Induced drag

[Crossunder]

When an aircraft is flying faster than M 1.0, do the wingtip vortices still develop add to induced drag? Does induced drag exist at all at supersonic speeds?

Also; in ground effect, is upwash increased or decreased? Please explain, as different books state differently!

Does anyone know if I can get hold of a wind tunnel video that shows streamlines / smoke pulses at different speeds and AoA?

Thanks a bunch!

[bookworm]

I'll leave the detail of the supersonic stuff to someone more expert than me but my understanding is that induced drag persists as vortex drag. Both are a consequence of having to provide the kinetic energy of the vortex wake. Prandtl's idea of the drag being induced by the wake doesn't really wash at supersonic speeds, but the drag is still there, even if it is often less than wave drag.

Back to subsonics. Ground effect comes in two flavours. The one that we're mostly concerned about is where the height of a wing is less than (or at least, comparable with) its span. In that case it's the trailing vortices that are most affected, and so the downwash they cause is reduced. That's a bit like increasing the aspect ratio of the wing. "Upwash" doesn't really get involved.

Much less commonly considered is a situation where the height of an aerofoil (assume it's very long so we're no longer much concerned about vortices) is less than, or at least, comparable with, its chord. The upwash as well as the downwash (both meant in general terms) is decreased. In that case there is a more comprehensive disruption of the flow around the aerofoil, and even the sign of the change in lift is more difficult to predict. At the same angle of attack, the circulation is increased but the lift per unit circulation is less than that in free space. More details in Kuethe and Chow, Chapter 4.

There's an awful bit in a book by Anderson and Eberhard if that's what's confusing you. They seem to get the signs wrong and in general their discussion of ground effect is... ahem.. of dubious validity.

Finally, no wind tunnel video sources, but you may find Marco Colombini's site (http://www.diam.unige.it/~irro/) of interest. It has some ideal fluid models of flow about a wing, shown as stream lines, streak lines (smoke simulation), velocity, pressure and force vectors.

[Crossunder]

Hmm, funny you should mention that, because it's actually the book by Anderson & Eberhart I'm looking at right now! Will do some surfing on Amazon.com instead... Thanks!

[bookworm]

I recommend Denker's critique (http://www.monmouth.com/~jsd/fly/lift.htm) of the Anderson & Eberhart paper (http://www.aa.washington.edu/faculty/eberhardt/lift.htm) (similar material to the book, I think.)

I'm not suggesting that the book is all bad, but I think it illustrates some of the pitfalls of the "we don't need no stinkin' Bernoulli" approach that seems to be popular in some circles these days.

If you want to know about aerodynamics, it's difficult to do much better than Barnes McCormick's Aerodynamics, Aeronautics and Flight Mechanics, ISBN 0471308994. Or any of John D Anderson's fine texts (no relation to the Anderson above) like Fundamentals of Aerodynamics ISBN 0071181466. Kuethe and Chow ISBN 0471129194 is more theoretical.

[error_401]

nothing to add about ground effect.

This site might be of interest for your flow patterns. Not really showing flow patterns but comprehensive shifts of forces with animations.

http://142.26.194.131/aerodynamics1/