In some of my first shut-off flight tests in the early spring 2006 I used a small reservoir with extra fuel under pressure connected to the fuel line between the engine and the shut-off.
It was made from a 5 ml syringe with an extra plunge.
First the free plunge is inserted as far as possible into the syringe housing.
Then the second plunge is inserted halfway into the housing and blocked by a piece of piano wire.
Between the two plunges we now have air under pressure.
When we open for the fuel from the bladder tank the fuel will press the free plunger back until the pressure of the air is the same as the pressure in the bladder.
When the shut-off valve close the fuel supply from bladder the compressed air will press the fuel from the syringe into the engine.
At my test flights in the spring 2006 this gave just enough runtime to get the model in the air.
Here are a couple of images of the reservoir system.
The "String-over-the-Wing" shut-off is an old and wellknown shut-off system.
Maybe this simple system could be used in a new shut-off for F2D models.
A shut-off system based on line tension must stop the engine when the line tension reaches a low level.
The worst case is a situation where a model flys away with a complete set of lines and handle.
This might happen if a pitman sends a model off without the pilot at the handle.
In the first test I tried to find out what influence the string pulling the lead out wires would have on
the control of the model. A simple spring could be adjusted to give different tension to the lead out wires.
A couple of test flights gave this result:
10 N was OK for flying but takeoff and landings were a little tricky.
20 N had a big influence on the control of the model in normal manoeuvres and is clearly too much.
For the next test the model was equipped with a simple shut-off system.
The model with the string over the wing and a adjustable spring at the centre section.
The "bar" on the two strings will make it be possible for the pilot to stop the engine.
If all forces from the model is hanging on one line and there is no tension on the other line, the bellcrank will turn to its most extreme position.
Since the bar is longer than the bellcrank the bar can turn even more.
This will let the shut-off have just enough extra string to stop the engine.
This bar system will also stop the engine if one line brakes during a flight.
See a video clip with a demonstration (9.4 Mb) or YouTube
The tension on the shut-off string could also be adjusted on this model. The ball bearing is just tfor reducing the friction in the system.
A small device in piano wire could pinch off the fuel line.
Here is a video clip from the test flight (3 Mb). See how the engine can be shut off by the pilot.
Test flight 6/9 2008
The strings over the wing can be a problem when a streamer is pulled over the wing.
Then there is a high risk that the streamer nut can catch a string and destroy the shut-off system.
Inverted landings will also be a problem.
A couple of strings over the inboard wing will probably also disturb the aerodynamics of the model.
To solve these problems the next step was to move the strings inside the wing.
The lead out wire on the model was a little short so first the bellcrank had to be moved into the inboard wing.
This gave 40 mm extra wire to play with. The "bar" was made from 2 mm plywood and the wire for the strings was just some spare combat line.
A lever made from piano wire transforms the big movement/low tension from the string system to a small movement/high force
for the device that pinches off the fuel line.
This piano lever does also have a spring to give some tension to the lines.
This spring was adjustable and the "shut off line tension" could be adjusted from 2 to 10 N.
See a video clip from the "String-inside-the-Wing" system (9.4 Mb).
The system was tested on a cold day with plenty of wind and rain.
With the shut-off line tension set to 10 N the engine could be stopped by pulling one line by hand.
Landing with the engine off up against the wind was a problem with this setting.
With 5 N there were no problems in controlling the model but the engine could not be stopped by pulling the line. The tension from wind resistance on the loose line was too high.
Simulating a fly-away
The ulimate test of a shut-off system is a let-go-of-the-handle test.
But this test is dangerus so it would be nice to simulate it in a way that is safer for both persons and equipment.
A shut-off system basede on line tension can be testet in flight by releasing the tension on the leadout wires.