Winch Launch Stresses on Vintage Gliders

Bill Daniels wrote:
"Brian" wrote in message
ups.com...

Someone please correct me if I am wrong,

But as I recall the weak link is supposed break before it carries twice
the gross weight of the glider.
So during the launch the wings must lift at a maximum the weight of the
glider plus up to 2 times weight of the glider through the weak link.
So the maximum load you should be able to put on the glider is 3G's.

I don't think I would want to fly a glider that could not withstand
3G's.

Brian



The 2X GW weak link figure is from the US FAR's (Max 200% and Min 80% of GW)
and references only air tow. The FAR's don't seem to know much about winch
launch.

If you examine glider handbooks, the strength of the recommended weak link
for winch launch is usually about 1.2 to 1.4 times the gliders gross weight
but sometimes it's less. In the case of my Nimbus, the GW is 650 kilos and
the recommended weak link for both aero and winch launch is only 600 kg
force. I suspect it has more to do with the hook mounting structure than
wing strength.

As a previous poster noted, RTFM.

Bill Daniels


The weak link is designed to limit the wing root bending moment to a safe
number. (Equivalent to 2-2.5G without gust loads) As previously posted, there is
no G relief on a winch launch, so the strength of a high aspect ratio glider
weak link will generally be a lower fraction of MTOW than for a low AR wing.
RTFM, do not assume.

The strength of the weak link is not 2xMTOW, it is the limiting force that will
not exceed the safe structural strength of the airframe when the aircraft is at
it's maximum winch speed, maximum coeficient of lift, and the cable is acting at
it's designed release angle (87 degrees if I recall correctly)These days this
takes into account the "standard" vertical gusts.

On some of the vintage gliders out there the limiting strength might be the
structure the hook is fitted to, I know of at least one case (Sagitta) where the
hook pulled out. However, I believe this is not generally the case.

You can be sure about compliance by the JAR-22 and newer gliders, but Remember
we are talking about vintage gliders here. Caveat aviator...
--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.