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Old Sep 29, 2002, 01:22 PM
soonerflyer is offline
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Throttling Non-Throttable Engines

I read in the latest issue of MAN that you can use a servo and piano wire to control the amount of smoke fluid into your muffler. You mount your servo on one side of a plate and drill a hole through a plate of balsa or plywood or whatever that you want to use. You bend some piano wire into a loop and slide your tubing through the loop and when the servo arm moves it will pinch the line therefore restricting flow.

I'm sitting here looking at towerhobby's cox c/l engines that are non-throttlable and wondering if the above approach could be taken to be able to control throttle on the engines. Is this crazy or would it work?
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Old Sep 29, 2002, 03:56 PM
steve lewin is offline
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Unfortunately you can't control the speed of an engine by restricting the fuel feed. That would just make it run very lean and stop, possibly after doing some damage.

In some engines without throttles you can do something by restricting the exhaust size, usually with a movable flap across the exhaust port. But I don't think that would work on a Cox due to the exhaust arrangement.

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Old Sep 29, 2002, 05:19 PM
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Most of the unsilenced Cox motors use sub-piston induction (at TDC the piston skirt is above the bottom edge of the exhaust port), so any kind of muffler causes exhaust gases to be sucked into the crankcase ... with dramatic loss of power.

They did actually make throttled versions of the 020 and 049 reed-valve motors that used a rotating sleeve fitted around the cylinder to restrict the exhaust ports, but these models had a deeper piston to avoid the problem mentioned above.

You could probably use a servo connected to the needle valve to richen the mixture and reduce power that way, but you'd be limited to about 60 degrees of rotation. Some bigger r/c motors have this feature on the carb (or available as an option) to fine-tune the mixture in flight via an extra servo.

You can throttle small diesels with a servo operating the comp. screw (I have seen this done with a Mills 75 and it was very effective).

But generally the best option is to get a throttle-equipped motor in the first place!
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