KGARS - Kiting Glider Automatic Relase System

On Wednesday, May 20, 2020 at 7:30:05 AM UTC-4, Sci Fi wrote:

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

Tom

Tom, if you made the load cell rupture when it sensed an overload would be
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 allow.


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 for
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 to
be made for safe aero-tow.

SiFi - if you check your POH under 'Weak Link', you will find a mfg recommended weak link breaking strength for winch launch AND one for aero-tow. They may be different, depending on the glider.
We must get away from this 80%-200% 'rule'! There is an FAR out there that states that if the glider has an approved POH at the time of certification, whatever is in this book takes precedence over the FARs.
So if we were to start using stronger (i.e. thicker) ropes like our European brethren do, instead of one from the aviation department of your local hardware store AND outfit it with the proper weak link, we may actually kill tow birds with one stone: reduce or eliminate rope breaks due to wear and tear and increase safety by having the proper weak link installed, which may actually break in a kiting event.

Uli
'AS'

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

I've had two kiting incidents as an aerotow pilot, both above 2000agl, when the glider didn't release as desired. In both cases it felt like a mild aerobatic maneuver: a smooth, uncontrollable pitch down, not alarming to me due to the safe altitude. My attention was so focussed on the pitch-down that I didn't notice any airspeed decrease, although it probably occurred. At that altitude, I was more curious than worried, and decided to delay releasing, and waited to see what happened. The glider released quickly, and I then recovered easily, although I didn't think to note the altitude loss.

From those experiences, I wonder if it might be a good thing for all aerotow pilots to experience this intentionally, at altitude, as part of training, akin to the 200ft 'ptt' practice we do. The practice could be pre-agreed, and take place after the tow pilot did a clearing turn and said (by radio) he was ready. The glider would start an abrupt climb, and the proper procedure would be for the towpilot to release or cut the rope as soon as full back elevator was reached and the pitch down had not stopped. The rope would however be lost, or dropped somewhere by the glider.

It would be best if the towpilot was comfortable with mild aerobatics before trying this. Chris Rollings did not mention any sense of danger in his kiting tests. Such intentional kiting exercises might build up a practical body of knowledge about best recovery methods, and height loss, and would increase the awareness of tow and glider pilots to the danger of low-altitude kiting.

On Thursday, May 21, 2020 at 4:53:54 AM UTC-7, wrote:
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

Doing proper state estimation would prevent any unexpected KGARS activation.. For example, your ground roll situation the towplane is obviously not flying (inadequate airspeed, still at ground elevation and no increase in pitch angle). I assume that you mean a slack line will break the weak link, which isn't an inadvertent activation of KGARS. This IS a solvable problem.

Tom

FAA and other CAAs seem to have no interest in the rope. So a KGARS could be installed between the towplane weak link and the main rope.

You would need angle sensors (as available for digital levels) on the towplane elevator and stabiliser to send a flash to the KGARS unit when release parameters are satisfied.

That would be when the stabiliser is pointing downhill while the elevator is at a steeper angle.

There are remote photoflash units that trigger on a flash from a master unit.

So you need a KGARS that on receiving the flash would let go as long as it's not from a Whelen strobe.

Alternatively Bluetooth can transmit over that distance. A stick switch could also signal via Bluetooth, but might need the transmitter at the tail.