Effects of Winglets on Handicaps


Theoretically winglets hinder the wing-tip vortices consequently reducing the induced drag. This has the same effect as an increase in aspect ratio would have on the induced drag. In fact the text books state that winglets have the same effect as an increase in wing span of 4/5 of the winglets height.
The induced drag coefficient = C**2 /pi*A
Where C= The lift coefficient and A is the aspect ratio.
That is the induced drag is inversely proportional to the aspect ratio.

Induced drag is predominate at low speeds so this would explain the pilot reports of better thermalling, stalling and lowspeed handling when fitted with winglets. The total drag is made up of the induced drag and the friction drag and at the minimum sink speed the two components of drag are the same.
drag diagram

To calculate the effect that the winglets might have on the handicaps I have chosen an example of 450mm wiglets on a 15M glider. This would effectively increase the aspect ratio of a typical sports class glider from 22.5 to 23.6. This would reduce the induced drag by 4.8% and consequently reduce the mimimum sink by approximately 2.4%. This improvement in minimum sink increases the cross country speed by approximately 1% - 2% using the Macready model.

Min Sink Speed
Standard Jantar -1.41 Knts 81.6 KPH
Standard Jantar with Winglets -1.37 Knts 82.6KPH

I therefore advocate a 1% additional handicap for gliders that have been retrofitted with winglets.

E Mail Graham Brown gbrown@zeta.org.au
Last Updated Thursday Dec 4th, 1997 - 10:30:00 PM