Flying a rotor system edgewise through the air creates a problem known as dissymmetry of lift. One side of the disc advances into the wind (headwind) while the other side is retreating (tailwind). For a fixed angle of attack, the lift on the advancing side is greater creating a lift imbalance that increases with airspeed. Early engineers came up with a way to equalize the lift by changing the angle of attack (flapping) as the blade moves around the rotor disc. This required increasing the angle of attack on the retreating side as the helicopter gained airspeed. As with any airfoil, the angle of attack can only increase so much before it stalls. Referred to as retreating blade stall, this problem limits a helicopter’s forward speed.
Ever since the development of the helicopter engineers have tried to figure out ways to get around the rotor system’s speed limitation. Although there are several ideas for this, one is a co-axial rotor design. This arrangement uses two stacked rotor systems with the same axis of rotation, but turn in opposite directions. This way when the retreating blade is losing lift the blade above (or below) it is advancing and gaining lift. This helps equalize lift across the rotor system. (There are other advantages as well like increased lift ability and no need for a tail rotor.)
In the 1970s Sikorsky demonstrated high speed flight with the S-69 Advancing Blade Concept (ABC) system. The S-69 used two rigid counter-rotating rotor systems with an auxiliary propulsion arrangement. Today, Sikorsky Aircraft is incorporating decades of research and development from the S-69 into its X2 technology concept demonstrator. The X2 aircraft uses two rigid counter-rotating rotor systems and a pusher propeller for auxiliary propulsion. Additionally, Sikorsky has included new technologies including an integrated fly-by-wire system, high lift-to-drag rigid blades, low drag hub fairings, and active vibration control.
The X2 is currently being test flown at Sikorsky’s West Palm Beach, Florida, development flight center. In October 2009 the X2 achieved a speed of 106 knots. This milestone moves the company a step closer to its stated goal of demonstrating that a helicopter can cruise comfortably at 250 knots while retaining such desirable attributes as excellent low-speed handling, efficient hovering, and a seamless and simple transition to high speed.
If Sikorsky is successful, the next 10 years could bring a new era of high speed helicopter flight.