McDONNELL DOUGLAS (MDC) and Rockwell have been selected to modify an MDC F-18 for a joint US Air Force/ NASA evaluation of an advanced control concept which makes use of wing warping.

Lockheed Martin had considered proposing an F-16 for the evaluation. It declined to bid after estimating an outlay of up to $36 million for extensive conversion of its production wing and flight-control system.

MDC and Rockwell will convert the high-alpha research vehicle (HARV) F-18 used until recently by NASA for high-angle-of-attack investigations.

The active aeroelastic wing (AAW) programme will demonstrate the use of wing warping to control aircraft at both transonic and supersonic speeds. It is expected to benefit designers of future combat aircraft by giving them greater freedom to use higher-aspect-ratio wings. The control-system research will also support the trend towards tailless aircraft designs.

The F-18 is suited for the AAW effort because the original wing design suffered from aileron roll-reversal and required stiffening and leading-edge modifications to meet the design specification.

The HARV F-18's wing will be restored to the original, more flexible, design by replacing wing panels with less-rigid skins. In addition, the leading-edge flap will be dived into two, each to be operated as an independent control surface.

The leading edge is now made up of two separate pieces, but is connected to a single torque rod. The inner flap section, will be controlled by the current rod, while a new actuator will command the outboard section.

The research flight-control system (RFCS), developed to control the HARV's thrust-vectoring paddles at high alpha levels, will be modified to govern the wing twist. At transonic speeds and above, the RFCS will command the leading edge to move up by 3° to begin the twisting motion. If the left-hand leading edge is moved up, for example, the aircraft will move to the right. Leading-edge deflection could eventually be extended to a maximum of 9°, depending upon the early results.

"Birds use internal muscles to do the same thing with their wings, and the Wright Brothers used cables to warp the trailing- and leading-edges of the Flyer," says NASA Dryden HARV programme manager, Denis Bessette. "In this case, we're using aerodynamic forces themselves to control the flight. The control is acting like a tab, while the wing acts like a control surface."

Preliminary design of the AAW system is due to begin in 1997, with final design planned for 1998. The HARV will be modified to AAW configuration in 1999, with flight tests expected to begin in the year 2000.

Source: Flight International