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free-lance designer, in business since 1975

 

 

 

 

 

 

 

 

 

 

 

"We didn't all use them, but I was one who did. When I had a problem on tow behind a Moyes' tug, I went for it and it worked flawlessly!"

Jim Zeiset
at the Hay Tow meet, Australia

 

 

 

 

 

 

 

"NOTHING works better than the Linknife."
Jeff Sinason

Former Region 6 Director

 

 

The light weight, low cost, sure fire, place anywhere,
Tow Release for Hang Gliders and Paragliders

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PITCH & LOCKOUT
LIMITER
———————diagram of movement

This diagram shows in scale a Hewett Center-of-Mass bridle with the apex ARS (weaklink attachment point) in a pendulum swing that covers positions from launch to top of tow. The arcs show 2 different bridle lengths. Notice how as altitude increases (pitch-up to the towline), the distance from the nose to the release will also increase. The 40% mark represents a release position just prior to the upper bridle line touching the base tube, a point where (in my opinion) a newer tow pilot could easily release on his/her own and have enough altitude to make it back to the LZ. But if an inadvertent, uncorrected turn occurs, the release would activate before the bridle begins pulling the nose of the glider down.

Static Limiter

April 22, 2000 - Static line towing at Cullom IL, using 3000' of 3/16" hollow braided poly towline. I set the nose line at the 40% length as described in the copy below. My bridle is 20 feet long, divide by 10 and multiply times 4 gives a noseline length of 8 feet. As I climbed on tow I could see the noseline tightening. At about 800 feet I could have easily pushed out to release but chose to see how high I'd get. At 850 feet I had to pull in slightly to keep it from releasing, but was still climbing at minimum tension of 110 pounds. At 960 the line was sufficiently tight and I just let the control bar out. It cut cleanly and I was off tow.

April 30, 2000 - Aerotowing at Leland IL. Arranged with the tug pilot to let him know what I would be attempting. I set the glider on the dolly where the nose angle is fairly high, pulled the apex of the bridle to just above ground level, measured the distance at that angle and reduced it slightly to 6 feet long... I really had no idea how long it should be. During the tow in very turbulent conditions I never climbed very high behind the tug until about 1200 feet when I hit a boomer thermal. As I eased the bar out and began climbing above the tug the noseline got noticeably tighter (as expected) but it was too long and triggered the tug's release when I was almost vertical to the tug. Next aerotow test will be with 5 feet of noseline.

After these two tests it's plain to me that the Pitch Limiter will work well if the length is correct. So as I continue testing, I'll post the results for you to see.

2002 - Pat Denevan is using a "nose line" as an automatic release with great success... also see Dave Broyles' note below.

More on the concept

In all of towing there is a potential for the dreaded LOCKOUT. This occurs when the pilot or conditions allow the glider to get so far off the towing direction that weight shift alone will not work to bring the glider back. The excellent book, Towing Aloft, by Dennis Pagen and Bill Bryden, goes into great depth on the causes and cures. Please read it as there's a wealth of information within the covers.

Bottom line: YOUR WEAKLINK WILL NOT SAVE YOU! Remember that fact and these simple corrective actions: 1) stay on tow by correcting as necessary; 2) recognize an out-of-line situation early; 3) release as quickly as possible; 4) throw your chute.

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 ARS (Apex Release Site, as described above) 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.

This arrangement will work with most any release that activates reliably with side pulls, not just the Linknife. It should work similarly with Aerotow launches that use a main V-bridle and ARS release position. With a little modification, it should also work with keel-mounted aerotow releases and with payout winch bridles.

Who would use it?

Every pilot new to towing could benefit from the Pitch Limiter setup, not just brand new Hang 1's but highly seasoned Hang 5's as well if they have never towed.

The needs of a new tow pilot, in my opinion, to learn the launch style and towing requirements outweigh the lure of a high altitude flight. The newest pilot, fresh from his/her tandem lessons and soloing for the first time, should be "protected" from his lack of muscle memory and short term brain/sensory translation and so could make use of a Pitch Limiter until 10-25 successful tows are completed, showing good control, staying on track, and no tendency for oscillation. It might take a Hang 4/5 only 2-5 tows (or more) to develop the necessary skills and understanding.

Note from Dave Broyles

I use a Linknife as an auto release with my scooter tow system. I am very pleased with it's effectiveness. I have also used the Linknife as a top release for aerotow. It worked well there too.

I hang the Linknife from the front wire ring or connector at the nose plate, and for low tows, I let it dangle down to just touch the control bar. I have a primary release attached to the pilot's waist and operated by a bicycle lever attached to the downtube. The tow rope is attached to it via a weaklink through the Linknife. If the pilot turns too much or lets the nose get too high, voila, free flight.

For tows, I lengthen the line so that the Linknife dangles about 4" below the control bar. It will reliably release a lockout before it gets severe. It should release the tow line if the pilot starts dragging it. (Adjustment length is critical here.) It also provides a backup release if the main release fails.

Thanks for reading this far. Do you have an opinion? Comment? Story of your own lockout?

Drop a note to Peter Birren if you have any comments on this proposal, want to discuss it or want to learn of the results.

 

DISCLAIMER: As with all aviation endeavors, your choice and use of equipment is totally up to you. It is assumed you are an experienced HG or PG tow pilot who is intimately familiar with the style of towing you will be doing. As such, YOU ASSUME ALL RISK AND LIABILITY in the use of the Linknife, as well as all other parts, functions and personnel involved in the towing and flight operations. If you do not have experience in towing, please contact an instructor for expert training. Trying to learn on your own can, and probably will, result in your injury and even death. Many pilots have paid the ultimate price so we may now tow as safely as never before possible. Please learn from their lessons.

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Page last update: July 8, 2015
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