What are the forces on a tied down glider?

On Aug 13, 8:13*pm, "Capt. Geoffrey Thorpe" The Sea Hawk @See My
Sig.com wrote:

Now, can someone explain why I can remember (and find) something I read
nearly 3 years ago, but I can't remember what I was doing on Friday when I
get back to work on Monday?

That's the way human brains work and why it never helps to cram the
night before a test. Your brain needs time to create links to new
information, making it easier to retrieve later. This all happens in
the background without your being aware of it.

-Doug

"Doug Hoffman" wrote in message
...

... it never helps to cram the night before a test.

That might be true for you, but certainly not for me. I have taken thousands
of tests over my lifetime (military, college, FAA). I consider myself very
good at taking tests and I can't think of once that I have failed to profit from
last-night and even last-minute preparation.

Vaughn

Vaughn Simon!
wrote:



"Doug Hoffman" wrote in message

...

... it never helps to cram the night before a test.

That might be true for you, but certainly not for me. I
have taken thousands
of tests over my lifetime (military, college, FAA). I
consider myself very
good at taking tests and I can't think of once that I have
failed to profit from
last-night and even last-minute preparation.


I envy your abilities. :-)

--
Regards,
Doug

** Posted from http://www.teranews.com **

"Doug Hoffman" wrote in message
...
I envy your abilities. :-)

Thanks, but I don't think that I have any special abilities.

I will say this: The items that I tend to study at the last minute are
probably the things that I will NOT be able to reliably recite two weeks after
the test. Passing a test is not proof that you will retain a satisfactory
percentage of the material, it is only proof that you passed the test.

Vaughn

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

Experts (?) in this area say that to remember something long term it should
be reviewed something like weekly for a month, then monthly, for a year,
and then yearly.

At 20:10 14 August 2008, Vaughn Simon wrote:

"Doug Hoffman" wrote in message
...
I envy your abilities. :-)

Thanks, but I don't think that I have any special abilities.

I will say this: The items that I tend to study at the last minute are

probably the things that I will NOT be able to reliably recite two weeks
after
the test. Passing a test is not proof that you will retain a
satisfactory

percentage of the material, it is only proof that you passed the test.

Vaughn

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

If the wing covers had extra material sewn in -infront and behind the wing proper- with tie-down points on them, would this help?
The result would be a ground bulge, not a wing shape at all.
This would prevent air going under the wing and taking control.
I cant imagine it would cost too much, there would be less rope to store as well, just peg the eyelets straight into the ground

just musing

bagger

On Aug 13, 1:01*pm, Eric Greenwell wrote:
...clip...
Question 1: Is there an analysis of the forces from wind on a tied down
glider, or maybe even measurements? ...clip...

Question 2: What is the effect on the wind forces when the glider has
wing covers on it, such as the Jaxida covers? .... I'm looking for real data and analysis.

I don't think anyone has put a model in the wind tunnel, and I don't
think you're likely to be able to talk an aerodynamicist to spend part
of the wind tunnel budget on such a project (I'd love to be wrong),
but I've thought about these same questions and would offer the
following analysis.

1: a tied-down aircraft is an aircraft "flying" in ground effect. The
normal aerodynamic analysis of the lifting surfaces applies,with that
proviso.
The possible wind directions are 'all those possible' and I expect
that all testing has been done assuming tailward orthogonal flow and
angles of attack that might occur during flight. In configuring your
tiedown, do think about the fact that the 90-knot-plus wind might be
blowing toward the ship's tail, or across its beam.
There are too many possibilities to permit testing that would
provide a clear answer, even for one ship - but I think one can use
the operating envelope and airframe design limits to reasonably state
the case for outer limits of tiedown strength.
First, ropes can be tested, though tensiometers might be hard to
locate. It's an instructive exercise to discover that how one fastens
the rope into the tensiometer affects the breaking strength - knots,
spices, and eyebolts affect strength differently; 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).
Second, as another writer inferred, the link into the ground is
crucial. A screw-in anchor holds much differently in drought-hardened
clay than rain-soaked clay.. 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.

1b: Now, what about the forces? I think that the analysis is easy,, if
we consider that the structure is somewhat likely to fail at 1.5x the
certified g limlit. If my aircraft has a max gross of 1000 lb and is
certified to plus/minus 5 g, then the structure is at real risk at 5 x
1000 = 5000 lb x 1.5 factor = 7500 lb. Divide this by 2 wings, and
4000 lb breaking strength on each wing is the max you'll need, as if
the wind is exactly right to generate enough lift to test the
structural limits of the aircraft, it matters less if the wings are
still affixed to terra firma with an intact rope at this point, except
that we also don't want the fragments of our glider to be flying
downwind into other aircraft, buildings, or people.

1c: The tailplane is simply not able to generate the lift of which the
wing is capable. I don't know the range of forces typical of aircraft
- perhaps one of the aeronautical engineers among us can tell us. But
the tail tiedown rope needs only to overcome the lift forces at the
tail.
The most important aspect of tail tiedown is to consider a tail
"tripod." Vern Frye of Air Sailing in Reno showed me that a milk
basket under the tail of a 1-26 reduces the angle of attack of the
tied-down glider sufficient to keep the glider from breaking out of
its moorings in wave conditions (sad experience was the teacher, I
believe). I'd guess that the optimum tripod height would be one that
would bring the mean wing chord to approximately level, to account for
tailwinds as well as headwinds.
Open spoilers and other obstructions to flow such as 2x4s will of
course decrease lift, but obviously one has to affix these in a safe
way. I can imagine a 2x4 coming partially loose in a stiff wind and
slatting around on the top of the wing. Not a happy thought.

Ropes.
I rather favor yacht braid, obtainable from yachting supply
companies. Typically, it will have a soft polyester jacket over a
hardy core such as Vectran. 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.

2: Perhaps you can talk Mark Maughmer into putting a section of your
wing, encased in a cover, into his wind tunnel at Penn State

I hope this stream of consciousness is useful...

Dan Johnson
N18UF