Electronic Tow Hook

This tow hook operates with Timer T10. It enables to close the latch by servo and repeat accelerate phase. Tension in the tow line is measured by two strain gauges. The measuring circuit is attached directly to the tow hook body.

Introduction

The moment when a model releases tow line is very important for successful flight. Any mistake at that moment can cause a poor flight time. For example, you are going to accelerate the model too early and then the model deflects from ideal launch trajectory. Another mistake occurs when the tow hook is unlatched unintentionally in gusty wind. If that happens, the model usually descends to a very low altitude, and the final result is very bad.

The ordinary tow hook unlatches always, when the tension in the line exceeds a pre-adjusted limit, usually defined by the stiffness of the tow-hook's spring. So there is no chance to repeat the accelerate phase. One way to fix this problem is electronic tow hook which can close the latch, if you don't release the line.

The Principle

The body of the tow hook is milled from one piece of duralumin. There are two strain gauges, which enables to cancel temperature error and increase sensitivity. The measuring circuit is attached directly to the tow hook body. Hence the output of the tow hook is already amplified voltage level. The tow hook itself has three cables: from the servo, from the measuring circuit, and from a switch that is attached on the back side of the tow hook body. The hook position in relation to CG can be adjusted by a screw in the fuselage that works as a front stop.

All parameters of the tow hook are stored in the Timer T10. In the first place, there is a factor between the output voltage and the line tension. This factor has to be adjusted in the calibration menu. When the tow hook is loaded with a known weight (5 kg for example), one can adjust the factor in order to obtain the right tension. Now, one can set arbitrary acceleration and unlatch tensions. There are two values of tension, so there is some kind of hysteresis. When the acceleration tension is exceeded, servos moves to the acceleration trims (it works as the zoom function for rudder). When the unlatch tension is exceeded, the hook is un-latched. The hook is latched again when tension remains below the acceleration tension for a pre-adjusted time. This hysteresis eliminates the problem when the tow hook is still closed in the accelerate phase with no wind conditions. There are also two adjustable times. The first one works as a filter for jerks in the line. The second one is the above mentioned time that if it is exceeded and tension remains below acceleration limit, the latch is closed again. There is also a function called "circle fail". It is a tension limit that if exceeded when the hook is in the back position, big rudder and wing deflections are executed. This should help if you are making a circle in strong wind, and you are not able to turn the model.