CG hook on aero tows??

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.