Crankshaft Counterweights on Boxer Engines - Why?

[plasticbaldy]

Thank you Guy. I'm open to all ideas and suggestions from subscibers here.

[Guy Croft]

If it was my task I would more-or-less copy the Zenoah! As all experienced engineers know it is always easier to refine a contemporary design and maybe improve it - than to start from scratch.

Inevitably any deviation from that basic layout will be a cost add-on. They pared the design to the limit quite obviously on weight and cost grounds and quite right too.

The only thing I would add is I would personally never develop an aero unit like this as a 2 stroke, far too 'thirsty' and prone to misfire, never mind 'ticklish' to tune (via exhaust) to get the powerband_u_want. But it's your project, not mine!

G

[WhizzMan]

It's a good thing Tom reads my posts and corrects me when I forget that boxers actually even out a lot of forces with their opposing cylinders.

Regarding the amount of weight you would put on a rotating assembly, remember this thing has to defy gravity and take off as well. I can understand you really don't want a lot of weight on this engine, so I assume having less weight further away from the center of the crank ("hollow" fly wheel with heavy metal on the outside) would probably give you more effect for the same amount of mass. On an engine that has to alter engine speed quickly like in a car or on a motorbike, you compromise much more between stability of the engine and quick acceleration, but on a (model) airplane other factors become more important.

[TomLouwrier]

Hi Andy,

I've worked out a sheet to do the calculations for the forces and balance in a boxer twin. Had to look up some theory, it's been over 15 years since I did one of these for real. Feels good though :-)
So far I've used some assumptions regarding the specs of your engine, but I'd like to check with the actual values before I post.

Could you confirm to me
- bore
- stroke
- piston weight (including rings and pin)
- rod length
- rod weight
- rod big end size (bore diameter x width)
Thanks.

regards
Tom

[plasticbaldy]

Sorry everybody. I checked back a couple of times but I didn't see page 2.
I'm having a conventional crankshaft made as pictured in my earlier post because I got lucky and a Chinese engine builder is in the process of building a Boxer aero twin with the same cylinders I'm using. He's machining some sample crankshafts with end shafts that suit my engine correctly.
This will give me a performance and weight benchmark. Then I'll build one with a lightweight centre flywheel disc and counterweights that only balance the crank pin side of the crankshaft and bottom of the rod and bearing. I'll have the crankshaft stuffed with resin to replace the counterweight volume.
Ideally our engines run at constant RPM and don't need a low idle.
To answer Tom's questions ;
- bore - 32mm
- stroke - 25mm
- piston weight (including rings and pin) - 28gm
- rod length - centre to centre 47.5mm
- rod weight - Can unly estimate at this stage 25-30gm ? I've asked for this spec & will advise the accurate weight soon I hope.
- rod big end size (bore diameter x width) 12.5 Bore approx X 8.9mm W - Crank pin 8.5mm approx - TBA.

[plasticbaldy]

If it was my task I would more-or-less copy the Zenoah! As all experienced engineers know it is always easier to refine a contemporary design and maybe improve it - than to start from scratch.

Inevitably any deviation from that basic layout will be a cost add-on. They pared the design to the limit quite obviously on weight and cost grounds and quite right too.

The only thing I would add is I would personally never develop an aero unit like this as a 2 stroke, far too 'thirsty' and prone to misfire, never mind 'ticklish' to tune (via exhaust) to get the powerband_u_want. But it's your project, not mine!

G

Thanks for your input Guy. The cylinder head I'm using resembles a scaled down Zenoah. It has to be smaller though because the 23cc Zenoah weighs too much and the 26 and 29 cc versions that weigh the same as the 23cc are much bigger than needed. A Zenoah style twin 52cc or 58cc in one of these helicopters would be like putting a Rolls Royce Merlin in a Cessna. The 20cc I'm using has nice big transfer ports with the flow web, nice big cooling fins and a suitable over square configuration. The porting closely resembles the Zenoah but is maybe proportionally bigger.

I'm not sure which motor you mean when you talk about misfiring ? All petrol 2 strokes ? Zenoah ? Boxer petrol 2 stroke ?

The alternative highly tuned Methanol/Nitro burning glow plug engines are more than twice as thirsty and more cantankerous to tune than 2 stroke petrol ('Gasser') engines. By comparison the 26cc single cylinder (badly vibrating) Zenoah that most of us have replaced the Methanol (commonly called 'Nitro') engines with are easy to tune and perform fine evern if run safely rich. The same can't be said for Nitro engines. Then there's the prohibitive cost of Methanol/Nitro fuel.