Users guide for Jacco’s  F2d-schut-off

There are a few important issues when you are considering the design of a swing-arm shut-off.

Centrifugal force as a trigger, or better, the absence of this force, is the most important.

Henning’s measurements are a very good lead in this case.

As line tension is also a result of centrifugal force, the results of Henning’s efforts are especially useful for his  “line tension based shut-off”.

A swing arm shut-off  however has no connection with line tension and that is an important difference. Because with a swing-arm shut-off the temporary lack of line tension, caused by a gust of wind, is not an issue.

        Figur made by Henning Forbech

The most important considerations are the three conditions that occur and determine the "positions" of the shut-off, namely:

    a)   Locked, the engine is running, ready to launch

    b)   Unlocked, the model is airborne, and the centrifugal force keeps it open

    c)   Closed, there is no more a centrifugal force, and the engine stops.

Especially position "b" needs attention.

There is a co-operation between forces to be managed to make the system work well.

The weight at the swing arm needed, that I found is 7 grams

Together with the arm ( 2,5 mm steel ) the effective weight is about 10 grams

Photo 1

Do'nt make any force to messure just the effective weight of the swingarm

.

The force to  "unlock" is about  85 grams

In  photo 2 the weighing scale gives a reading of 95 grams but we should reduce that with 10 grams of its own weight to get the nett force.

Photo 2

Make a torque with thumb and forefinger to mesure the force of the spring

That means that unlocking will occur at about 8,5G and that happens at a speed of about 132 km/h. Even unlocked and at a lower speed, say 80 or 90 km/h, the fuel line stays open.

 Is the spring really strong enough to squeeze the tube until it is closed ?

We can easily test this while not airborne (position c)

If this function is not satisfactory, we have to look at the geometry of de "pincher" and the fuel tubing being used.

I based the design on 5 x 2 mm silicon tube, but even when the specifications of the tubing are the same, it may behave differently in conjunction with the shut-off

Measure your fuel line with a caliper and squeeze the tubing until it is closed.

The gap that remains is what we need to know.

Foto 3

By squeezing the tank we can feel when it is closed

If we look at the gap in the shut off in the position that the sides of the gap are parallel, the size of the gap should be the same as we measured with the caliper in the closed position.

If the gap is not parallel at about the right size that the tube should be closed, then you can be sure the force needed to close the tube, will be greater.

A trick to measure this, you can use a drill of the corresponding size and move it in the gap of the pincher.

If the swing arm doesn’t move, it's ok. If it is not ok, the pincher should be re-adjusted.

Increasing the force of the spring is not the best way because in that case you also have to increase the weight at the end of the swing arm. Accurate gap-sizing is the best option.

Success,

Jacco

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