Warping wings readied for flight

Engineers at NASA’s Dryden Flight Research Center will soon be beginning the first research flights in a project to demonstrate that twisting or warping flexible wings can enhance aircraft performance.

Engineers and technicians at NASA’s Dryden Flight Research Center are wrapping up the last major ground tests this month before beginning the first research flights in a project to demonstrate that twisting or warping flexible wings can enhance aircraft performance.

The ground vibration and structural mode interaction tests on the ‘Active Aeroelastic Wing’ (AAW) F/A-18A test aircraft began in late August, and should be completed by the middle of this month, according to Dryden’s AAW project manager Denis Bessette.

Following final pre-flight checks, control room training of project staff and updating of mission rules and flight plans, the modified jet fighter could fly in mid-October.

A joint program of the US Air Force Research Laboratory (AFRL), Boeing’s Phantom Works and NASA Dryden, the Active Aeroelastic Wing developers hope to demonstrate improved aircraft roll control through aerodynamically induced wing twist on a full-scale manned supersonic aircraft.

‘The project reflects both a return to aviation’s beginnings, when the Wright Brothers devised a primitive wing-warping method to control the Wright Flyer, and a gateway to the future – a future where aircraft will sense their environment, morph, and adapt their shape to the existing flight conditions,’ said Bessette.

AAW research could also enable thinner, higher aspect-ratio wings on future aircraft, which could result in reduced aerodynamic drag, allowing greater range or payload and improved fuel efficiency. Data obtained from flight tests at Dryden will provide benchmark design criteria as guidance for future aircraft designs.

‘Active Aeroelastic Wing technology is important to the Air Force because it represents a new approach to designing wings, and is applicable to a wide variety of future air vehicle concepts that are under study,’ said Pete Flick, AAW program manager for the AFRL Air Vehicles Directorate.

During the current tests, the F-18 rests on three large airbags, while electro-mechanical shakers induce vibrations into the wings at varying amplitudes and frequencies. Test instrumentation measures how the structure reacts as these vibrations propagate through the aircraft to determine potentially adverse effects.

In the ground vibration tests, the F-18’s hydraulics were powered up, but the control surfaces were inactive. The structural mode interaction tests take the process one step further, with the flight controls operating and the interaction of the flight control surfaces with the aircraft structure observed. This test assures that vibrations caused by the actions of the flight controls are damped or suppressed, rather than reinforcing each other to cause large, uncontrolled vibrations or ‘flutter’ that could lead to catastrophic failure of the aircraft structure.

The testbed F/A-18A, provided by the US Navy, has been modified with additional actuators, a split leading edge flap actuation system and thinner wing skins that will allow the outer wing panels to twist up to five degrees. The traditional wing control surfaces – trailing edge ailerons and the leading and trailing edge flaps – are used to provide the aerodynamic force needed to twist or ‘warp’ the wing.

Project engineers hope to obtain almost equivalent roll performance of production F/A-18s at transonic and supersonic speeds without using the horizontal stabilators and with smaller control surface deflections.

A six-month long structural loads testing program on the F/A-18’s modified wings – one of the most extensive tests ever performed in Dryden’s Flight Loads Laboratory – was conducted in 2001. As part of those tests, the wings were subjected to loads up to 70% of the design limit load of the aeroplane, with load distribution over the wings a particularly critical item.

The two-phase AAW flight tests will begin with a series of about 30 to 40 parameter identification flights. Boeing’s Phantom Works will use data obtained from the first series of flights to refine wing effectiveness models and design the AAW flight control laws. The second phase of research flights to demonstrate the AAW concept with effective control laws should begin in mid- to late 2003, almost 100 years after the Wright Brothers’ first powered flight on December 17, 1903.

‘We’ve been successful to date, but the real test of this technology is when we start flying, and we see how the flight data correlates to our predictions of aircraft response,’ Flick added.

The AAW program receives its funding from the US AFRL’s Air Vehicles Directorate and NASA’s Office of Aerospace Technology. The Boeing Company performed the AAW F/A-18 modifications under contract with the AFRL Air Vehicles Directorate. The eight-year program’s total cost is about $41 million, of which about $25 million is in direct costs and about $16 million in in-kind services.