Has Sikorsky's X2 seen its highest speed?

A new paper by a Sikorsky technical fellow involved in the X2 technology demonstrator program reveals that an attempt to reach 250kt (the target speed for the advancing blade concept, coaxial rotor vehicle) in August ended with significantly less.

Published by the IEEE this month, the paper, written by Thomas Lawrence (the technical fellow) and David Jenney reports that engineers had planned to reach the 250kt (463km/h)speed goal during a flight in August, but the LHTEC 800-powered pusher quit accelerating at 235kt (435km/h). In July, Sikorsky announced that the aircraft had reached 225kt (417km/h).

..Most recently, we set our sights on 463 km/h.

On the day of that flight, in August of this year, the test team got started at the crack of dawn. To ensure that the pilots would be flying in smooth air, the crew had to be on site by 5:00 a.m., before the sun had a chance to heat the air enough for the wind to pick up. The crew rolled the aircraft out onto the runway, where a dozen safety officers in bright orange jumpsuits and noise-canceling headsets were on patrol. Two chase vehicles were there to observe the test flight--another helicopter and a fixed-wing turboprop. The latter would be needed to keep up with the X2 as it accelerated to higher speeds. The test team was on high alert as it orchestrated flight activities to keep the three vehicles a safe distance apart.

With everyone's nerves on edge, the X2 started up its engine at 6:30 a.m., and the helicopter took off. Within a few minutes the X2 had reached a speed of 350 km/h. A dozen people watched from the ground as the airspeed crept up, first to 400, then 410, and finally topping out at 435 km/h--not quite the goal we'd set, but good enough for this round. Cheers and applause broke out on the ground. The pilot slowed the X2, turned it around, and flew back to land on the runway....

Even so, Sikorsky broke the long standing official speed record for a helicopter set by Westland Lynx in 1984 at 216kt (400km/h). The company's own XH-59A coaxial demonstrator in the early 1970s however recorded an unofficial speed of 263kt (487km/h), though technology at the time couldn't handle the types of vibrations required to go that fast.

The company has said it will continue pushing the X2 until the end of September, with  simulations putting the ultimate top speed between 251kt and 264kt, just a tad faster than the XH-59A.