CG hook on aero tows??

C of G hooks do not create a problem on tow - for anyone
who does not find himself with a significant degree
of pitch up (sorry can't give a figure in degrees,
it will vary according to type) relative to the tow-plane's
flight path.

I agree that competent pilots (whatever that means)
are less likely to find themselves in that position.
However, given that it takes less than 0.5 seconds
for the necessary amount of pitch up to occur, and
that pitch up can be caused by either a gust, an inadvertant
elevator input by the glider pilot, or an unexpected
attitude change by the tow-plane, I don't think anyone
is 'competent' enough to guarantee that it can never
happen to them.

Changing the subject just slightly; the use of 'low'
high tow, refered to several times in this thread,
does not really help because:

1. The 'few feet lower' position adds less than half
a second to the time the 'upset' takes (given a 'normal'
lenght rope of just under 200 feet); usually not enough
to make any difference to the outcome.

2. The slight upward angle of the force on the rope
increases the instability of the situation and means
that a slightly smaller pitch angle is needed to set
the whole thing off.

3. The close proximity to the tow-plane's prop wash,
and occasional indvertant excursions into it, may well
provide the trigger mechanism that causes the unwanted
pitch movement.

I will post no more on this subject until and unless
someone actually comes up with some new flight test
reports.

However, I am prepared to offer my services as an expert
witness, to the estate of any tow-pilot who is killed
in a 'tug upset' accident whilst towing a glider on
a C og G hook.

Chris Rollings

At 03:18 09 January 2004, E. A. Grens wrote:
CG hooks on tow do not create a problem for competent
pilots. It is only
necessary to fly the towed glider, as in close formation
flying, without
depending on the tow hook to do a lot of the work for
you. Of course, very
short tow ropes can make any tow difficult. Both my
own glider, a Phoebus
A-1, and the DG-101 I frequently fly in my club, have
only CG hooks. With
reasonable tow-rope length I have never found CG hooks
to present any
problems. But, do keep the trim full down on take-off.

Ed

I think it is interesting to look at what types of glider have been involved in
aerotow upsets in the past.

Most of the incidents/accidents cited here have been in older generation
gliders with low wingloadings and stalling speeds. They have a natural tendency
to want to rear up behind the tug and the aerotow is generally flown with quite
a large amount of forward stick, possibly outside the trim range.

Compare this with the later high wingloading glass gliders, especially when
flown fully ballasted. Unless the tug accelerates to 70Kts or so you are left
dangling below, trying to get out of the slipstream.

I find the most worrying thing about towing from a belly/winch hook is the
possibility of a back release at an awkward height. When I do a long tow I
disable the back release.

I find towing off the rear hook more comfortable in some gliders, as the nose
doesn't get dragged into the turn so much. I have a '27 and the nose hook is a
bit of an abortion so I don't use it unless I have to (slope, crosswinds etc.)

As a tug pilot I would be quite happy with someone in a modern glider behind
me, towing on the belly hook, as long as they were trained to aerotow properly
and paid attention to what they were doing. If in doubt, pull the release!

Anyone who has aerotowed in gliders from the 60's or before, using modern tugs
and their higher speeds, will know how 'on the limit' the whole thing feels and
what a relief it is to come off tow. Compare that to modern heavy machines
where you have a large margin of control.

To summarise my opinion: In modern gliders the hook position is almost
irrelevant when discussing the possibility of an aerotow upset - the main
factor is the competence of the guy on the back and how much concentration he
has on the task in hand.

Edward Downham wrote:
I find the most worrying thing about towing from a belly/winch hook is the
possibility of a back release at an awkward height. When I do a long tow I
disable the back release.

As one who has had a back release 50 miles from home, over the desolate
terrain of central Nevada, just after sunset, I can relate to that!

Marc

Some years ago I had a back release on the belly hook of my Pik-20D while
being towed in Gap Tallard France. I was still pretty low when it happened
over unlandable terrain in mountanous area (near the Petite Céuse). Luckily
enough I could just make it back to the airfield.
Since then I use a small piece of spongie rubber between the outer ring of
the Tost hook and the fuselage at the back side of the ring. In this way the
outer ring can still move backwards when necessary, but not very easily
anymore. I have had no back release problems since then. I use it also on my
later gliders.

Karel, NL

"Marc Ramsey" schreef in bericht
m...
Edward Downham wrote:
I find the most worrying thing about towing from a belly/winch hook is
the
possibility of a back release at an awkward height. When I do a long tow
I
disable the back release.

As one who has had a back release 50 miles from home, over the desolate
terrain of central Nevada, just after sunset, I can relate to that!

Marc

I must say that I am a bit confused by all this discussion.
When I was trained back in the 70's, the rules were simple, nose tow for aerotow, belly tow for
winch, NO exceptions.
The reason for nose tow on aerotow was to keep the "line of forces" in as near as possible to a
straight line, ie, from the tug, the pull force is on the line, the force is applied to the glider
at the nose with the "projected line of force" being pretty much through the glider cg, and from a
yaw point of view, there was a continuous restoring couple since even if you got out of line a bit,
the resulting "side force" was applied far forward of the cg, and it was in the direction of the
required correction, ie, it was a "self restoring" force.
If on the other hand, you towed with the belly cg hook, the "line of force" was way below the cg,
and, worse still, it is virtually directly below the cg, with virtually no yaw restoring force
available, and under some conditions, can produce pitch control difficulties with hook way below cg,
which may be well below the aerodynamic centre, particularly on older high wing gliders with fairly
"deep" fuselages (ie, upright seating) compared to modern high performance gliders, which tend to
have mid or shoulder wings and reclining seating with shallow fuselage depth. In other words, if
you take a "side view" of the three positions, ie, hook, cg and aerodynamic centre, on the older
gliders, the distance hook to cg and the distance cg to aerodynamic centre is substantial, setting
up the possibility of significant "divergent pitch couples" under some conditions, often requiring
substantial forward stick on tow to control. These two distances are much reduced on modern
gliders, hence the possible "pitch couples" are reduced, hence the possible control problems are
reduced.
The second issue is "best towing speed" under "normal conditions". Older gliders tended to have
lower wing loading and lower min sink and best l/d speeds. Usually, a well designed glider has
minimum trim forces required at best l/d. The further you move from best l/d speed, faster or
slower, the more trim or stick force you need. Older gliders had fairly steep polars, modern
gliders have fairly flat polars. Thus the available speed range for towing with reasonable and safe
control characteristics is narrower for older gliders than modern ones.
Then there is the length of the tow rope to consider. A long rope reduces the workload on the
glider pilot substantially, since everything is far less "twitchy" and there is more time to correct
divergent trends as the tug goes through the edge of a thermal or turbulence or whatever. Short
ropes reduce reaction time and generate greater "relative tug / glider" station keeping
displacements, requiring faster and more positive corrections by the glider pilot to return to
position. We always used longish ropes, 150 to 200 feet. I can not understand why people want to
use short ropes. To me, anything under 100 feet is insane, just asking for trouble.
Now this bit is my own personal observation, but the best towing speed for older gliders always
seemed to me to be about 1.2 to 1.3 times best l/d speed, simply because of trim and control issues,
newer gliders say 1.2 to 1.4 best l/d.
For the sake of illustration only, lets create some numbers. Say old glider X has best l/d at
45KIAS, I found it towed best at 55 to 60 KIAS, no more. Say modern glider Y has best l/d at
55KIAS, I found it towed best faster, ie, around 65 to 75 KIAS. Note, that both the speed is
higher, and the speed range is greater, for the modern gliders.
This then puts the requirement back on the tug pilot, to tow at the appropriate speed(s) for the
glider he has on the rope at the time.
Unfortunately, I get the impression, that most tugs were/are operated to try and make the tow most
efficient for them, to reduce costs etc, which can create the situation where a powerful tug is
towing too fast for an old glider, which means that even if the tow is stable, the pilot may be
using a lot of stick just to stay there. If conditions are turbulent, he will have to be "on his
toes". If he is on a short rope as well, he will have to be "very toeie", and if he/she is a
relatively inexperienced pilot to boot, then perhaps the club secretary should start checking for
those insurance claim forms.
Under powered tugs trying to drag a fully ballasted open ship is another story, but I never actually
had that experience, so I will leave that issue alone.
In short, I think club CFI's and Tugmasters/captains need to give towing configurations a total
rethink, from first principles.

At 14:18 11 January 2004, Edward Downham wrote:
I think it is interesting to look at what types of
glider have been involved in aerotow upsets in the
past.

I seem to remember a BGA poster which highlighted the
dangers of the combination of a CG hook, high wing
glider and a pilot who had little aerotowing experience.

The recommendation to tug-pilots and instructors was
not to let any more than two of these factors meet
on the same launch!

I don't think the positition of the horizon should
make any difference. In mountains it's irrelevant
and different tugs will climb at different rates -
a powerful tug will be more pitched up and adopt a
higher position relative to the horizon than a low
power one (relative to the thrust line, the prop-wash
and the best position of the glider just above the
prop-wash).
One of the advantages of being as low as possible is
not just the extra time that it takes to get to 'upset'
position (which is small) but the fact that you are
less likely to lose sight of the tug in the first place.
Once you can't see the tug, things can go wrong very
quickly.
I've flown in Australia with their low tow and whilst
I'm sure each method has its merits, I am personally
much more comfortable with the UK position, don't like
having to transition through the propwash at low level
and find that teaching a 'correct' position that looks
almost identical to the position on the ground before
All Out is easier too.
Rob

At 16:48 09 January 2004, Eric Greenwell wrote:
Andy Durbin wrote:

As a US instructor I have flown with many pilots that
received their
initial training from other instructors. I have often
been surprised
at the tow position taken by such pilots. I usually
urge them to keep
lowering the tow position until they feel the wake,
then to move just
high enough that the wake is not a factor. That tow
position may be
10 or 15 ft lower than that initially used.

Where is the towplane, relative to the horizon?

A pilot flying a high high tow has less time to react
to a potential
upset than one flying the UK recommended low high
tow. The fact that
US pilots seem to be trained to use high high tow
may explain why tug
upsets continue to happen even when a nose or forward
hook is used.

10 or 15 feet doesn't sound like it would give much
extra time, not like
the low tow position Australia uses.

--
-----
change 'netto' to 'net' to email me directly

Eric Greenwell
Washington State
USA

Robert John wrote:

I don't think the positition of the horizon should
make any difference.

It's the position I typically use, so I'm trying to get an idea of how
different it is from what I normally do. Unfortunately, I can't go out
and experiment right now, with a foot of snow on the runway!

So, with a 180 hp Super Cub or Pawnee, say, is the tug canopy on the
horizon, the wing root, wheels, or maybe the tug is an entire "tug"
distance above the horizon?

In mountains it's irrelevant
and different tugs will climb at different rates -
a powerful tug will be more pitched up and adopt a
higher position relative to the horizon than a low
power one (relative to the thrust line, the prop-wash
and the best position of the glider just above the
prop-wash).

--
-----
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Eric Greenwell
Washington State
USA

As a US instructor I have flown with many pilots that
received their
initial training from other instructors. I have often
been surprised
at the tow position taken by such pilots. I usually
urge them to keep
lowering the tow position until they feel the wake,
then to move just
high enough that the wake is not a factor.

I also agree with Andy that the optimal tow location is just above the
wake. I like it because it's more efficient for the tow plane since less
up elevator is required to maintain proper airspeed. In the flat lands,
if the tug wings are on or above the horizon, you're too high, IMHO.

I've also been known to fly a little to the left on tow to give the tow
pilot's right leg a little rest.

Tony V.

Tony Verhulst wrote in message ...


In the flat lands,
if the tug wings are on or above the horizon, you're too high, IMHO.


I think you got something reversed. The lower the glider goes, the
higher above the horizon the tug will appear to be. When I tow behind
a 235 Pawnee all the tug is above the horizon.


Andy