Max Tow Weight and Rope Strength

I am trying to find (remember) the standard formula for tow pressure on the
tow line in a standard smooth tow.
I forget if it is weight of the glider divided by L/d or a percentage of
L/d.

Initial tow pull to get rolling, is high drag on the line. I can tell when
the skid comes up on the 2-33 or when the 2-33/Grob/Janus break ground. The
reduction in rolling resistance can be felt in the tow plane, even on a
paved runway. We never notice a 1-26 on tow. Unless the weak link has been
further weakened by use, we normally don't break the link for training slack
line recoveries.

We have a Standard Schweizer tow set up on our Pawnee and it is placarded to
1200# Max.
Our weak links are to the about the 1000# configuration to fit the 80-200%
CFR requirement for most of our gliders. We routinely tow one glider at
about 1500# with no issues. This glider POH specifies a weak link tolerance
higher than our standard links and CFRs. We have a special weak link for
that one and still stay within the 1200# max.

Yes the Tow rating is limited to being able to effect a release if it had
the max pressure holding mechanism, as in the case of the glider kiting and
lifting the Pawnee tail.

B

"150flivver" wrote in message
...
On Apr 13, 8:01 am, wrote:

The basis of this specification, if I recall correctly, was force on
the release handle.
Rope strength is specified with limits related to the max certified
gross weight of the glider being towed, that is NTL .80 of gross wt
and NGT than 2.0 times gross wt. Weak links can be used, as
appropriate to saty within these limits.
If the approved installation of the tow hook has other limits, they
are in addition to those related to the glider defined limits.
If you are using a hook not produced for towing gliders(there are lots
of home made banner towing hooks made to AC 43-13 which may not be
suitable for glider towing), consider moving to a proven tow hook
installation.
Good Luck
UH

So in your example, a Schweizer hook is limited to 1200 lbs and it
would be acceptable to tow a 1500 lb glider as long as the rope (or
weak link setup) was rated at 1200 lbs? Unfortunately, the wording on
the placard doesn't refer to rope strength limits, but to maximum
glider weight. If it referred to rope strength, I could see it being
legal to tow a 1500 lb. glider by using a 1200 lb setup but because it
specifies a max gross weight for the glider/banner, wouldn't anything
heavier exceed the legal limit?

Yes the Tow rating is limited to being able to effect a release if it had
the max pressure holding mechanism, as in the case of the glider kiting and
lifting the Pawnee tail.

I can't remember the name of the company right now, but they are
making SGS style tow hooks that will release under considerably more
pressure than std SGS hook. It has precision bearings in it where the
SGS is just plain metal on metal so the release friction is reduced by
multitudes of force.

-Paul

On Apr 13, 8:06*pm, sisu1a wrote:
Yes the Tow rating is limited to being able to effect a release if it had
the max pressure holding mechanism, as in the case of the glider kiting and
lifting the Pawnee tail.

I can't remember the name of the company right now, but they are
making SGS style tow hooks that will release under considerably more
pressure than std SGS hook. It has precision bearings in it where the
SGS is just plain metal on metal so the release friction is reduced by
multitudes of force.

-Paul

well Spruce has'm but they're not STC'd... (hook at bottom of page)
http://www.aircraftspruce.com/catalo...towBanners.php

-Paul

On Apr 14, 2:23*pm, "BT" wrote:
I am trying to find (remember) the standard formula for tow pressure on the
tow line in a standard smooth tow.
I forget if it is weight of the glider divided by L/d or a percentage of
L/d.

Well, yes, for straight and level flight at constant speed it would be
the weight of the glider divided by the L/D of the glider at that
speed. So if it's 400 kg and 40:1 then the tow force is 10 "kg" (not a
proper SI unit of force), or about 100 N.

But if you're climbing then you also need to look at the climb angle.
Technically you'll want the sine of the climb angle times the weight
of the glider but for real world tugs it'll be plenty close enough to
use:

(rate of climb / tow speed) * weight of the glider

Making sure of course that the first two are in the same units e.g.
both in knots or both in fpm or whatever.

So, for example if you tow the above 400 kg glider at 70 knots at a
rate of climb of 700 fpm (7 knots) then you've got another 40 kg or
400 N of tension on the rope from that, for a total of 50 kg of
tension.

Plus, as you point out, if you're accelerating then you need to take
that into account as well, but once you're into a steady climb you're
obviously not.