Starter for prop engine

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
HMEM Supporting Member
Jun 24, 2010
Reaction score
Slowly inching towards test day of my (50cc) 5-cyl radial. I'm not expecting a miracle like pop-pop after a few flicks of the propeller. Rather, likely many flicks required & I don't have the best shoulder. I have RC experience & still have a Sullivan electric starter with the typical silicone cup to engage a spinner. But I think it spins way to fast for my liking until I'm familiar & comfortable. I'm not keen on bump starting & the cup doesn't really fit the nut although I could modify things. But the bigger issue is starting direction would also tend to loosen the nut, conventional RH thread.

I was thinking of something like this, a prop driver with 2 rubber sleeved pegs to engage the prop blades & a centering dimple. The thought being chuck it in my cordless drill which seems a better, slower rpm & sufficient torque. But I've never seen anything like it in my years & maybe there is a reason, such as safely removing when it fires. Any thoughts from those of you who went through the process?


  • SNAG-30-05-2023 5.48.49 PM.jpg
    SNAG-30-05-2023 5.48.49 PM.jpg
    17.2 KB · Views: 14
  • SNAG-30-05-2023 6.18.32 PM.jpg
    SNAG-30-05-2023 6.18.32 PM.jpg
    30.9 KB · Views: 4
I built similar with three pegs for a hit & miss with a six-spoke flywheel. It worked ok, but your centering dimple is an improvement that would help keep the thing centered.
I made an adaptor so I could fit the Sullivan cup to my cordless drill and made modified "spinners" for the engines for it to engage with. The good thing with the cup is it will slip if something locks up.
starting a multi cylinder engine with propeller is easier than you'd imagine, and easier than I imagined. I went through similar mental anguish trying to figure out a way to adapt an electric starter motor, but to the rear, of my Merlin model engine, and after much designing, re-designing, and re-re-designing, finally decided to give hand starting a try and if that didn't work then I'd get back to the hassle of a starter motor. funny thing is it did start !!!, and has continued to start by hand, so still haven't finished the starter motor design !

search for "first pop, merlin v12, peter lawrence" on youtube, you can avoid the boring narrative by jumping to the end, BTW I don't bother with gloves any more.

it will be interesting to see if I can repeat this with any of the inline-4 projects I have going

what the Merlin has going for it is the prop is geared down, so every complete revolution of the propeller all 12 cylinder fire, by hand prop'ing you put from 4 to 6 cylinders through ignition, if only one actually fires it makes a very distinctive sound and you know it will roar to life on the next try or two, I'm guessing because enough mixture has been drawn through the intake manifold that its getting to all the cylinders and if more than one fires its got enough power to keep going.

so my guess is that with an inline-4 or a radial-5 you're only going to be able to push one cylinder through ignition by hand prop'ing, so it will be marginal whether that's enough to get it going or not, you might get to test this out before me (lots to do before my Cirrus I4 is ready to test), so let us know how it turns out if you decide to try hand prop'ing.
Petertha, A guy at my local club has a simple cone lined with rubber that fits (approximately) onto the cone on the prop... He uses his battery drill/screwdriver in drill (high speed gear) mode and just touches the trigger for a fraction of a second to start his engines. Each cylinder is about 4 ccs? - so not a lot of torque needed to get through compression. (I think he has 9 cylinders at about 37ccs total?). The prop cone never comes off (so he told me) because it is screwed on tighter than the pistons at compression can resist the torque. - so the engine turns before the cone can become unscrewed.
On another engine, he had made a "quick-release" double notch - like engine crankshafts would have on a hand-crank engine. (My 1970 Volvo was the last hand cranker I owned! - And it worked when the battery was flat!). Then the starter handle disengages automatically when the engine fires. - Have a look next time you see a pre-1970 car with ability for hand cranking?
I built similar with three pegs for a hit & miss with a six-spoke flywheel. It worked ok, but your centering dimple is an improvement that would help keep the thing centered.
Actually years ago a guy named miller created a tooth belt drive starter almost exactly as you are proposing Nnicad battery was the bat of choice then We just got an Rc aircraft electric motor assembly using an out runner style electric motor these are three phase the entire motor rotated prop drive hp can be as high as 15 hp depending on Lopo battery the out runnervisccapablevofvincredible rpm with a speed controller ours looks to be about 3:1 reduction actual prop speed could be about 6,000 or more so maybe something on this order might be used you might need a sprag butvtheyvareceasyvtobget. You could simply leave it engaged full time let your ic engine just over run the storage .

Second choice is to use parts drone batt electric drill most have more than enough power to spin the ic engine . As for hard starting something iv advocated for a long time is some knobs of scroll blower pulling through the carb while it would have to spin pretty fast to cause supercharging it would provide well mixed fuel and air plus a gentle push down the small intake passages. Air fuel mix does not like to flow down small passages easily small ic engines done have a lot of vacuum due to small cylinder displacement .
I'm falling way behind in my posts again. But this is what I ended up buying. Its a planetary gear reduction motor which I married to a 4S (14.4v) 6.5A-hr RC Lipo on a makeshift plywood frame. Works just right for my application. For reference, the compression was approaching stall on my 18v cordless Makita, but some of that may be related to how these engines are started. When the starter is spinning & applied to the spinner/nut it works better vs a standing start, probably related to the torque curve. I'm not fond of bump starting, but with this one I just engage the nut with the rubber cone & away it goes.


  • SNAG-27-10-2023 4.00.34 PM.jpg
    SNAG-27-10-2023 4.00.34 PM.jpg
    78.1 KB · Views: 8
  • SNAG-27-10-2023 4.00.44 PM.jpg
    SNAG-27-10-2023 4.00.44 PM.jpg
    119.6 KB · Views: 6

Latest posts