R/C Engine Models

by Paul Butler

Mercedes D.III Engine

Kit: Scratch (1:5)

The attached images are of a 1/5 scale Mercedes D.IIIa engine project I started a few years back for eventual use as a dummy engine in 1/5 scale R/C models.
The prototype depicted is way too heavy for a flying model but it proved the concept.
The next stage is to improve some of the patterns (notably the engine cylinder) and use 'hollow' (where possible) balsa wood cores to reduce the wall thickness of the urethane plastic, 'hopefuly to 1 - 1.5 mm'.
Of the models in my project list, only the Albatros D.III and D.Va will require more than the valve gear, air pump and the very top of the engine cylinders so the added weight should be acceptable.
Note: urethane plastic has a specfic gravity slightly more that unity, ie. it is a little heavier than water.

For those who might be interested, the salient details are:

• All patterns where turned, milled or shaped on an engineers lathe out of aluminum, steel or plastic
• Flexible silicon rubber molds were cast around the patterns using petroleum jelly 'Vaseline' thinned with a lot of white mineral spirit as a parting agent
• All molds have a single parting line
• Up to three molds had to be made for some engine components to get a satisfactory flow of the liquid plastic, eg. no voids or bubbles
• Small parts such as the valve rocker arms are cast together as a 'rosette' in the one mold
• The urethane plastic was the water thin variety that has a pot life of about 30 - 45 seconds
• I believe urethane plastic can be tinted (don't know how strong the color may be and am not certain just what the tinting agent may be)
• I constructed an electric centrifuge to distribute the plastic in the mold for the engine cylinder
• Other parts are centrifuged by placing the filled molds in a bucket swung at the end of a length of rope (exciting)
• The valve springs are actually a very deep 'thread' turned to a taper on the outside. Note that two valves are depressed (open) on the engine
• All hexagonal 'nuts' were filed on the lathe using a filing rest
• Pieces of the plastic soaked in glow fuel for about 2 years without apparent structural detriment. The appearance of the plastic degraded but it also does when left exposed to sunlight for a similar time