What are the forces on a tied down glider?

jb92563 wrote:
Here is some simplified logic on the matter.

If you fly a 750 lb gross weight glider and it keeps you aloft above
stall at say 40 mph then its fair to say generally that it is
generating close to 750 lbs of lift.

The typical nylon 5/8 tow rope will break at somwhere beyond 400 lbs

Did you mean 3/8"? Typical *breaking* strength of 5/8 nylon rope is in
the 10,000 pound range, and even 3/8" nylon rope can have 3000 pound
breaking strength. The *working* load limit of 5/8" nylon can range from
900 pounds to 3000 pounds, depending on construction.

and if you use a pair of these for tiedowns it should be adequate for
800 pounds tension in 40 mph winds if you have a secure anchor.

Because the glider weighs 750 pounds in this example, it will counter
the 750 pounds of lift. Simplistically, the ropes can be just large
enough to steady the glider; with a 56 mph wind, the wing would develop
1500 pounds of lift, and then the ropes would need to restrain 750 pounds.

I suspect weight is a big reason even the apparently minmal glider
tiedowns work so much of the time: the wing isn't optimally positioned
for generating lift (lower angle of attack than stall, wind not coming
head on, spoilers out, etc), so the weight of the glider exceeds the
wing's lift.

I use chain myself since it does not weaken much over time in the sun
and should be good way past what my glider could withstand.

I often see gliders tied down with ropes/straps that can restrain many
times the weight of the glider, but they are on the wing tips. How much
force can a wing tip take before something breaks?

Decades ago, we had a club Blanik effectively tied down only at the wing
tips when a 70+ mph wind lifted it enough to bend the wings (downward)
at about two thirds of the way out on the wing. I think it would have
survived the wind with no damage if the factory ring tiedowns at the
wing midspan had been connected to a ground anchor instead of a cable.


At 100mph winds I doubt your ground anchor would hold against the lift
and you should just forget about anything outside the trailer
surviving at the tie down at that speed because if the rope and anchor
don't break then something else probably will.

And at 100 mph, it's probably time to worry about how well the trailer
is tied down!

--
Eric Greenwell - Washington State, USA
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