Lift-induced drag
In aerodynamics, lift-induced drag, or more simply, induced drag, is a drag force arising from the generation of lift. It is a small, but proportionate component directly arising from lift - the greater the lift, the greater the induced drag.
 |
Another source of induced drag is at the wingtips. The higher pressure below the wing and lower above cause the air to "roll around" the wingtip, creating a vortex which is shed behind the aircraft. The strength of the vortex is directly proportional to the lift force generated by the wing, and the energy absorbed by the vortex contributes to the induced drag. Various devices have been tried to combat this effect, the most successful of which are winglets, as seen on a number of modern airliners such as the Airbus A340. Winglets do not prevent the vortex, but they control its formation such that the vortex energy actually adds to the aircraft's thrust, rather than to its drag. Winglets can yield very worthwhile economy improvements on long distance flights.
Induced drag must be added to the form drag to find the total drag. In a practical aircraft, induced drag becomes less of a factor the faster the aircraft flies. This is because the requisite lift can be obtained at a lower angle of attack with a faster airspeed. Thus induced drag reduces with speed. The opposite occurs with form drag (the drag caused simply by pushing the aircraft through the air), which increases with speed. The combined overall drag curve therefore shows a minimum at some airspeed - an aircraft flying at this speed will be at or close to its optimal efficiency. Pilots will use this speed to maximise endurance (minimum fuel consumption), or maximise gliding range in the event of an engine failure.
