Thomas Lawrence, a technical fellow at Sikorsky, made a career of peculiar projects before successfully developing the fastest helicopter on Earth, the X2:
First he helped design an airship lifted by four conjoined helicopters. Next came the XH-59A, a futile effort to break the helicopter speed record. He then focused on the X-Wing, an aircraft that could take off like a helicopter but switched midair to fly like a fixed-wing airplane. None succeeded. Before turning to the very successful X2, Lawrence says, “I wasn’t sure what my career path was.”
The X2 has unofficially reached 435 km/h, and should soon officially break the 400 km/hr record set by a modified Westland Lynx. A typical helicopter can reach 270 km/h — with some difficulty:
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.
There’s more than one way to skin a cat:
In the 1950s, aircraft designers began to look at other configurations to achieve vertical takeoff and landing and reach forward speeds greater than 450 km/h. One approach was to design an aircraft whose thrust could be tilted vertically for takeoff and landing and horizontally for forward flight, during which time the vehicle would produce its lift from fixed wings. Some, such as the Bell X-22, used tilting ducted fans, while others used tilting propellers—for example, the Vought-Hiller-Ryan XC-142A and Canadair CL-84. Some had tilting jet engines, like the EWR VJ 101C. All handled poorly when hovering and produced downward air velocities high enough to blow a house down and uproot trees.
A more successful variation on this theme is the tilt-rotor, which uses helicopter-like rotor blades instead of ducted fans or propellers, with the axis of rotation switching from vertical to horizontal after takeoff. But this dual use is awkward: The rotors are too small for the aircraft to hover efficiently and larger than optimal for forward flight. The complicated tilting mechanism and excessive amount of power it requires also make the tilt-rotor option costly and complex.
Another approach to high-speed vertical takeoff used specially designed lift engines that worked only during takeoff and landing. These heavy engines produced large thrust for vertical takeoff and landing; in forward flight they were shut down and covered by doors. One such vehicle, the Dassault Mirage IIIV, was able to reach Mach 2, twice the speed of sound. Unfortunately, if you were on your roof waiting to be rescued, such an aircraft might not only fail to retrieve you, it could set your house on fire.
Aircraft engineers have also tried out jet engines with adjustable exhaust nozzles. The Hawker Siddeley Kestrel, which first flew in 1960, could aim its thrust in different directions, making the aircraft quite agile in forward flight and able to take off and land vertically. But the exhaust was again too hot and fast for the helicopter to be suitable for rescue purposes.What we at Sikorsky settled on almost four decades ago was a design we called the Advancing Blade Concept. It uses two counterrotating rigid rotors that spin around the same axis, which is why they are known as coaxial rotors. In forward flight, each rotor produces a surfeit of lift on its advancing side, freeing the retreating side from having to do any heavy lifting, all while maintaining good balance. Sikorsky patented the concept in 1964, but considerable engineering was needed to actually get something like this in the air.