What are the forces on a tied down glider?

[Eric Greenwell]

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" at www.motorglider.org

[John H. Campbell[_2_]]

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

[JJ Sinclair]

Just some facts, Eric. At Stead 2 gliders that were tied down at wing
tips and tail with 3/8" poly rope, broke all 3 ropes and performed a
near perfect half loop landing upside down after experiencing a
reported 100 knot wind (ships were tied down facing the wind).

At Minden 2 ships did much the same maneuver after experiencing about
the same wind (Winter storm frontal passage).

Raising the tail to get a negative angle of attack and locking the
spoilers open can go a long way in preventing the above OR, put it in
the box!
JJ

[Andy[_1_]]

On Aug 13, 11:01*am, 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

[5Z]

On Aug 13, 2:49*pm, Andy > wrote:
> Others would say why worry - put it in the box!

Kinda hard to do that if:

1) You've landed away from home and are waiting for the box to arrive

2) Touring in a self-launcher without a chase crew

[Eric Greenwell]

JJ Sinclair wrote:
> Just some facts, Eric. At Stead 2 gliders that were tied down at wing
> tips and tail with 3/8" poly rope, broke all 3 ropes and performed a
> near perfect half loop landing upside down after experiencing a
> reported 100 knot wind (ships were tied down facing the wind).
>
> At Minden 2 ships did much the same maneuver after experiencing about
> the same wind (Winter storm frontal passage).
>
> Raising the tail to get a negative angle of attack and locking the
> spoilers open can go a long way in preventing the above OR, put it in
> the box!

Does anyone know the force required to break those ropes; for example,
were the ropes tested afterwards for breaking strength?

Was there any analysis of the forces on the gliders? A 100 knots might
destroy a glider, even if the ropes held. That happened to our club
Blanik years ago in winds of about 70 mph.

--
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

[JJ Sinclair]

On Aug 13, 1:58*pm, 5Z > wrote:
> On Aug 13, 2:49*pm, Andy > wrote:
>
> > Others would say why worry - put it in the box!
>
> Kinda hard to do that if:
>
> 1) *You've landed away from home and are waiting for the box to arrive
>
> 2) *Touring in a self-launcher without a chase crew

Williams Soaring is selling JJ's little tie-down kit that can be
carried in the ship.
JJ

[Bill Daniels]

"John H. Campbell" > wrote in message
...
> One sample has been in evidence for years on the back page of the SGS 2-33
> POH. --JHC

Yes, and some would say unfortunately, it works. Just look at the
improbable population of 2-33's still in existance.

Bill D

[Bill Daniels]

There's a tiedown technique I developed 40 years ago when I decided that
steel stakes were too heavy and often didn't work in soft ground.

I bought some stout 10 gallon nylon bags with 1/2" nylon rope closures and a
lightweight folding shovel "trenching tool" from a military surplus store.
Three bags and one shovel were much lighter and took less space in the
glider than the steel screw-in stakes they replaced. The idea was to dig
holes and put the dirt in the bags and push the filled bags into the holes,
then tie the glider to the nylon ropes.

After using them, I tried to pull the bags out of the ground with a truck
but gave up and left the bags in the ground when the truck almost got stuck.
Even if the glider was able to pull a bag out of the ground, the
considerable weight of the dirt filled bag would still hold the glider. On
the other hand, steel stakes pulled out easily.

I've just heard that these bags are still available through military
sources.


"5Z" > wrote in message
...
On Aug 13, 2:49 pm, Andy > wrote:
> Others would say why worry - put it in the box!

Kinda hard to do that if:

1) You've landed away from home and are waiting for the box to arrive

2) Touring in a self-launcher without a chase crew

[[email protected]]

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

[[email protected]]

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.

[Nyal Williams[_2_]]

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.
>

[Capt. Geoffrey Thorpe]

"Andy" > wrote in message
...

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


"Glider trailers parked on the field were crushed from being tossed about
like dice."

(to be fair "The hangar right across the alleyway had it's roof and walls
blown off.")

http://iac78.org/newsletter/NL%20Fall-Winter%2005%20Color.pdf Page 8.



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?


--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Don Johnstone[_3_]]

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.

[Eric Greenwell]

Doug Hoffman wrote:

> Seems like wing and tailfeather covers should help some. Maybe
> a lot. The full covers I've seen (Jaxida) hang down a lot of
> covering beneath the wings and horizontal stabs/elevators. To my
> eye the covers might be far more effective than even open
> spoilers in destroying lift.

My observation is the the wind puffs up the covers so they stand
completely off the top and bottom of the wing. In a strong wind, the
covers look like they are inflated to a high pressure, making the wing
look more like a sausage than anything else. My guess is this is a high
drag, low lift "airfoil", but I have no documentation for it.

It also provides protection from hail, which just bounces off the taut
wing covers and never gets near the wing skin.

--
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

[bumper]

"Eric Greenwell" > wrote in message
news:x7Mok.925$xv.899@trnddc02...
> My observation is the the wind puffs up the covers so they stand
> completely off the top and bottom of the wing. In a strong wind, the
> covers look like they are inflated to a high pressure, making the wing
> look more like a sausage than anything else. My guess is this is a high
> drag, low lift "airfoil", but I have no documentation for it.
>
> --
> Eric Greenwell - Washington State, USA

My observation is similar . . . except to me, the puffed up covers look like
the thick, high lift wing on a trainer.

I'd feel a lot better about covers in high winds if there were spanwise
fabric tubes sewn in to allow pipe foam insulation to be inserted for full
length spoilers.

bumper
zz
Minden
USA

[Darryl Ramm]

On Aug 13, 7:55*pm, "bumper" > wrote:
> "Eric Greenwell" > wrote in message
>
> news:x7Mok.925$xv.899@trnddc02...
>
> > My observation is the the wind puffs up the covers so they stand
> > completely off the top and bottom of the wing. In a strong wind, the
> > covers look like they are inflated to a high pressure, making the wing
> > look more like a sausage than anything else. My guess is this is a high
> > drag, low lift "airfoil", but I have no documentation for it.
>
> > --
> > Eric Greenwell - Washington State, USA
>
> My observation is similar . . . except to me, the puffed up covers look like
> the thick, high lift wing on a trainer.
>
> I'd feel a lot better about covers in high winds if there were spanwise
> fabric tubes sewn in to allow pipe foam insulation *to be inserted for full
> length spoilers.
>
> bumper
> zz
> Minden
> USA

The standard Jaxida covers look like this in the wind...

http://www.flickr.com/photos/darrylramm/517165653/
http://www.flickr.com/photos/darrylramm/517140648/

(maybe it was really faster than 20 knots in these photos).

Does Jaxida offer a sewn in place to insert a foam tube?

Darryl

[Doug Hoffman]

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.

Seems like wing and tailfeather covers should help some. Maybe
a lot. The full covers I've seen (Jaxida) hang down a lot of
covering beneath the wings and horizontal stabs/elevators. To my
eye the covers might be far more effective than even open
spoilers in destroying lift.

--
Doug

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

[Eric Greenwell]

Darryl Ramm wrote:
> On Aug 13, 7:55 pm, "bumper" > wrote:
>> "Eric Greenwell" > wrote in message
>>
>> news:x7Mok.925$xv.899@trnddc02...
>>
>>> My observation is the the wind puffs up the covers so they stand
>>> completely off the top and bottom of the wing. In a strong wind, the
>>> covers look like they are inflated to a high pressure, making the wing
>>> look more like a sausage than anything else. My guess is this is a high
>>> drag, low lift "airfoil", but I have no documentation for it.
>>> --
>>> Eric Greenwell - Washington State, USA
>> My observation is similar . . . except to me, the puffed up covers look like
>> the thick, high lift wing on a trainer.
>>
>> I'd feel a lot better about covers in high winds if there were spanwise
>> fabric tubes sewn in to allow pipe foam insulation to be inserted for full
>> length spoilers.
>>
>> bumper
>> zz
>> Minden
>> USA
>
> The standard Jaxida covers look like this in the wind...
>
> http://www.flickr.com/photos/darrylramm/517165653/
> http://www.flickr.com/photos/darrylramm/517140648/
>
> (maybe it was really faster than 20 knots in these photos).
>
> Does Jaxida offer a sewn in place to insert a foam tube?

My covers are not Jaxidas, but are similar in design using a lighter
material (a type of Tyvek). They bulge more evenly on the top and bottom
than Darryl's Jaxida covers. Perhaps Darryl could slide one cover off
it's wing to determine if the other wing is lifting or not? It's not so
easy for me to do that because my covers attach to each other, not a
center section like the Jaxidas.

--
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

[Bruce]

How interested you were in Friday?

Capt. Geoffrey Thorpe wrote:
> "Andy" > wrote in message
> ...
>
>> Others would say why worry - put it in the box!
>
>
> "Glider trailers parked on the field were crushed from being tossed
> about like dice."
>
> (to be fair "The hangar right across the alleyway had it's roof and
> walls blown off.")
>
> http://iac78.org/newsletter/NL%20Fall-Winter%2005%20Color.pdf Page 8.
>
>
>
> 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?
>
>

[Doug Hoffman]

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

[Vaughn Simon]

"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

[Doug Hoffman]

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 **

[Vaughn Simon]

"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]

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

[Nyal Williams[_2_]]

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
>
>
>

[Eric Greenwell]

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

[Bill Daniels]

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

[bagmaker]

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

[danlj]

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

[danlj]

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

[Bob Kuykendall]

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.

[Eric Greenwell]

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

[Bill Daniels]

"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

[Gilbert Smith]

"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

[rlovinggood]

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 -

[Cats]

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.

[Derek Copeland[_2_]]

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
>

[Eric Greenwell]

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 Daniels]

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

[Derek Copeland[_2_]]

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.
>>