Ever since a near miss with a glow starter getting caught in a prop back in the nineties I’ve used on-board glow drivers in any model big enough to carry an extra ‘C’ cell around.
As well as the safety aspect, not having to play with a glow starter around spinning props, most glow motors idle more reliably with the glow plug lit. This is even more so with four strokes due to the time between induction strokes being longer than with a two stroke.
The incoming methanol has a catalytic reaction with the platinum on the glow plug coil which causes the coil to get hot, which ignites the mixture. The longer time between charges of methanol, oil and air entering the cylinder mean the plug can cool between compression strokes and the idle gets a bit rough around the edges.
The benefit of on-board glow is greater if the motor is inverted where oil and combustion by-products can collect around the plug making the idle worse and the transition to high speed lumpy as the gunk burns out from around the plug.
A few weeks back I was lucky to add a beautiful quarter scale fly baby to my collection.
There is an unwritten law that large, slow flying, classically shaped models have to have a four stroke Engine. You can’t fight it, they are just not right with a screaming two stroke or the sterile urgency of brushless electric.
I bought a shiny new 1.2 ASP noise maker to fasten to the engine mount which of course needs an on-board driver. No problem; In a past life I ran a business that made 1,000’s of glow drivers so I was sure I’d have another one lying around somewhere.
After turning my workshop and garage upside down it appears the last functional one I had was destroyed in the violent arrival to terra-firma of my 1/5th scale cub. This allowed the model to fit in a much smaller space than it did before the start of the flight.
Back to the drawing board.
So, 20-ish years after I designed the first Ohmark on-board glow switch I went back to the drawing board.
Primarily because I didn’t want to wait for postage but also because I’m an electronics nerd and it would erk me to have to buy something simple that I can make.
In the original design I used a small dip switch and pot to set the point and direction of the throttle at which the glow would come on but that is so last century.
These days transmitters are capable of collating the census results for a small nation, never mind controlling a glow switch. Along with the transmitter improvements, eight plus channel receivers are now cheaper than a good servo rather costing more than the air-frames we put them in.
This means the magic can be handled from the comfort of the transmitter and the switch itself can be simpler.
Into the bits box I delved and came out about an hour or so later with a functional prototype using a small 8-pin micro, a few discrete components and MOSFET rated to switch 55 volts at 29 amps which is far more than the 1.5 volts and around three amps which is the maximum you’d expect from a cold glow plug.
For a battery I found a tagged 3.3AH NiMh from Jaycar which was a refugee from another project and although the finished unit is a bit bulky because it’s built on strip board it’s only 50x10mm or so and weighs maybe two grams. If I need another one I’ll get some PCB’s done for them and surface mount it.
The previous owner had already fitted a remote glow lead so all I had to was put the arming switch and status LED through the cockpit floor, wrap things in foam and cable tie them into the model. Job Done.
On the transmitter side I used one of the three position switches of my Taranis 9XD to control the glow driver. The three positions are effectively engine off, engine run and engine with glow. Some would refer to the later as ‘engine start’ but in reality I fly with the glow always enabled and it turns on / off at a low throttle position.
In my experience a good quality NiMH C cell will give you a solid hour of flying with 5-6 starts and you could extend that by going to the ‘run’ position if you had an extended idle / taxi period.
Programming for the Taranis
The programming for this is to set up the throttle throw without trim you have the idle you want and the switch position at ‘Cut’ applies an override value to the the throttle servo which completely closes the throttle no matter where the stick is.
In the middle ‘Run’ position the switch does nothing at all allowing the the throttle to move through it’s full normal range.
The glow driver is plugged into Channel 6, which is set to ‘MAX’ and has a weight of -100% which means that is is held in the ‘off’ position unless something specifically overrides it.
The glow-on position is slightly more fiddly if you’re not familiar with the Taranis style of programming. It uses a logical Switch that is ‘on’ when the throttle is below 25% and a special function maps that logical switch to override Channel 6 to +100% which is where I’ve plugged in the glow driver.
For the sake of convenience I also linked an ‘Engine Off’ voice prompt to the off position and a beep for when the glow comes on so if you’re standing out at the flight line and my transmitter is beeping occasionally it’s because the glow has come on, not because my lunch is cooked.
Now that I’ve typed that and read it back I’m not even sure it makes sense, so some photos of the programming on the radio are included and if you’re curious catch up with me at the field and I can explain it in gratuitous detail.
Build it yourself
For those interested in the gory details I used an AT-Tiny85-20P micro and IRFZ44N fet with a 3V9 Zener regulator so it can run on higher than 4cell NiMH/NiCD.
Here’s the schematic for those really curious, the funky mosfet is the default library file in kicad on Ubuntu Linux, don’t blame me. 🙂
The source code is not a good example of coding by any standard, but can be found on github at this address: https://github.com/kiwichrish/on-board-Glow under the MIT license.