What is the Mu-1 barrier?

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This story is sourced from Flight International
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The ratio of the forward speed of a helicopter to the tip speed of its rotor is expressed as µ (Mu). Conventional helicopters fly at a Mu of around 0.3, which means the rotor tip speed is about three times greater than the forward speed of the aircraft. It is difficult to achieve a higher ratio, and therefore greater overall speed, because in forward flight the retreating rotor blade moves in the opposite direction to the aircraft.

At a Mu of 1, the aircraft forward speed equals the tip speed of the retreating blade, and airflow over the blade is reversed over its entire length, making it very unstable. For this reason, sustained flight at Mu-1 or higher was thought to be impossible. Carter’s design tries to overcome this by offloading lift from the rotor to the wing, and by minimising the drag penalty of the rotor in forward flight by slowing it.

Normally, slowing the rotor would reduce its stiffness, increasing its tendency to “flap” and making it more unstable. But the Carter design increases the bending and twisting stiffness in forward flight, even at slower rotational speeds. This is achieved by mechanical links, and by adding a forward-projecting lead weight and a trailing-edge extension that gives the blade tip a forward centre of gravity and an aft centre of lift. This, theoretically, will allow flight speeds up to a ratio of Mu-5.