Faster helicopters
飞得更快的直升机
More rotors, more speed
螺旋翼的多少决定速度的快慢
A new type of helicopter breaks speed records
新型直升机打破速度记录
Sep 9th 2010
Catch me if you can
THE ability of a helicopter to hover and land almost anywhere makes it an enormously useful machine. But helicopters have their limitations, particularly when it comes to flying fast. In a recent series of test flights, a new type of chopper has begun smashing speed records.
The X2 is an experimental helicopter being developed by Sikorsky, an American company, at a test-flight centre in Florida. It recently flew at more than 430kph (267mph), according to a report in Spectrum, published by the Institute of Electrical and Electronics Engineers. The present record is held by a souped-up Westland Lynx helicopter, which managed 400kph in 1986. But most helicopters can’t fly at anything like these speeds and are typically flat out at 270kph.
To make an official attempt on the record, Sikorsky will need to have the flight monitored by the Fédération Aéronautique Internationale, which compiles airspeed records. But that is unlikely to happen until the X2 is going even faster. Later this year, Sikorsky hopes it will be zipping along at more than 460kph. The company, however, is interested in more than just breaking speed records. It plans to use the technology developed for the X2 in commercial helicopters.
What limits the speed of a helicopter is the same thing that allows it to hover—the air flowing over its spinning rotor blades. The rotor blades work like the wings of an aeroplane, with an aerofoil shape providing lift. But unlike an aircraft, when a helicopter is flying forwards the air passing over its rotor blades does so at different speeds. The air passing across the blade that is advancing to the front of the helicopter and into the oncoming air is going faster than the air passing over the blade that is retreating to the rear of the helicopter. And the faster the helicopter goes, the greater this difference. At 300kph, the air passing over the advancing blade could reach 1,100kph while that over the retreating blade would be about 500kph. This difference in lift can make it hard to maintain level flight. And to make matters worse for the pilot, as the tips of the rotor blades approach the speed of sound (around 1,200kph at lower altitude temperatures), shock waves produce huge vibrations.
The X2 gets around these problems in a number of ways. First, it uses two counter-rotating rotors that spin around the same axis, one positioned above the other. So in forward flight each rotor can produce an equal amount of lift on each side, thus providing balance. The idea has been around for some time, but it proved difficult to make it work properly.
What has changed are technological advances in aircraft engineering and control systems. Now, vibrations can be reduced using “active control”, which involves placing sensors around the helicopter to detect the onset of vibration and then using force generators on various parts of the frame to vibrate in such a way that they cancel out the original tremors. Advanced computer modelling has also made it possible to design more efficient rotors. A pusher propeller has been fitted at the rear of the X2 to provide extra oomph. According to the engineers, this propeller can also be used to slow the helicopter snappily. And computerised “fly-by-wire” controls allow the X2 to be flown relatively easily.
Sikorsky reckons that future helicopters built using the X2 technology would be extremely versatile machines. They would dash to and from a medical emergency a lot faster. They would also be very agile in flight, which would increase their capabilities in combat. Sikorsky has already produced a simulator so that potential customers can experience what these fast helicopters will be like to fly. Plenty of whirlybird pilots will be keen to get their hands on the real thing.
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