On Board Glow Questions

[airbusdrvr]

I am considering building instead of buying of an on board glow system for a 1.20 sized OS four-stroke twin cylinder motor. The most commonly sold on board glow systems use a 4.8v-50v battery that pulses the power to the 1.5v glow plug so that the effect is the same as a continuous 1.5v. I was considering using one or two Sanyo 1.5v Candica rechargeable batteries(3000 to 4000 mah) and a switch operated by the unused gear channel on my transmitter/receiver. Several questions for this endeavor.

1) A trip to Radio Shack revealed they have only two very small "lever" switches(what I would call a micro switch). What is a good source for a switch that could be activated through a linkage to a servo? The engine owner's manual states that the glow plugs could use up to 6 amps each. So the switch would need to rated for this type amperage.

2) The simple diagrams below depict the basic thoughts I had for the system. Comments are welcome.

3)If there are any difinitive threads here on RCGroups that would be helpful, please post a link.

I'm thinking I would just like to keep the system very simple and manually activated by the transmitter. That way I can use it for start and then activate it in flight were a cylinder to drop off due to the glow plug going out.

[Chippie]

Why not use a mosfet for switching the 1.5v supply to the plug?

A google search should reveal info on how to......

[Tomapowa]

Here's a couple of ideas...
First one connects to receiver for on/off control...
Second one is a modified version that uses a simple mechanical switch (micro switch engaged by servo arm). You can even get the +5v to control the FET from a spare channel on your R/C receiver, and you can use a seperate Nicad for glow power. There are larger capacity Nicads out there (D size even) that should be plenty for powering two glow plugs at one time (or you could parallel a few smaller ones...your choice).

Here's another thread on remote glow but... you are right... not many threads re: DIY remote glow sytems here in RCGroups...
http://www.rcgroups.com/forums/showthread.php?t=144205

[darkith]

My Dad's Telemaster 40 has a FEMA remote electric starter, and it basically used the micro switch idea and a 1.5v NiCad. We ended up switching to a RAM RC switch (Ram 30) because it was easier to mount and had less risk of vibration. It's basically a printed circuit board that glues to the top of a servo, and a "V" shaped contact that screws onto the servo wheel and bends down on either side to make various contacts with the segments of the PCB as the wheel turns

Another "simple" option could be to use a micro switch to trigger a relay.

D.

[dmccormick001]

I'm thinking I would just like to keep the system very simple and manually activated by the transmitter. That way I can use it for start and then activate it in flight were a cylinder to drop off due to the glow plug going out.
Maybe I'm misunderstanding this statement, but if your glow plug "goes out" in flight, and by this I'm assuming you mean "burns out", putting power to it won't make it start working again. Once a glow plug is burned out, it's gone. If, however, a cylinder quits in-flight because it's flooded or something, then switching the plug on might fire the cylinder again and get it running, but most likely only if the condition is gone that caused it to stop running in the first place. If it's due to anything with the fuel system, and if a cylinder drops out in-flight about 99% of the time it will be, I'm not sure I'd bet on being able to re-start it. That's just my opinion, anyway.

Of course, I'm sure you already know this, but on board glow-drivers are primarily designed to assist an engine in idling smoothly, especially in mounting situations where the cylinder is inverted. Fuel tends to fall down onto and around the plug cooling it off, and making smooth idle hard to achieve, so keeping the plug "lit" at low throttle settings helps keep it from being flooded out. One simple and effective method is to locate the micro switch near the throttle servo, and place a wheel collar onto the throttle control rod so that at a certain point, as you reduce your throttle, the collar hits and closes the switch, which powers the plug, and keeps it closed until the throttle is advanced again. If you do it this way, be sure to add another (external) "safety" switch somewhere on the plane that can be used to disconnect the glow driver circuit completely until just before you're ready to fly. You don't want the throttle stick down and the plug hot while you're fueling it or working on it in the pits.

Tomapowa:

What does the trim pot in that first diagram do? Does it adjust the point at which the plug is powered? I can't figure it out. :o

[Tomapowa]

Tomapowa:

What does the trim pot in that first diagram do? Does it adjust the point at which the plug is powered? I can't figure it out. :o

Yes... it adjusts the pulse width (ouput of spare channel on RC receiver) required to turn on the switch/FET. When the potentiometer is in center position, the turn on point of the switch should be around 1/2 stick (1.5ms). If you want to reverse the direction at which the switch turns on, you simply use pin#12 or #13 on the 4013 chip. You can also do this with a pic (no potentiometer/moving parts): http://www.laureanno.com/RC/picaxe-maxiswitch.htm

Funny though... I also sometimes like the not-so-elegant solutions. I remember years ago I rigged up a friend's twin engine Cessna with remote glows that used a simple brass finger that rubbed up along the metal-braided throttle cable for glow connection (part of it had insulation on it to break the contact during 1/4 throttle & higher). It worked for years until the cable wore through the soft brass fingers!