Demoiselle boxer twin

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Intake manifold is now complete.

The carb is "borrowed" from an ASP25 ( 4cc).

I finished the last parts: pistons.

Remaining: all the fiddly jobs like test fitting ,airtightness of the valves testing, and last but not least , final assembly.
This will take some time.....

At last, the engine is assembled and fitted to the test stand.

However, initial starting attempts were not succesful. The prop came loose every time the engine gave a pop. It turned out that angle of the cone ring and the propdriver were not exactly the same, causing insufficient grip on the crankshaft.


It also seemed that the compression was a bit too high, I calculated the CR at about 1: 9,3 where, for example, the Edwards radial has 1: 8,5.

Now both problems were easy to tackle but there was another problem.

From the beginning. I considered the clampingsystem of the crankshaft as a possible weakness, and indeed it was.
After a number of starting attempts the crankshaft was turning heavy so I took the engine apart an the crankshaft was a bit distirtes at the front crankpin.
So , back to the drawing board. It was obvious that the front crankpin had to be silver soldered. Since the rear crankpin does only transmit a very small amount of torque there is no need to solder this one.

I re-aligned the crankshaft by fitting it in the two bearing covers together with one half of the crankcase.

Afther that, the solder joint was made, job done.

With the front crankpin fixed, ther was of course need for a split big-end.
Now there are various types.

conrod types.jpg

A is the most common, B and C are less common, as far as I know, that is.
Now there are a few things to consider. I have a philosophy to use as many materials and parts I already have. The gears for the distribution were laying about in a drawer before I decided to use them. Gears are very expensive so this was an attractive option.
The only drawback was that they were rather small , causing the camshaft turming very close to the crankwebs. There is also the risk of the cam lobes hitting the big-end.
This rules out conrodtype A because the rod bearing cap had to be as small as possible.This leave type B and C. Giveen the fact that there is not much space in the crankcase , type C is not very attractive, leaving type B.
So,I came up with the following design.
split big end.jpg

First I made a dummy of " ordinary "aluminium to explore fabrication methods and possible interference with cam and crankcase.
conrod dummy.jpg

Yes, it looks a bit shabby but it's only for fitting purposes. Besides, if it was the real thing, no one would ever see it ( except you folks, of course...)

Drilling and reaming the crankpin hole I found that is is very difficult to make this hole right in between the already drilled boltholes for the bearingcap with only 0,25mm to spare on each side . So instead of drilling and tapping the boltholes an then machining the crankpinbearing I decided to reverse the machining sequence. This involved the fabrication of a fixture to hold the conrod and bearingcap blanks in place during drilling and reaming.

The same setup was used to drill and ream the wrist pin hole.

Next step was to puch mark the boltholes on the bearing cap,pre-drill the holes, and transfer the holes on the conrod, keeping the bearingcap centered to the conrod .


Boltholes drilled and tapped ( M2 thread)

Excess material removed...

I had to remove some material from the inside of the crankcase with the Dremel because the big-end took a bit more space inside the engine.

Last job... assemble everything again...fingers crossed...

Today was a fine day for test running. Everything assembled with the split bigend conrod, rigged up and ready to go.
It took a few attempts to find the right needle valve setting ( wide open..) and then.. well, see for yourself:

The carb internal dia seemed still a bit too big because it tended to starve at full throttle so maybe I should look in my two-stroke engine collection if I can find an even smaller carb. This one being from a 3,5 cc ASP, maybe I should use one from a 2,5 cc.... The engine ran well without glow power once the engine had started, even at slow idle so that's good.

Apart from the carb issue I consider this build complete. Next step would be the build of the plane but there a few other odd jobs to do, amongst them the overhaul of my MZ ETZ 251 motorbike engine ( 250 cc two-stroke)

I think this will keep me busy for the time to come. Furthermore, since this forum is about engines and not about planes, I will give only very short updates once the Demoiselle is taking shape.

But.. all in due course.

Today was a fine day for test running. Everything assembled with the split bigend conrod, rigged up and ready to go.
It took a few attempts to find the right needle valve setting ( wide open..) and then.. well, see for yourself:

Engine runs great . Congratulations !
You might consider making your engine a 2-stroke. It would look good, and with each cylinder firing each cycle it would produce more power for the same size, and fewer parts.
Hi kf2qd, I have been operating model two-strokes for more than half a century and I still like them but, apart from balancing properties, I see no advantage of boxer twin two-strokes over single cylinder two-strokes of the same swept volume, since the twin cylinders ignite at the same time. Furthermore there is the noise issue. In our tiny little country there is no place where you are far away from wildlife, neighbourhoods, nature parks etc. This is why two-strokes are not allowed by some RC flying clubs. There are even clubs that have banned all IC engines. Besides, I like visible moving parts......

I am 90% electric with my RC models but still love IC engines, especially multi-cylinder 4 strokes. Sadly many clubs are banning them. I can still fly them around the farm but I suspect that the day is coming when those of us with meth(anol) addictions will have to run our engines on the bench.

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