GRAHAM WARWICK / WASHINGTON DC
Excess reaction-drive rotor-hub friction could delay start of forward-flight tests
Boeing Phantom Works plans to resume flight testing of the X-50A Dragonfly canard rotor/wing (CRW) technology demonstrator in February, after completing analysis of data from the 80s first hover flight on 4 December. A range conflict at the Yuma proving ground in Arizona has prevented flights resuming this month as planned.
Boeing is assessing whether further work is required to reduce friction in the reaction-drive rotor hub before beginning forward flights. The next test is planned to involve a transition from the hover to forward, sideways and rearward flight at 10kt (18km/h). Boeing then intends to increase speed in 10kt increments until reaching the 120kt target for conversion between rotary-wing and fixed-wing flight, when the rotor is stopped and locked to become a wing.
The unmanned X-50 differs from previous, unsuccessful, stopped-rotor designs in that the rotor is unloaded during conversion, with lift provided by a canard foreplane and aft wing and thrust by a cruise nozzle. The vehicle will operate as a pure helicopter up to 60kt, when it will transition to compound mode as thrust transfers from the rotor to the cruise nozzle, says programme manager Clark Mitchell. Between 60kt and 120kt the tip-drive rotor will be slowed from 100% to 70% RPM. At 120kt it will be stopped.
In hover mode, 70% of the mass flow from the Williams F112 turbofan goes to the tip-drive rotor, the remainder going to a directional nozzle used for yaw control. The cruise nozzle begins to open at 40kt and by 60kt makes a significant contribution to thrust, says Mitchell. Between 60kt and 120kt, airflow is progressively diverted from the tipjets to the nozzle, reducing rotor speed and increasing cruise thrust.
"The flight control system (FCS) is the key," says Mitchell. "Despite all the interplay, the engine always sees the same back pressure and stall margin. It is all scheduled with airspeed in the FCS control laws." At the 120kt conversion speed, Boeing expects the canard and aft wing to be generating enough lift to offload the rotor so that it is not producing lift or thrust and can be safely stopped and locked perpendicular to the fuselage. Conversion will be controlled autonomously by the FCS. "The pilot will push a button to convert, then sit back and watch," he says. "If things are not right, the FCS will do its best to reconvert [to helicopter mode]."
Boeing plans two flights involving conversion to fixed-wing mode. In the first, the aircraft will remain in 1g level flight to avoid loading the rotor/wing. "For the first we will not load the rotor. On the second, we will apply lift and fly as a fixed-wing aircraft for 5min," says Mitchell. Although the rotor/wing has a pitch-change mechanism for use in helicopter mode, incidence will not be varied in fixed-wing mode. "That's an interesting approach for future CRW tests," he says.
Conversion is the next major risk in the already much-delayed programme, says Mitchell. First flight was delayed several months by issues including excess friction caused by leakage in the carbon seals that direct engine airflow through the rotor. Friction was reduced enough to allow a stable first hover, preceded by a 23s "pop-up" flight, but was still higher than expected, he says.
Source: Flight International
