Induced drag is associated with the shedding of vorticity along the span of a finite wing, especially its tip region; for most subsonic aircraft configurations, induced drag constitutes about 50 percent of total aircraft drag throughout the flight envelope. NASA and the U.S. aircraft industry have aggressively studied induced-drag reduction methods. The state-of-the-art CTOL commercial aircraft wing is as a result of these efforts virtually optimal, with a total induced drag lying within a percent of the theoretical minimum. Many of the devices currently under study for induced drag reduction are added to wingtips, yielding benefits through their effects on the wake vortex as well as through forces generated in the flowfield.

Induced drag - Historical perspective

: The formation of a strong tip vortex has an undesirable effect on the drag of a lifting surface. Firstly, it induces a strong downwash field locally which increases the vortex drag and secondly introduces a strong cross flow around the tip causing separation on the top surface. Tip devices seek to remove these effects. Sails unwind the tip vortex and in doing so experience a thrust, like stators behind a fan. Their nose to tail arrangement gives them the ability to carry high lift coefficients, in the manner of cascades or slotted aerofoils. Winglets, or tip fins attenuate the tip vortex by dissipating the shed vorticity along their length, thereby reducing the downwash induced at the wing tip region. In order to asses these effects, it was proposed that a 1/10th scale model of a BAe Jetstream (Figure 1) should be tested at Cranfield Institute of Technology in the 8' x 6' low speed wind tunnel with sails and winglets. The first objective of the tests was to demonstrate a measurable drag reduction for a Jetstream fitted with tip devices over the range of operational lift coefficients. The second objective was to estimate the increase in wing root bending moment due to either device in order to compare their relative merits with simple tip extensions. In the past this had been done with wing only half model with encouraging results, but an estimate for the complete aircraft configuration was highly desirable.

J. J. Spillman

Papers

The Effects of Wing Tip Devices on the Performance of the BAe (British Aerospace) Jetstream