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Shut-off for F2D-Combat
Line tension shut-off with delay

Last update: 26/5 2009

Shut-off with delay. Ready for the "Hard Core Rules"
Design by Bjarne Schou and Henning Forbech



Henning Forbech and Bjarne Schou
A moving bellcrank shut-off that will be armed after start by a mini swingarm seems to be a good solution for a shut-off system.   -   But there is a problem!!!
The problem is that the shut-off will not only be blocked on launce but also for a short period after takeoff.
The system will arm as soon as the line tension and the centripetal acceleration build up. The rule book does not allow the shut-off to be blocked at any time at all.

      "The device must remain functional for the entire flight period .." (4.4.5).

This rule is very clear and there is no doubt that flying with a blocked shut-off can not be allowed. Even to fly a few meters flight right after take-off with a blocked system is prohibited and should not be allowed.

If a pilot uses a system that is blocked after take-off he may be disqualified. If his opponent or any other pilot puts in a protest on this blocked shut-off they will have a good chance of putting that pilot out of the competition. To avoid these problems you have to use a shut-off system that is always active.

An easy way to determine if your system is blocked is to see what will happen if the pitman lets off the model without a pilot at the handle or even without any lines at all.
If the shut-off allows the engine to keep on running in this situation your system is not active. It is blocked and your shut-off is not fulfilling the demands in the rule book.
An active system must be able to stop the engine even in this situation.

You might find that a pitman letting off a model without the pilot at the handle is an extreme situation. Unfortunately it is more common than one would think. It happened at the World Champs in France last summer and at the competition in Phoenix this spring.
Pitmen under stress do make mistakes and the new shut-off systems will put the pitmen under even higher stress. I think we will see more of these dangerous mistakes in the future.

The practical problem with an always active system, based on line tension, is to keep the engine running long enough to get the model flying.
Some pilots solve this problem by setting the shut-off line tension very low. This will make the shut-off open shortly after take-off and the engine will survive the short break in the fuel supply. The problem with this solution is that the shut-off will not be able to stop the engine if the model flies away with a complete set of lines.

Another solution could be to make the shut-off react slower. If there was a delay from the moment the line tension turns low to the moment the fuel supply is stopped the engine would be able to run just long enough to build up enough flying speed to re-establish the fuel supply. This slow reaction - or delay - in the shut-off system will not cause problems with the rules. All systems will have some reaction time and there is no time limit in the rules.

The delay that is necessary to take-off with a line tension based shut-off is about half a second if the shut-off line tension is set to 10 Newton. This delay can be made by inserting a small reservoir between the shut off valve and the engine. The reservoir can be made by replacing some of the fuel line with a piece of latex tube.

At the World Cup competition in Bitterfeld, Germany, both Bjarne Schou and I were using shut-off systems with a delay function based on a small reservoir.

The system I used was based on my "2009 system" but without the activation mechanism (the mini swing-arm).
The reservoir was made from a latex tube. To let the pitman start the engine some extra features have also been added. At the moment all this tubing is quite confusing but the system is very easy for the pitman to handle.

This slower reaction from the shut-off in the event of a fly-away will reduce the safety benefits from the mandatory shut off systems. A model that is cut loose of the lines will fly a little longer and will be more dangerous to pitmen and officials close to the circle.

Now I am working on a new design for a reservoir system with a delay that is only active at take off. This new delay system would allow a model to get airborne with a shut-off system that is always active. When the model is in the air this system will have no delay and will react as fast as the "old" system that was blocked during the take off.

I know that a " Clarification" was sent out last year. I find that this clarification is quite problematic and to avoid all the discussions and problems it might curse I have chosen to use a shut-off that will full fill the "Hard Core Rules" in the rule book.
At the moment this gives me a lot of problems with the design but I think it is easier to solve the technical problems than to start an endless discussion on the right interpretation of the rules and the clarification.

More information on the rules for shut-off systems and the clarification can be found here.

First version of the reservoir was made from silicone tubing. Now we are using latex.

Bjarne Shou and André Bertelsen adjusting the volume of the reservoir

André Bertelsen and Henning Forbech preparing models for Bjarne Schou. Bitterfeld 2009.

The Danish team for Serbia: André Bertelsen, Anders Kudsk and Henning Forbech.

Test flight:
A video (2.5 Mb avi) from a takeoff where the engine is only running on the reservoir.
The engine is running full speed for less than a quater lap.


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