Are Weak Links really Necessary for Aero Tow?

Oh, my, where to start?

KM wrote:

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.

No, I'm saying the airplane was designed to handle flight and landing
loads, based on it's max gross weight. These loads on the tail are only
a fraction of it's weight. The glider can transfer most of the lifting
force developed by the wing to the rope if a C.G. hook is used for
aerotow. An aircraft with a design load limit of 4.4 G's will have an
ultimate load limit 6.6 G's so a glider with a 1000 lb gross weight
could deliver over 6000 lb of force, before the glider's wings failed.

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.

You can't pull out of the dive if the glider is still attached to your
tail by a rope that won't break. And if you have a Schweizer hook on
the tail, it may not release after the upset because the pull is
greater and may no longer be straight back. There have been several
cases of upset where the tow pilot could not make the hook release, and
the dive would have been unrecoverable if the rope did not break.

Even if you tow with a Tost hook, you still need to react and operate
the release, and this will take more time than a rope needs to break.
If the glider pulls your tail up fast enough, you could be in negative
G's which will take your hand off the knob. How fast can you find and
operate the corect knob when the stuff hits the fan? That time could be
the difference between a low recovery and a splat.

KM wrote:

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.


Happy Landings
KMU

KM,

I think what you're missing here is the difference between the limiting
load factor on the towplane as a system (ie. the 4.4Gs) vs. the
limiting load factor on a component. Typically, the primary
load-bearing components in flight are the wing spars and the
carry-through structure; this is carrying the majority of the load.
The tail section will never sustain anything near those sorts of loads
in normal flight.

Think of this way. Suppose your towplane weights 1,500 lbs (just an
illustration). It's pretty easy to imagine it supporting 6,000 lbs of
sandbags spread along the wings (just think of those impressive ads
showing proof-testing of spars). Now, imagine piling those same
6,000 lbs of sandbags on the empennage (assuming you could find the
space to stack them :-))

Anyway, I can probably come up with some guesstimates on what the tail
section of a typical welded steel cluster could withstand, but why
bother?

P3

On 16 Sep 2006 13:30:15 -0700, "Doug Haluza"
wrote:

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.


my old auster J1b which is under a long slow restoration was used for
towing gliders at on stage of its life by the Narrogin Gliding Club in
Western Australia. You can clearly see a weld repair to the bottom two
longerons just in front of the tailpost where a glider did its
damndest to tear the back end out of the aircraft.

no weak link seems on evidence to warn of a funeral in the making.
Stealth Pilot
Oz

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

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

Been there, done that (damaged a towplane). I got my CFI-G ticket in
2-33s and had about 100 hours instructing in these. Then, I
volunteered with another club that used Grob 103s for training. A very
different animal from the 2-33. Things get out of whack much faster.
On my second flight with a fairly new student, I let her get a fairly
impressive PIO going in both pitch (vertical position) and roll
(lateral position). On about the 3rd oscillation, she had a huge loop
develop and, before I could reach the release, she applied full
opposite stick. A godawful sproingggg and we were in free flight. I
noticed the towplane (an L-19) in a fairly impressive rolling dive to
the left and saw the rope trailing free from the towplane end (ie.
attached to the glider) . Once I got my nerves under control, we
dropped the remaining rope over the airport and had an uneventful
landing...

.... Uneventful, that is, until the towpilot came walking over. It
didn't take a genius to figure out that he was not in the best of
moods. I noticed he was carrying something in his hand. The
something was the mangled release mechanism. We had broken it from
the towplane at the mounting bolt; it stayed attached only thanks to
the release cable.

So, in answer to your question, it is possible to damage a towplane
with a glider. At this operation, they used 5/8 braided poly with
Schweizer rings for the main rope. The club which owned the G103 used
"adapters" for the Tost release. I believe (can't recall for sure)
that these were also 5/8.

Erik Mann
LS8-18 (P3)

Papa3 wrote:
... Uneventful, that is, until the towpilot came walking over. It
didn't take a genius to figure out that he was not in the best of
moods. I noticed he was carrying something in his hand. The
something was the mangled release mechanism. We had broken it from
the towplane at the mounting bolt; it stayed attached only thanks to
the release cable.

You are lucky you broke the tow hook attachment bolts. This engaged the
tow hook release by pulling on the release cable from the other end.
That is why the hook stayed attached to the towplane, and the rope
stayed attached to the glider (and the tail stayed attached to the
towplane).

P.S. I saw a similar failure on an L-19 tow hook installation due to
metal fatigue. A bolt broke, and the tow hook released during a normal
tow. It happened early in the tow, so no accident. But it showed an
obvious design flaw with that installation. The hook was mounted
directly to the leaf spring with two plates on either side, all
sandwiched together by two 1/4" bolts. The plates were separated by the
spring thickness, and put a lot of shear loading on those tiny bolts,
both of which are critical. I would rather see a 4-bolt installation,
so you get a chance to catch a broken bolt on pre-flight.

A simple question to all the participants in this thread
- How many off you have actually experienced piloting
a tow plane when the glider has kited behind you?
As a tow pilot who has had it happen at 500 feet agl
and did survive I can state that it is not just the
angle of the rope but also the highloads imposed that
prevent release of the rope by the tow plane. This
affects all types of release. You have a tow plane
trying to dive at a steep angle and a glider virtually
winch launching of the back of it - quite a high loading
I can assure you and one that decellerates the tow
plane rapidly. Secondly the upset occurs because of
the upwards load from the glider overcoming the maximum
down load able to be applied at the tow planes tailplane.
This effectively stalls the tailpane in an inverted
sense and the tow plane is actually pitched rapidly
nose down with very little acceleration and indeed
speed. If the rope breaks or releases it is actually
necessary to accelerate the tow plane in its nose down
attitude to gain sufficient speed to pull out of the
nose down attitude.
And NO, I really don't want to experience it again.
With reference to the original question I would strongly
advise the fitting of weak links to all glider tow
ropes regardless of the supposed breaking strength
of the rope used.
PS I do operate at both ends of the tow rope.




At 16:30 17 September 2006, Km wrote:

Doug Haluza wrote:
Oh, my, where to start?

How about with your condisending tude?

No, I'm saying the airplane was designed to handle
flight and landing
loads, based on it's max gross weight. These loads
on the tail are only
a fraction of it's weight. The glider can transfer
most of the lifting
force developed by the wing to the rope if a C.G.
hook is used for
aerotow. An aircraft with a design load limit of 4.4
G's will have an
ultimate load limit 6.6 G's so a glider with a 1000
lb gross weight
could deliver over 6000 lb of force, before the glider's
wings failed.

OK now focus here Doug, the math is not in dispute.
The question is
whether a glider could exert this force while 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.

But what makes you think the dive would be 'Unrecoverable'
just because
the tow plane is past its manurering speed?

You can't pull out of the dive if the glider is still
attached to your
tail by a rope that won't break. And if you have a
Schweizer hook on
the tail, it may not release after the upset because
the pull is
greater and may no longer be straight back. There
have been several
cases of upset where the tow pilot could not make
the hook release, and
the dive would have been unrecoverable if the rope
did not break.

Completely true statement.But, as the nose of the tow
plane drops, this
would change the angle on the release would it not?A
couple of local
pilots tried this (At altitude of course) and found
this to be the
case.Now at low altitudes, all bets are off of course.This
is the
beauty of the tost hook.

Even if you tow with a Tost hook, you still need to
react and operate
the release.

Doesnt this go without saying?
I have to take issue with your previous post where
you implied that a
pilot could get away with aerobatics in a Super Cub
as long as he wasnt
doing 'Tailslides'.To coin your phrase this is 'Ignorant
Thinking'.You
should read my response to Baron 58Yankee on this one.I
think that any
aerobatics in a Super Cub should be discuraged.
Most Respectfully Yours,
KMU

The document at the following reference may be of interest
- in particular pages 28 - 30

https://www.gliding.co.uk/bgainfo/cl...uments/aerotow
notes.pdf

also see my edit re the tow plane diving

At 17:24 17 September 2006, John Smith wrote:
A simple question to all the participants in this thread
- How many off you have actually experienced piloting
a tow plane when the glider has kited behind you?
As a tow pilot who has had it happen at 500 feet agl
and did survive I can state that it is not just the
angle of the rope but also the highloads imposed that
prevent release of the rope by the tow plane. This
affects all types of release. You have a tow plane
'most likely at full power' trying to dive at a steep
angle and a glider virtually
winch launching of the back of it - quite a high loading
I can assure you and one that decellerates the tow
plane rapidly. Secondly the upset occurs because of
the upwards load from the glider overcoming the maximum
down load able to be applied at the tow planes tailplane.
This effectively stalls the tailpane in an inverted
sense and the tow plane is actually pitched rapidly
nose down with very little acceleration and indeed
speed. If the rope breaks or releases it is actually
necessary to accelerate the tow plane in its nose down
attitude to gain sufficient speed to pull out of the
nose down attitude.
And NO, I really don't want to experience it again.
With reference to the original question I would strongly
advise the fitting of weak links to all glider tow
ropes regardless of the supposed breaking strength
of the rope used.
PS I do operate at both ends of the tow rope.




At 16:30 17 September 2006, Km wrote:

Doug Haluza wrote:
Oh, my, where to start?

How about with your condisending tude?

No, I'm saying the airplane was designed to handle
flight and landing
loads, based on it's max gross weight. These loads
on the tail are only
a fraction of it's weight. The glider can transfer
most of the lifting
force developed by the wing to the rope if a C.G.
hook is used for
aerotow. An aircraft with a design load limit of 4.4
G's will have an
ultimate load limit 6.6 G's so a glider with a 1000
lb gross weight
could deliver over 6000 lb of force, before the glider's
wings failed.

OK now focus here Doug, the math is not in dispute.
The question is
whether a glider could exert this force while 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.

But what makes you think the dive would be 'Unrecoverable'
just because
the tow plane is past its manurering speed?

You can't pull out of the dive if the glider is still
attached to your
tail by a rope that won't break. And if you have a
Schweizer hook on
the tail, it may not release after the upset because
the pull is
greater and may no longer be straight back. There
have been several
cases of upset where the tow pilot could not make
the hook release, and
the dive would have been unrecoverable if the rope
did not break.

Completely true statement.But, as the nose of the tow
plane drops, this
would change the angle on the release would it not?A
couple of local
pilots tried this (At altitude of course) and found
this to be the
case.Now at low altitudes, all bets are off of course.This
is the
beauty of the tost hook.

Even if you tow with a Tost hook, you still need to
react and operate
the release.

Doesnt this go without saying?
I have to take issue with your previous post where
you implied that a
pilot could get away with aerobatics in a Super Cub
as long as he wasnt
doing 'Tailslides'.To coin your phrase this is 'Ignorant
Thinking'.You
should read my response to Baron 58Yankee on this one.I
think that any
aerobatics in a Super Cub should be discuraged.
Most Respectfully Yours,
KMU