What are the forces on a tied down glider?

One sample has been in evidence for years on the back page of the SGS
2-33 POH. --JHC

On Aug 13, 11:01 am, Eric Greenwell <flyguy...@to> wrote:
> I see gliders tied down in a bewildering variety of ways, all apparently
> "adequate" in the owner's mind.

Perhaps the real question is how well does it have to be tied down to
satisfy the insurance company that you were not negligent.

Another significant factor is what is it tied too. We seem to take
for granted that tie down wires etc that we find at the airport are
secure, but I've seen a whole line of cable and the anchors pulled out
of the ground. I used to trust the rings on the ramp at Hobbs until
one year I cleared all the dirt out of the hole and found the rings
corroded almost all the way through. Last time I flew there I dug
around until I found a good one and tied the CG hook to it.

Others would say why worry - put it in the box!

Andy

There is no hard data on tie down strength. Just plan for the worst.
PolyPro will lose most of its strength rapidly from UV. Good covered
climbing rope and locking carabiners are the best bet.

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
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.

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.

At higher speeds the lift would be greater depending on your airfoil
and someone with that knowledge could figure it out as well.

I'd say that to be safe then double the tiedown rope strength.

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.

Ray

Five (U.S.) Gallons is only 0.67 cubic feet.

Typical weight of concrete is about 150 lbs per cubic foot.

So, a five gallon bucket of concrete should only weigh about 100 lbs
plus the weight of the bucket.

Not much weight at all.

But I've found the easiest way to add weight is by drinking beer. :-)


Ray Lovinggood
Carrboro, North Carolina, USA


> Our favourite tiedowns used to be 5 gallon drums filled with concrete,
> until one day we saw that both of them (one at each wingtip) were 3
> inches off the ground.
>
> Gilbert- Hide quoted text -
>
> - Show quoted text -

On Aug 13, 10:03 pm, Eric Greenwell <flyguy...@to> wrote:
<snip>
>
> Does anyone know the force required to break those ropes; for example,
> were the ropes tested afterwards for breaking strength?
<snip>

The weakest point is where the problem is, and that is often the knot
not the rope unless the rope is old and tatty.

However, the breaking strength of ropes sold for marine applications
is tested and known, and some kinds of rope (probably not the best for
tie-down applications) are very, very strong.