Max Tow Weight and Rope Strength
150flivver
Weight: 1000 lbs." You are going to tow a glider with a max gross
weight of 1000 pounds. Are you supposed to limit the tensile strength
of the tow rope used to 1000 lbs? The regulation specifies that the
tow rope should be no less than 80 percent and no greater than 200
percent of the max towed weight. This would allow you to use a rope
that has a strength of 2000 pounds but wouldn't that endanger the
towing aircraft which has the placarded 1000 pound limit? Would you
use a weaker tow rope or a weak link or would you just tow the 1000
pound glider with the rated 2000 pound rope?
uncl...@ix.netcom.com
It would be worthwhile to look at the origin of the placard to
determine what it really means.
Example. Schweizer hooks for many applications have a maximum rope
strength specified of 1200 lb. This effectively limits the max weight
of the glider to be towed to 1500 lb based on min rope strength of 80%
of max gross weight of the glider..
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
150flivver
> 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?
BT
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" <timot...@hotmail.com> wrote in message
news:dd52faa4-61dd-44ec...@w9g2000yqa.googlegroups.com...
sisu1a
> 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
sisu1a
well Spruce has'm but they're not STC'd... (hook at bottom of page)
http://www.aircraftspruce.com/catalog/appages/towBanners.php
-Paul
Bruce Hoult
> 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.
uncl...@ix.netcom.com
To repeat myself another way: You should look through the records for
the aircraft and determine where the limit on the placard came from.
It may be as simple as someone putting their own saftey factor on top
of one established by the hook supplier.
A 1000 lb limit seems really low. This would preclude towing most 2
seaters including 2-33's(1040 gross wt).
Having determined origin, it can be either corrected, or if not,
consider a hook installation more suitable for your needs.
If it is someone's home made hook, it may be placarded that way
because it was only tested to that force when it was installed. AC
43-13 specifies these testing requirements and is commonly used as
method of compliance by banner tow guys. The placard you describe
mentioning both makes me suspect this could be the case.
If it is not a commonly used glider tow hook(Schweizer type or Tost),
I would seriously consider going to a proven glider tow hook which has
been demonstrated to work at the expected loads and proven to release
when you need it to.
FWIW- the highest loads I've experienced have all been from hanging
the rope in the trees and getting the "big yank".
Good Luck
UH
150flivver
Are any hooks rated to more than 1200 pounds?
uncl...@ix.netcom.com
>
> - Show quoted text -
Tost hooks go higher- not sure what limits are- Tim Marra would know.
What kind of towplane is this?
UH
150flivver
It's a 180hp Cessna 150.
uncl...@ix.netcom.com
>
> - Show quoted text -
What kind of hitch?
UH
150flivver
> What kind of hitch?
> UH
Looks like a Schweitzer hook. The hook matches the towhook that's in
the parts manual for the Cessna and is attached by bolting the tiedown
ring through the tow hook assembly. The tow hook assembly then is
free to swivel around the tiedown ring. The cable to release the tow
rope of course limits the amount the tow hook can freely swivel.
uncl...@ix.netcom.com
>
> - Show quoted text -
Schweizer hooks are good for 1200 Lb rope as mentioned before.
Limitation may be related to installing on aft end of sheet metal
fuselage. It would pay to follow the installation info and STC
backwards to see if placard is correct interpretation of approved
data.
UH