Are Weak Links really Necessary for Aero Tow?

A lot of the clubs here tow with thickish (5/8 inch and some 1/2inch)
poly and nylon ropes, without weak links, and pilots with years of
experience say that no load exerted by the glider can damage the tug -
and in the worst of cases even a 5/8 rope will break before damaging
the Tug (Most often a supercub)

Comments from knowlegable people please ---

Thanks and kind regards

JS

There was a recent thread on this topic...a search
of the archives should turn it up. FWIW 1/2' poly
typically is 1,000lb strength new, which allows it
to be used here in the USA for most gliders without
weak links.

I think this is really ignorant and misguided thinking. Yes, absolutely
the glider can damage the tug. The tail structure of most airplanes was
not designed to handle the load of a glider on tow. It was only
designed to handle aerodynamic and landing loads. Considering that the
breaking strength of a 5/8" rope is greater than the weight of the
towplane, it's not hard to imagine that it is certainly strong enough
to damage the tail. A 1000 lb glider can deliver well over 6000 lb of
pull before it's wings come off.

We have had an ongoing problem with stress cracking of the longerons
near the tail on our Super Cub towplane. It has been dsicovered from
time to time over the years during annual inspections because we have a
very good IA who knows how to look for this. And we tow with a 5/16"
poly rope, 1/4 the breaking strength of a 5/8" rope.

There have been a number of towplane upsets that would have resulted in
accidets had the tow rope not broken. The rope will not break when the
glider pulls the towplane's tail up, because the forces are not that
great. But after the upset, the forces are much greater, and if the
rope breaks, the towpilot has a chance to recover if there is enough
altitude remaining.

Most tow hook installations in the US are supposed to be placarded for
1200 lb breaking strength maximum. Most gliders also have maximum
breaking strength limits on their tow hooks as well (check your
manual). As has been pointed out in another thread, if you conduct
operations outside these limits, your insurance could deny coverage for
any resulting accident, especially since there will be a causal
relationship.

wrote:
A lot of the clubs here tow with thickish (5/8 inch and some 1/2inch)
poly and nylon ropes, without weak links, and pilots with years of
experience say that no load exerted by the glider can damage the tug -
and in the worst of cases even a 5/8 rope will break before damaging
the Tug (Most often a supercub)

Comments from knowlegable people please ---

Thanks and kind regards

JS

1/4" poly rope is between 1000 and 1200 lbs breaking strength when new,
not 1/2" which is much too strong. Note that the breaking strength will
reduce with use, so you would not want to use 1/4" poly to tow a double
seater or motorglider with water ballast.

Stewart Kissel wrote:
There was a recent thread on this topic...a search
of the archives should turn it up. FWIW 1/2' poly
typically is 1,000lb strength new, which allows it
to be used here in the USA for most gliders without
weak links.

At 20:36 16 September 2006, Doug Haluza wrote:
1/4' poly rope is between 1000 and 1200 lbs breaking
strength when new,
not 1/2' which is much too strong.

My bad...yes you are correct, I had my numbers wrong.

If the towrope has a breaking strength more than twice the maximum
certificated operating weight of the glider being towed, a safety link
has to be installed at the point of attachment of the glider and the
tow plane with the following breaking strength requirements.
Safety Link (Weak Link) Requirements
Safety link (Weak Link) at the glider end:
o Minimum Strength = 80 percent of the glider maximum certificated
operating weight
o Maximum Strength = twice the maximum certificated operating weight
Safety link (Weak Link) at the tow plane end:
o Strength Requirements = Greater, but not more than 25% greater than
that of the safety link on the glider end, and not more than twice the
maximum certificated operating weight of the glider

Typical Rope Strengths in Pounds
Diameter Hollow Braid Polypropylene Polypropylene
Nylon Dacron Polyethylene Monofilament Multifilament
3/16 " 960 720 700 800 870
1/4 " 1,500 1,150 1,200 1,300 1,200
5/16" 2,400 1,750 1,750 1,900 2,050

Doug Haluza wrote:
I think this is really ignorant

No need to call people names here.I think the guy was just looking for
an honest opinion.

The tail structure of most airplanes was
not designed to handle the load of a glider on tow. It was only
designed to handle aerodynamic and landing loads. Considering that the
breaking strength of a 5/8" rope is greater than the weight of the
towplane, it's not hard to imagine that it is certainly strong enough
to damage the tail. A 1000 lb glider can deliver well over 6000 lb of
pull before it's wings come off.

This is maybe true at high airspeeds.But at typical towing speeds, the
glider would probably stall before it could exert 6Gs of load.Most
towing is done well below the max maneuvering speed of a
sailplane.Therefore, it would stall well before the wings came
off.Another thing to consider (those of you who auto tow or winch
launch can relate), The tailplane would probable stall before you could
exert 6K load on tow.


We have had an ongoing problem with stress cracking of the longerons
near the tail on our Super Cub towplane.

Do you know for certain this is caused by towing?I flew for a company
with a fleet of tow ships, some with 8000 hours of nothing but tows and
we didnt have this problem.Check to see what your tow pilots are doing
on the way down.

The rope will not break when the
glider pulls the towplane's tail up, because the forces are not that
great. But after the upset, the forces are much greater, and if the
rope breaks, the towpilot has a chance to recover if there is enough
altitude remaining.

Now you are contradicting yourself.You just wrote that a sailplane can
apply 6000 LBS of pull, and here you say the force is not that
great.Why would the forces increase after an upset.

Most tow hook installations in the US are supposed to be placarded for
1200 lb breaking strength maximum.

This depends on the type of hook.Also, I think this guy was posting
from another country.Also, he was aking about tow ROPES and not hooks.


Fly Safe,
KMU

KM wrote:
Doug Haluza wrote:
I think this is really ignorant

No need to call people names here.I think the guy was just looking for
an honest opinion.

I was not referring to the poster, I was referring to the supposedly
experienced people he was quoting.

The tail structure of most airplanes was
not designed to handle the load of a glider on tow. It was only
designed to handle aerodynamic and landing loads. Considering that the
breaking strength of a 5/8" rope is greater than the weight of the
towplane, it's not hard to imagine that it is certainly strong enough
to damage the tail. A 1000 lb glider can deliver well over 6000 lb of
pull before it's wings come off.

This is maybe true at high airspeeds.But at typical towing speeds, the
glider would probably stall before it could exert 6Gs of load.Most
towing is done well below the max maneuvering speed of a
sailplane.Therefore, it would stall well before the wings came
off.Another thing to consider (those of you who auto tow or winch
launch can relate), The tailplane would probable stall before you could
exert 6K load on tow.

After an upset, the towplane will enter an unrecoverable dive, and if
the rope does not break, the speeds will quickly increase beyond
maneuvering speed.

We have had an ongoing problem with stress cracking of the longerons
near the tail on our Super Cub towplane.

Do you know for certain this is caused by towing?I flew for a company
with a fleet of tow ships, some with 8000 hours of nothing but tows and
we didnt have this problem.Check to see what your tow pilots are doing
on the way down.

They are stress cracks, probably from a combination of vibration,
landing, and towing loads. They are not doing tail slides on the way
down.

The rope will not break when the
glider pulls the towplane's tail up, because the forces are not that
great. But after the upset, the forces are much greater, and if the
rope breaks, the towpilot has a chance to recover if there is enough
altitude remaining.

Now you are contradicting yourself.You just wrote that a sailplane can
apply 6000 LBS of pull, and here you say the force is not that
great.Why would the forces increase after an upset.

Because the airspeed will increase.

Most tow hook installations in the US are supposed to be placarded for
1200 lb breaking strength maximum.

This depends on the type of hook.Also, I think this guy was posting
from another country.Also, he was aking about tow ROPES and not hooks.

The rope and the hooks on both end work as a complete system. All of
the parts must work together. And they will work the same in any
country--the laws of Physics know no political boundaries.

Doug Haluza wrote:
Considering that the
breaking strength of a 5/8" rope is greater than the weight of the
towplane, it's not hard to imagine that it is certainly strong enough
to damage the tail.

Are you telling us that the tail can only handle its own weight?You are
using apples and oranges here because you used the FLIGHT loads of a
sailplane and the actual weight of the tow plane.In other words, if a
towplane can sustain 4.4Gs (In the utility category) shouldnt the tail
of said towplane ALSO sustain 4.4Gs.


After an upset, the towplane will enter an unrecoverable dive, and if
the rope does not break, the speeds will quickly increase beyond
maneuvering speed.

Where do you come up with this?I have NEVER flown an airplane that
could not be pulled out of a dive.Another thing to consider is that the
tow pilot would just release by this point.The tost hook will release
at vitually any angle, and even if the plane had a Schweitzer hook, by
forcing the tail up you will change the angle on the rope and the pilot
could then release it.

They are stress cracks, probably from a combination of vibration,
landing, and towing loads. They are not doing tail slides on the way
down.

Are you a metalurgist?When you say "Probably" it kinda implies that you
are guessing.If your Cub has Jack screw trim, take a look at what the
tail is doing on the take off roll.Also, I never mentioned a tail
slide.There has been a tow pilot or two who thinks acro wont hurt a
plane as long as you are carefull.How long have you been hanging out at
gliderports?

The rope and the hooks on both end work as a complete system. All of
the parts must work together. And they will work the same in any
country--the laws of Physics know no political boundaries.

Boy you are a sharp one Doug!I would have never guessed (G).What I
actually meant was that the laws in this pilots country could be more
conservitive than the US.Take a look at the rules in germany (Where
most of our gliders come from).
Happy Landings
KMU