Drag Reduction from Formation Flight. Flying Aircraft in Bird-Like Formations Could Significantly Increase Range.

摘要

The Air Vehicles Directorate is currently studying a novel form of formation flight. For centuries, flocks of migratory birds have flown in large formations. One reason for this is the drag reduction that is obtained by flying in close proximity to wakes generated by other birds. Photographic studies of Canadian Geese indicate the average spacing between adjacent birds is very close to the optimum predicted by simple aerodynamic theory. Small heart monitors implanted in White Pelicans show reduced heart rates while flying in formation compared to individual flight. Recent advances in automatic control theory combined with the ability to accurately determine the location of aircraft may now make this practical for aircraft. Aircraft wings generate strong tip vortices (like horizontal tornadoes) that generate large downward velocities ('downwash') between the wing tips and upward velocities ('upwash') outboard of the tips. For some aircraft, the velocities at the edge of these vortices can exceed 100 miles per hour. By properly positioning the wing of another aircraft within this upwash, the effective velocity vector of the aircraft is rotated downward. This rotates the lift vector forward and the drag vector upward, giving the impression of flying downhill. The net effect is a decrease in drag as measured with respect to the flight path. The phenomenon is not 'drafting', which bicycle and automobile racers use to reduce wind resistance. The upper limit on the theoretical benefit in range increases with the square root of the number of aircraft in the formation. For example, the range of nine aircraft in formation would by three times the range of a single aircraft. Introducing only a single constraint, that the formation cruises at the same altitude that single aircraft currently use, reduces the benefit for a nine aircraft formation to an 80% increase. Other considerations like engine performance and atmospheric turbulence reduce the value even further.