What are the forces on a tied down glider?

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

Bill,

As you are keen on winch launching, I wouldn't recommend a longer axle
bolt. We have had a couple of potentially serious incidents in the UK
where the winch cable has got caught around protruding mainwheel or
nosewheel axle bolts and then hung up. I suppose that this could also
happen to an aerotow rope.

Del Copeland


At 02:29 21 August 2008, Bill Daniels wrote:
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.

On Aug 13, 2:01*pm, Eric Greenwell wrote:
I see gliders tied down in a bewildering variety of ways, all apparently
"adequate" in the owner's mind. AT one extreme I see multiple ropes,
straps, wing stands, and load spreaders, while at the other extreme it's
just a piece of light rope (barely more than clothesline) on each wing
tip. Two questions have slowly formed in my mind...

Question 1: Is there an analysis of the forces from wind on a tied down
glider, or maybe even measurements? Perhaps there is a "best practices"
document somewhere, derived from surveys of what's worked and what's
failed? I'm not looking for lists of personal preferences (I've seen
lots of those at airports and have the pictures to prove it!), but real
data and analysis.

Question 2: What is the effect on the wind forces when the glider has
wing covers on it, such as the Jaxida covers? I assume it reduces the
lift the wing can produce, but another pilot I talked to thought it
would increase the lift. Again, I'm looking for real data and analysis.

--
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" atwww.motorglider.org

If at all possible, add a nose tie down. This limits the angle of
attack available and can be a big safety margin adder.
Good rule of thumb is at twice stall speed, wing is generating 4 times
gross weight in lift.
For 2-33's, we use 2 tiedowns on each main, a tiedown on the tips, and
a nose tiedown. Smaller ships use tips and nose.
We had a blow away due to a failed chain in 80 mph winds about 15 yr
ago. Once you have one, you realize extra tiedowns are cheap.
Heinz W had his ASH-25 off the ground in gust front and was saved by
nose tiedown.
UH

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.

Back in the 60s someone wrote in Soaring about tying two-by-fours
along the tops of wings to act as spoilers. I've never seen it, but this
is strictly a "for at-home" solution if it works. Boards would need to
be stabilized in some way, of course.

At 23:05 13 August 2008, wrote:
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.

I used to leave my ASW17 with a trestle under each wing, to stop them
moving and ties the tail down. I also left it in full negative flap which
meant that it would not lift.
If the wind was forecast to be more than 30 knots it went in it's box
although I was caught out a couple of times with unexpected wind speeds of
up to 60 knots, it never moved.
I suspect with speeds of 100kts you would be luck to find the glider
intact even if the tie down held, and even in the trailer it would be at
risk.

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

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
* 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

The 15 or so gliders tied down at the Boulder, Colorado airport encounter
100MPH winds routinely - usually in winter. 100MPH is above most gliders
maneuvering speed and above a 2-33's Vne. Tiedowns are a very serious
matter.

The strategy is to orient the gliders N-S since the prevailing strong winds
are from the west. The nose, tail and, if possible, two ropes on each wing
are secured to strong ground anchors. Some use wing stands at each wing
rope so it can be very tight without bending the wing. Many use Jaxida or
other covers but they deteriorate fairly quickly in the intense high
altitude UV.

Each tiedown bay is fenced with chain link with plastic tubes inserted into
the fence. The purpose of the fencing is to strain out wind borne debris
and reduce wind velosity around the glider itself.

Trailers are oriented E-W with tongue and tailgate tiedowns.

It's been several decades since a well secured glider has suffered wind
damage so it must work.

"Eric Greenwell" wrote in message
news:Fkjpk.9$w51.1@trnddc01...
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
* 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