The designer of the unusual CarterCopter made history last week during a test flight of a prototype slowed-rotor compound helicopter by breaking a barrier considered crucial to high-speed rotorcraft flight.
Carter Aviation Technologies of Wichita Falls, Texas, said test pilots flew the CarterCopter above the so-called Mu-1 barrier (in which the speed of the rotor tip matches forward airspeed) for 7 seconds and above Mu-.96 for 7 minutes and 32 seconds, reaching a true airspeed of 200 mph (174 knots) at 10,790 feet.
The Mu-1 barrier is the optimal condition for a compound helicopter featuring a wing and slowly turning main rotor. With the rotor turning very slowly, rotor drag all but disappears. And with very long, thin wings, the CarterCopter's efficiency is claimed to be better than most general aviation airplanes and about four times better than the most efficient helicopters, notes designer Jay Carter.
"By being able to safely slow the rotor in flight, the technology allows for forward speeds as high as 500 mph, the cruise speed of some business jets, without the tip speed of the advancing blade going over Mach 0.90," he said.
In 2005, Carter managed to break the Mu-1 barrier with an initial prototype hybrid gyrocopter, but a crash of that aircraft and shortcomings in the design led to the creation of a second test aircraft. The latest CarterCopter test article is capable of taking off and landing vertically like a helicopter and flying at reasonably high forward speed thanks to its slowed rotor, which eliminates problems associated with retreating rotor blade stall – although for the craft to reach 500 mph, turbine propulsion would be needed.
The CarterCopter uses a rear-facing, five-blade propeller for fast forward speed. Eurocopter's X3 compound rotorcraft – the fastest helicopter in the world, which set a speed record of 255 knots earlier this year – uses similar technology but with two forward-facing propellers mounted on short-span wings.
Next up for the CarterCopter will be demonstrations of its range and endurance capabilities.
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