Are Weak Links really Necessary for Aero Tow?

KM wrote:
Doug Haluza wrote:
I think this is really ignorant

No need to call people names here.I think the guy was just looking for
an honest opinion.

I was not referring to the poster, I was referring to the supposedly
experienced people he was quoting.

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.

This is maybe true at high airspeeds.But at typical towing speeds, the
glider would probably stall before it could exert 6Gs of load.Most
towing is done well below the max maneuvering speed of a
sailplane.Therefore, it would stall well before the wings came
off.Another thing to consider (those of you who auto tow or winch
launch can relate), The tailplane would probable stall before you could
exert 6K load 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.

We have had an ongoing problem with stress cracking of the longerons
near the tail on our Super Cub towplane.

Do you know for certain this is caused by towing?I flew for a company
with a fleet of tow ships, some with 8000 hours of nothing but tows and
we didnt have this problem.Check to see what your tow pilots are doing
on the way down.

They are stress cracks, probably from a combination of vibration,
landing, and towing loads. They are not doing tail slides on the way
down.

The rope will not break when the
glider pulls the towplane's tail up, because the forces are not that
great. But after the upset, the forces are much greater, and if the
rope breaks, the towpilot has a chance to recover if there is enough
altitude remaining.

Now you are contradicting yourself.You just wrote that a sailplane can
apply 6000 LBS of pull, and here you say the force is not that
great.Why would the forces increase after an upset.

Because the airspeed will increase.

Most tow hook installations in the US are supposed to be placarded for
1200 lb breaking strength maximum.

This depends on the type of hook.Also, I think this guy was posting
from another country.Also, he was aking about tow ROPES and not hooks.

The rope and the hooks on both end work as a complete system. All of
the parts must work together. And they will work the same in any
country--the laws of Physics know no political boundaries.

Doug Haluza wrote:
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.

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.


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.

They are stress cracks, probably from a combination of vibration,
landing, and towing loads. They are not doing tail slides on the way
down.

Are you a metalurgist?When you say "Probably" it kinda implies that you
are guessing.If your Cub has Jack screw trim, take a look at what the
tail is doing on the take off roll.Also, I never mentioned a tail
slide.There has been a tow pilot or two who thinks acro wont hurt a
plane as long as you are carefull.How long have you been hanging out at
gliderports?

The rope and the hooks on both end work as a complete system. All of
the parts must work together. And they will work the same in any
country--the laws of Physics know no political boundaries.

Boy you are a sharp one Doug!I would have never guessed (G).What I
actually meant was that the laws in this pilots country could be more
conservitive than the US.Take a look at the rules in germany (Where
most of our gliders come from).
Happy Landings
KMU

KM wrote:

Doug Haluza 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.

Then you haven't flown tow planes much (if at all), if you can't imagine
that scenario.


Jack

baron58y wrote:

Doug Haluza 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.

Then you haven't flown tow planes much (if at all), if you can't imagine
that scenario.

Jack, Why dont you go back and read my post?I never said I could not
imagine ANY senario.Are you just trying to be stupid?Doug wrote that a
sailplane could impose a 6000 LB. load on the tail of a towplane.Now if
you want to split hairs THAT would be a scenario I would have a hard
time imagining.Jack, with all your experience towing, have YOU ever
seen such a thing?
Jack
With Warmest Regards,
KMU

KM wrote:

Are you just trying to be stupid?

Sorry, KM, I'm just too stupid to tell when you mean what you write and
when you don't. I'll try harder, I really will.


Doug wrote that a sailplane could impose a 6000 LB. load on the tail
of a towplane....

Jack, with all your experience towing, have YOU ever
seen such a thing?

I don't know if a sailplane can impose a 6000 lb load on a tow plane.
I have just enough experience towing to know it takes only a fraction
of that loading for the tow plane to run out of pitch control, and it
didn't take much experience to find that out.


Jack

baron58y wrote:
Sorry, KM, I'm just too stupid to tell when you mean what you write and
when you don't. I'll try harder, I really will.

What?Jack, I think you and I are arguing about two different things
here.I took exeption to you questioning whether I had any
experience.Now I am gonna be honest with you, I dont have much
experience in soaring.I have about 450 hours of towing, and about 200
hours in sailplanes, half of which is in an ASW20 that I own.In
reference to Doug's post,and using my 20 as an example, I dont see how
I could pull 6Gs while on tow.Also, in reference to Dougs post, If a
tow pilot was losing pitch authority, why wouldnt he just pull the
release?I know I would, in fact, some tow pilots I know keep their free
hand on the release till they are through pattern altitude.Now Jack,
lets get back to this experience thing.I notice from your nickname that
you fly a Baron.And that you have to make sure everyone is aware of
this .Why dont you fly that Baron down to Atlanta and we can get into
the cockpit of my 737-800 and talk more about this experience thing.
Or better yet, lets just get back to the post that started this
thread.And the short answer is; Keep a week link in the cockpit of your
sailplane and use it, and two, a sailplane, given a pretty extreem (And
remote) scenario could cause damage to a tow plane.
I don't know if a sailplane can impose a 6000 lb load on a tow plane.
I have just enough experience towing to know it takes only a fraction
of that loading for the tow plane to run out of pitch control, and it
didn't take much experience to find that out.


Jack
Best Wishes, KMU

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


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.


...some tow pilots I know keep their free
hand on the release till they are through pattern altitude.

Pretty smart. So, whatever the g-load of which a sailplane might be
capable while on tow, it's more than enough to upset the tow plane.


...lets get back to this experience thing. I notice from your nickname that
you fly a Baron.

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


...we can get into the cockpit of my 737-800 and talk more
about this experience thing.

Oh, excellent! A "mine is bigger than yours" retort. We rarely get those
on r.a.s. -- unlike most of the rest of USENET. When we do it's usually
just some lurking wannabee.

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.


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:

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


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?



Jack

On Sun, 17 Sep 2006 00:54:22 UTC, "KM" wrote:

: Where do you come up with this?I have NEVER flown an airplane that
: could not be pulled out of a dive

Started vertically downwards at an altutude of 300 feet?

Ian
--

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.

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