Induced Drag

[FDX8891]

Could someone explain to me the relationship between wingtip vortices and induced drag?

I often hear other pilots say that wingtip vortices are the cause of induced drag, but to me it seems like the vortices are a natural byproduct of the wing producing lift, and the true cause of induced drag comes from the rearward component of lift when flying at high angles of attack (this is how I currently explain it to my students).

 

[PGT]

Wingtip vortices force the air behind the wing down, which means that the lift vector is titlted back as lift is perpendicular to airflow. Drag is the difference between the actual lift vector and a theoretical one (straight up in the usual scenario). Stronger wingtip vortices angle the air down at a greater angle so the actual lift vector points rearward more, which means more drag.

 

[tgrayson]

and the true cause of induced drag comes from the rearward component of lift when flying at high angles of attack (this is how I currently explain it to my students).

Sounds like you may be thinking that the lift vector is tilted backwards because the wing is. Not so. Without the existence of wingtip vortices, the lift vector would actually be perpendicular to the relative wind, not the wing itself.

However, the wingtip vortices distort the relative wind in the vicinity of the wing so that it approaches the wing from slightly above the direction of flight; the lift vector will instead be perpendicular to this local relative wind, rather than the true relative wind, which means that the lift vector is tilted slightly backwards with respect to the direction of flight.

 

[Xiph]

However, the wingtip vortices distort the relative wind in the vicinity of the wing so that it approaches the wing from slightly above the direction of flight;

Would it then be correct to say that the vortices cause a decrease in angle of attack from what it would otherwise be?

 

[tgrayson]

Would it then be correct to say that the vortices cause a decrease in angle of attack from what it would otherwise be?

Yes, but it would be more correct to say the "local AoA". AoA, by definition, refers the the angle between the chordline and the relative wind far enough ahead of the airplane to not be influenced by the presence of the airplane. That's why many books use the infinity subscript on "V" when referring to the aircraft's velocity.

The fact that the local AoA changes in the presence of wingtip vortices is why aircraft tend to balloon when in ground effect...the local AoA is suddenly larger.

 

[dlcmdrx]

http://www.youtube.com/watch?v=WTTKFqm20RY

 

[R Banger]

The easiest way is to imagine lift as a whole across whole wing as opposed to a single linear vector.

http://www.pilotfriend.com/training/flight_training/aero/images2/15.gif