On Tuesday, December 13, 2016 at 5:53:34 PM UTC-8, Casey wrote:
An owner of a 15m with 18m tips told me his glider was designed as 18m and the manufacture cut the tips to make removable and that it did not fly well as a 15m.

My question is this common or glider specific? How do manufactures design a 15/18m? Do they start with design of 15m and add 3m and then test performance or design 18m and cut down? Or do different manufactures have different technics? Can a 15/18m have equally performance or does one configuration suffer and that's the price one pays?

A few semi-random points of perspective from one who has BTDT and has the many epoxy-stained T-shirts and carbon splinters to show for it:

* There is no one way to design a 15/18 glider. You can start with a 15m and stretch it, or you can start with an 18m and cut it. Neither is perfectly optimized for both spans, but in both cases the optimization is not really all that bad.

* Especially for motorgliders, wing area can be an issue. Carrying a self-launch engine and systems on the area you get with 15m tips can result in sink rates you don't want on a weak day.

* As another poster points out, there are structural penalties at issue. If you are going to have 18m tips, you must provision the wing to accommodate the extra bending moment. The difference can be pretty big; adding span at the tip makes a huge increase in the stresses at the inboard end of the wing. One side effect to this is you can end up with a glider that rides nice in 18m config, but beats you up with the 15m tips.

* One thing I observed from studying the wings of the ASG29 is that you can improve the area distribution optimization by moving the 15/18m separation joint inboard. That gives you a head start on matching the local taper to the lift distribution. But as another poster points out, it also increases the amount of glider you have to buy and schlep around. With the separation line around the 5.5m semispan as on the ASG29, the short tips are 2m each and the long tips are about 3.5m each. That means you are buying and carrying around in the trailer about 22m of wing for a glider that is at most 18m span.

* I designed the HP-24 as a non-serious racing machine, so its span optimization leans towards the 15m end, but there will still be a substantial performance gain from the optional 18m tips. It's not meant to run with the ASG29, but it still has flaps, winglets, autoconnecting controls, water ballast, front-hinged canopy, and other advanced features at a price about 1/3 that of the '29.

Thanks, Bob K.
https://www.facebook.com/HP-24-Sailp...t-200931354951