What are the forces on a tied down glider?

On Aug 14, 3:17*pm, jb92563 wrote:
...clip...
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 our airport a thunderstorm with downbursts came through about
this time last year. The C182 that was tied down with 1/4" aged
polyester broke all 3 strings and flipped on its back. (I'd feel worse
about this except that guy who tied it down, our local mechanic, had
listened to me give a speech on proper tiedown ropes at our airport
commission meeting not too many months before, and had chimed in with
support.)

And the C152 that was tied down with chains, in the lee of the FBO,
broke 2 of 3 chains and had damaged wing ends and tail.

The problem with chains is that you can't get all the slack out, and
in the turbulence of high surface winds, the variable lift generated
jerks the chains violently. Each sharp jerk generates powerful
transient tensions, and metal fatigue with repeated jerks is a very
real phenomenon. As our mechanic found out (yes, he too had faith in
chain until then).

Meanwhile, it obviously depends on the chain's metallurgy and
size...

Dan Johnson

On Aug 18, 3:00*pm, danlj wrote:
... First, ropes can be tested, though tensiometers might be hard to
locate...

Well, yes and no. A simple 8000 lbf (35 kN) tensiometer can be
assembled for about $200 worth of Harbor Freight stuff. Adding a 10000
lbf (44 kN) electronic load cell gives you near lab-level accuracy for
another $300. Search YouTube on "breakotron" to see some tests of
climbing gear I've done with my cheap pull testers. I'm hardly unique
in having built such machines; I've encountered similar rigs in the
hands of hobbyists in several metro areas.

Eyebolts cut the rope quickly (the eyebolt is a proxy for the tiedown
ring on your glider, which is essentially a dull but effective knife at,
say 2000 lb tension).

Well, again, yes and no. Yes, the turn through the eyebolt will
definitely weaken the rope somewhat, and depending on a lot of factors
may be where the rope breaks. But my experience with climbing gear is
that unless the rod diameter of the eyebolt is significantly smaller
than the rope diameter the weakening effect is not likely to be
critical. Observe that with climbing gear, most lead ropes are around
10mm in diameter, and that the contacti radii at the business ends of
most carabiners are also around 10mm.

...I happen to have just made a couple of sets of airplane tiedown
ropes from 16mm vectran with a listed breaking strength of
28,000 lb. Talk about overkill - and there's no way I'm going
to put that rope into a tensiometer myself, to see what
happens when it breaks. But I'm confident that the failure
will be at the tiedown ring.

I'd be pretty confident that I couldn't break that rope even bent over
a rod half its diameter (8mm) within the 10000 lbf capacity of my
machine!

One caution I will add to this thread: Also be careful about the
strengths of the metal hardware you use on tiedowns. I've used 5/16"
screw links, usually rated to 1560 lbf working load, to loads up
around 4500 lbf without failure. The 3/8" screwlink is rated to 2000
lbs, and I've used them in tests up to around 6000 lbf. No problem
there.

However, beware that snap links like these are a lot less strong than
screw links of the same rod diameter:

http://www.hobbytool.com/steelsnaplinks.aspx

Observe that the working load of the 5/16" snap link is only 240 lbf,
as opposed to the 1560 lbf working load of the 5/16" screw link. When
I accidently tested a 5/16" snap link to destruction, it gave up at a
relatively pathetic 800 lbf. After that, I stopped using snap links
for anything heavier than keyrings.

Thanks, Bob K.

Bob Kuykendall wrote:


However, beware that snap links like these are a lot less strong than
screw links of the same rod diameter:

http://www.hobbytool.com/steelsnaplinks.aspx

Observe that the working load of the 5/16" snap link is only 240 lbf,
as opposed to the 1560 lbf working load of the 5/16" screw link. When
I accidently tested a 5/16" snap link to destruction, it gave up at a
relatively pathetic 800 lbf. After that, I stopped using snap links
for anything heavier than keyrings.

I've taken pictures of tiedowns with "carabiners" that are really just
hardware store snap links. I've always assumed they were adequate only
for times when you didn't need to tie down the glider in the first
place, and just wanted to steady it in the wind. Assumption validated.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

"danlj" wrote in message
...


The ten-gallon trash-bag anchor is the
most robust portable solution I've ever heard about. And if your crew
takes a long time arriving, you'll have plenty of time to dig a grave
for the bags. But remember, the strength relates to the strength of
the fabric of the neck of the bag and the durability of the link
between filled bag and tiedown rope. And a wing pulling the bag
vertically is a different situation than a truck pulling obliquely.

Dan, If you are referring to my post on this subject, I didn't mention
'garbage bags'. I described tough nylon bags of the "ditty bag" type with a
1/2" nylon rope draw. These things are STRONG.

Most tiedown failures I've seen result from a combination of vertical pull
and horizontal. Ten gallons of dirt is very heavy and probably overkill -
something else will fail first. I think five gallons will probably match
any type of stake for holding ability and probably fail whatever is
attaching the rope to the wing.

Once a stake pulls out, it's useless. Even if bags are lifted out of their
holes, they will still work to some degree. You can fill the bags with
whatever is avaiable like rocks.

The neat thing is that a folding trenching tool is about 8oz and bags are
less than that. Unlike stakes, they won't impale you in a hard landing or
crash. I'd still include a couple of motorcycle straps in case you wind up
on an airport ramp.

Land out tie downs aren't meant to be convienient they are meant to work in
unusual situations - even if they take some work to put in place. You'd
probably only tie the glider down if you planned to walk away from it.

Bill D

"Bill Daniels" bildan@comcast-dot-net wrote:


"danlj" wrote in message
...


The ten-gallon trash-bag anchor is the
most robust portable solution I've ever heard about. And if your crew
takes a long time arriving, you'll have plenty of time to dig a grave
for the bags. But remember, the strength relates to the strength of
the fabric of the neck of the bag and the durability of the link
between filled bag and tiedown rope. And a wing pulling the bag
vertically is a different situation than a truck pulling obliquely.

Dan, If you are referring to my post on this subject, I didn't mention
'garbage bags'. I described tough nylon bags of the "ditty bag" type with a
1/2" nylon rope draw. These things are STRONG.

Most tiedown failures I've seen result from a combination of vertical pull
and horizontal. Ten gallons of dirt is very heavy and probably overkill -
something else will fail first. I think five gallons will probably match
any type of stake for holding ability and probably fail whatever is
attaching the rope to the wing.

Once a stake pulls out, it's useless. Even if bags are lifted out of their
holes, they will still work to some degree. You can fill the bags with
whatever is avaiable like rocks.

The neat thing is that a folding trenching tool is about 8oz and bags are
less than that. Unlike stakes, they won't impale you in a hard landing or
crash. I'd still include a couple of motorcycle straps in case you wind up
on an airport ramp.

Land out tie downs aren't meant to be convienient they are meant to work in
unusual situations - even if they take some work to put in place. You'd
probably only tie the glider down if you planned to walk away from it.

Bill D

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

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 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.

I have heard of a case where a Blanik was wrecked by a violent gust of wind
during a storm, even though it was very well tied down. The lift on the
wings was sufficient to lift the glider. The wingtips were both tied down
and the mainspar was damaged beyond economic repair by the resulting
downwards bending loads!

Del Copeland


At 20:49 13 August 2008, Andy wrote:
On Aug 13, 11:01=A0am, Eric Greenwell 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

Derek Copeland wrote:
I have heard of a case where a Blanik was wrecked by a violent gust of wind
during a storm, even though it was very well tied down. The lift on the
wings was sufficient to lift the glider. The wingtips were both tied down
and the mainspar was damaged beyond economic repair by the resulting
downwards bending loads!

That might have been our club's Blanik in the '80s. In retrospect, it
wasn't "very well tied down". Only the tips were tied down well to solid
ground anchors; the fuselage was not restrained directly, and the center
of the wing, using the factory tie down rings, was tied to a cable that
could lift a foot or so with 100-200 pounds of force.

It was that experience that makes me question a lot of the tie down
methods I see that use just the wing tips. I'm now of the opinion the
best situation has the fuselage restrained using the towhook, or perhaps
the landing gear. If that straps and ground anchor can take 5+ Gs, it
doesn't matter much how well the wings are restrained. The tail
restraint is probably important if very strong quartering winds are
encountered.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Way back I needed to tie down an old TG-3 at Boulder, CO.

Like you, I decided the wings weren't strong enough but the structure around
the main wheel was. I bought a longer bolt to replace the one that served
as the wheel axle. Using spacers and washers much like those used on axle
extensions seen on tail dolly wheels to engage tow out gear, I added
"spools" on each side of the main wheel.

I then made a deeply anchored concrete pad with two 1/2" steel plate "ears"
that engaged the extended axle spools like hooks when the glider was rolled
backwards onto the pad. The tail was chained so the glider couldn't roll
forward enough to disengage the hooks from the spools. Just for good
measure, the nose was anchored with the tow hook. Wing stands and ropes
kept the wings from rocking.

The old TG-3 didn't even budge in a strong wind.


"Eric Greenwell" wrote in message
news:K34rk.397$lf2.251@trnddc07...
Derek Copeland wrote:
I have heard of a case where a Blanik was wrecked by a violent gust of
wind
during a storm, even though it was very well tied down. The lift on the
wings was sufficient to lift the glider. The wingtips were both tied down
and the mainspar was damaged beyond economic repair by the resulting
downwards bending loads!

That might have been our club's Blanik in the '80s. In retrospect, it
wasn't "very well tied down". Only the tips were tied down well to solid
ground anchors; the fuselage was not restrained directly, and the center
of the wing, using the factory tie down rings, was tied to a cable that
could lift a foot or so with 100-200 pounds of force.

It was that experience that makes me question a lot of the tie down
methods I see that use just the wing tips. I'm now of the opinion the best
situation has the fuselage restrained using the towhook, or perhaps the
landing gear. If that straps and ground anchor can take 5+ Gs, it doesn't
matter much how well the wings are restrained. The tail restraint is
probably important if very strong quartering winds are encountered.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org