Does anyone use a tug tow hook that releases automatically whenglider kites?

The more I think about it I think we are going down the wrong path. Move the tow-hook to the CG of the towplane and the problem pretty much goes away.

Back to some sort of bridle from the wingtips so any pulling vector is through the CG (or close).

Or an "outrigger" on each side of fuselage sticking out far enough for the bridle to clear the tail, again installed close to the CG. That might be an easier structural solution, but the geometry would make the outriggers pretty long!

Kirk
66

Just looking to properly understand and maybe differently explain the dynamics of the situation, Chris. What you described before was:

"Third test: Terrier Tow-Plane, K 8b on C of G hook. I pitched the glider about 25 degrees nose up. The glider continued to pitch up fairly rapidly (as at the start of a winch launch) and substantial forward movement of the stick only slightly slowed the rate of pitch. The glider achieved about 45 degrees nose up, speed increased rapidly from 55 knots to about 75 knots and the glider was pulled back towards level flight (again as at the top of a winch launch). I released at that point. The entire sequence of events occupied a VERY short period of time (subsequently measured as 2 - 3 seconds). The Tow Pilot reported a marked deceleration and start of pitching down which he attempted to contain by moving the stick back; this was followed immediately by a very rapid pitch down accompanied by significant negative "G". The tow-plane finished up about 70 degrees nose down and took about 400 feet to recover to level flight. We both found the experience alarming, even undertaken deliberately at 4000 feet. Our conclusion was that the combination of the initial pitch down and the upward deflection of the elevator caused the horizontal stabilizer/elevator combination to stall and the abrupt removal of the down-force it provided caused the subsequent very rapid pitch-down and negative "G"."

Not to be to "chicken or egg" here, but, I don't think the horizontal tail of the towplane stalled. I think the glider provided an upward force, creating a nose down pitching moment far greater than the elevator could counter (obviously, right?). Once the glider has pitched the plane sufficiently nose down, the wing of the towplane is actually pushing DOWN (even though the pilot is pulling the nose up), hench the negative g felt in the towplane. The glider wing on its long moment arm can produce a far greater pitching moment than the horizontal tail on its short moment arm. The towplane transitions from steady, upright, one g flight, to negative g doward arcing flight because the glider changes the angle of attack of the wing of the towplane from positive to negative.

Your pictures taken later show this is likely. The glider has a large pitch change before it starts its vertical displacement. Similarly, the towplane will get a large pitch change nose down before it starts to deviate from its climbing or level flight path. And it will happen so rapidly, the pilot will not be able to tell if it happened because he pulled back and the tail stalled, or because the glider created such a large nose down pitching moment. I strongly suspect the results would be the same if the towpilot did nothing to try and keep the nose up in the CG hook kiting event. Glider pitches towplane, towplane responds due to lift vector changing magnitude initially, then direction.

We know that this event does not happen often, but when it does, it often has catastrophic results. I don't think there is a practical way to attach the rope to the CG of the towplane. So, we are left with training (which we have seen does not eliminate the problem) or some sort of automatic system (since it has been documented that it is unlikely a pilot can react fast enough to be able to save his or her own life if it goes really bad).

Time to start working on a secondary pull system (sorry, Kirk. No explosive bolts or missles. Would be more fun, though.) using attitude and elevator positon. Or maybe attitude and pitch rate. Need to keep it as simple (fewest inputs and software) as possible. I do like the idea of initial tests of set values to turn on a light in the cockpit for the pilot to see that the automatic system will have operated. Got any reasons not to start at 10 degrees nose down pitch attitude and half way to max nose up elevator travel?

Steve Leonard

On Thursday, February 20, 2014 1:21:58 PM UTC-6, Steve Leonard wrote:

Time to start working on a secondary pull system (sorry, Kirk. No explosive bolts or missles. Would be more fun, though.) using attitude and elevator positon. Or maybe attitude and pitch rate. Need to keep it as simple (fewest inputs and software) as possible. I do like the idea of initial tests of set values to turn on a light in the cockpit for the pilot to see that the automatic system will have operated. Got any reasons not to start at 10 degrees nose down pitch attitude and half way to max nose up elevator travel?

No pyrotechnics? How are we going to grow the sport without pyrotechnics (and a TV reality show...).

How about:

1. Tow gliders vertically with helicopters (It's been done..)
2. Have the towplane push the glider with a reverse towbar setup.
3. Mount the glider on top of the towplane - space-shuttle/747 style.
4. Two words: Electromagnetic catapults.
5. Automatic BRS on towplane triggered by sudden pitch-down.
6. Custom design towplane made from 2 Pawnee fuselages, F-82 style; would make it easy to mount tow hook near CG on center wing.
7. "Tail-gunner" facing backwards in towplane with both hands on tow release. Better not **** that guy off before you launch!
8. Use F8F Bearcats for towplanes - they would climb so steeply that if the glider kited, it would only level off the towplane (and then the glider's wings would come off as you accelerated through glider's Vne - Better release quick!).
9. Use webcam on tail of towplane to track glider and release him when he gets out of position (displacement, rate, or combination of both). As added incentive, use bluetooth to set off smoke bomb in glider cockpit if release is triggered.
10. Take all glider pilots up in a two-seat towplane (with a briefed student and instructor in the glider) and at a safe altitude demonstrate what an upset looks and feels like from both ends of the string.

I like #8, myself, but I think #10 would work pretty darn good.

Kirk
66
Tuggie

On Thu, 20 Feb 2014 09:55:38 -0800, kirk.stant wrote:

The more I think about it I think we are going down the wrong path.
Move the tow-hook to the CG of the towplane and the problem pretty much
goes away.

Back to some sort of bridle from the wingtips so any pulling vector is
through the CG (or close).

I'm sure that would work well in the air, but isn't there a potential
problem during the landing roll-out? With no tension on the tow rope it
seems to me that you've got a U-shaped loop of rope dragging on the
ground behind the tow plane which would provide considerable drag on a
grass field if it isn't recently mown. Would this also be a problem at
the start of the take-off roll?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

At 14:36 20 February 2014, Chris Rollings wrote:

To repeat one point, our tests left us convinced that the tail-plane on
the
tow-plane stalled quite early on in the kiting event, it would need some
tests to check exactly how early. It might be that the nose down angle
would have to be quite small, something that could happen in turbulence
perhaps.

Which is why I suggested speed camera technology to detect a faster than
normal rapid climb of the glider in relation to the tug. Measuring the
height or even the angle is just not going to give the system time to work.
Detecting the rapid vertical acceleration early will do that. Even if the
pilot in the glider detects the early acceleration, inertia probably means
that he will be unable to correct in time.

Chris Rollings wrote:

At 07:23 19 February 2014, Surge wrote:
On Tuesday, 18 February 2014 12:07:20 UTC+2, Chris Rollings wrote:
That might work but I suspect it would be impossible, in the real
World,
to
get a sufficiently high degree of reliability that we did not get more
accidents from innocent gliders be jettisoned at low altitudes than we
currently get from kiting accidents.

Hi Chris

This is not a personal attack - I appreciate your valuable input but ...

With modern technology we can reliably detect pitch, roll, acceleration
of
a tug plane IMO.
e.g. My Note 2 has a magnetometer, electronic gyros, accelerometers,
barometer, and GPS. I downloaded an artificial horizon app for my Android
phone and when it blends all the sensor data you get a very reliable
pitch
and roll output.
It doesn't even experience gyro drift because it is able to compensate
using the magnetometer to sense gravity.

Another example are all the Ardupilot projects.
If they are capable of flying DIY autonomous drones around for under $300
then I think the technology is available and good enough for us to use.

Even so, I'd rather have a 1 in 1000 chance of accidentally being dumped
than kill a tug pilot and myself through a momentary lapse of
judgement/attention.

I happen to fly at the club in South Africa which had the low altitude,
tug
upset on Sunday 16th Feb 2014, resulting in the death of the tug pilot in
a
Cessna 182A.
I simply refuse to believe that we can fly to the moon and back but can't
devise a reliable automatic tow release mechanism.
How many more tug pilots around the world do we have to kill before we
devise a mechanism which works?

To detect a tug upset reliably I think one would only need:
1. To sense the tug elevator at the back stop +
2. The pitch of the tug (say 20 degrees down and increasing) +
3. The load on the rope (glider still attached).
I can't imagine the above being present in any normal operational
situation.

Paul


Paul, by the time your items 1, 2 & 3 are present it is probably too late,
the tail-plane is almost certainly stalled and even if the glider were
jettisoned at that instant, the tow-plane would probably still pitch down
another 40 or 50 degrees before recovery (I say probably, could someone
please test it with a camera plane alongside at safe altitude, done
deliberately it should be possible to pull the release at exactly the right
moment - might take three or four goes to get on exactly right).

The thing that is constantly under-estimated by those that haven't
experienced it is how quickly it all happens. LESS THAN 3 SECONDS FROM ALL
NORMAL TO ALL OVER. People take at least that long to react to something
unexpected unless it is something they have practiced frequently, certainly
not the case in kiting incidents.

This is correct, except that the duration can be sub one second. Your
only possible reaction is to shut the throttle. I am sure you can use
today's technology to detect the effect, but certainly not the cause.

Brian wrote:

I think the Gyro, and full back stick switch has some potential but has some issues, such as calibrating the Gyro, and possible Test/fault issues that would need to be overcome.

Another one to consider would be a time delay switch on the Throttle. Once the throttle has been set to climb power for more than say 5 seconds (time might need to be adjusted) then if the stick is moved to full aft, a release is initiated.
The throttle delay is to prevent releases during the 1st part of the ground roll. I can't think of any other time when the tow plane would have full back stick and full throttle that the pilot would not want the glider to be released. One advantage of this system is that the tow pilot should know this combination will cause a release and bring a higher awareness to the possibility of the glider kiting.

Another issue to be addressed by either of these systems is that it will require a powerful actuator to ensure a release actually occurs when needed.

Just brain storming some idea's

Brian

Full back stick is not a parameter. If it occurs at all it is after
the event. On the other hand, full back stick can be reached in non
life-threatening situations.

Steve Leonard wrote:

Just looking to properly understand and maybe differently explain the dynamics of the situation, Chris. What you described before was:

"Third test: Terrier Tow-Plane, K 8b on C of G hook. I pitched the glider about 25 degrees nose up. The glider continued to pitch up fairly rapidly (as at the start of a winch launch) and substantial forward movement of the stick only slightly slowed the rate of pitch. The glider achieved about 45 degrees nose up, speed increased rapidly from 55 knots to about 75 knots and the glider was pulled back towards level flight (again as at the top of a winch launch). I released at that point. The entire sequence of events occupied a VERY short period of time (subsequently measured as 2 - 3 seconds). The Tow Pilot reported a marked deceleration and start of pitching down which he attempted to contain by moving the stick back; this was followed immediately by a very rapid pitch down accompanied by significant negative "G". The tow-plane finished up about 70 degrees nose down and took about 400 feet to recover to level flight. We both found the experience alarming, even undertaken
deliberately
at 4000 feet. Our conclusion was that the combination of the initial pitch down and the upward deflection of the elevator caused the horizontal stabilizer/elevator combination to stall and the abrupt removal of the down-force it provided caused the subsequent very rapid pitch-down and negative "G"."

Not to be to "chicken or egg" here, but, I don't think the horizontal tail of the towplane stalled. I think the glider provided an upward force, creating a nose down pitching moment far greater than the elevator could counter (obviously, right?). Once the glider has pitched the plane sufficiently nose down, the wing of the towplane is actually pushing DOWN (even though the pilot is pulling the nose up), hench the negative g felt in the towplane. The glider wing on its long moment arm can produce a far greater pitching moment than the horizontal tail on its short moment arm. The towplane transitions from steady, upright, one g flight, to negative g doward arcing flight because the glider changes the angle of attack of the wing of the towplane from positive to negative.

Your pictures taken later show this is likely. The glider has a large pitch change before it starts its vertical displacement. Similarly, the towplane will get a large pitch change nose down before it starts to deviate from its climbing or level flight path. And it will happen so rapidly, the pilot will not be able to tell if it happened because he pulled back and the tail stalled, or because the glider created such a large nose down pitching moment. I strongly suspect the results would be the same if the towpilot did nothing to try and keep the nose up in the CG hook kiting event. Glider pitches towplane, towplane responds due to lift vector changing magnitude initially, then direction.

We know that this event does not happen often, but when it does, it often has catastrophic results. I don't think there is a practical way to attach the rope to the CG of the towplane. So, we are left with training (which we have seen does not eliminate the problem) or some sort of automatic system (since it has been documented that it is unlikely a pilot can react fast enough to be able to save his or her own life if it goes really bad).

Time to start working on a secondary pull system (sorry, Kirk. No explosive bolts or missles. Would be more fun, though.) using attitude and elevator positon. Or maybe attitude and pitch rate. Need to keep it as simple (fewest inputs and software) as possible. I do like the idea of initial tests of set values to turn on a light in the cockpit for the pilot to see that the automatic system will have operated. Got any reasons not to start at 10 degrees nose down pitch attitude and half way to max nose up elevator travel?

Steve Leonard

Most people posting here do not understand the word "instantaneous".
Chris Rollings, and the CofG hook exponents, are the only people who
get it.

Here's another idea that probably has something wrong with it.
Attach the tow rope to c/g, maybe the turtle deck and have a fair
lead at the tail which releases at the tail only under some set of unfavorable circumstances.

The glider can zoom without upsetting the tow plane nor will the glider be underemoniously dropped in turbulence.

Not sure about fouling the tail feathers with the tow rope.

On Thursday, February 20, 2014 9:36:03 AM UTC-5, Chris Rollings wrote:
To repeat one point, our tests left us convinced that the tail-plane on the
tow-plane stalled quite early on in the kiting event, it would need some
tests to check exactly how early. It might be that the nose down angle
would have to be quite small, something that could happen in turbulence
perhaps.

1)It might be possible to recognize a pattern of stick movements and vertical acceleration (and possibly AOA and position relative to the tug) in the GLIDER that would provide a high confidence signature of an imminent kiting event prior to upsetting the tug. The fatal control input is made by the pilot a second or two before the glider kites. Perhaps stage one would trigger an alarm and stage two would trigger a release.

Discovery of the 'kiting signature' would be based on analysis of detailed logs of normal tows, including tows in severe turbulence. Logs would include the parameters noted above and perhaps a video of the tug from the glider. The release device might have three modes 1)normal turbulence 2)severe turbulence mode with higher threshold 3)alarm only mode with simulated release tone. AGL and distance to landable area might lower the release threshold.

Mode 3 would allow me to confirm calibration of the device to my particulars and to gradually increase glider pilot confidence in the device. If it were based on sound science and engineering, I would install a device like this on my glider to reduce my chances of killing a tow pilot even if it somewhat increased my risk of an unwarranted release. Kiting is rare, but I would rather die than possibly kill a tow pilot (especially a young one). I would take my chances on a sound device.

I know that this approach sounds sophisticated, but we live in a time of rapidly advancing technology and dropping costs. There are some good starting points that might only require software extensions once the 'kiting signature' is known, see http://www.diydrones.com/notes/ArduPilot

2)The clubs that I know find it nearly impossible to build a consensus about the most simple and inexpensive things, let alone an automatically activated tug hook. Spend money on a 1:1000000 chance?? There are also good arguments against an auto-release tug hook that might lead to the death of a glider pilot in a hard to justify/defend scenario. Lawyers would have a field day in the USA. Deployment would be spotty and slow. Lots of 'wait and see' decisions. On the hand, I could put a device on my glider and the question would be settled for me behind 100% of tugs.

3)What happens in rotor? Is it possible to stall the horizontal stabilizer of the tow plane? Perhaps it would be good to automatically released in severe rotor, especially if my AGL and distance to a landable spot were within predefined limits. Someday I think I might stay on tow in rotor longer than I should. A mode 3 alarm might provide timely and objective advice to the pilot in rotor. The device could confidently detect inverted flight on tow and auto-release immediately.

4)If you had this kind of sensor and computation power in a glider, it could detect and warn of things like 'imminent spin on turn to final' and other mistakes. Once you had the hardware platform installed a lot of useful safety features could be added.