Use of weak links

On Mon, 07 Jun 2010 13:54:50 +0200, Andreas Maurer wrote:

On Mon, 07 Jun 2010 05:20:48 -0500, brian whatcott
wrote:

Brian wrote:
On Jun 6, 5:54 pm, John Smith wrote:
Brian wrote:
True the 90deg AOA was justa theoritical way to look at the loads
on a stalled wing.
As soon as the wing stalls, the load decreases.

Yes it decreases, but there is some load on it still. Even a stalled
wings produces some lift just not nearly as much as a flying wing.

Brian C.

How about drag?

Forget the drag - you cannot go fast enough with 90 degrees AoA to
produce any significant amount of drag.

There's another point that hasn't been mentioned - many winches can't
pull in cable at anything like Vwinch for most gliders, e.g. most single
seaters have a Vwinch of 65kts or higher but I'm told our Supacat's
maximum cable speed is 55 kts.

This doesn't mean that it can't break gliders: try this thought
experiment. Launch without a weak link, wait until the glider is at 60-70
degrees from the winch and ramp the winch up to full power to cause a
gross overspeed. This is the situation that will put maximum load on the
wings and it is the situation where the weak link is designed to fail.

At least one of these things is very likely to happen:
(1) the pilot pulls the bung
(2) a back release, if the glider gets far enough overhead of the winch
(3) the hook gets pulled out of the glider
(4) the wings break
(5) the cable snaps

arranged in increasing severity and (my guess) decreasing probability
since I think that one of the less severe events will happen before the
more severe ones.

Some time back during a rainstorm our then CFI gave a talk about how the
various airframe limits are set. I remember him saying that Vwinch is 95%
of the speed at which the wing's maximum design loading is reached when
the glider is overhead the winch with the stick fully back.


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