Rotorcraft Breaks Mu-1 Barrier

A prototype rotorcraft reached a speed and efficiency milestone earlier this month. Carter Aviation’s slowed rotor/compound (SR/C) prototype broke the Mu-1 barrier during a test flight.

Carter has designed an aircraft that combines the speed of an airplane with the vertical takeoff and landing (VTOL) features of a traditional helicopter.

“Mu” is used to express the rotor tip advance ratio of a rotorcraft. At Mu-1, rotor drag almost disappears and the aircraft’s efficiency improves significantly compared to a helicopter. According to 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 9.”

The company says the prototype craft flew for 49 seconds at Mu-1 and above, and flew above Mu .96 for more than seven minutes.

Conventional helicopters usually fly at Mu 0.3, with the rotor tip speed being three times greater than the forward speed of the aircraft. The movement of the rotor blade in the opposite direction of a forward-moving aircraft makes it difficult to achieve a higher Mu ratio. Flying at Mu-1 also makes flight unstable, a factor Carter has overcome by offloading lift from the rotor to the wing and slowing the rotor in forward flight. Carter last exceeded Mu-1 with its first-generation SR/C aircraft back in 2005.

In September, Carter was selected for a phase-one study contract as part of DARPA’s Tactically Exploited Reconnaissance Node (TERN) unmanned aircraft system program.

Source: Carter Aviation Technologies