Over the past 40 years I have made perhaps 1,000 auto tows. Some techniques I had to learn the hard way but eventually it all came together. The end result is very similar to the principles practiced today by the hang glider tow clubs. Seven significant items come to mind which make auto towing a safe simple operation. They are:

1, An instrument for reading the towline tension constantly throughout the tow by the driver of the tow vehicle.

2, A 2-way radio communication between the driver and the pilot .

3, A tow release hook operational at anytime during the tow by the towing vehicle.

4, A suitable towline for your gliders gross flying weight.

5, A weak link at the glider end.

6, A center of gravity tow hook installation on the glider.

7, A tow vehicle which has adequate power to satisfy the needs of the particular glider being towed.

By far the most important saftey device is an instrument that lets you know the line is overloaded before it is too late. The opposite is also true if a downdraft, or a drop in wind speed is encountered shortly after takeoff. If tow speed is not increased quickly the pilot is left in a potentially dangerous situation. If the driver sets his autos speed by the speedometer only, even if he allows for the headwind, he does not guarantee the pilot a safe tow.

If its a hot day a greater tow speed is required due to the airs lower density. Installing an aircraft airspeed indicator is a smart move to overcome changes in air density as well as make allowance for any headwind. However, a sudden increase in airspeed on the autos airspeed indicator does not mean that the glider some distance behind you also sees that same change in airspeed. The longer your towline the more this holds true. Again, if a line tension meter is utilized the tow driver will immediately know when to increase or decrease power. Even a beginner tow driver will provide a safe tow.

There are two types of line tension meters. The most common meter is an oil filled cylinder about 1-inch in diameter. If you can find a cylinder with a diameter of 1.28 inches the pressure gage will read in pounds force as well as psi. The major problem with an oil cylinder is that they often leak. A second type of tension meter is an electronic load cell such as found in digital scales. Reading the scale digitally requires too much concentration so an analog scale is much prefered. One can tell at a glance if the load is increasing or decreasing and that you are pulling at a safe load. The scale is not identified in pounds force but rather by a green, yellow or red zone as indicated by the needle. The top end of the green area, maximum tow tension, is set by a calibrated pot. One simply dials in the gross weight of the particular glider being towed. The first portion of the tow may be pulled at the max green area ( 50% of the gliders gross weight). As the tow progresses the tension is backed off so that it is in the minimum green area (about 30% the gliders gross weight) just prior to release. Where do you get such a tension meter? Well, we had to make our own. Fortunately, I have a few creative friends for whom I am extremely grateful who did the circuitry for me.

A towline release mechanism, similar to the glider release, should be installed immediately downstream to the line tension sensor. If the glider gets off to a bad start it is easier and quicker to release the towline than it is to stop the moving tow vehicle. The release line should be placed next to the tow drivers side with a loop in the end. Often the loop in drapped around the inside rear view mirror.

Communication between the glider pilot and the tow car driver is vital. The longer the towline the more important the communication. The driver needs to know when to start the tow, take up slack and perhaps some request from the pilot during the tow. We have found 5 watt CB tranceivers adequate for our needs. The glider unit is powered by a motorcycle battery to insure all day operation. The auto unit is plugged into the cigarette lighter recepticle. Ground signals are always used in conjunction with radio transmission and is the responsibility of the wing tip runner. Basically only one signal is used. If the wings are held level the tow driver knows he is free to proceed with the tow procedure. If one wingtip is placed on the ground the tow driver is to stop everything and wait for further tow instructions.

The towline we use for towing is the twist type polypropylene line. While the braided line is easy to splice it does not seem to be as strong as the twist type. Strength need not be more than 1.5 times the gross weight of the glider. Stronger lines must use a weak link of some sort to break at 1 to 1.5 times the gross weight of the glider. There are different qualities of poly line. Check the rated breaking strength of the line before you buy it. Often a low cost line means low strength.

Polypropylene line has a bit of elasticity to it. I find this beneficial at the start of the tow. The tow vehicle prestretches the line during acceleration before the glider begins to move. I like to put a 2 x 4 chock in front of the glider wheel to insure a good towline prestrech. The longer the towline, the more prestretch, and the shorter the takeoff run. The towline will strech 5 to 8% of it's length. This elasticity will help pull you even higher at the end of the tow after the tow vehicle has stopped. You will have to judge when maximum altitude has been reached.

You may want to purchase a larger diameter line than is necessary for longer service life due to ground wear. If you use lines shorter than 600 feet this may be a wise move if a weak link is provided. However, when longer lines are used release altitude will be lessened due to the high drag of the larger diameter line. Gliders weighing less than 450 pounds gross weight may use 3/16" line. Gliders 450 to 900 pounds gross weight may use 1/4" line. Gliders 900 to 1,500 pounds gross weight should use 5/16" line

The use of a glider center of gravity tow hook gets 10% higher tows over the conventional nose hook location. A nose hook tries to pull the nose down while you are attempting to keep the nose up by carrying full up elevator. Should the towline break in this condition the nose will pop up requiring a fast response by the pilot to get the nose down immediately. On the other hand, a line break on a glider using a c.g. hook the nose will not pop up. However, the pilot must recognise the drop in line tension and immediately lower the nose to maintain flying speed. The most likely time a rope break will occurr is shortly after takeoff when the pilot starts his climb.

The whole point I'm trying to make here, is that if proper power was being applied during the tow, stall accidents would not happen regardless of wind conditions. The only way to know the line tension (power to the glider) is to have an instrument to inform you. Hence we have a strong case advocating the use of a line tension meters. Personally, I would never auto tow without one.

On further suggestion when towing gliders with wing loadings above 5 psf. The ground vehicle speeds may exceed 50 mph which may make driving unsafe especially if the surface is uneven. In such a situation I have found the use of an auto-pulley tow device very helpful. The tow vehicle speed is cut in half and higher tows will result. This type tow is especially helpful when towing off runways shorter than 4,000 feet. To set up an auto pulley tow you drive a stake into the ground, about half way down the length of the runway and to one side. Tie one end of the towline to the stake and run the line, about 50 feet upwind, to the tow vehicle which has a pulley secured to the bumper. Run the towline around the pulley and all the way back to the glider. The tow car will have to pull twice as hard but the required speed will be cut in half. The towline tension meter is positioned between the car bumper and the pulley. The first half of the tow should read the gliders gross weight on the scale and then gradually reduce the power to almost half gross weight at the end of the tow.

There is no need to go into all the involved mathematics of auto towing which indeed are present and have often been presented by others. All that is necessary is to keep the power to the glider during tow at the correct level. This can only be controlled with a line tension meter. I'll say this over a hundred times if necessary to get my point across.

Jim Marske