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Peter Birren
PITCH & LOCKOUT LIMITER
The lockout starts with a bit of roll away from the tow direction. This rolling ultimately makes the glider want to behave like a tail-less kite and turn 'round the line. A short way into the turn there is a high degree of pitch-up attitude relative to the towline, so having a release at the point of too much pitch could automatically release the pilot and hopefully provide the pilot with sufficient recovery time.
How it works:
With the release at the apex release site, a second release line will be attached to the glider's nose. As the glider pitches up relative to the towline, the release gets farther from the nose and tightens the line.
Peter Birren - 2009/05/09
If you want a truly foolproof release, it's got to be one that eliminates the pilot from the equation with a release that operates automatically. Certain tradeoffs would have to be made, like limiting the top altitude of a surface-based tow or having it release within certain limits of pitch, but if you want to legislate safety.... See:
http://www.birrendesign.com/LKOpinions.html
for more details. I am not trying to push the automatic release, definitely not the way you're pushing your setup, but it works.
And here:
http://www.youtube.com/watch?v=IjncKQ02FJ8
we see an actual pitch and lockout limiter in action with an actual pilot in an actual tow situation.
And when there's a high degree of pitch-up attitude relative to the towline it does INDEED automatically release the pilot.
And the pilot IS - along with his driver - TOTALLY eliminated from the equation.
And there IS a tradeoff in that the top altitude of the surface-based tow IS (severely) limited.
But the problem is that when it automatically releases the pilot at the point of too much pitch it DOESN'T provide him with the hoped for sufficient recovery time. In fact, it does the PRECISE OPPOSITE.
Manned Kiting
The Basic Handbook of Tow Launched Hang Gliding
Daniel F. Poynter
1974
"The greatest dangers are a rope break or a premature release." - Richard Johnson
Can you possibly see how Pitch and Lockout Limiter might instantly convert an easily manageable situation into a fatality?
Donnell Hewett - 1981/04
WEAK LINK
Every tension limiting device discussed up to now consists of mechanical components, has a limited range, or relies upon human operation. Every one of these tension limiting devices is subject to failure. Please correct me if I am wrong, but it is also my understanding that there are a large number of tow pilots today who are depending upon smooth air, rope stretch, boat speed, mechanical devices, and ground crews to provide the tension limitation control for their flights. Well, in the author's opinion that is just not good enough. Skyting requires the use of an infallible weak link to place an absolute upper limit to the towline tension in the unlikely event that everything else fails.
Now I've heard the argument that "Weak links always break at the worst possible time, when the glider is climbing hard in a near stall situation," and that "More people have been injured because of a weak link than saved by one." Well, I for one have been saved by a weak link and would not even consider towing without one. I want to know without a doubt (1) when I am pushing too hard, and (2) what will break when I push too hard, and (3) that no other damage need result because I push too hard.
Furthermore, I will not use a mechanical weak link no matter how elaborate or expensive because there is always the possibility that it may fail to operate properly. In skyting we use a simple and inexpensive strand of nylon fishing line which breaks at the desired tension limit. There is no possible way for it to jam and fail to release when the maximum tension is exceeded. Sure, it may get weaker through aging or wear and break too soon, but it cannot get stronger and fail to break. If it does break too soon, so what? We simply replace it with a fresh one.
A properly designed weak link must be strong enough to permit a good rate of climb without breaking, and it must be weak enough to break before the glider gets out of control, stalls, or collapses. Since our glider flies level with a 50 pound pull, climbs at about 500 fpm with a 130 pound pull, and retains sufficient control to prevent stalling if a weak link breaks at 200 pounds pull, we selected that value. Of course, a pilot could deliberately produce a stalled break at 200 lbs, just as he can stall a glider in free flight. But if he is trying to limit his climb rate and the forces exceed the break limit, the glider simply drops its nose to the free flight attitude and continues flying. If the weak link breaks (or should the towline break) at less than the 200 pound value, the effect is even less dramatic and controlled flight is still present.
Peter Birren
These are Donnell Hewett's original 12 elements of a good tow system. They are as viable today as they were in the early 80's when he wrote them.
Towing Aloft - 1998/01
A weak link is the focal point of a safe towing system.
Speed controlled towing is when the speed of the device doing the towing is maintained at a reasonably constant value. Controls, such as throttle, are used to keep the speed of the tow vehicle or tow winch operating at a constant speed. Towline tension can vary dramatically in response to thermals, sink, pilot corrections, etc. Aerotowing is clearly in this category as the tug needs to maintain a minimum speed to prevent stalling. Many of the early towing efforts of the '70s where the vehicle drove at a fixed speed would also fall into this category. Weak links very clearly will provide protection from excessive angles of attack, high bank turns and the like for this form of towing.
Any chance that those pioneer guys mostly knew what they were talking about and Donnell and the a**holes who wrote Towing Aloft mostly didn't/don't have a freakin' clue?
