Once I started thinking about kiting accidents as a total energy transfer that robbed the towplane of airspeed, it definitely changed how I felt about the opportunity for release and recovery. Below a few hundred feed, it may not matter if the release happens after you have lost flying speed.

I've often felt that the 80%-200% rule for weak links was faulty. At least for protecting the tow pilot.

At least one element of a kiting accident is the transfer of energy from the towplane to the glider. In the simplest form, we want to restrict the gliders ability to strip the towplane of flying energy. This should be the job of the weak link. F=MA provides some guidance here. We have a pretty light towplane with a Cessna 150/150. Probably in the 700kg range with fuel and a pilot.

If a kiting accident decelerates the towplane 6kts every second. That's maybe 2-3 seconds from the start of the incident to complete loss of flying airspeed.

Go metric so the units are easier and round a bit. 3m/s^2 of acceleration. F=MA. F=700kg*3m/s^2 = 2100N.

That's about 214kg of force. ~471pounds. That's not far off of 80% of gross of a 1-26. But way less than 80% of say a 2-33.

But the issue is that weak links seem to be built around protecting the glider, when maybe they should be around protecting the tow pilot. Regardless of whether the tow pilot is hauling a light 1-26 aloft or a ballasted open class glider, it's the force required to stop the towplane from flying that is what worries me.

Turbulence and slack line can certainly impart higher forces on a line than is required for steady state towing and climbing.

Has anyone ever explored the idea of a short bungee as a shock absorber? Something to slow the shock load, but allow the weak link to be more appropriately sized to the towplane?

I have thankfully never had anyone kite behind me in the towplane. I have been yanked all over the place by students in a 2-33 though. Low and inside on a turn and they decide that's the time to correct. Woof, there goes 5kts of airspeed in a heartbeat when my pitch attitude hasn't changed.

I've also had people get very high on tow because of a thermal or they went wide on a turn and that extra speed resulted in extra altitude. When it happens slowly and the energy isn't really coming from me in the towplane, it's not that noticeable. Even with the glider 40ft high, the pitch force if they aren't climbing is negligible.

I'm glad to see so many people pouring some thought into this problem. It's scary to have to acknowledge that my life is entirely in the hands of the person I'm towing for the first minute or so of the tow. A simple fix like shock absorbers and appropriate weak link rules sure would be nice versus automation and complexity and those failure modes that come with that.

Morgan


On Wednesday, May 20, 2020 at 8:45:06 AM UTC-7, Sci Fi wrote:
At 15:11 20 May 2020, 2G wrote:
On Wednesday, May 20, 2020 at 4:30:05 AM UTC-7, Sci Fi wrote:

It is true that increasing tow rope angle precedes a towplane upset,
it
is
=3D
not exclusive to upsets. That is why I suggested putting a load cell
on
the=3D
tow release to measure the force on the tow rope which is a better
indicat=3D
or of a kiting event.

Tom

Tom, if you made the load cell rupture when it sensed an overload would
b=
e
good... Don't we have them now.. they are called Weak-Links. All
you
need to do is correctly calculate the maximum tension that you could
allo=
w.
=20
=20
As a starting point.. A 350kg Glider with a 35:1 L/D ratio just
requires
10kg of pull to keep it flying. So factor in a safety margin to allow
fo=
r
turbulence, of say 10x, and what you need is a 150kg Weak-Link. This
is
much less than the lowest (White.) Link made so far. But as you know
those links are designed for Winch (Kiting.) Launches. Our Link needs
t=
o
be made for safe aero-tow.

I simplified the criteria in the spirit of understandability: the
decision
=
would not be based on a static load level, it would be based on the input
o=
f several sensors using a technique called state estimation. An upset
state=
or event is characterized by a rapidly increasing load on the tow rope,
a
=
rapidly increasing pitch down attitude of the towplane, and a decreasing
to=
wplane airspeed. Note: a weak link is not a sensor.

Tom

Sorry Tom, but I was brought up in the Horse-drawn era, where some very
elegant and simple solutions were found for problems. We engineered
systems in the most uncomplicated ways, because there were no 'Apps' or
'Actuators' or Hydraulics to make things complicated.
What could be simpler than a 'Weak-Link' of 150 kg breaking strain..?
Maybe the BGA needs to amend the rules to state that for aero-tows the
weak-link needs to be 1/3 the breaking strain of that used for winch
launches.
Glider owners would then need to have two lead ropes, one for winching with
a white or blue link. and the other with a green Tost link.