Towing Elements: Parts

by Peter Birren, with lots of help from Dan Hartowicz, Warren Seipman and others

This is mostly a how-to on putting together the parts of a static towing system. I've tried to "hide" it from casual web browsers but still make it available to serious pilots ready to make the move to Static Towing. If you as a reader are here and have no experience towing a hang glider, much less flying one, seek instruction before attempting to fly. We (all the participants in the sport) don't want you to die because of your doing something stupid like buying a glider at a garage sale and going to a hill and trying this all on your own. Be smart.

Before I go into the parts of a system similar to mine, a little history: Donnell Hewett realized in the early 80's that towing without knowing the towline tension was causing fatal accidents and breaking gliders. His first job, though, was to devise a bridle to allow for more control of the glider, especially compared to the previous method of just tying the rope to the glider's base tube. This was accomplished with a 2:1 "pulley" bridle that attaches to the center-of-mass of both pilot and glider. The bridle attachment points, materials, etc., could be the subject of a different article, but others have done admirably, namely Bill Bryden and Dennis Pagen in the book Towing Aloft.

During the testing of a variety of bridle setups, Don would have his wife, Helen, and daughter hold a short towline while he soared the dunes near Corpus Christie TX. They could be seen "walking" their dad along the beach much like a kid plays with his kite. Eventually the bridle was settled on and Don went about figuring how to sample the towline tension.

His original, and very functional, device used a coiled garage door spring inside a PVC tube. The spring had a tab on it that stuck through a slot in the tube. This all was attached to their car's front bumper with Helen holding the tube at an appropriate place. When she drove, she could feel for when the tab hit her thumb, then the tension was not too high. Don progressed, with the help of a few others, to using a simple hydraulic system.

Over the next few months various tensions were tried and minimums and maximums were found. These are in fairly wide use today, and Don wants to be known for this development more than for the bridle which carries his name. He feels that the tension sensing device is a much more important contribution to towing safety than anything else.

If, after reading the details below, you don't feel up to building one, you might contact Bill Bryden to see if he has any pre-assembled units (about $125.00 complete).

Somewhat of a misnomer, this is actually a pneumatic cylinder but it pushes hydraulic fluid (see #3). The type suggested is a Double-Action, Pivot-Mount, Stainless Steel-body Air Cylinder. Bore = 1.25" (gives about 1" square surface area for what amounts to a 1:1 reading at the gauge). Larger bore = lower reading at the gauge. Stroke = 1" to 2" (shorter is better to limit shaft bending)

The "pivot mount" means it has a hole on the back end that will accept 1/4" Perlon or 1" tubular webbing for a flexible mount. The shaft is threaded on the end to accept any of several types of line attachment devices.

W.W.Grainger shows these in their 1998 catalog on page 2949 and cost about $33.00, WWG #6D842 (1" stroke) McMaster-Carr has 'em, too.

These sketches are from my system. As long as yours works
it doesn't matter what it looks like. But style does count, ya know.

Split rubber tubing in several layers was added
to help prevent crimping of the pressure hose.

Must work with in concert with the cylinder. Should be large 4" face with polycarbonate (Lexan) cover plate. 300 pound rated. You can replace the dial faceplate with your own after hanging weights and testing (see #8 below). A decent gauge is Grainger's "Mechanical Contractor's Lead Gauge" from Ametek. It's got a 4.5" face and 1/4" ANPT fitting. Stock number (WWG) = 1X734, Cost = $16.00. Speaker magnets epoxied to the bottom hold it fast to the vehicle hood or to a plate as shown above.

I wasn't able to find a stock number on the hose, but I've been told it's a 400# test clear hydraulic line. You'll need a little more than 20 feet (better to have too much than too little). This will allow the gauge to magnet mount on the hood of the car or on another base of your choice (I'll show my mount in the line art) and the cylinder to attach to the rear bumper.

Johnson's Baby Oil is what most reel owners I know use. If there's a leak in the hose, the fluid doesn't eat the paint as hydraulic or brake fluid will. Saw a break happen once when someone set up the hose strangely and it got caught in the car door.

Do not use "tool-less" fittings anywhere in the system. These have a tendency to come undone at the most inopportune time. Use the regular threaded brass fittings available wherever you buy the other stuff. They're pretty easy to work with and are quite inexpensive, so buy more than you need to make up a repair kit. Use your imagination to mount the gauge to a CB-antenna magnet; a speaker magnet will do also. The hose should come out horizontally above the mag-base. On the cylinder, make sure the "rear" has a screen filter (or better) fitted to prevent dust and dirt from entering. I suggest you also use some old fuel line or similar rubber hose to layer on as a strain relief at both the gauge and cylinder. These are the areas that take the most abuse, especially when rolling up the hose to put away after a day of flying.

Bill Bryden has found some snap-in fittings that he reports working great with very little loss of fluid. I have yet to see or work with 'em but look forward to reporting back when I do.

This is /my/ system setup. Yours can certainly be different as long as it works.

Gauge should be mounted in the driver's line of sight, preferably so s/he doesn't have to look anywhere but straight ahead.

Cylinder needs a couple of attachments: one to the bumper to provide complete freedom of movement – up, down, sideways – without interference. The other is a safety to some point above in case the bumper attachment comes loose for whatever reason. This secondary also should prevent the cylinder from dragging on the ground, but should not interfere with its normal operation.

To allow the towline to easily be put on and taken off the cylinder shaft, I use a spring-loaded 1/4" snap link welded to a bolt, then double-bolted on the shaft. Another pilot has a sailing shackle that doubles as an emergency release, similar to the one used at Wallaby/Quest for a glider keel release that opens all the way. Again, be creative but make it bulletproof and functional.

When the system is placed on the car/truck, route the hose under one windshield wiper arm to keep it from getting caught in a door or anything else.

Once the system is assembled and all bubbles have been removed from the hose, now comes the fun. By using a variety of weights, including your body weight, hang test and record the main and secondary tension points on the gauge, then other weight amounts inbetween. When using any given weight, bounce it and/or let it sit for a while so you don't get just the first reading. Remove the weights to let all go back to zero, then add more. Or use a light weight, then a max weight, then another test on a light weight, all to make certain that you're reading as accurately as possible.

I tape a sheet of clear acetate to the gauge face and use a grease pencil for the marks, then use a protractor and computer drawing program to make a replacement face plate that gets taped behind the needle. Here are the weights/tensions and colors as I use 'em:

WHITE –- 0 - 20 Take up slack
RED –- 20 - 100 pounds for launch tension: GO, GO, GO
YELLOW – 100 - 120 Average launch tension
GREEN –- 120 - 180 Stay on the low side until MAX is requested
YELLOW – 180 - 200 Upper warning area
RED –--- 200 - 265 (obvious?) weaklink break area, uncomfortable.

The Reel Pilots suggest some form of release at the car be used when training newer pilots, those with less than -say- 25 successful, PIO-free tows and obviously in full control. There are a couple of drawbacks, like if the release is used, the towline can snag on something on the ground and possibly be more of a hazard than if it was not released. There have been maybe 3 instances in 20 years of towing with our group where it might have been handy to have this setup.

Previous situations/problems
One situation in 2000 resulted in the fatality of a woman on her 3rd solo tow. No one may ever know what really happened, but she seemed to just freeze at 200 feet (using 1500 feet of towline), turned slowly into a classic lockout and dove straight in without ever reaching for the release or correcting for the turn. Tragic to the max.

Another fatality: a pilot (flying only a couple of times per year, if that) had a release failure while on a pulley tow, had no hook knife and no radio – he came in from 750' – and it's possible that releasing from the car might have helped... maybe not as the line could possibly have snagged in the pulley.

Another incident involved no less than 7 errors (radio problems, no weaklink, almost-hang-3 pilot, launching during on highly thermic day, launching through slot in trees, homemade 1-string release, fast-handling glider), so dropping the line might or might not have helped – resulted in the pilot being hospitalized but surviving.

The jury's still out on the absolute need for a release at the vehicle for tows involving other than newer or low-time pilots. No reel owner I know has used one, ever, as the vast majority of tows involve highly experienced pilots. However, we're getting more new pilots involved and may make an emergency release an SOP. If you have any questions or suggestions to improve any of the above, please write and let me know.

12V long shaft starter motor, welded commutators and end-shaft bearings through continuous duty golf cart solenoid. Plugs into car battery using #2 welding cable with fork lift charging plugs to connect/disconnect the reel. Cables stay connected to the battery for the entire flying season but disconnected during the winter as road salt causes heavy corrosion. Spool has aluminum side plates bolted to a 6" PVC spool base with internal bearings on the solid axle - spool could be mounted solidly to the axle if pillow blocks are used on the frame. Bottom bracket is semi-permantently mounted to the truck. Reel is held in place at the top with welded-on bolt heads on the reel which fit into slots on the mounting brackets and, at the bottom, with 2 trailer pins.

This is one of my early gauge templates. Every year I hang weights and, if necessary, make a new paper faceplate and apply with SprayMount.

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.