--= Back   Start   2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50   F2D.dk   Next   =--

Shut-off for F2D-Combat
Bubble shut-off

Last update: 23/5 2009

Bubble shut-off

Idea by Henning Forbech

A simple, light and cheap shut-off
- but with very low reliability!!!
Maybe it can be used for noise reduction?


Henning Forbech
This is a very simple shut-off. Weight and price is close to nothing and everybody can make in a few minutes.

The problem is that it is based on the same principle as swing-arm shut-offs but with the unfortunate difference that it has an even lower success rate.

The idea it to use the fuel bladder as shut-off.
Normally the end of the fuel tube is placed just inside the latex tube of the bladder tank. By inserting the fuel line almost to the end of the latex tube the bladder tank will be able act as a simple shut-off.

The trick will be not only to fill the tank with the fuel necessary for the flying but also with a good amount of air. As long as the model is flying with lines the fuel will be located in the outboard part of the bladder. The air will be at where the fuel line comes in. Normally this air bubble would be the first to leave the bladder but since we have extended the fuel line inside the bladder this air is trapped. As long as there is line tension the air will remain this position. If the lines get cut the centripetal acceleration will disappear and the fuel and air will be free to move around. There is now a good chance that the engine will get air instead of fuel. If the amount of air is big enough this will now stop the engine.

The problem is simply that we can not be sure that the fuel line inside the bladder will pick up air in the event of a fly-away. This makes the reliability of the system very low. Probably much lower than what we now see with swing-arm systems. For this reason the bubble shut-off can not be an acceptable stand alone shut-off for combat.

Because the system is so easy to make it might also be used in a different way.
All shut-off systems will some times fail to operate. With a bubble shut-off as backup the total reliability of a model's ability to stop in the event of a fly-away could be enhanced. Since this shut-off operates from the same physical laws as swing-arm shut-offs it will probably not improve their success rate significally.

The air in the bladder may also be used for another function.
When flying on an ordinary bladder tank the engine will start to slow down at the end of the flight. Right before the engine stops it will speed up and then run lean and stop. Normally you will know that your engine is about to stop 5-10 bouts before it actually stops. This is 10-20 seconds or 5-10 ml of fuel.

If the bubble shut-off could be used to stop the engine when it is running at full speed some fuel and weight could be saved on the model. More importantly the run time of the engine could be set lower since the slowing down and speeding up at the end can be avoided.
If every flight could be reduced by 15 seconds this simple trick would reduce the noise load by 5%!

Test flight:

A test flight with a bubble shut-off was set up on the 25. May in Aalborg, Denmark.

First test flight with the Bubble shut-off. Pilot: André Bertelsen, Denmark

As you can see from this video the engine did not stop any quicker than with an ordinary bladder. This might be improved by the shape and positioning of the fuel line inside the bladder.

An idea could be to let the fuel pass a small tank with the air bubble.

Tank and photo: Brent Allwood, Australia

The british vintage combat pilots are using a front feed tank. This tank do also work as a simple anti fly-away device. These solutions might give other pilots inspiration for new and better designs.

Personally I don't think there is any future for the bubble shut-off.

/Henning Forbech

--= Back 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Next =--
Shut-off start - F2D.dk