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The steam engine itself can have nearly all of it's non moving parts made of aluminium alloy and most of the working parts made of steel. The stresses in a steam engine are generally lower than an equivalent sized model diesel, glow or petrol engine as there is no explosive pressures to withstand and usually the compression ratio of a steam engine is lower. Yes, steam engines, particularly high performance ones benefit from a degree of compression on the return stroke. Most model engines have massive bronze or cast iron cylinders, solid pistons and thick cast cylinder heads and trunk guides. l try all the time to think about how thin l can make the cylinder wall, how light l can make the crankshaft and how many holes 1 can drill in any flat areas of metal. l wish I could lose weight as easily as my steam engines! ! ! !
Modern engines are so much more powerful than their earlier counterparts because measures have been taken to make them turn out the power at higher and higher speeds. All my current steam engine designs are based on short stroke cranks and BIG valves and BIG ports for inlet and exhaust. These designs are in themselves lighter because they use less metal. The conventional steam engine uses either a piston valve or a slide valve. All my new ideas revolve around the use of poppet valves and uniflow porting which are again lighter, more efficient, and no disadvantage in operation as we do not need reverse! ! ln case you are not familier with the uniflow principle it is a bit like a two stroke IC. engine in that the exhaust ports are cut into the sides of the cylinder wall (see drawing). There is only one valve and in my engines it is a cam operated poppet valve made from an old motor cycle exhaust valve, tough to machine but it takes the heat!
A series of tests publicised in the English magazine |'The Model Engineer' concluded that the uniflow engine in the smaller sizes is the most eff|icient kind of reciprocating steam engine ever developed. Steam inlet cut off is as early as 5 and 10% of stroke but Tiddlers cut off is 25% (90 degrees) which is not efficient but that is not my concern I need all the power I can get!
The Parker Comet engine was designed as an integral part of the airframe and it would be difficult to better the combination just as it is. I have changed the covering under the left wing and used the thinnest litho plate instead of lightweight model covering. This fire resistant material is essential just above the fire door where flames can quickly catch fire to the wing. This area is the worst fire hazard on the whole model during those few moments as you light the fire. For future models I will certainly use a pusher layout which will prevent a wing being over the top of the fire and the rear engine position will protect the mechanics from the worst effects of a seriously misjudged landing. A CLEAR access to the engine fuel tank and boiler is essential with quick release one piece cowlings, you have to be able to get at it all with steam engines---they are more in need of attention at all times this is a fact of life with steam.
I think that the Aeromodeller Plans Service F.F. design Pushy Cat would be a reasonable place to start for inspiration. If this design were to be used there would have to be some serious weight reduction performed as the fuselage is a sheet slab sided affair and a close eye kept on the CG position when fitting an engine and boiler. Pushy Cat is not a bad place to start but it is only a start.
If I do get to the point where a really aerobatic performance becomes a reality I will opt for a pusher, twin boom layout.