As the month of May came to a close, a team of Boeing engineers were putting the finishing touches to a one-of-a-kind flying machine at an outpost of that company’s “Phantom Works” just outside the sun- and sand-blasted southwestern Arizona town of Yuma. Unpiloted and powered by a Williams International F112 jet, the X-50A Dragonfly will, if successful, knock off one of the last of aviation’s landmark goals–the successful flight of a wing that can transition from rotary to fixed airfoil flight and back without landing.
The advantage? Elimination of the helicopter’s “roll over” tendency at high speed, an effect caused by the asymmetric lift that results when a helicopter gains significant forward speed. As it does so, the airflow over the rotor blade advancing forward in its rotational arc speeds up, becoming the sum of its advancing rotational speed and the forward speed of the entire aircraft. At the same time, rotor blades on the retreating side develop less lift, since the flow of air over their surface is the forward speed of the helicopter minus the retreating rotational speed.
Various rotor designs have included flapping or teetering mechanisms to cope with this “rollover” asymmetric lift tendency, and most succeeded at low speed. But as speeds increased so did the asymmetric lift, with the rollover forces becoming so pronounced that rotor systems either tore themselves apart or the aircraft became uncontrollable. The highest rotorcraft speed yet recorded (for a pure rotor-driven rotorcraft) is 249.10 mph, set in 1986 by a specially modified Westland Lynx.
Sikorsky’s S-69 advancing blade concept (ABC) testbed aircraft countered asymmetrical torque via a pair of contrarotating three-bladed rotors turning in such a way that a blade on one rotor was always advancing while another on the other rotor retreated. In addition to the forward speed produced by its 1,825-shp PT6T-3 TwinPac powerplant, the ABC was propelled by a pair of Pratt & Whitney J60-P-3A turbojets. Top speed for this hybrid aircraft in level flight was 309 mph, but by that time the ABC had become a compound helicopter– thanks to its auxiliary power–and its achievement was discounted from the helicopter ranks.
X-wing aircraft try to beat the asymmetric lift barrier by avoiding it altogether. At low speeds, lift and thrust is provided by a rotating rotor, in the X-50A’s case a rotor propelled by hot gases generated by its turbofan engine and vented through rotortip jets. The tipjet-driven rotor eliminates the level of torque that would require a mechanically complex and drag-inducing tail rotor. As the rotor slows and locks into fixed-wing configuration at higher speed, propulsion comes from the Williams International F112 in conventional jet form.
Featuring a 12-foot-long rotating rotor/wing, the X-50A will take off like a conventional helicopter (although it could also take off with its wing in fixed position). At a forward airspeed of 60 knots, the Dragonfly’s forward canard and aft horizontal stabilizer will begin to generate lift; at 120 knots those surfaces will develop enough lift that the rotor/wing will be able to be safely slowed, stopped and locked perpendicular to the fuselage. Now all three sets of surfaces will be developing lift, as will the fuselage of the X-50A itself, which has something of the lifting- body shape of those NASA “flying flatiron” test aircraft that proved the feasibility of the space shuttle.
Subsequent, higher-speed Dragonfly variants still on the drawing board are designed to cruise at speeds of 450 knots and above. Top speed for the X-50A is a hoped-for 150 knots.
Present plans call for a modest flight-test program of about 10 flights.