Wing lets are replacing wing tips on larger aircraft and high altitude aircraft. There is more to winglets than just a means of reducing wing tip vortices. Birds, such as crows and vultures, have been using a variation on the theme with their individual wing tip feathers for eons.
Winglets were originally fitted to Learjets, which made use of their enhanced lift at high attitude. Since the early days some twenty years ago, the aviation industry has refined the design, and they are now being retrofitted to older airliners. Given the major structural alteration and the cost of this work, it is evident that these items must have a significant effect on the performance of the aircraft. Expect future air transport aircraft to have winglets as standard equipment, as designing the winglet into a wing is easier than the retrofit.
The advantage of wing lets comes from their aerofoil cross-section (see diagram 3.26), and the way they are mounted. Wing lets are normally swept back, with the tip inclined slightly outward. The leading edge is angled slightly outward, so that the winglet’s chord line does not run parallel with the aircraft’s longitudinal centre line.
The flow of air from the bottom surface of the wing to the upper surface and the inward flow of air across the top surface of the wing react with the normal free-stream airflow to create a small positive angle of attack for the winglet. This results in lift created over the inner surface of the winglet.
Lift is always at right angles to the airflow. A component of wing let lift acts in the direction of flight. This has the effect of negating some of the aircraft’s drag. The downwash from the trailing edge of the winglet blocks or counteracts the flow of the main wingtip vortices, and even the vortex from the tip of the winglet is positioned to affect a portion of the main wingtip vortex.
The amount of lift that the wing let produces depends on the angle of attack at which the aircraft is being flown. High angles of attack, which produce the largest vortex effect, produce the maximum lift from the winglets.
As high co-efficient of lift conditions are encountered in take-off, landing, and high altitude cruise, one of the first aircraft to fit wing lets as standard equipment was the Lear Jet 55. This aircraft was designed to be efficient at high altitudes
For commercial aircraft, wing lets may offer the operators:
~ A decrease in the take-off and landing distance required for a given mass
~ An increase in payload for a given take-off I landing distance available
~ A reduction in specific fuel consumption
Wing lets have certain disadvantages:
:>- At low co-efficient of lift values, they add to the aircraft’s overall drag by increasing the parasitic drag. Induced drag also increases with the additional lift created by the winglets.
:>- They increase the aircraft’s mass due to the need for them to be “manufactured so that they can withstand the aerodynamic loads that they could expect to be subject to”.
:>- They impose an additional structural load that requires the wing to be strengthened, increasing the aircraft’s mass.
The advantages outweigh the disadvantages for a commercial aircraft. To gain the same lift by increasing its overall span instead of the adding wing lets would result in the addition of a much greater mass of material, as the structural complexity needed to support the extension would be greater than that of the winglet. Also from a commercial point of view, an aircraft with wing lets still fits into the existing stands at airports around the world, whereas wingspan extensions would not