After successful first flight, Sikorsky cautiously expands X2 flight envelope

By Andrew Healy

NBAA Convention News >> October 2008
Aircraft

At a small airfield near Horseheads, N.Y., Sikorsky is slowly expanding the flight envelope of its X2 technology demonstrator. After a first flight at the end of August, the coaxial rotor helicopter is currently midway through the first of four flight-test phases that should enable it to reach a forward speed of 250 knots by the middle of next year.

Sikorsky chief test pilot Kevin Bredenbeck was at the controls for the 30-minute summer test flight at the Schweizer Aircraft rapid-prototyping facility. The former U.S. Army officer, a 14-year veteran with Sikorsky, put the single-seat aircraft through hover, low-speed forward flight and pedal-turn maneuvers at heights of up to 40 feet. The milestone capped more than four years of design, development and testing of a suite of technologies intended to push the conventional helicopter into performance areas currently occupied by fixed-wing and tilt-rotor aircraft.

Bredenback told NBAA Convention News that after a four-year development process, the final build-up to first flight took little more than three weeks. “We put the blades on and started 25 hours of tethered PFAT [pre-flight acceptance tests] on August 6.
It quickly became clear we were achieving our goals ahead of schedule, and were soon looking at removing the tether, enabling us to adjust stability and try out control inputs. Three hours of that and we were ready to fly,” he said.

Before the first flight, Bredenbeck practiced taking off in a fixed-base simulator at the company’s Stratford, Conn. headquarters. He described the takeoff in the X2 as “a straightforward maneuver,” requiring minimal input on the sidestick and rudder pedals to lift off.

“Without any stabilization or autopilot I was prepared for a basic ‘stick-to-head’ experience, but the fly-by-wire control inputs were tremendous and it was clear from the start that we have designed a very responsive aircraft. Everything went according to plan, and I spent the time trying out [mostly] hovering maneuvers and gathering mechanical control data,” he said.

Program manager James Kagdis paid tribute to the 25-strong permanent cadre
of engineers–plus the raft of specialists drafted in as required–who worked toward the milestone. “Today’s achievement is the result of dedicated efforts by the entire X2 team,” he said. “It is proof of the complete commitment by Sikorsky to this program and to the exploration of innovation in aviation. We look forward to expanding the flight envelope and will continue to analyze the market to determine the next steps for this important program.”

Sikorsky first announced the self-funded initiative to develop the X2 in June 2005. At one point the demonstrator was slated to fly before the end of 2006. Kagdis said that the X2 is designed to establish that a conventional helicopter can make a simple and seamless transition to a “comfortable” 250-knot cruise, while retaining traditional characteristics such as good low-speed handling, efficient hovering and safe autorotation.

In its conventional low-speed configuration, X2 technology provides lift and directional control through the efficiency of its coaxial rotor, eliminating the need for a tail rotor. At high speeds, the X2 takes advantage of the full potential lift of the rigid coaxial rotor and a tail-mounted, six-bladed thruster or “propulsor” to drive it toward the 250-knot goal. Bredenbeck said the propulsor is configured to start contributing to forward speed at between 80 and 100 knots.

Future Applications Undetermined

Sikorsky has not yet decided whether to launch the X2 as a new civil aircraft. The company will use data from the flight tests to make a decision, expected in 2010.
Kagdis pointed out that the technology “can be applied to a wide variety of designs, up and down the weight scale,” and he sees evolving models making an impact in first-response situations such as EMS and search-and-rescue, as well as in the energy sector, “where it can offer significantly higher productivity than the conventional helicopter.”

Kagdis said he is also excited about the aircraft’s military potential–perhaps as an armed escort for the equally speedy V-22 that is currently making such an impact in Iraq. At the moment, during long-range missions U.S. Marine Corps crews have to refuel their conventional Huey Cobra escorts from desert fuel stop-offs or, sometimes, from the tiltrotor’s own tanks.

Like the V-22, the X2 will need a lot of fuel. It is an evolution of the Sikorsky ABC (advancing blade concept) demonstrator from 20 years ago, and that aircraft’s excessive fuel consumption was one of the reasons that the Department of Defense favored the XV-15 tiltrotor, which evolved into the V-22. Project chief engineer Steve Weiner predicted the X2 would offer better fuel consumption “at conventional helicopter speeds” because its overall drag and drag-to-lift ratio is better than that of a conventional helicopter.

U.S. Army Brig. Gen. Stephen Mundt, who directs the Army Aviation Task Force, wants to see the X2 reach 250 knots before making a judgment. “Do you still have the same level of payload capability? Did my fuel consumption go up so I get one-third the distance? All those things will have to be balanced,” he explained.

Despite these concerns, Sikorsky president Jeff Pino said he is pleased that the X2 project crossed this major threshold in August and is confident about its future. “The team’s achievement sets the stage for the next series of tests, eventually leading to [the demonstrator achieving] maximum speed. It also sparks the imagination for what, ultimately, the technology can mean to the future of the rotorcraft industry.
We are far from having a product, but closer than ever to realizing its potential,” he said.  

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