KGARS - Kiting Glider Automatic Relase System

On Wednesday, May 20, 2020 at 9:45:59 AM UTC-7, Morgan wrote:
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.

I am a firm believer of the KISS principal: Keep It Simple, Stupid. If a bungie shock absorber and a weaker weak link worked, fine. But it seems to me that we have had 80 years to perfect the weak link and it just hasn't happened. Since then we have had a progression in computer technology from zero to self-driving vehicles. Thus it seems natural to me to apply this technology to this problem.

Tom

I am a firm believer of the KISS principal: Keep It Simple, Stupid. If a bungie shock absorber and a weaker weak link worked, fine. But it seems to me that we have had 80 years to perfect the weak link and it just hasn't happened. Since then we have had a progression in computer technology from zero to self-driving vehicles. Thus it seems natural to me to apply this technology to this problem.

Tom

Bungie isn't really much a of a shock absorber, the exponential nature of the spring force isn't the best here. Combine that (or a spring) with an actual shock absorber and autorelease when the shock is compressed enough, you can set the limit of stolen energy via the tow-rope to pretty much anything within a certain time.

As a software engineer who's done plenty of embedded stuff also, I would much prefer an analog solution.

I'm neither a software nor mechanical engineer, but I think the relationship between force and deflection might be linear, not exponential


On Wednesday, 20 May 2020 19:49:22 UTC+1, wrote:
Bungie isn't really much a of a shock absorber, the exponential nature of the spring force isn't the best here. Combine that (or a spring) with an actual shock absorber and autorelease when the shock is compressed enough, you can set the limit of stolen energy via the tow-rope to pretty much anything within a certain time.

As a software engineer who's done plenty of embedded stuff also, I would much prefer an analog solution.

On Wednesday, May 20, 2020 at 2:57:36 PM UTC-7, wrote:
I'm neither a software nor mechanical engineer, but I think the relationship between force and deflection might be linear, not exponential


On Wednesday, 20 May 2020 19:49:22 UTC+1, wrote:
Bungie isn't really much a of a shock absorber, the exponential nature of the spring force isn't the best here. Combine that (or a spring) with an actual shock absorber and autorelease when the shock is compressed enough, you can set the limit of stolen energy via the tow-rope to pretty much anything within a certain time.

As a software engineer who's done plenty of embedded stuff also, I would much prefer an analog solution.

I know you can upset a towplane w/o breaking the rope or weak link because it has happened. The towplane's elevator may be able to generate 10-15% of its all-up weight, or 250-300 lb, and it is probably already using half of that in normal flight. And even if the rope or weak link breaks that does not guarantee that the towplane can recover. The rope did break in the 1999 towplane upset accident at Ephrata, WA.

Tom

On Wednesday, May 20, 2020 at 5:57:36 PM UTC-4, wrote:
I'm neither a software nor mechanical engineer, but I think the relationship between force and deflection might be linear, not exponential

Yeah, was thinking energy, ie. stored energy that's going to come back also.
With a linear-ish shock absorber, the energy dissipated is linear.

With a bungee/spring alone relying on a weak(er) link, not much happens until you get very close to the breaking condition. With linear energy dissipation you can tune the release condition much better, and you won't have the springback effect.

On Wednesday, May 20, 2020 at 5:42:13 PM UTC-7, wrote:
On Wednesday, May 20, 2020 at 5:57:36 PM UTC-4, wrote:
I'm neither a software nor mechanical engineer, but I think the relationship between force and deflection might be linear, not exponential

Yeah, was thinking energy, ie. stored energy that's going to come back also.
With a linear-ish shock absorber, the energy dissipated is linear.

With a bungee/spring alone relying on a weak(er) link, not much happens until you get very close to the breaking condition. With linear energy dissipation you can tune the release condition much better, and you won't have the springback effect.

Show me the data that any such tuning is even possible. I have just described how it isn't.

Tom

On Wednesday, May 20, 2020 at 11:09:55 PM UTC-4, 2G wrote:
On Wednesday, May 20, 2020 at 5:42:13 PM UTC-7, wrote:
On Wednesday, May 20, 2020 at 5:57:36 PM UTC-4, wrote:
I'm neither a software nor mechanical engineer, but I think the relationship between force and deflection might be linear, not exponential

Yeah, was thinking energy, ie. stored energy that's going to come back also.
With a linear-ish shock absorber, the energy dissipated is linear.

With a bungee/spring alone relying on a weak(er) link, not much happens until you get very close to the breaking condition. With linear energy dissipation you can tune the release condition much better, and you won't have the springback effect.

Show me the data that any such tuning is even possible. I have just described how it isn't.

Tom

The only comment I've seen from you wrt shock absorbing was about bungees. I should make it clear that that's not what I'm talking about when referring to shock absorbers. I'm talking about the shock absorbers you use in car/motorcycle suspensions. The racing stuff is fully tunable via preload setting, compression resistance and rebound.

For towing gliders, the preload should be set to somewhere near expected 'normal' tow load on the rope. Compression set so that by the time the absorber has travelled the full length (and releasing) the energy dissipated is less than would cause the tow-plan to stall. Rebound setting should be relative to the energy gain for the ow-plane in 'normal' tow. With such settings, it should be impossible to pull on the rope enough to cause the tow-plane to stall.

This doesn't solve the pitch issue, but I'd rather be nose down not stalled than stalled.

Unfortunately, I think we can discount any load sensing concepts (weak links, load cells, etc) for the following reason. Kiting events are rare and so for any system to be widely adopted, the frequency of any nuisance releases of any system need to be similarly rare or hopefully less rare. Otherwise the system will be a PIA and will be removed because of the nuisances.

Two situations that come to mind that would/could occur that would lead nuisance releases are the following

1. This spring the ground was soft and it took 100% power from the Pawnee to get a 2 seat glider to start to move.

2. Slack line situations, either real or simulated for training. The impact load when the slack comes out will likely overpower any weak link and then we've created a PTT.

As such, I suspect for a KGARS system to be workable, it needs to go back to a parallel operation of the mechanical tow release in the cockpit, but only when very specific upset conditions are detected. (airspeed, pitch angle, elevator position, etc.)

KISS is nice, but if weak links worked reliably for upsets, we'd probably be doing it already.

With all due respect.

Mark