Life glider hours with winch

On Friday, March 22, 2019 at 9:15:42 AM UTC-4, José Ignacio Otero wrote:
Hi Bob.

Thanks for the reply, it was what I looking for !!

I think that the pilots how said that, were thinking that because the 3.52 g that the glider "suffer" during the wich start.

Have a nice day, José


El jueves, 21 de marzo de 2019, 23:02:51 (UTC-3), Bob Kuykendall escribió:
What I've read is that the bending forces applied to the wings and tail during a typical winch launch are roughly equivalent to 3.5g flight. That is comfortably below the load limit of every common glider. However, it is high enough that repeated launches will eventually shorten the service life of an aluminum glider. The Blanik designers understood this very well, and the original service documentation was quite explicit about logging operations carefully so the service life could be tracked. Unfortunately, most operators did not do so.

Normal winching operations probably has no appreciable effect on the composite parts of a composite glider. However, I believe it was BGA analysis of repeated winch launch stresses that initiated the service bulletin on the steel end-pins ("spigots") on Grob 102, 103 and 109 series sailplanes.

As for wood sailplanes, my concern would mostly be around the age and condition of the structure, since many wood gliders are now half a century old. Since you don't feel the added stresses of winch launching the way you feel the g of vertical acceleration, it would be easy to get into the gray area between what the structure was originally built to sustain, and what it can actually sustain after fifty years of operation, repairs, and deterioration.

--Bob K.

Guys - let's stop making the winch launch sound like it is a glider shredding method! The force introduced into the glider's structure is limited by a weak-link. The max. breaking strength of these weak-links together with the Vw are determined by the designer and documented in the POH.
I am just referencing two popular two-seat trainers he
ASK-21 - MTOW of 600kg, black weak-link of 1,000kg, ratio 1:1.66
G103 Twin-II - MTOW 580kg, red weak-link of 750kg, ratio 1:1.29
(may the gods of physics forgive me for mixing mass and force units here!)
There is a load added during the launch when the glider transitions from the initial shallow climb into the full climb (akin to performing a small segment of a looping) but all this will not get you near the mentioned 3.5g!
However, all this is predicated on the use of proper weak-links. If you ignore these safety devices, then all bets are off. To the best of my knowledge, there hasn't been a properly maintained glider launched with the correct weak-links, that got pulled apart during the launch!

Uli
'AS'