A compound helicopter is a vehicle that combines standard vertical rotors with one or more horizontal-facing propellers, much like an airplane’s. They are also called gyroplanes or heliplanes. Compound helicopters are capable of greater forward speed than conventional helicopters but offer less stability overall for hovering.
A coaxial helicopter has two sets of counter-rotating vertical blades. The blades counter-rotate, as opposed to both rotating in the same direction, to counter each other’s angular momentum, which could make the aircraft unstable in flight. The counter-rotating blades eliminate the need for a stabilizing tail rotor. This configuration offers superior hovering power and stability.
A compound coaxial helicopter combines both features in an attempt to provide both a dependable stable platform for hovering as well as high forward speed. Since coaxial helicopters do not need a conventional tail rotor, it is replaced with one or more larger, more powerful, airplane-like propellers for forward propulsion. A rear-mounted ‘pusher’ propeller is usually used, though more standard propellers may be mounted on outrigger wings.
The Sikorsky X2 is an experimental testbed for the technology that has gone through a number of critical test flights starting in 2008. Besides the advanced configuration, the X2 also uses advanced computer controls ("fly by wire" controls, in aviation jargon), high lift-to-drag rigid blades, low drag hub fairings, and active vibration controls.
The X2’s big promise is in combining speed and hovering power in the same aircraft. Usually, a helicopter has to trade off one for the other. The Sikorsky hopes to alter that equation with the compund-coaxial configuration. The X2 is expected to be able to obtain top speeds of over 280 miles per hour. Compare to a current ‘fast’ helicopter such as the Apache attack helicopter, which usually tops out at about 180 mph.
The computer controls slow the rigid, carbon-fiber blades’ rotation at high speeds, keeping them from going supersonic and creating stress damage. They also help with left and minimize drag by adjusting the pitch of the blades.
Also, unlike another helicopter-airplane hybrid, the V-22 Osprey, the X2 does not go through a potentially hazardous ‘transition’ phase as it changes over from vertical to horizontal flight. A compound coaxial helicopter can transition between vertical and horizontal flight relatively seamlessly.
The concept is said to be scalable, meaning that larger and more robust versions of this configuration are possible. Besides the standard civilian and military versions, Sikorsky envisions big heavy-lift descendants of the X2, which will be able to lift payloads of up to 40 tons and still fly at speeds of 250 knots or better.
http://en.wikipedia.org/wiki/Sikorsky_X2http://www.sikorsky.com/vgn-ext-templating-SIK/v/index.jsp?vgnextoid=40c96eb78fa78110VgnVCM1000001382000aRCRD http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=busav&id=news/SIKOR02258.xml http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=busav&id=news/SIKOR02258.xml
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