Mini Buddy .021 ci engine

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Sydney Australia
Project of the Month Winner
Jun 14, 2013
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Sylvania, Sydney Australia
Just a quick report on a new motor just finished.

I have called it the Mini Buddy as a follow on from the Holly Buddy and Holly Buddy inline twin.

The design had to incorporate these design features
  • Easy to build
  • Enough power to fly a free flight model around 24 to 30 inches
  • Long lasting
  • Easy to start
  • Conservative timing
  • Light weight
  • Radial mount
  • Piston port
  • Some tolerance to the piston bore fit.
  • Use a ball bearing on the inner end of the shaft
It is a simple square crankcase design with a solid cylinder that allows for the bypass within.

On test it swings a 5 ¼ x 4 prop at just over 7,000, so it should have enough power to fly even an RC assist model of intended dimensions.

Re long lasting – time will tell, but in the first 20 minutes of running it has actually improved in both feel and compression.

Easy to start – a long piston helps piston seal with not ideal piston / bore fit. The compressing gases have to sneak further beside the piston to “escape” with subsequent loss of compression.

Conservative timing helps easy starting. Maybe the exhaust timing is a little less so and experimentation might show a slightly lower exhaust port could improve performance a little more – but it runs beaut the way it is.

Light weight – it weighs 47 grams so not too bad. A DC Dart weighs 41 grams and would probably be at least 50% more powerful.

The radial mount uses a solid backplate, the rear cylinder hold-down bolts go through into the solid part as the crankcase is too thin there for enough thread. The bottom of the backplate uses normal countersunk screws. In practice this has worked fine.

A piston port design seems to give reasonable performance with the benefit of being easier to start.

Some tolerance in the piston to bore fit. This engine has around 1/10th thou tolerance but a long piston would normally allow a bigger tolerance as described above. On test I used a 40:40:20 ratio for fuel – the 20 being kero – this seemed to work just fine and the added oil not only keeps things well lubricated but helps sealing too.

Using a ball bearing on the inner end of a small motor seems to make a lot of sense. The components are pretty fragile and from experience wear in the crankshaft tunnel can be a problem with small motors, especially if the shaft is a bit short. This shaft is ample long enough to not have that problem.

In the finalizing of the build I found that I had cut the transfer port a bit short and in correcting the error I then machined it too long. This meant my very nice fitting cast iron piston was useless and faced with making another from 30mm round cast iron bar – I thought just maybe I could get away with using some 18% silicon alloy I was given by Geoff Potter some time ago. It was about 20mm round but one end was down to small diameter almost the right length so I made the piston out of that. I have used alloy before in larger motors and it will seize as it gets hot and relieved enough to run it is soft in compression cold. BUT in this instance I figured with such a small 7mm piston diameter the expansion would be negligible. N fact it has turned out perfect and hot comp is the same as cold – but no hint of slowing down as it gets hot with running. The other thing this allowed me to do is correct to a degree the too big timing for the exhaust – I made the piston 11.8 mm long rather than the 11 mm originally and put a .83 step in it instead of the 2.0 step in the first piston. Again seems I have “stumbled” across a happy resolution to my mistake in making the transfer too long ( by 2mm not allowing for that step in the original piston.)

I have some rather rudimentary plans for it – if someone want to transfer to CAD would be good.

Here are a few photos and a link to it running on its 2nd ever start. And no I didn’t prune the video – it started first flick for it !


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