NASA has begun test flights of an adaptive control-surface experiment which it hopes could lead to drag reductions of up to 3% for commercial aircraft, worth roughly $140 million a year in reduced fuel savings.

The tests are taking place on the Lockheed L-1011 TriStar operated by Orbital Sciences (OSC). The aircraft has had its outboard ailerons modified under the Adaptive Performance Optimization experiment. The outboard aileron works as a follow-up to the inboard section through a series of bell-cranks and push-rods. "We've taken one of those push rods and inserted an electronic actuator," says NASA Dryden principal investigator and flight-test director Glenn Gilyard. Using the additional control of the actuator, the aileron can be fine-tuned by an extra 20mm to suit the flight profile.

Controllers on board the L-1011 can adjust the position, depending on the flight condition. Although the initial sortie, from Bakersfield, California, was aimed mainly at checking systems, initial data suggested that "-it looks as if we got about 1% [drag reduction] for a couple of degrees trailing edge down", says Gilyard.

Near-term goals for the experiment, which forms part of NASA's Advanced Subsonic Transport Aircraft Research programme, include the development of steady-state algorithms to reduce drag based on the aircraft's normal performance envelope.

Future flights will concentrate on assessing drag reduction at different weights (for various lift/co-efficient conditions) at speeds of around Mach 0.83 and altitudes between 30,000ft (9,150m) and 40,000ft. Drag reduction in the climb will also be evaluated.

Initial contact has been made with United Airlines and Northwest Airlines, which have fleets of fly-by-wire Airbus A320s, deemed to be the most promising early applications. "They won't require any hardware modifications to the outboard aileron, and it's not much of a burden to put the algorithm into the flight-management computer," says Gilyard. Retrofit kits for conventionally controlled aircraft would then follow, he adds.

Source: Flight International