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Aircraft Drag

Induced Drag

When the wings of an aircraft are producing lift induced drag is present; in short no lift, no drag. This drag results from the down-wash by the wingtips and trailing edge vortices which tilt the lift backwards and increase the induced drag vector.

The aircraft builder has some limited influence in these forms of drag but by clever thinking he can reduce this as much as possible and count on more performance or fuel savings.

Induced drag will be at its maximum when airspeed is slow, thus with a large angle of attack. It decreases with increasing speed where parasite drag will increase to form the total drag of an aircraft.

Armed with this knowledge the pilot can use this to increase the performance of his aircraft when flying the longest distance or remaining airborne for as long as possible.

Induced drag factors

A number of factors are to blame for causing induced drag, namely: the wing shape or planform, aspect ratio (AR) and coefficient lift (CL) which the pilot can influence by extending flaps and slats.

Wing plan-form

The elliptical plan-form has the least amount of drag but its a bit more difficult to build. Contrary to that fact: The Spitfire, build in considerable numbers. Following the elliptical wing, we see the tapered wing and then the more common easy to build rectangular wing shape.

The difference between a rectangular, straight wing compared to an elliptical wing is about 10% more induced drag for the straight wing.

Aspect Ratio

This is the wingspan / chord ratio and the higher this number the lower the amount of induced drag the wing creates. Gliders have small chord, long wing thus implying high AR. Down-wash is also less compared to low AR wings.

Coefficient Lift

Coefficient lift, CL

The CL is something a pilot can influence and the most important factors are angle of attack (AOA), airspeed and aircraft weight. You will also find the coefficient lift in our well known lift formula.

A higher AOA means more induced drag and the strength of the generated vortices depends on the pressure difference above and below the wing. When the wing is at the zero AOA there are no vortices. When the AOA is increased up to the maximum CL (near the stall) induced drag is also at its maximum.

A lower weight means a lower AOA for the same airspeed compared to an aircraft with a higher TOW and this will result in lower induced drag.

Increasing airspeed while maintaining altitude results in the AOA being lower and thus less induced drag generated by the wing. Although, parasite drag will go up. An aircraft just after take-off has a high AOA, low speed and as such induced drag also high and it can be some 75% of total drag.

Reducing induced drag

This can be accomplished by wing washout, this means that the angle of incidence is lower at the wingtips than at the root. It also favors a stall which starts at the root and this keeps the ailerons more effective throughout the stall. Wing fences, drooping wingtips, vortex generators all help to reduce wing vortices and induced drag.

Total Drag

Lift/Drag ratio

This is the sum of parasite and induced drag and called CD. Drag can also be expressed in a formula, which looks remarkably like the lift formula: L = 1/2 ρ V2 x S x CD, but S stands now for frontal area instead of wing area.

The drag curve (top of the page) shows that total drag is high at slow (high AOA and mainly induced drag) and high airspeeds (low AOA and mainly parasite drag). Minimum drag is experienced somewhere in the middle where the lift/drag ratio is at its highest.

Lift/drag ratio

This is an indication of aerodynamic efficiency of the aircraft. The highest L/D ratio is around 4° AOA for common general aviation aircraft. Some manufacturers sell a device which measures the pressures above and below the wing and indicating the AOA on a instrument on the panel.

Important flight speeds are obtained from the L/D ratio: maximum cruise range and maximum gliding speeds.

Written by EAI.

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