Are Weak Links really Necessary for Aero Tow?

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

KM wrote:


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.



I recall that John Campbell did a detailed analysis of this issue
(loads on a Schweizer towplane release mechanism) when he was a
postDoc. There were a couple of pretty sobering conclusions IIRC:

1. There is a critical angle (not a particularly steep one) beyond
which the vertical component of the force applied by the towrope will
overcome the force available from the emergency release cable on the
towplane side. In other words, once the glider kites up to a certain
angle, the towpilot may (probably won't) be able to release. Anyone
who has ever eyeballed the Schweizer release will immediately be able
to see why this is the case.

2. The force required for this was significantly less than the
breaking strength of a typical towrope. I could probably rough out
the numbers for this, but it is intuitively makes sense.

Maybe somebody has a copy of this analysis handy?

P3

Papa3 wrote:
Doug Haluza wrote:
Oh, my, where to start?

KM wrote:


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.



I recall that John Campbell did a detailed analysis of this issue
(loads on a Schweizer towplane release mechanism) when he was a
postDoc. There were a couple of pretty sobering conclusions IIRC:

1. There is a critical angle (not a particularly steep one) beyond
which the vertical component of the force applied by the towrope will
overcome the force available from the emergency release cable on the
towplane side. In other words, once the glider kites up to a certain
angle, the towpilot may (probably won't) be able to release. Anyone
who has ever eyeballed the Schweizer release will immediately be able
to see why this is the case.

2. The force required for this was significantly less than the
breaking strength of a typical towrope. I could probably rough out
the numbers for this, but it is intuitively makes sense.

Maybe somebody has a copy of this analysis handy?

P3

Here's a very good analysis: http://home.att.net/~jdburch/Towstudy.htm

Oh, now why did you have to inject facts into the discussion? ;-)

A very good analysis. The AC 43.13-2 referenced can be found at:

http://tinyurl.com/25oz7

The information on tow hook installatioin is at the end.

Papa3 wrote:
Here's a very good analysis: http://home.att.net/~jdburch/Towstudy.htm

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.

baron58y wrote:
KM wrote:

I dont see how
I could pull 6Gs while on tow.

Fair enough -- I don't either, but I'm not going to argue about it.

Dude!We agree on something!But I do think you want to argue about it or
you wouldnt keep posting marginally coherent responses.

Unless I find some reliable info that states otherwise, Doug might be
right. Got some?

Well consider this.If you take a look at the report (Elswhere on this
thread) about Shchweitzer tow hooks, it states that there were 3
accidents in this 12 year period that were caused by loss of pitch
control.During this same time frame there were at least 4 accidents
where tow planes came apart during aerobatics after the release of the
sailplane.So you have to ask yourself, What causes more damage to
towplanes? I hope you are reading this Doug.
I was flying in So Cal back in the 90s, and right at release the tow
pilot applied (What appeared from my vantage point) full left rudder
and snap rolled the tow plane upside down.He then split S'ed out the
bottom (With a Pawnee).Now I know alot of people on this list are gonna
counter that this wont hurt a towplane if you are slow enough, but I
beg to differ.I used to fly for an aerobatics flight school, and the
types of cracks Doug is descibing showed up on planes that were NEVER
used for towing.I have no idea why Doug had to make a silly remark
about tailslides.You have to also consider that most of our tugs are
tired old ships to begin with.


Also, in reference to Dougs post, If a tow pilot was losing pitch
authority, why wouldnt he just pull the release?

Would, if he could -- if he didn't wait too long.

Jack this is an excellent point, and I would ad that if this happened
low to the ground, a weak link would not matter.

Yet another unfounded assumption on your part. I fly a 1-26.

Now Jack you silly goose!You cant sign yourself "Baron 58Y" and not
have people assume you fly a Baron.

Oh, excellent! A "mine is bigger than yours" retort. We rarely get those
on r.a.s. -- unlike most of the rest of USENET.
There are probably some folks here whose aviation careers could put both
mine and yours to shame, if we wanted to talk about experience -- so
let's not embarrass ourselves. If "my 737-800" means you own it,
congratulations are in order! But if Delta just pays you to fly it, I
ain't that impressed.

Jack, you would be even less impressed if you saw my paycheck
lately.But more to the point, I threw this in because you had to make
a "unfounded assumption" about my experience level.Now once again I
will be the first to acknowlege that 9/10s of this list has more time
in tow planes and sailplanes than I do.


Or better yet, lets just get back to the post that started this
thread.

First smart thing you said. What was that post all about? Oh yeah, here
it is:
Jack, was that a compliment? Iam touched!

So civil, not contentious at all -- what a guy that JS is.

So, what do you think KMU -- now that you have reviewed the original post?

???? Once again for you and Doug.Keep a week ling in your plane.And if
your tow pilot is pulling any shenanigans, kindly ask him or her to
knock it off.

Jack

Your Pal,
KMU

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

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

Note that the drawing on p.28 of the document referenced by John Smith was
taken from a series of photos taken with a motor drive camera during a
deliberate upset of a Supercub towing a K8 during trials many years ago.
Note that the rope angle to the tug hardly changes.

See http://www.glidingmagazine.com/Featu...cle.asp?id=327 , where Chris
Rollings who was flying the K8 describes what happened. The PA18-180 was
being flown by Brian Spreckley.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


"John Smith"
wrote in message ...

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

https://www.gliding.co.uk/bgainfo/cl...rotownotes.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 high loads 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 decelerates 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 tailplane 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.

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

If both aircraft are in a steep dive from a high altitude upset, and
the glider pilot panics and pulls the sitck, it certainly can. But it
really doesn't matter--using a dockline as a tow rope means it won't
break before one of the aircraft does.

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

The dive after an upset will be unrecoverable as long as the glider
stays attached to the towplane.