Sunday, May 1, 2011
Testing the X Factor
The “X” factor has become symbol in current rotorcraft development: Sikorsky’s X2 technology demonstrator (now transforming into the S-97 Raider) verses Eurocopter’s X³ high-speed hybrid demonstrator. While these are not competing airframes in the widest sense, they are trail-blazing each company’s drive towards new dimensions in rotary capability.
Sikorsky’s X2 Moves Toward the S-97
Actual voice transmission during the Sept. 15, 2010 test flight when the Sikorsky X2 broke the 250-knot objective: “245, 250, 251 knots... pretty much level... lower the nose... nice... 260, 262... bringing the nose up, bringing the prop back at the same time... wow, that was nice, I can’t thank you guys enough, that was amazing.”
As Kevin Bredenbeck, Sikorsky’s chief test pilot and director of Test & Evaluation admitted at HAI’s Heli-Expo recently, it is a rare occasion that the experience of a test flight gets the better of the consummate professional pilot at the controls. But when Sikorsky’s X2 demonstrator exceeded the test team’s expectations during that test flight at the company’s Development Flight Center at West Palm Beach, Fla., Bredenbeck said that the achievement, with a sense of aviation history behind it, just got the better of him. He had something written down on his kneeboard to say, but busting over 250 knots at less of a power setting than predicted blew all other thoughts away.
“I am having engineering discussions prior to the point, during the point, after the point. The telemetry is talking back to me, my team back and forth about what just happened. We intended to turn around and go back again and validate the data coming back, but due to the fact that we hit the targets for the test card, we called it a knock-off and came home to evaluate the day. I have to redeem myself,” he added, “you just don’t say: ‘Wow, that was nice.’”
Bredenbeck keenly felt the sense of occasion and this was the first part of what was on the kneeboard to say: “It has been 71 years since Igor Sikorsky first flew the VS-300 the day before, Sept. 14, 1939, and set the stage for rotary wing flight. This X2 flight is a rebirth to Sikorsky. We build helicopters, but it has been a long time since we developed a prototype to push the sciences and technologies. We are back to our institution foundation.” (They actually intended to conduct the flight test on the day before, but had some weather issues that delayed it for 24 hours.)
Sikorsky originally unveiled the X2 at Heli-Expo 2008 in Houston. “It was 47 months since the inception of the program and only 25 months after its first flight in 2008 that we hit 250 knots on its 17th flight,” stated Bredenbeck, acknowledging that this timeframe was relatively short for such an ambitious project.
“We basically took a rapid prototyping concept—similar to Kelly Johnson’s Skunk Works model. We got a small team and gave them a responsibility to design, build and test the aircraft—fast and safely. We also had great partners [in technologies that included] vibration control, the speed prop, rigid rotor blades and a smart airspeed probe.”
Bredenbeck continued: “We concentrated on four objectives: the advancing suite of technologies from a fly-by-wire system that we adapted for a rigid rotor co-axial, we took an active vibration control system, a propulsor (sic) and integrated it within the transmission and the rigid rotor blades, and we morphed those into our objectives and called those our key performance parameters. We did not want to lose the great attributes of a helicopter: great hover performance and low speed maneuverability.” The aircraft was designed to be low vibration, high speed at 250 knots, low pilot workload (one pilot) and low noise.
“During the 25 months of flight testing, the team encountered some challenges along the way, recalls Bredenbeck. “On Flight 12 we ran up against the pitch stability issue. On Flight 12B [there is no Flight 13] we solved this issue—there is actually a little additional tail surface, which resolved this. On Flight 14 we moved out to 225 knots, we had plenty of power remaining to go faster. What is really interesting is when we solved the stability issue, we had permission to change the tail number of the aircraft, N525SA—which signifies Sikorsky’s birthday.”
Finally, he said, “we get to September 15th, Flight 17, we are set up for the objective of looking at the parameters to get to 250 knots. We thought we could probably get there. Not installed on the aircraft was the hub fairing that goes between the two rotors that cuts down about 44 percent drag on the whole aircraft, and we thought we would need that to get to 250 knots, but from our predictions, we thought we could get out to the limiting areas of the engine and the rotor to approach 250. Part of the test card was dive out to 260, recover, turn around and do a secondary speed run and make adjustments. Again we hoped the predictions that we had would get us there. As it turned out, we did it with approximately 200 hp less than we expected, so we could have got to 265 or even 270, but the protocol of testing is you do what is on the card—you get the data then come home.”
|Sikorsky S-97 Raider|
But Sikorsky is now using its success to move on to the S-97 Raider, its new light tactical helicopter (LTH), very much being connected with the U.S. Army’s Armed Aerial Scout (AAS) requirement. The initial decision to self-fund two prototype LTH airframes was made on Oct. 20, 2010. The first flight of the S-97 is projected in four years’ time with a potential fielding date around 2018 if the program is accepted quickly.
Without doubt, both Sikorsky and Eurocopter are breaking through the traditional boundaries set by Igor Sikorsky’s VS-300 aircraft 71 years ago. Both companies are well positioned to bring new capabilities to a world market, both civil and military, that will be very responsive new dimensions in flight characteristics and performance. Bell/Boeing/AgustaWestland be warned—the tiltrotor is starting to look its age.
Eurocopter X³: Race for Productivity
Eurocopter’s X³ (X cube) hybrid demonstrator is undergoing its second round of flight tests at Marignane in France as this feature goes to press. The X³ uses a Eurocopter Dauphin airframe. Its propulsion system is described as: “two turboshaft engines that power a five-blade main rotor system, along with two propellers installed on short-span fixed wings. This hybrid configuration creates an advanced transportation system that offers the speed of a turboprop-powered aircraft and the full hover flight capabilities of a helicopter. It is tailored to applications where operational costs, flight duration and mission success depend directly on the maximum cruising speed.”
Having achieved the first speed objective of 180 knots (at a reduced level of engine power, according to the company) on Nov. 29, 2010 at the DGA Flight Test base in Istres, France, the second round of speed tests hope to extend the cruise speed out to 220+ knots. At the controls the first time around were Eurocopter test pilot Herve Jammayrac and flight test engineer Daniel Semioli. Jammayrac said that during the test, the X³ performed as expected, “demonstrating handling and flight qualities that are exactly in line with our ground-based simulator evaluations.”
The maiden flight of the X³ occurred on Sept. 6, 2010, so in a similar way to Sikorsky’s X2 (S-97), this is a project that seems to be racing ahead on a priority basis as well.
Lutz Bertling, Eurocopter’s CEO, maintains that the ambition for the X³ is for the aircraft to fly at just over 220 knots. “I don’t wish to enter a speed race with other helicopters because I don’t believe it (speed) is a target,” he stated earlier this year. “As yet nobody goes with the top speed because it is uneconomical. What I am entering into is a race for productivity.”
As the second phase of testing begins, Rotor & Wing talked to Jean-Michel Billig, executive vice president of research and development for Eurocopter, and put to him several questions regarding the X³ program and its direction.
Rotor & Wing: Is there anything more you can say about the current tests?
Billig: First of all, I have to say that this concept is now becoming a reality. The X³ benefits from the lessons we’ve learned after more than 20 years of research in the field. This includes the tiltrotor technology that we studied as part of the Eurofar project [a European Future Advanced Rotorcraft project for a vertical take-off passenger airplane]. One of the main challenges with this technology was the extreme technical complexity, not to mention major concerns regarding its cost-effectiveness.
The key points we focused on when designing the X³ were to maintain the multifunction capabilities of the aircraft. It has kept all helicopter capabilities such as hover flight, autorotation and a large versatility of missions. The first test phase was successfully completed at the end of November 2010. All objectives for this first phase were either met or were exceeded. A cruise speed of 180 knots, a bank angle of 60 degrees, and flying at an altitude of 12,500 feet. The second test phase will start in the coming weeks.
Rotor & Wing: What are Phase 2’s objectives?
Billig: For test phase 2, the objective is to demonstrate a cruise speed in excess of 220 knots, while confirming the cost effectiveness of the concept. We are also looking at exploring stability and handling qualities during this phase.
Rotor & Wing: How many flight tests are planned?
Billig: Let me just say that tests in phase one were completed in less than 15 flight hours. This demonstrates the robustness of the concept and the quality of the engineers and technicians working on the project.
Rotor & Wing: How long will this second phase last, and where are flights being held?
Billig: Test phase 2 will be completed by end of 2011. Tests are being held in southeast France. Primarily at the French military base in Istres, but also at Eurocopter’s Marignane facility.
Rotor & Wing: What does Eurocopter want to get out of the X³ as a maximum speed?
Billig: The X³ concept is not aiming to break a speed record. The X³ concept will demonstrate a cost-effective high speed around the range of 230 knots. Cost effective means that our target is to reduce mission-costs by 20 percent for those missions with little hover time and high mission duration.
Rotor & Wing: How many X³ aircraft have been built and how many will be built? Is it likely that the X³ will become a production aircraft or is it strictly limited to a technology demonstrator?
Billig: The X³ is a demonstrator. Only one demonstrator has been built. X³ will never enter into production. Once the concept is validated, its conceptual elements will be included into Eurocopter’s future product range.