Originally Posted by
sellener
I would like to understand induced drag a bit better.
I am fully capable of spitting out phrases I have heard in handbooks, and circulers. For instance.... for a checkride oral I might say induced drag is the penalty an airfoil pays anytime its generating lift.
Or
Wing tip vortices are the source of induced drag
Or
Vortices create downwash behind the trailing edge which effectivly increaseing the angle of the relative wind which also directs the lift vector (which acts perpindiculer to the relative wind) back.
Or
Something about upwash in front of the wing which also changes the relative wind a bit bending the lift vector back
bottom line, I dont truly understand what all causes induced drag....
my best understanding to date is.....
air travels high to low from under the wing towards the wingtip and then combines with the air their to impart a rotation to the air (vortice) the spiraling down motion over the trailing edge makes the effective relative wind angle a bit more and thus the drag vector tilts back to stay perpindiculer to the relative wind.
But I think there's a bit more to it than that if thats even an accurate statement.
You pretty much have most parts of it. If you really want to break it down technically there are two parts of induced drag: Inviscid and viscous.
...But... to start simple....most cases lift is 90 degrees from the relative wind. The airfoil's force of lift may not be generated 90 degrees from the relative wind, it may be slightly aft angle - inclined (like at 95 degrees say). The difference of the 90 degree lift and the actual airfoil force created is an
induced angle of attack - it is induced by lift caused by circulation flow (upwash, vortices,etc).....
It is a lift component in the drag direction, thus called
induced drag
I'm not sure if the examiner wants you to explain circulation flow - but if he does.. you could probably be pretty simplistic about it. Key factors are tip vortex strength depends on angle of attack and wing design and are a result of lateral flow. It is basically a convergence of higher pressure from the bottom moving laterally and upwards to the lower pressure on the top which is moving downward and lateral
at the wing tips. The energy contained in that flow determines the induced angle of attack. The greater the pressure differential, the greater the energy in the vortex. Aircraft with higher aspect wings generally have lower induced drag. High altitudes require high angle of attacks so generally the induced AoA (and induced drag) is high... that's why the U-2 has a high-aspect wing.