How much washout at wingtip?

I have never used washout on any model that I have designed, on others that are not originally my own, I tend to follow what they say.

Personally do not like the idea of the wing operating at different angles of attack, as it seems inefficient to me.

I seldom use tip chords of less than 6" but before someone points it out the Watts Up does have a tip of 4" chord, on the 100S & 120" versions, but this is covered by having sweep forward on the mean chord line that in theory stops tip stalling. Seems to work OK

Further to this discussion, I had to get a piece that was written some 60 years ago in the Aeromodeller Annual 1956/7, on Buzzard Wings page 69, quote:-

With negative sweep(buzzard wings) the wing tips are relieved of static stress, as lift generation is higher at the centre section of the wing. This produces the same effect as washout does on a normal straight wing.. Effects of buzzard wing layout on spiral dive(peel off) & lateral stability(yawing) is quite favourable. Wing washout can be eliminated through use of the buzzard wing lay out and if lateral stability were sufficient, one can imagine giving wash in to both tips. This would bring about an optimum of overall lift generated.

I would not go for the last bit IMHO The whole article runs for 7 pages and details a lot of interesting stuff, that sadly we no longer get in current publications. A lot of 2 seat glider K13's etc mainly used it to accommodate the second person up front, conversant with CG position. I used it on my first glider design for radio in 1975, Summer Wind, after reading this article, and the bonus was that the CG was well forward so ballast could be loaded easily from the canopy area.

Picture of of Nigel Dell with his Summer Wind which shows the plan form and how big it was then