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|Category:||Theory of Flight|
Induced Drag is an inevitable consequence of lift and is produced by the passage of an aerofoil (e.g. wing or tailplane) through the air. Air flowing over the top of a wing tends to flow inwards because the deceased pressure over the top surface is less than the pressure outside the wing tip. Below the wing, the air flows outwards because the pressure below the wing is greater than that outside the wing tip. The direct consequence of this as far as the wing tips is concerned is that there is a continual spilling of air upwards around the wing tip a phenomenon called ‘tip effect’ or ‘end effect’. One way to appreciate why a high aspect ratio for a wing is better than a low one is that with a high aspect ratio, the proportion of air which moves in this way is reduced and therefore more of it generates lift.
For the wing more generally, since the streams of air from above and below the wing which meet along the trailing edge are flowing at an angle to each other as they meet, they combine to form vortices, which, when viewed from the rear, rotate clockwise from the left wing and counter clockwise from the right. The tendency is for these vortices to move outwards towards the wing tip joining up as they do so. Eventually, by the time the wing tip is reached, one large wig tip vortex has formed and is shed.
Most of these vortices are of course completely invisible but, in very humid air, the central core of a vortex may become visible because the air pressure within its centre has reduced - and has therefore cooled - sufficiently for condensation to occur. The higher wing loading in a turn will also increase the strength - and the degree of reduced pressure - so that visible vortex cores are even more likely during turns. If close up to these vortices, they can also sometimes be audible!
Most of the air flowing off the top of a wing - ‘downwash’ - continues more or less horizontally towards the empennage because it is balanced by a corresponding upwash in front of the wing leading edge. In contrast, the upwards air movement which leads to vortex ‘consolidation’ at the tip is just outside the tip whereas the corresponding downward movement is just at the extremity of the wingspan so that the net direction of airflow past the wing is downwards. The lift created by the wing - which is by definition at right angles to the airflow, is therefore inclined slightly backwards and thus ‘contributes’ drag - induced drag.
Although there must always be at least some induced drag because wings have a finite thickness, design attempts wherever possible to reduce this flow. A required wing area can be achieved using different wing span-to-chord ratios (aspect ratios). The larger the wing aspect ratio, the less air disturbance is created at the tip. However, for most aircraft, there are both practical limits to maximum wing span for ground manoeuvring and structural issues which mean that eventually, the weight penalty to adequately strengthen a long thin wing becomes excessive. The fact that aircraft carry most of their fuel in the wings is also a factor in wing design. Typical transport aircraft aspect ratios range between 6:1 and 10:1.
Other ways to reduce induced drag and tip vortex strength in a wing design are also based upon reducing the quantity air movement upwards at the wing tip by aiming to generate relatively more of the lift away from tips. Wing taper towards the tip assists this as does wing twist. The Boeing 767 is a example of a twisted wing. The inner wing is set at a higher Angle of Attack than the outer wing and thus generates proportionately more lift whereas the tip, at a very small Angle of Attack generates very little. Winglets (sharklets) have also become popular, both the usual up-turned versions and the Airbus A320 series two-way ‘wingtip fence’ versions. Well designed winglets can prevent about 20% of the airflow spillage at the tip - and therefore 20% of the induced drag.
So, induced drag and its wing tip vortices are a direct consequence of the creation of lift by the wing. Since the Coefficient of Lift is large when the Angle of Attack is large, induced drag is inversely proportional to the square of the speed whereas all other drag is directly proportional to the square of the speed. The effect of this is that induced drag is relatively unimportant at high speed in the cruise and descent where it probably represents less than 10% of total drag. In the climb, it is more important representing at least 20% of total drag. At slow speeds just after take off and in the initial climb, it is of maximum importance and may produce as much as 70% of total drag. Finally, when looking at the potential strength of wing tip vortices, all this theory on induced drag must be moderated by the effect of aircraft weight. Induced drag will always increase with aircraft weight.