PAUL LEWIS / MESA, ARIZONA
For 60 years the aerospace industry has been trying to stop and start a rotory-wing aircraft's rotor in flight. A new concept from Boeing may be the answer
Tucked in the corner of a Boeing hangar in arid Arizona is a porpoise-like research craft, the peculiar design of which represents a hybrid of helicopter and fixed-wing jet. The newly designated X-50A Dragonfly demonstrator has one main goal: to be the first rotorcraft to stop and start its rotor in flight. The canard rotor wing (CRW) concept, if proved successful, promises to open the door to development of a new type of high-speed vertical take-off and landing (VTOL) air vehicle.
The unmanned X-50A is the direct descendant of two distinct genealogical lines of experimental rotorcraft. One side can be traced to the Herrick HV-2A convertiplane, which in 1937 made the first fixed- to rotary-wing transition: the other ancestor is the 1952 Hughes XH-17, with its monstrous tip-burning, reaction-driven rotor system. There has been a succession of other test vehicles since, including the McDonnell XV-1 compound helicopter, the Hughes XV-9A hot-cycle research craft and, more recently, the unflown Sikorsky S-72 X-Wing with its complex blown-air circulation control system.
In this context, the CRW demonstration, jointly funded by Boeing and the US Defense Advanced Research Projects Agency (DARPA), is the culmination of more than 60 years of technological trial and error. "They have never successfully stopped and started a rotor in flight. That is what this entire vehicle is designed around. I don't think it's been a technological issue, it's been an integration issue. We're not really adding any new technology, we're really repackaging existing technology," says Clark Mitchell, Boeing Phantom Works CRW systems integration team leader.
The principal lesson learned from the X-wing and the earlier 1960s Hughes Rotor/Wing programmes was the need to unload the rotor during conversion between rotary- and fixed-wing modes. Windtunnel testing of the XV-19 showed extreme oscillation during conversion as a result of constantly shifting pressure on the rotor/wing. The X-wing relied on blowing air over the blades to maintain lift during conversion, as well as a convertible turbofan/turboshaft engine.
The X-wing programme was terminated in 1988 before the rotor could be attached. "If you look at all the previous stop-rotor designs, without fail they always loaded the rotor during conversion mode. The definitive feature of CRW is the unloading of the rotor system, which we consider our ace in the hole," says Mitchell.
The X-50A has a 2.71m (8.9ft)-span forward canard and 2.47m-span aft horizontal stabiliser, which above 60kt (110km/h) forward airspeed will start to generate lift. By 120kt, these surfaces will provide lift equal to the vehicle's weight, unloading the 3.66m-diameter rotor/wing and allowing it to be safely slowed, stopped and finally locked perpendicular to the fuselage. The demonstrator is not intended to go beyond 150kt, but in higher-speed operation the stopped rotor/wing will generate lift, improving the vehicle's manoeuvrability. Above 450kt the rotor/wing could be stowed obliquely to reduce drag.
The CRW uses a "daisy lobe mixer" on the exhaust of the X-50A's Williams International F112 engine to power the rotor via a warm-cycle reaction drive. Temperatures are around 440°C (825°F), compared to 720°C on the hot-cycle XV-9A. This permits the use of titanium rather than more exotic materials for propulsion ducting and has helped minimise weight. The rotor/wing tipjet is a one-piece superplastic-formed titanium structure. The rotor system houses four gas ducts running out to the tips from a hub-mounted transition duct.
"It all comes down to handling heat loads and gs," says Mitchell. "A rotorcraft is always sized by tip speed, which in this case is 735ft/s [224m/s], the same as the [Boeing AH-64] Apache but with only one-quarter the blade diameter. As a result we're spinning four times as fast and have a lot more gs - 2,800g at the tip - and that's a real design challenge." On the other hand, the CRW dynamic system has been simplified by eliminating the mechanical transmissions and drive train normally associated with a helicopter, and by using off-the-shelf engine technology.
In rotary-wing mode, most of the engine exhaust flows up the mast and along the rotor/wing ducting to eight tip-jet nozzles. The remainder is ducted to two fuselage-side directional control (DC) nozzles that replace the conventional tail rotor. During conversion to fixed-wing mode, a valve will progressively divert airflow away from the rotor/wing to an aft cruise nozzle, while the DC nozzles will close off, with yaw, pitch and roll authority transferring to the flying surfaces as forward speed rises.
The Dragonfly advanced technology demonstration (ATD) programme has been under way since 1998, with an initial phase of fixed- and rotary-wing windtunnel testing with a 75%-scale model, cold-flow propulsion and full-scale engine testing completed by May 1999. The ATD effort is to conclude in October with an in-flight conversion from rotary- to fixed-wing mode. Boeing has mapped out an ambitious schedule of 11 flight-tests, starting this summer at the proving grounds in Yuma, Arizona, using a single vehicle. A second identical X-50A will be certified for flight and kept in reserve.
The first four flights will be used to validate rotary-wing mode and provide limited envelope expansion up to 60kt. The next five planned flights will be a blending of fixed- and rotary-wing modes, with airspeed steadily increasing up to the conversion point at 130kt. The final two flights will be complete conversions to aircraft mode. There is margin in the schedule for additional test flights. "But right now the indications are we could achieve our objectives in 11 flights," says Mitchell.
At the conclusion of the current ATD phase, the two Dragonfly demonstrators will become the property of the US government. Boeing is under contract to support any follow-on flight testing, but none has yet been funded or scheduled. The focus instead is on this summer's demonstration and proving the concept works. Only then will DARPA and the services be willing to entertain any more funding. The next logical step would be an advanced concept technology demonstration (ACTD), which could then move into full development and production of an operational system.
Based on projected technology readiness levels, the earliest application for CRW would be either a manned or unmanned air vehicle (UAV) in around 2008, suggests Boeing. With designers looking at speeds of 400kt and beyond, CRW promises a huge leap in capability over current propeller-driven fixed-wing and tiltrotor UAVs in terms of faster transit times and smaller areas of target location uncertainty. "It could be a very good fit for the US Army's planned Unmanned Combat Armed Rotorcraft, depending on where the requirements fall," says Mitchell.
In the manned arena, the CRW's promised performance positions it between slower-flying tiltrotors such as the Bell Boeing V-22 Osprey and fixed-wing V/STOL jets like the Boeing AV-8B Harrier II. In the hover, the CRW is expected to have a 60-160kg/m2 (12-15lb/ft2) disc loading, closer to that of a conventional helicopter than the 100-110kg/m2 of a tiltrotor. Among the roles envisaged for a manned vehicle are an armed escort for the Osprey and eventual replacement for attack helicopters such as the US Marine Corps' Bell AH-1Z Cobra.
The X-50A has been scaled to 70% of the US Navy's tactical VTOL UAV requirement. It is clear an ACTD or operational vehicle could differ in size and shape. Increasing the size of the rotor/wing while maintaining the current tip speed would result in a simpler design, but there remains the perennial challenge of weight and drag. This would require more extensive use of composites than in the X-50A, while the tip nozzles may have to be redesigned to allow them to be closed in fixed-wing mode to stop ram-air recirculation. Researchers are also looking at smart materials in an effort to improve on the present elliptical-section rotor/wing design.
"CRW in terms of aviation development is still very much in its infancy," says Mitchell. "Things like trying to move beyond elliptical blades to lower-drag aerofoils and closing the tip jet nozzles are basically all in the camp of either weight reduction or drag reduction. The ATD ignores most of those areas out of simplicity, but an operational vehicle cannot afford to ignore them and we need to start looking at optimising the design."
Source: Flight International