In the aeronautical mountain range, a few peaks are still unclimbed. Some goals, once thought impossible, are an everyday fact. Jet propulsion, supersonic flight, a practical helicopter and the miracles of manned and unmanned spaceflight all whiz past our bemused faces while we scarcely register a raised eyebrow.

But there are still some peaks to be scaled. Among them is the apparently absolute speed limit for helicopters. The current top speed for a rotorcraft is 216 kt (249 mph), set by a specially modified Westland Lynx on August 11, 1986.

The helicopter speed barrier comes in the form of asymmetrical lift. As the rotorcraft gains forward speed, the airflow over a rotor blade advancing forward in its rotational arc is the rotational speed combined with the forward speed of the entire airframe. Thus that rotor blade generates more lift than it would in hovering flight, the sum of its rotating speed and additional lift due to the entire helicopter’s passage through the air.

When that rotor blade completes the forward portion of its arc, and begins to retreat aft toward the tail boom, it loses the added lift imparted to it via the airflow generated by the helo’s forward flight. If the lift generated by the entire rotor disc can be visualized from straight above, it’s as if one side is pulling up and one side is pushing down. The pioneers of the helicopter world–Igor Sikorsky, Arthur Young and Frank Piasecki–devised various flapping or teetering mechanisms to cope with asymmetric lift and the resulting “rollover” tendency, and they succeeded–at low speed. But as speeds increased so did the asymmetric lift and at some point the mechanisms designed to equalize the lift of the advancing and retreating blades reach their limit. The retreating blade stalls and the rollover forces become so strong that the rotor systems either tear apart or the aircraft becomes uncontrollable.

The Answer?
Manufacturers have tried various combinations of wings and rotors to beat this speed barrier. The most recent attempt was Sikorsky’s X-wing, a radical design that was intended to use a rotating main rotor for low-speed lift, then locking that rotor into place to serve as fixed supporting airfoils for predicted flight to nearly 260 kt.

The trick to locking and unlocking the rotor for in-flight conversion between fixed- and rotary-wing flight has never quite been worked out. Engineers foresaw a corner of the flight envelope in which the aircraft would be neither helicopter nor airplane and could instead become a paperweight. In the X-wing’s case, that particular technical trick of carrying over lift between the two flight regimes was to have been accomplished through use of air blown over the slowing and stopped blades as the conversion was made and the airframe accelerated to flight speed by conventional jet engines.

Alas, the X-wing never converted to helicopter mode, nor was it flown as a rotorcraft from a ground start. As the test flights in basic airframe form without the experimental X-wing racked up the data points, Sikorsky engineers backed away from the sheer complexity of the task they had proposed. Defense Department money dried up and the program was eventually scratched.

Today, a small unmanned aircraft seeks to do what Sikorsky’s X-wing never did. A product of the Boeing Phantom Works (that company’s hush-hush flight-test center), the X-50A Dragonfly canard rotor wing (CRW) looks like one of Burt Rutan’s line of EZ homebuilts as if assembled by a junior high-school shop class–a not very gifted shop class at that.

Featuring a 12-ft-span wing (or should that be diameter, since this wing will rotate as a rotor?), the X-50A will take off like a conventional helicopter (although it could take off with lift coming from its wing in the fixed position). At a forward airspeed of 60 kt, the Dragonfly’s forward canard and aft horizontal stabilizer will begin to generate lift; at 120 kt those surfaces will develop enough lift that the rotor/wing will be able to be safely slowed, stopped and locked perpendicular to the fuselage. Subsequent, higher-speed Dragonfly variants still on the drawing board are designed to cruise at speeds of 450 kt and above. Top speed for the X-50A is a hoped-for 150 kt.

Looking at the 17.7-ft-long, 6.5-ft-tall X-50A prototype, one may wonder: “Why no tail rotor?”

“Tipjets” comes the answer. The Dragonfly’s unique rotor is powered by hot gas produced by a Williams International F112 jet. This hot exhaust is processed through a daisy-lobe mixer to cool the gas to a still lively 825 deg F.

It is then passed through plumbing carrying it from the engine through the rotor hub to eight tip jets (four on each rotor) that power the rotor system. Cooling the gas allows the use of conventional titanium rather than more exotic heat-tolerant hybrid alloys. In an effort to keep the X-50A parts count low, the entire rotor/wing/tipjet assembly is molded as one single structure.

In addition to its heat-resistant qualities, the brute strength of titanium is also needed in this unique rotor system. Tip speed for the X-50A rotor is 735 fps, roughly the same speed as that of the main rotor on a Boeing AH-64 Apache, but with one-fourth the blade diameter and four times the rpm. That works out to a staggering tip g-loading of 2,800 g. On the other hand, much of the complexity of a conventional helicopter has been eliminated by doing away with the transmission and drivetrain linkages.

Disk loading is predicted to be somewhere between 12 and 15 lb/sq ft, far closer to that of a conventional helicopter than the 67 lb/sq ft disk loading for a hovering MV-22 Osprey tiltrotor.

While tipjet main rotor propulsion is famous for dramatically cutting the anti-torque forces needed to control a helicopter, such a propulsion plan does not completely eliminate them on the X-50A. Drawing a small amount of exhaust air away from the Williams International F112, a pair of directional control nozzles will handle anti-torque control duties during low-speed, rotary-wing flight. As the X-50A accelerates and converts to fixed-wing flight, valves progressively divert hot gas away from the directional nozzles to the aft cruise nozzle, which provides the thrust for forward flight. Control in fixed-wing cruise (yaw, pitch and roll) is by means of the X-50A’s aerodynamic control surfaces.

Flight Test
Following first flight later this summer, a quartet of envelope expansion flights is aimed at getting the X-50A up to a forward speed of 60 kt in helicopter mode. Throughout the autumn, five more flights will combine various aspects of fixed- and rotary-wing flight, building to an airspeed of 130 kt. Finally, Boeing hopes the full conversion from rotary- to fixed-wing flight will take place this fall. A full test program of just 11 planned flights provides little room for mistakes, although the use of a pair of X-50A airframes should provide some buffer against delays due to mishaps and malfunctions.

Further development could result in either manned or unmanned military variants of the X-50A by 2008. Should a follow-on version make the quantum leap to 400 kt, both the military and possibly civil market could wake up to the long-awaited realization of a cherished dream: simple, reliable high-speed flight with the vest-pocket convenience of rotary-wing flight.