Demoiselle boxer twin

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Well-Known Member
Apr 24, 2016
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Rotterdam, the Netherlands
For a very ( very, very) long time I have been considering the build of a Demoiselle model RC plane. The original was built at the beginning of the 20th century by the Brazilian aircraft pioneer Alberto Santos Dumont. A replica was constructed for the film "Those Magnificent Men In Their Flying Machines. It was powered by a Volkswagen four cylinder boxer engine. The original version seems to be flown both by two and four cylinder opposed twin engines.

Following this , I made a design for a two-cylinder boxer engine . I already have a home-built boxer twin ( see avatar) but the main goal was to make a running engine (which it eventually did , 40 years after the first chips..) but it was't meant to "deliver" . To begin with, I started looking what gears I had laying around because they are ridiculously expensive to buy and difficult to make. I found some gears with a reduction of 1:2 and a center distance of 18 mm . Initial bore and stroke dimensions are 18 mm x 20 mm.

The general idea of the engine looks like this:

dwarsdoorsnede 2b.jpg

Crankshaft will be the build-up-type with toolbit crankpins, silver soldered together. Bearing covers on both sides for crankshaft and camshaft bearings

Here are a few pre-machined parts.
onderdelen 1.jpg

I have been working on crankshaft parts. The crankshaft consists of three main parts, the front part, the intermediate crankweb, the rear part and two crankpins made of 6 mm HSS toolbits. The front an rear crankweb have a clamp bolt for the crankpins. The idea is to assemble the crankshaft with all parts centered with a centering pin. Clamp bolts on the front and rear crankweb are tightened and the crank pins are silver soldered to the intermediate crankweb.
krukas  assy center pin clamp bolt.jpg

After soldering, everything is taken apart, cleaned , assembled with the conrods , and aligned to minimun runout.
After that, the shafts are machined to final diameters.

So far the theory, practice will show if it all works out.
I continued with fabrication of the actual crank, front rear and intermediate web.

before sol.jpg

After soldering I found that there was a considerable amount of runout so I did some pre-machining before fitting the conrods.

pre pre.jpg

After cleaning up the crankpins the conrods were fitted.

before f.jpg

Before finish machining I taped in the conrods to protect them from swarf.



I have been working at the crankcase. Machined the holes for the cylinders and a lot of drilling and tapping M 2,5 .
crankcase cpl 1.jpg

And, at last, the icing on the cake: test fitting of the crankshaft assembly in the crankcase.

crankcase cpl 2.jpg

crankcase cpl3 .jpg

It still looks a bit lumpy, but since this build is an ongoing process of fabricating and designing I will remove excess material after the main dimensions are finished.

First of all I would like to wish all of you a prosperous and healthy 2022. May the chips be with you....

I have prepared the camshaft gears. I made a flanged hub and fitted the two gears with four M2 screws. See sketch and pictures below.

I made keyways in the pinion and crankshaft by drilling a 1,5 mm hole half in the shaft, half in the pinion, see the arrow. I will use a 1,5 mm needle as a key. I did a test assembly with a dummy camshaft to check everything, in particular the gear mesh which was fine.


It's been a while since I've been working on the boxer twin. Last in-between project was the rebuild of a 49 cc Solex moped engine.

So, now on with the Deemoiselle engine.
I started to work on the cylinder heads, pre-machining and drilling pilot holes for the valve cages.


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I think is is always handy to make a translucent model of a cylinder head to see where all the holes are going, ant to avoid unwanted interference these holes. Rocker arm support is also fitted.


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really like that acrylic head !!!, I always make clear acrylic distributors (on a recommendation from a very wise old machinist) so I can see what I'm adjusting. "may the swarf be with you" :) !
I think is is always handy to make a translucent model of a cylinder head to see where all the holes are going, ant to avoid unwanted interference these holes. Rocker arm support is also fitted.
Great idea, will do that for my 2VD5. Not only very helpfull to check the design, but later also as a gimmic to show the internal structure of the engine.
After a break due to some other projects I went on with the cyinder heads. I milled the slot for the rocker arm support.
milslot 2.jpg

After that , drilling of pilot holes for the intake and exhaust ports.

And planing mounting surfaces for in- and exhaust pipe flanges.

Having completed this, I went on milling the top cooling fins.


Checking the fin distance with my trusty improvised DRO...


At last, cutting the side fins, a rather elaborate operation with a funny mounting plate ( leftover from the Edwards project)
and a parting tool.

sidefin com.jpg


Well so far, so good. In and exhaust ports will be drilled to final diameter after fitting the valve cages which will be the next job.

Hi Jo!
Congratulations on your project!
As being a brasilian, I will follow your building with close interest.
Santos Dumont was the first person in the world to fly with a machine using the 14BIS. The Deomoiselle was the second flying machine he built. After several atempts he only got the sucsses flying 2.500 meters in April 1909 using a Dutheil-Chalmers engine with 18 CV. Later he change to a Darracq engine with 30 CV. With this configuration he was able to fly 8 quilometers from Saint-Cyr to Bucin France. He is considered th Father of the Aviation.

Good louck with your building than flying.

Hello Edi,
As far as I know Alberto Santos Dumont is still considered an all-time hero in Brazil like , for instance, Ayrton Senna. He was also a pioneer balloonist with gas balloons.
My engine will not look like any of the original engines such as the Dutheil-Chalmers.
NASM-a19690250000cp08 (1).jpg

This also applies to the plane itself such as this one.

Having said that, I'll cross that bridge when I come to it...