strongest wing tip vortices when slow, clean and heavy. BUT WHY?

[Amen_Brother]

I think I was the author of the comment in Wikipedia saying wingtip vortices are strongest with flaps extended.

Good old wikipedia. My favourite scientific read!

conventional wisdom I was taught is that clean slow and heavy produces stronger tip vortices.

heavy - increased AOA required for a given speed - stronger circulation at the tip driven by the greater difference in pressure above and below the wing which results from the higher AOA.

slow - the reduced forward speed promotes greater spanwise flow, ergo stronger vortex at the tip.

clean wing vs flaps extended - the span wise lift distribution is alters with flap, the centre of lift moves inwards towards the fuselage. less lift is being produced at the tip, so there’s a smaller vortex at the tip in the dirty config. Palou89's explanation with CL vs AOA is different way of saying the same thing.


its all a bit academic though since the plane will either be going fast and clean.....or very slow and very dirty, so in real life the speed and configuration are not really independent variables - for reasons more important than worrying about vortices. So although flaps out might tend to produce smaller tip vortices, the lower speed you will definitely be flying at with your flaps extended will tend to produce stronger vortices, so which effect wins?? This is why the effect of flap setting on vortex formation could be mis-understood.

The following argument doesn’t stack up..

If it were true that trailing vortices are strongest in the clean configuration, why wouldn’t airplane designers create airplanes with fixed trailing edge flaps so they would benefit continually from weaker trailing vortices and therefore weaker induced drag?

they wouldn’t do that because flaps INCREASE the coefficient of drag - your plane wont get all the way to Ibiza with its flaps out. think in terms of form drag - those big bulky flaps sticking out behind your nice smooth thin wing. or the extra skin friction, resulting from the increased wetted area with your fowler flaps extended. Also, saying that ‘induced drag’ is just the result of whats going on at the wingtip with the vortex is an over-simplification.

changing flap setting causes a significant change in angle of attack but no significant change in lift coefficient.

Extending the flaps will certainly change the lift coefficient. That is the only reason they are there.