Hi Bruce

thank you for noting there is extra tension in the aerorow rope due
to the angle of climb. If the climb rate is 3kts (which is typical for a
2-seater at my club) at a towing speed of 60kts, the angle of climb
(theta) = 3/60 radians = 3 degrees, and the extra tension in the
rope is (weight of glider)*(sin theta) = typically 1000*(sin 3
degrees) = 50lbs. This would add to the tension in the rope due to
glider drag, as you say, to make a typical aerotow rope tension of
35+50 = 85lbs.

My estimate of the tension in the aerotow rope during initial
ground roll, assuming acceleration of a 1000lb glider to 60kts in
10secs, is about 315lbs.

Whilst the numbers can be juggled for different sailplanes and
towplanes, I agree with you that the greatest rope tension is likely
to be during initial acceleration i.e. during the ground roll. So
aerotow ropes are 'proof tested' on every ground roll, to a useful
degree. This does not assure that the rope meets the full rated
breaking strain however.

I read Bill's comment of June 2 about the rope perhaps taking
some longer time to actually break. This is a new idea to me, and I
don't know what too make of it. I'd like to hear the evidence for
this effect.

The more important discussion, is whether it is a good idea to train
or teach 200ft turn-backs to our students. Despite it being long
accepted practice, in the USA anyway, I doubt that it is a good idea
in terms of reducing serious accident rates. I note nobody suggests
we teach students to do final turns under 200ft. I wonder why.

I was interested in the suggestion from others that Germany has a
much lower PT3 accident rate, due to their stronger ropes and
weak links, and that this could perhaps be allowed by the FAA.


At 07:04 02 June 2014, Bruce Hoult wrote:
On Monday, June 2, 2014 4:50:10 PM UTC+12, Andrew wrote:
My experience seems logical when one considers that after
liftoff, the tension on the rope should be close to the drag on
the
glider, i.e. about weight divided by L/D, i.e. about 35lbs or less.

No, that's not the case unless you're not climbing.

With a tug flying at 65 knots and climbing at 6 knots (typical for
our
glass two seaters) somewhere around 9% of the weight of the
glider (up to
600 kg or 1300 lb) is being borne by the rope. That's about 120
lbs in
addition to the 35 lbs from drag.

With a 300 kg all up single seater (PW5, Libelle etc) flying a bit
slower
and climbing at over 1000 fpm there is actually even more strain
on the
rope.

I do agree that if it doesn't break on initial acceleration then it
probably won't.

I'm not going to go into the turn back or not question again other
than to
say if you can land safely more or less straight ahead then of
course do
so, but you should also be competent to turn back if that's best.