On Wednesday, 14 December 2016 03:53:34 UTC+2, 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?

Glider that have wing parting well inboard of 15m tips are usually quite well optimised for two classes. Such gliders are ASW 28/ASG 29/ASH 31, Ventus-2c/Discus-2c and LS10 (?). Downsize is that you are paying 20k euros for set of 15m tips (large section of wing) instead of 2,5 k euros (just short tips with winglets).

Most if not all 18m gliders come with engine, and they are usually too heavy for serious comp. pilots, at least in Europe. ASG29E-15 wingloading is easily 45kg/m2 without water. Try to thermal in 0,2 m/s with that...

Older generation of glider usually were optimized for 15m and then span was increased with narrow wing section, LS6/LS8/Ventus c, for example. You can also do a not-very-well-optimized smaller span glider by cutting wings shorter (Mini-LAK).