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What is the induced drag ?

Always directed parallel to the relative wind. This is the main cause of trailing edge vortices : the air that passes over the upper surface of a wing tends to flow inward. lt is so because the pressure on the upper surface is lower than the pressure outside of the wing tips. On the other hand, the air below the wing flows outwardly because the pressure on the lower surface is greater than that which prevails at outside wingtips. The air continually seeks to circumvent the wingtips, of the intrados to the extrados. perhaps the manner to explain why high aspect ratio is better than a low aspect ratio would be to say when aspect ratio is more , the greater the amount of air that escapes through the wingtips is low. The air that bypasses the wingtips is no longer there to produce lift, this is sometimes called a "marginal loss".

As the two flows, which of the upper surface and which of the lower surface , meet at the trailing edge at a certain angle, they forments vortices that rotate clockwise (viewed from the rear) behind the left wing and in the counterclockwise direction behind the right wing. All the vortices of the same side tend to join to form a single large vortex which escapes from each wingtip. These two large eddies are called marginal vortices

Most pilots saw these vortices, or more precisely, the central part of those made visible by condensation. The humidity of the air condenses due to the pressure drop in the core of the vortex. It should not be confused these drags with the condensation produced by ejected gas of the engines at the high altitude.
If we now consider the direction of rotation of these vortices, we see that there is a flow of air up, outside the wingspan and a current toward the down in the back of the trailing edge . Do not confuse this current toward the down with the deflection that occurs normally. In the latter case, the downwash is always accompanied by upward deflection in front of the wing so that the final direction of the flow is not changed. But in the case of the marginal vortices, the upward deflection occurs outside of the wing, and not in front of it, so that the flow leaving the wing is ultimately directed to the low. Therefore, the lift, which is perpendicular to the flow, is slightly inclined rearwardly and contributes to the drag. This part of the drag is called induced drag.

This induced drag is inversely proportional to the square of the speed, while the remainder of the drag is directly proportional to the square of the velocity.

How to calculate the coefficient of induced drag (Cxi)
First calculate the aspect ratio / Chord
Then calculate the coefficient of lift (Cz): Cz 1/2 r V².S
Then divide the coefficient of lift to the square by the aspect ratio.

How to calculate the resistance (Rx) in Newtons of the induced drag:
Multiply the induced drag coefficient by a 1/2, by the density of the air, by the speed in meters per second to the squared, and by the wing surface.
(Cz˛/p A). 1/2 r V². S, the result is in Newtons
(N = equal to the force which a body having a weight of 1 kilogram an acceleration of 1 meter per second squared).

How to calculate the required power (in Watts) to oppose to this induced drag:
Multiply the resistance (in Newtons) by the speed (meters by seconds ) Rx.V.
It only remains to convert the Watts into horses.
horses (hp = 736 Watts or 75 kg per second).

Last update : Agust 11th 2019
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