Mike Schumann wrote:
Our club is exploring switching from aero tow to winch launching. Some of
our members own older wooden gliders and have expressed concerns about the
increased stress on the glider's structure resulting from winch launching
vs. aero tow.

Is this a valid concern?
We fly almost exclusively vintage aircraft at our club. All launches are winch,
and the safety and maintenance record is excellent. (30,000 vintage trainer
launches at last count) The two principal trainers are a 1965 Bergfalke III and
a 1955 Bergfalke II/55. with 13,000 and 19,000 launches respectively.

Like with any mechanical system it is possible to damage an aircraft with winch
launches if you do not follow correct procedures. Conversely, simple procedures
can ensure safe operation.

My Advise is -
1] some engineer put a lot of work into designing and specifying the weak link
system. Use the right weak links and your chances of overstressing are minimal.
2] make sure your winch drivers are pilots and visa versa. That way there is a
lower risk of over exuberance...
3] Teach a progressive rotation into the steep climb, slamming the tail of a
wooden glider into the ground with full up elevator can result in a lot of work.
(if you are lucky) One pilot who did this at our club flew for 2:30 with a
cracked rudder and broken frame under the elevator hinge. He didn't think it was
a "loud bang..."
4] The point on the launch that exposes the structure to greatest stress is the
top of the launch where you have maximum downward force from the winch and
cable, often together with higher speed if the winch driver is insensitive. Add
a pilot who is bending the stick to wring the last bit of height out of a launch
(last year someone - different club - managed to spin off the cable) and you
have the limiting force being the down force on the elevator.

Respect the speed limits and use appropriate equipment and winching is no
problem for vintage types. Conversely, the sustained flexing on the wings of
some of the newer glass ships can result in premature gell coat crazing, and
some owners refuse to winch launch for this reason.

--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.