Are Weak Links really Necessary for Aero Tow?

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.