Darracq 25HP 1/4 Scale Model

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kenr

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My research for the Anzani Fan Type engine model exposed a lot of prospects for other engine models that have not been built by anyone yet. Among these was the opposed two cylinder water cooled, 25HP Darracq engine of 1909. While not as popular as the Anzani, this small engine was used by several of the pioneers of flight. Alberto Santos-Dumont used it in one of his Demoiselle aircraft (See l'Aerophile, page 435, 1 October, 1909) Examples of the 25HP Darracq are on display in museums in Paris, Budapest, Toronto, and Oslo. Several people have posted images of these in blogs and there are many articles about Alexandre Darracq and his enterprises on the net.

I found patents for this engine at the US (961938, June 21, 1910) and French (407283, September 23, 1909) patent offices. The engine was described in "Some Recent Light Aeroplane Engines" in The Horseless Age, July 13, 1910 and a detailed illustrated technical description of it can be found in The Aero magazine of March 8, 1910.

I made a set of drawings for this model by tracing the Patent drawings. I will post these in PDF format in the Plans section of this forum.

After finishing some, but not all the ultimate tasks on the Anzani I longed to return to making chips. The drawing for the Darracq crankcase was finished and I found a couple of pieces of aluminum for the case halves in my stock box.

I glued a drawing of the case to each halve and roughed out the exterior shape and drilled holes for the through bolts and cylinder center holes and then roughed out the inner recesses of the crankcase and timimg gear cavities. These cavities will be finished with a dremel tool. They do not require any great precision, just enough to give clearence for the crankshaft and timing gears. I left a .020" raised ring on one case and a corresponding relief on the other so that the halves will register correctly together and with a fixture that will hold them during machining.

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I made a couple of gauges attached to dummy shafts to measure the interior clearences for the crank and rod and the beveled timing gear.

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I milled across on the centerline of the crankshaft with a round nose endmill as a guide for boring the hole for the crankshaft. This hole is the same size at the front and center bearing journals and smaller at thr rear journal. My plan was to drill and ream for the front and rear journal and then to bore the center journal with a piloted boring tool that fit the front and rear journals and would make a precise hole in the center established by the pilot section. This didn't work out very well as the strain from the boring caused the tool to break at the small pilot section.

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This problem was overcome by a new tool drilled to accept the smaller diameter fittable rear journal pilot and with a fairly agressive cutter to cut the center journal bore. After this operation was finished and the bushings made, a dummy crankshaft fit quite well with minimum clearance and revolved with no drag when the cases were clamped together.

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After the boring operation and the interior cavities were finished with the dremel, needle and riffler files and emery cloth, work commenced on the external features. I wrote a simple C program to produce tables of offsets for milling the ball shape of the case half. The offsets were computed for just 1/4 of the circumference as the shape is symetrical. I refered to my drawings to determine the starting and ending points of each cut to accomodate the bosses for the cylinders, oil lines and breathers and such. I made a set of appropriatly sized buttons to screw onto the various round bosses. Then it was just a matter of stepping around the exterior of the cases bolted to a fixture on the mill table.

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During all this, I mistakenly cut away a portion of the magneto mount boss. I fixed this by drilling and tapping a hole in the case to accept a 1/2-20 aluminum plug that was glued into place with Locktite super glue. After the glue set I finished the inside with the dremel and the outside with the mill using the correct offsets this time. The plug is not too obvious. Compare this repair with the riveted patch on the engine in the Buapest museum. And imagine going up in the air in a pine and linen craft with such an engine.

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Eventually the outside was finished with the dremel and files and emery cloth.

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Next step was the cylinders. These are a simple cylindrical shape with a mounting flange and a boss to mate with the copper waterjacket. The water jacket is to be soldered to the cylinder at the boss and I thought it might be good to have a matching copper ring at that joint. I experimented a little and found that I could build up copper (from household romex cable) on the boss with a small OA torch using Boraxo as a flux. This is the strange blob like black mess in the image. Eventually the cylinder was finished with the copper ring firmly brazed onto the boss and the flange cut to thickness and the bore finished. The upper end of the cylinder is closed with bosses and threaded holes for the intake and exhaust valve cages, the spark plug and the rocker arm support stud.

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The water jackets are fabricated from .015" cooper sheet. I formed a tube over a mandrel and with an overlapped seam like the prototype, and silver brazed the seam. This was pretty accurately made and fit the copper boss on the cylinder just right for a soft solder joint. The upper end of the water jackets is a cup shape pressed from the sheet copper with a punch and die and the bench vice. It takes several tries to get a good part but when it's done the cup fits over the tubular section just fine. The cup is silver brazed to the tube and tested for leakage. The siver brazing is a little tricky to get right with no leaks but no excess silver filler either.

The end cup part of the water jackets is bored to match the threaded holes in the cylinder for the valve cages, spark plug and rocker arm stud. I believe that when the water jackets are soldered to the cylinder boss and the valve cages and other parts are screwed in place at the head with some Locktite sealer they will be leakproof.

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Thanks Steve. I was inspired to try this by your and George Britnell's posts a long time ago, maybe near 2008 when I first joined the forum.
 
Nice project Ken! Do you intend to fly it on a R/C plane? I'm building a boxer twin to fly it on a R/C Demoiselle but it's not anywhere near a scale Darraq.

I'll be following this build with great interest.

Jos
 
Great work on your engine! Although it takes many hours to whittle out the shapes I find it most satisfying.
 
Nice project Ken! Do you intend to fly it on a R/C plane? I'm building a boxer twin to fly it on a R/C Demoiselle but it's not anywhere near a scale Darraq.

I'll be following this build with great interest.

Jos
Thanaks Jos. I'm not brave enough to try to fly this. I will be satisfied if it runs.

When I started this post I found your's about the Demoiselle. That is a great start on a very ambitious prokect.

Ken Rector
 
After the boring operation and the interior cavities were finished with the dremel, needle and riffler files and emery cloth, work commenced on the external features. I wrote a simple C program to produce tables of offsets for milling the ball shape of the case half. The offsets were computed for just 1/4 of the circumference as the shape is symetrical. I refered to my drawings to determine the starting and ending points of each cut to accomodate the bosses for the cylinders, oil lines and breathers and such. I made a set of appropriatly sized buttons to screw onto the various round bosses. Then it was just a matter of stepping around the exterior of the cases bolted to a fixture on the mill table.
I was trying to figure out from the pictures if you do CNC or not.
The answer is C N H C, Computer numerical human control.
It is amazing how the parts look, a CNC would be a step up or down from here?
 
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