Innovative X-2 rotor setup overcomes problems of the past

Innovative X-2 rotor setup overcomes problems of the past
With all the advantages, helicopters still possess a number of distinct operational constraints. They are excellent hovering machines, and can do so for long periods of time, allowing them to be used in a variety of roles from search and rescue to supporting troops on the ground. But when it comes to forward flight they have limited speed and range, and can have issues hovering at high altitudes.

These were the design issues that Sikorsky set out to overcome with its X-2 development aircraft. Visually the X-2 is not conventional. It uses coaxial rotor blades which rotate in opposite directions. This mechanism balances the torque and effectively gets rid of the need for a tail rotor.

Steve Weiner, Sikorsky's director of engineering sciences and chief engineer for the X2 programme, says: "The drive shaft that goes to the back can now be used for more important things, in this case a propeller. This allows the forward speed of the aircraft to be much faster than usual."

The X-2 takes its design philosophy and flight principles from an experimental research aircraft called the XH-59 advanced blade concept, built and flown in the early 1970's. The XH-59 possessed a lot of the features that Sikorsky wanted to incorporate, specifically the coaxial rotor setup which provided excellent manoeuvrability, high altitude performance and speed.

"The XH-59 had a lot of problems though," says Weiner. "The technology at that time did not provide a means for keeping vibration in check. It also required two pilots as there was nothing like the modern fly-by-wire systems available. So we wanted this type of vehicle but in the modern context."

One key enablers for the improved functionality was down to the use of Computational Fluid Dynamics (CFD). This enabled the engineers to 'dial in' the lift to drag ratios on the rotors and really understand the aerodynamics of such an innovative rotor setup. The sheer amount of virtual analysis that went in to this program meant that the aircraft only flew for around 20 hours, in comparison to 2500-2600 hours in the simulator. Despite limited flying hours, the aircraft set the unofficial speed record for helicopters flying at 250knots.

The other major innovation comes from the ability to tame the vibration that plagued its predecessor, the XH-59. Active vibration is a system that consists of a series of sensors that are connected to a computer and force generators. The force generators are basically eccentric masses on a common shaft which rotate at a particular rpm to provide the frequency of force needed to eliminate vibration. They can produce 700, 800, 900lbs of force in a particular direction and are placed on the airframe directly to the transmission and rotor system, counteracting the forces produced by the rotors.

"Vibration control has really improved quite a bit," says Weiner. "We use active vibration control on most of our production helicopters and we proved that by using these systems and having them acting directly on the source of vibration on the rotor system, we could keep it to a point where we could get a very comfortable ride and cruise at 250knots."

The X-2 is the forerunner of the planned S-97 Raider by Sikorsky which is now undergoing detailed design. It will take the same rotor setup, flight principles and engineering concepts that have be proven on the X-2 and take them closer to production.

"It will have the capability of carrying six passengers, have a pilot and a co-pilot, armament, and be a larger aircraft," says Weiner. "It will have extremely good hot day hover and specific mechanical changes to improve its operational capabilities. We expect to be flying the S-97 from 2014."

Justin Cunningham

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