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Stop at Will for swing arm shut-off
Rebuild your shut-off for pilot activated stop
First published: 24/12 2012 - Last update: 17/1 2013
Swing arm shut-off rebuild with stop at will

by Henning Forbech

Here is a simple way to rebuild your swing arm shut off to stop you engine after a strong pull on the lines.

The idea is to replace the simple weight at the end of the swing arm with a similar weight that consists of two parts. One part that is fixated on the swing arm and a free part that the pilot can separate from the swing arm.

The free part is connected to the fixated part by a strong magnet. The force from the magnet is just enough to hold the free part in place during normal flight. If the pilot makes a hard pull to the lines the free part will drop off.

When the swing arm loose most of the weight the spring will pull the arm back, pinch off the fuel line and stop the engine

This system has not been tested yet but it should be easy to build a prototype. You might be the first pilot to test this system!

Here are some CAD models to demonstrate the system.
First we have a swing arm shut-off like this system from Aerohobby in Canada.

Now we replace the old weight at the end of the swing arm by the new weight (the scale may be a little wrong in this photo manipulation).

The free part is made as a cylinder that fit around the fixed part. All forces from maneuvers and hard landings will be transfer directly between the two parts. Only force in the direction of the pilot will depend on the strength of the magnetic force.

To prevent the free part from flaying away it must be secured by a wire to the engine or model.

After a flight the mechanic only have to put the free part back on the fixed part to be ready to fly again.

Small neodymium magnets can be very strong but the force is highly related to the distance between the magnet and the steel part it is acting on. In this design this air gab is adjustable and it should be easy to adjust the force of the magnet to a value where the free part will only come free if the pilot gives a very hard pull to the lines.

A cut through the new weight:

Drop Stop cad-files:
Download 3D model (unzip and run)
3D model (exe-file)
3D model (eDrawing)
More info on eDrawings
More info on 3D CAD models
  Tech info:
Magnet used in this CAD model:
Neodymium disc, ø 5 mm x 3 mm
from RS Components
RS Stock No. 695-0166
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