AAW project uses modified Boeing F/A-18 demonstrate warping technology
NASA has essentially completed flight testing of an active aero-elastic wing (AAW), demonstrating that a wing can be made lighter and more flexible and still provide adequate roll control and acceptable handling.
The tests involve a Boeing F/A-18 modified to use wing warping instead of an all-moving tail for roll control at transonic speeds. In aircraft like the F/A-18, a differential horizontal stabiliser is used for roll control at transonic and supersonic speeds because wing flexing makes the ailerons ineffective.
For the AAW project, the F/A-18's wing was made more flexible, and the inboard and outboard leading-edge flaps were modified to operate separately, allowing the outboard section to be moved from 20° down to 10° up to actively and aerodynamically twist the wing for roll control. The modified F/A-18 was able to achieve roll rates with wing warping alone of over 200°/s, says Larry Meyers, AAW project manager for NASA's Dryden Flight Research Center at Edwards AFB, California.
From the lowest and slowest test point at 5,000ft (1,520m) and Mach 0.85 to the highest and fastest at 25,000ft and M1.3, roll rates were typically 20% lower than for the standard F/A-18, but provided adequate control and acceptable handling qualities, he says.
After an initial phase of 51 parameter identification flights to develop aerodynamic and loads models and design control laws for the modified F/A-18's research flight control system, 25 flights tested the AAW at nine subsonic and nine supersonic test points. Control was "robust", says Meyers, although at M0.95 at 5,000ft roll rate was enough to provide lateral control, but not to perform a full 360° roll safely. "It was not a real snappy roll rate," he says.
The final few flights will look at control-law "tweaks" to improve roll control at M1.2 and 20,000ft and provide load alleviation in wind-up turns, says Meyers. In a 5g turn at 10,000ft and M0.95, the control system will actively unload the wing by twisting it to shift the aerodynamic load inboard and reduce bending strain.
GRAHAM WARWICK/WASHINGTON DC
Source: Flight International