Winglets are vertical extensions added to each end of a wing that reduce induced drag by inhibiting turbulence and diminishing the strength of wing tip vortices. Acting as a barrier between the high-pressure flow under the wing and the low-pressure flow above it, winglets force the spanwise air aft, making the overall flow more two-dimensional and thereby refining the lift-to-drag ratio of the wing. This aerodynamic refinement increases performance, allowing designers to extend an aircraft's range and simultaneously decrease fuel burn. NASA tests of a Boeing-707-type model showed a 6.5 percent saving with winglets fitted.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
Why do some aircraft have winglets?
Aircraft have winglets to reduce drag created by wingtip vortices. Winglets are nearly vertical airfoils at an airplane's wingtip that reduce drag by inhibiting turbulence. They act as a barrier between two different air flows that form on the top and bottom of a wing, deflecting the wingtip vortex away from the lift-producing part of the wing. By opposing the drag that wingtip vortices create, winglets diminish vortex strength and increase the effective wing-aspect-ratio, granting the increase in effective wingspan without the added form drag of actually making the wing longer.
These devices refine the lift-to-drag ratio of the wing, leading to fuel savings and a performance improvement of between five and seven percent. They boost directional stability of the aircraft. Winglets generate some forward force from the wingtip vortex by bouncing spanwise flow aft, which generates a small amount of thrust, and they reduce drag when the aircraft is at a high angle of attack. For larger aircraft that fly long distances, the proportion of aircraft weight due to winglets is much smaller than for small aircraft, making them especially beneficial for long-haul operations.
Why don't all planes have winglets?
Many smaller, slower planes do not benefit enough from winglets to justify the additional weight, cost and complexity. For them the contribution of tip vortices to overall drag is modest, so the point of diminishing returns is reached quickly. Most of these aircraft are designs from a pre-winglet era, and retro-fitting requires a new type certificate.
Larger jets face different obstacles. On long-haul flights where cruising speed is extremely important, winglets offer less advantage and add extra weight. The Boeing 777 was therefore designed with specially raked wingtips that avoid both the added mass and the increased wingspan normal winglets bring. Maintaining the distance below 65 m (213.25 ft) preserves ICAO airport code-E compatibility and prevents gate-clearance issues that compel operators to construct new gates. The forthcoming 777X will use a folding wingtip so that the same aircraft can exploit a larger wingspan in flight yet retract on the apron to fit existing infrastructure.
Why don't military aircraft have winglets?
Winglets add weight, and on military airframes every extra kilogram competes with fuel, weapons or sensors. A winglet interferes with antennas or sensor equipment on military airplanes, so designers prefer clean wingtips that leave room for pods and aerials.
Military aircraft have to operate in a wider range of conditions, including large sideslips at high dynamic pressure. Under those manoeuvres, winglet loads exceed loads of conventional extensions. Whitcomb claims winglets hurt at high speed, and fighters often cruise and fight close to the speed where induced benefits collapse while profile drag rises.
Improvement in cruise performance is not worth the reduction in loiter time: a tanker or patrol aircraft gains a few percent in range, yet loses endurance when the heavier wing demands more thrust in a holding pattern. Others like the KC-135 tanker have been evaluated for winglet modification, but analysts concluded that wingtip modifications impact other wing requirements, like low-speed handling for refuelling contact or formation flight.
