Gull wings have usually been used on aircraft for mechanical reasons - to combine the biggest propeller with the shortest landing gear on the F4U Corsair, for example. But Dr Nhan Nguyen, a scientist at NASA Ames Research Center, believes a seagull-like drooped wing could reduce drag.Graphics: NASA
Nguyen's bio-inspired elastically shaped wing
concept was selected for a NASA Innovation Fund award in 2010, and NASA has worked with Boeing to study the idea in more detail.
The four-month study looked at three concepts: a drooped-wing vehicle (above), offering a 5.3% drag reduction; an inflected-wing vehicle (below), with 3.5% less drag; and a "squashed-fuselage" drooped-wing vehicle (bottom) promising a 15% drag saving.
Why drooping the wing reduces drag is not quite clear, but it's possible that negative wing curvature increases the local "alpha" (angle of attack), which causes the aircraft's trim alpha to move closer to that for minimum drag, according to Nguyen. The downward droop also reduces the tendency for high-pressure air under the wing to flow round the tip to the lower-pressure upper surface. In the third design, the squashed fuselage generates lift, offsetting wing lift and reducing induced drag.
Maintaining the optimum shape as fuel in the wing tanks is consumed in the cruise will require active wing shaping control - Nguyen's idea is to use a variable-camber continuous trailing-edge flap, as conventional flaps would generate too much drag. Downward flap deflection would be used to control the shape of the wing. In the drooped concepts, the outer wing panels would rotate upwards to provide ground clearance for take-off and landing.
Studies of the elastically shaped wing concept by Boeing and NASA showed the potential for drag reductions, but highlighted several potential issues requiring further investigation. Aeroelastic bending and twisting of the flexible wing in flight, for example, could adversely affect aerodynamic performance and would require dynamic aeroelastic control by the wing-shape control system.
The study estimated 17% fuel savings over 4,500nm range due to elastic wing shaping control, increasing to 23% when the variable-camber continuous trailing-edge flap is used for dynamic aeroelastic tailoring to maintain optimum wing shape in the cruise,
Boeing's assessment of the initial study points out the gulled wing shape adds weight and cost, may limit space for fuel, would require a heavy wing fold and could introduce handling-quality issues. The failure modes of the multiple variable-camber flap actuators could be a certification concern. The recommendation: more study needed.