Kill Switch

[Chris Lane]

I am now fitting out both my Hostetler 30% Cessna F152 and my 1/4 scale DHC1 Chipmunk. Both have magneto ignition Zenoahs - a 45cc in the Cessna and a 38cc in the Chipmunk. Both have two Futaba R617FS 2.4gHz receivers and have one aileron to each receiver and a mechanical mixer for the rudder using one servo from each receiver. The remaining "twinned" controls are arranged differently on the two models.

The problem is the engine, when considering a single receiver failure. If the receiver that controls the throttle fails then there is no power control available. I have concluded that a "kill-switch", on a channel which is unused on both receivers, could be used on the receiver that doesn't control the throttle servo. This would at least allow an engine, stuck at high power, to be stopped allowing a safe "dead-stick" recovery to land.

I assume that a kill switch earths the magneto. Now that Terry has retired and there are no more SMS kill switches are there alternatives available because switching megavolts seems a bit specialist! How do the guys do this please.

Chris Lane

[Rob Buckley]

Put a servo on the choke on the other receiver. It saves a manual choke linkage & you can stop the engine on it if needs be.

[Chris Lane]

Thanks Rob - that's the clue I needed. It also allows the choke to be used during start-up without needing to open cowls near the propeller! The Chipmunk ZD38 is closely cowled and the carb is on an elbow so the levers are in unusual orientations.

I was having visions of contacts pulled off old Post Office relays and capacitor and inductive spark suppression. . .

Chris

[Rob Buckley]

Chris, There's nothing to stop you fashioning something out of an old welder contactor, some oil filled caps out of a 3 phase switch cabinet and a big old blade switch driven by a sailwinch servo & an ingenious cable & pulley system if you need the noseweight!

[Dave Hayfield]

Sorry to disillusion you Rob but I don't think your big blade switch will be man enough for the job. I suggest a pick-up shoe off a third rail rail way system which will clamp to a short length of railway line via a release pin and gravity. This will also provide even more noseweight if required.
Actually, we must stop being silly!.....your idea of a servo operated choke is good but on my Zenoahs it doesn't always stop the engine. I use a microswitch operated by a servo to earth the ignition system to completely stop the engine.

[Chris Lane]

I think I'll forget the blown arc job and just have a servo that pushes a length of hardened silver steel into the (wooden) propeller!

In reality I think the choke control is the way to go. I'm guessing that, even if the engine doesn't actually stop on full choke, it won't be giving any real power and the model will land. Although the kill switch is the most certain I fancy the start-up advantages of the remote choke control.

Chris