discrepancy. If the speed increases too much, the machine tends to roll to one side and the retreating blade will stall, generating no upward force at all. In order to solve this problem, Sikorsky’s engineers opted to replace the conventional single rotor design with twin, eight-metre-long rotors that spin in opposite directions on the same axis. The rotors both produce dissymmetry of lift, but in counteracting directions. This setup is referred to as a coaxial rotor system and brings with it numerous advantages. Firstly, there is no longer a need for the helicopter to have a side-facing tail rotor, as the counter-rotating blades no longer produce a torque
can have catastrophic consequences. In the case of the X2, Sikorsky has replaced the side-facing tail rotor with an aft-facing pusher propeller, much like that found on a boat. Called the propulsor, this added thrust lies at the heart of pushing the X2 to speeds as yet unheard of in the helicopter world. In addition, the Sikorsky X2 incorporates and demonstrates several new technologies in the rotary wing flight environment. These technologies include an integrated Fly-by-Wire system that allows the Rotor/Propulsor/Engine control system to operate efficiently, with full control of the rotor rpm throughout the flight envelope, high lift-to-drag rigid blades, low drag hub fairings,
Sikorsky doesn’t know whether its first clients will be military or commercial or both, but for now, the X2 team has set the speed bar high – at a staggering 460km/h.
the hover. Sikorsky recently shifted its focus towards creating an aircraft able to offer greater speed without compromising the essential attributes that make helicopters valuable. But what lies behind the complexities that have made it so hard to breach the limiting barrier that has plagued helicopter designers for decades now? The speed limit for any conventional helicopter is about 300km/h. But this was just not good enough for Steve Weiner, the X2’s Chief Engineer who “just wanted to go faster.” Now, the problem with going fast in helicopters is a phenomenon called “dissymmetry of lift.” When a helicopter starts to fly forward, the advancing blade cuts through the air faster, thereby generating more lift. At the same time, the retreating blade’s relative velocity and lift decrease. The faster the helicopter goes, the greater the
effect that tries to turn the helicopter’s nose in a direction opposite to the blade rotation. This improves the safety around the helicopter, which is a particularly important advantage for military and tourist applications, where circumstances may not allow time to stop rotors before personnel or passengers approach or disembark the aircraft. Secondly, helicopters powered by coaxial rotors are generally capable of higher speeds and able to bear greater weights than single-rotor aircraft. This superior performance is due to the sheer physics of how rotors work. Every conventional helicopter has a maximum speed at which it can no longer produce sufficient laterally balanced lift and maintain controllability on the rotor due to retreating blade stall. Furthermore, power is taken from the engine to operate the tail rotor, which in certain extreme circumstances can lead to either loss of power to the main rotor system or the tail rotor, both of which
and Active Vibration Control – all of which combine to make for smooth, safe and efficient flight and, more importantly, higher top speeds. The X2, is the descendant of the XH-59A, a machine with the same stacked rotor configuration that the company built with NASA and the US Army in the 1970s. The craft was thought unwieldy and the project shelved, though Sikorsky engineers never gave up on it. Sikorsky doesn’t know whether its first clients will be military or commercial or both, but for now, the X2 team has set the speed bar high – at a staggering 460km/h. “But the physics say we can probably go to 560km/h,” says Weiner. At that speed a medical transport helicopter could fly 250-odd kilometres, pick up a patient and return to the hospital in the same amount of time it would take a conventional helicopter to just get to the scene. Impressive. Visit www.sikorsky.com for more information.
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Published on Mar 8, 2010