| Revision as of 08:57, 28 June 2005 editMolloy (talk | contribs)290 edits Phew, spent several hours re-writing this article. The theory in the old version was disproved long ago, and is only used today as a simple (but incorrect) way of explaining induced drag.← Previous edit | Revision as of 09:00, 28 June 2005 edit undoMolloy (talk | contribs)290 editsm Spelling mistakes correctedNext edit → | ||
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| In ], '''lift-induced drag''', or more simply, '''induced drag''', is a ] ] arising from the generation of ] by ]s or a ] during flight. | In ], '''lift-induced drag''', or more simply, '''induced drag''', is a ] ] arising from the generation of ] by ]s or a ] during flight. | ||
| Induced drag will be present whenever the wings are producing lift. To that extent, it is often said that induced drag is a part of lift. It arises from the downwash induced by the wingtip and trailing edge vortices which, for a given amount of |
Induced drag will be present whenever the wings are producing lift. To that extent, it is often said that induced drag is a part of lift. It arises from the downwash induced by the wingtip and trailing edge vortices which, for a given amount of lift being produced, tilts the total reaction force further backwards through the induced downwash angle. This extra rearward tilt, in effect, increases the length of the drag vector and it is this increase in drag which is know an induced drag. The angle through which the Total Reaction tilts towards the rear is determined by the pressure distribution and the direction of the effective airflow. By definition, the lift and drag components of the Total Reaction must be resolved with respect to the remote relative airflow which reflects the direction of flight. Obviously, the smaller the angle of induced downwash, the lower will be the induced drag. | ||
| There are a number of factors affecting induced drag: | There are a number of factors affecting induced drag: | ||
Revision as of 09:00, 28 June 2005
In aerodynamics, lift-induced drag, or more simply, induced drag, is a drag force arising from the generation of lift by wings or a lifting body during flight.
Induced drag will be present whenever the wings are producing lift. To that extent, it is often said that induced drag is a part of lift. It arises from the downwash induced by the wingtip and trailing edge vortices which, for a given amount of lift being produced, tilts the total reaction force further backwards through the induced downwash angle. This extra rearward tilt, in effect, increases the length of the drag vector and it is this increase in drag which is know an induced drag. The angle through which the Total Reaction tilts towards the rear is determined by the pressure distribution and the direction of the effective airflow. By definition, the lift and drag components of the Total Reaction must be resolved with respect to the remote relative airflow which reflects the direction of flight. Obviously, the smaller the angle of induced downwash, the lower will be the induced drag.
There are a number of factors affecting induced drag:
Aspect Ratio:
- High aspect ratio wings produce smaller vortices and, in comparison with a wing of lower aspect ratio, proportionally less of the airflow swept by the longer span is affected by the vortices. Consequently, the induced downwash angle when averaged over the whole of the high aspect ratio wing, is smaller and the induced drag is low.
Wing Planform Shape:
- For a wing of given span, an elliptical planform shape produces the smallest vortices and therefore the lowest induced drag. Because of their difficulty in construction not many aircraft have been built with thes planform shape - perhaps the most famous example being the World War II Spitfire. However, for wings with straight leading and trailing edges, the fudicious use of taper and washout of the wing sections toward the tips can produce a similar reduction in induced drag. Most straight wings produce between 5 to 15% more induced drag than an elliptical wing and this is accounted for by the wing efficiency factor.
Coefficient of Lift:
- Front the pilot's point of view, where the aspect ratio and planform shape of the aircraft are fixed, the important factors in determining induced drag are angle of attack, airspeed and aircraft weight. These are incorporated in the induced drag formula which can be seen to have a powerful effect on the amount of induced drag generated.
- Angle of attack. Induced drag increases as the angle of attack is increased. The strength of the vortices is determined by the pressure difference above and below the wing. When the wing is at the zero-lift angle of attack (AoA -2° / CL = 0) there are no vortices and therefore no induced drag. As the angle of attack is increased, vortices form and increase in strength up to the angle of attack for CL Max, typically 16° (varies between aircraft). Induced drag therefore increases with angle of attack to be at a maximum at the stalling angle.
- Airspeed. It can be shown that induced drag is inversely proportion to the square of Indicated Airspeed (IAS). This is the opposite to the effect of airspeed on parasite drag, which is directly proportional to IAS². When the factors of angle of attack and airspeed are combined, induced drag is greatest at low airspeeds and at high angles of attack. FOr an aircraft just after take-off for example, induced drag can be as high as 75% of total drag. When an aircraft is manoeuvring at high speed, although induced drag is proportionally lower, it is still significant because of the high angle of attack being used.
- Weight. Increased weight means that higher angles of attack must be used to produce a given amount of lift at any given speed. Induced drag increases in proportion to weight squared (W²).
An alternative way of looking at induced drag is as follows. The production of vortices is an inevitable consequence of the production of lift with a wing of finite span. THese vortices result in an induced downwash which is over and above the downwash necessary to produce lift. TO producea rotary motion of any fluid requres energy - an example is the energy requred to stir a large volume of water in a drum with some sort of paddle. In flight, the energy required to create the vortices must be sourced from somewhere. Ultimately, that temad is placed on the engine by requiring hihger power to be used to offset the induced drag when it is desired to maintain a given speed.
Spanwise flow creating vortices can be reduced by certain methods, wingtip tanks and winglets being the most effective of them all.
Induced drag must be added to the parasitic drag to find the total drag. As discussed above, induced drag becomes less of a factor the faster the aircraft flies because at higher speeds a smaller angle attack is required for the same amount of lift. The opposite occurs with parasitic 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.