The U.S. Marine Corps' sword gets a brand-new edge.
- By George C. Larson
- Air & Space magazine, November 1998
(Page 2 of 5)
The mid-1950s through mid-1960s saw hybrids like the British Fairey Rotodyne, which combined a traditional lift rotor and a wing with propellers for forward thrust. Bell's XV-3 and X-22A were tiltrotors, the latter with ducts around the blades, while Boeing's VZ-2 and the LTV-Hiller-Ryan XC-142 were tiltwings in which the entire wing, together with engines and propellers (four of them on the -142), rotated as a unit. All these machines were conceived because the helicopter, a machine designed to hover, has an irreparable flaw when it tries to fly forward at high speed.
Hovering in still air, the helicopter is happyeverything is balanced and symmetrical, with the rotor blades creating equal lift throughout the circle of their revolution. To move forward, the pilot pushes the cyclic control, or stick, toward the nose, which causes the swash plate in the rotor head to increase the pitch of the rotors when they are swinging through the aft portion of the circle. The effective lift of the rotor system tilts forward, and the helicopter gently begins to accelerate. As its airspeed increases, the air flowing over the rotor blades begins to create an imbalance. The rotor blade that is advancing into the relative wind encounters a net gain in lift due to higher speed over its airfoil. The retreating blade encounters a net loss, and this divergence eventually increases until the retreating blade can no longer create sufficient lift and the aircraft rolls off toward the retreating blade even if the pilot tries to counter the roll with the cyclic control. This phenomenon, retreating blade stall, limits the top speed of traditional helicopters like the RH-53.
The tiltrotor effectively trumps this effect by turning the rotor into a propeller. No more advancing blade, no more retreating blade, just simple thrust, and all of it moving the airplane forward, supported on its wings. As Bill Leonard, Bell Boeing senior test pilot for the integrated test team from all three services that will operate the Osprey, puts it: "Don't think of this thing as a helicopter. Think of it as an airplane that hovers."
It is also important to think of it as an invention and to recognize the V-22 as the first wholly new aircraft configuration in the sky since the Harrierthe British jet that can take off and land verticallywhich entered service in 1970. (Accordingly, last August, the Federal Aviation Administration issued its first powered-lift ratingsspecifically for tiltrotor aircraftto eight V-22 test pilots.) While much of the inventing was accomplished by research aircraft, most significantly Bell's XV-15 for NASA and the Army, the final design of the V-22 has benefited enormously from the march of technological development in the broader fields of materials, electronics, and computer software. In fact, the aircraft that will be fielded first to the Marines as the MV-22 (the Air Force version is the CV-22; the Navy's, the HV-22) represents a wiser design that has benefited from some of the delays that have affected the program. The first prototypes were all-composite, but the decision to mate advanced composite materials with more traditional aluminum in the production aircraft, to cite just one example, appears smart: The most recent engineering and manufacturing development (EMD) aircraft are lighter and cheaper to build than earlier prototypes.
After the XV-15 showed that a tilt rotor aircraft could evolve into something more than a research platform, the next step was to build and fly six full-scale development (FSD) prototypes of the V-22. Although the program began as the JVX in 1981 under management of the Army, the Navy and Marines took over early in the game, with the Air Force as an interested partner looking at a smaller number of a special long-range V-22 version for the U.S. Special Operations Command, a combined force that would be called upon if something like the Tehran situation arose today.
Bell and Boeing formed a joint venture to build the craft in 1982, and a year later received a contract. Peering into the future, the Marines were especially anxious to replace their medium-lift Boeing Vertol CH-46 Sea Knights, a large, tandem-rotor helicopter designed and built in the 1960s, battle-tested in Vietnam, and refurbished, renovated, and rebuilt more often than George Washington's hatchet. The average Sea Knight is about 30 years old.
Crashes claimed two of the FSD prototypes. Following uneventful first flights in 1989, initially as a helicopter and later as a fixed wing, the first loss occurred on June 11, 1991, when one aircraft crashed due to incorrect wiring in the flight control system. The loss of a second aircraft and its crew on July 20, 1992, dealt the project a severe setback. While on approach to Quantico, Virginia, a series of component failures in one engine nacelle ignited a fire. Testing was halted pending an investigation and a redesign of the failed parts.
The V-22 program hit another rough patch in 1989, when Secretary of Defense Richard Cheney canceled it during the budget balancing battles of the early Bush administration. While the Reagan administration had allocated almost $1.5 billion for research and production, Pentagon analyst David Chu convinced Cheney that the V-22 was unaffordable. Chu and Cheney wanted more helicopters instead.