Sikorsky Aircraft's quest to produce the world's fastest helicopter raises one question above all: How fast?
As of Tuesday, the date of the X2 prototype's most recent flight, the answer was 235 knots, the latest increase in a steady advance toward the avowed goal of 250 knots, or 287 mph. That's nearly 100 knots faster than conventional modern helicopters' top speed.
Another, more philosophical question has also been popping up lately: Is the X2 really a helicopter?
Stratford-based Sikorsky, one of the world's biggest, most famous helicopter makers, says yes. But in the wonky world of aviation enthusiasts, there's debate. And Sikorsky's steady stream of public announcements about the X2's boundary-pushing progress has prompted some observers to raise the question — and answer it with a resounding no.
"The X2 is a compound aircraft, not a true helicopter," said Elfan Ap Rees, the editor of Helicopter International magazine in Great Britain, who objected after Sikorsky's July 26 statement that the X2 had reached 225 knots, setting an unofficial speed record. "…The X2 is a fine technological achievement with great promise and Sikorsky should be proud of that and not belittle its success with inaccurate claims."
The 5,000-pound, single-pilot X2 has one engine, two counter-rotating main rotors on top — and a rear "pusher" propeller to give it extra thrust.
Geoff Russell, a spokesman for AgustaWestland, a Sikorsky competitor in Europe, also said the X2 does not qualify as a "pure helicopter" because it generates power from a device other than the main rotor atop the aircraft.
"The X2 therefore will not be able to claim the speed record set by the Lynx helicopter in 1986," he said, referring to the souped-up version of a Westland helicopter still on record as the fastest.
(According to the
Washington-based National Aeronautic Association, a non-profit U.S. organization that certifies aviation records, a Westland Lynx remains the record-holder for speed, at 216 knots, or 249 mph. Sikorsky plans to invite an NAA representative to attend a test flight of the X2 once it reaches 250 knots, expected later this year.)
But Sikorsky is making no apologies for its innovations, and confidently insists the X2 is no mere rotorcraft, a broad term that includes helicopters, but a bona fide helicopter.
"We stand by our claims," company spokeswoman Marianne Heffernan wrote in an e-mail.
The Fédération Aéronautique Internationale — an international organization based in Switzerland that certifies world aviation records (and which X2 critic Elfan Ap Rees serves as honorary president of the rotorcraft committee) — defines helicopter in a way that would appear to include the X2.
Section 9 of the group's Sporting Code for rotorcraft, available on the NAA website, says a helicopter is a "rotorcraft which, in flight, derives substantially the whole of its lift from a power-driven rotor system whose axis (axes) is (are) fixed and substantially perpendicular to the longitudinal axis of the rotorcraft."
By the FAI's own definition, Sikorsky's X2 seems to fit. It generates lift from its main rotors — the ones on top — which spin on an axis perpendicular to the lengthwise axis of the aircraft's body.
While the axis of the pusher prop is horizontal, like the aircraft's lengthwise axis, the pusher prop does not generate lift — only thrust, Sikorsky said. The X2 does not rely on wings for lift, or anything else; it has none.
"The X2 demonstrator is considered a pure helicopter because all of its lift is derived from its rotor system rather than being augmented by wings," Steve Weiner, Sikorsky's director of engineering sciences and head engineer for the X2, said in a statement. "No other helicopter meeting these criteria has cruised at 250 knots to date."
Other helicopter-like aircraft have traveled at 250 knots and faster, such as the V-22 Osprey, a high-speed aircraft made by a partnership of Bell Helicopter and The
Boeing Co. But it has large wings and two dramatically adjustable rotors. The experimental Bell 533, which traveled at more than 270 knots, had stub wings and jet engines.
Whatever the squabbles among helicopter purists, advances in aircraft technology are blurring the lines between long-recognized types of flying machines — even for people who have been flying them for decades.
"It's actually a good question," said Jay Brown, a former Army helicopter pilot in Vietnam who is the executive director of the Combat Helicopter Pilots Association. "It's come up before when we talk about people who are eligible for membership in our organization."
Brown said his group has concluded that the V-22, for example, a so-called tilt-rotor helicopter, should not count as a helicopter, because its rotors swivel from a horizontal to a vertical orientation for forward travel.
"The V22 is not a helicopter," he said in an interview. "When it shifts from hovering flight to forward flight, it becomes an airplane." (Boeing, one of the V-22's producers, agrees, describing it as a tiltrotor aircraft that takes off and lands like a helicopter, but converts into a turboprop airplane.)
Brown says the X2, despite the pusher propeller at its rear, meets his concept of a helicopter, because its top-mounted rotor system "stays where you expect a helicopter rotor system to be."
"If you were in theory to remove the rotor system, it would stop flying," Brown said of the X2. "It has to have the rotor system spinning in order to maintain flight. I think it's still a helicopter."
The American Helicopter Museum and Education Center, near
Philadelphia, keeps things simple by opting for a flexible definition.
Said president Sean Saunders, "It changes with every new invention."