NASA tests could pave the way for formation flying by freighters and manned and unmanned military aircraft

Flying in wingtip-to-wingtip formation can produce fuel savings of over 15%, NASA has demonstrated. The savings have been measured in tests of two Boeing FA-18s under the Autonomous Formation Flight (AFF) programme inspired by the familiar 'V' of migrating geese.

Formations of cargo aircraft crossing the Pacific are among applications that have been envisaged, but AFF chief engineer Brent Cobleigh expects military uses to be the first to emerge, possibly involving formation operations by manned and unmanned aircraft.

In the first phase of tests nearing completion at the Dryden Flight Research Center, Edwards AFB, California, NASA is mapping the effects on the trailing aircraft of the leading aircraft's wingtip vortex. The manually flown tests are generating data for use in the design of a formation-keeping autopilot for the next phase of flights, which is scheduled to begin next July.

Cobleigh says that consistent reductions in trailing-aircraft drag of almost 20% have been measured on the instrumented flights, conducted at 25,000ft (7,600m) and Mach 0.56. "This equates to a fuel flow reduction of over 15%," he says. The AFF goal is to achieve fuel savings of more than 10%.

Drag reductions result from the trailing aircraft flying in the upwash from the lead aircraft's wingtip vortex. "It's like flying downhill," says Cobleigh. "The aircraft can be retrimmed nose down by about 1í in pitch, which rotates the lift vector forward and counteracts drag on the aircraft. The net result is that drag along the flightpath is lower."

Best results have been achieved with the trailing FA-18 positioned about one aircraft length - 17m (55ft) - behind and 1m below the lead FA-18, wingtips overlapping by about 2m, Cobleigh says. Flights with the trailing aircraft 35m and 60m behind the lead have shown only slight degradations in the drag reduction, while flying closer does not increase the savings, he adds.

To allow accurate station-keeping, both FA-18s are equipped with a formation flight instrumentation system. This is a differential global positioning system that measures the relative position of the two aircraft with sub-metre accuracy and then shares the data between the FA-18s via a wireless local-area network. Realising the savings requires the trailing aircraft to hold formation to within 1m.

Source: Flight International