor could not be balanced and the vehicle wouldn't be able to maintain level flight. To complicate matters further, portions of the tip of a fast-flying helicopter's advancing blade can exceed the speed of sound, producing shock waves that cause large vibrations and generate considerable noise. For these reasons, most helicopters just don't like to go fast.