Are Weak Links really Necessary for Aero Tow?

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

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

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.

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