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Engine size rules of thumb?

Old 01-05-2004, 09:23 AM
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jchumley
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Default Engine size rules of thumb?

I seaarched but found little in the way of guidance. I have a particular plane in my head that I want to figure engine size for but I would really like a more general discussion of determining the correct engine size for a new plane. I know there are many variables.

THe plane I am thinking about is a Caproni Ca.42 triplane bomber from 1918. 1/12 scale puts it just about 8 feet on the wingspan of all three wings. It has twin tail booms, three full flying rudders and a single, long stabilizer. Here are ball park figures:

Wing span - 96"
Wing Chord - 7.5" (Long, narrow wings)
Wing area - 2160 sq. in.
Length overall - 42" (SHort coupled)
Engines - 3 (2 tractor and one pusher.)
Speed - 78 mph

Lots of spars and wires.

What displacement should each engine be?

Is there a rule of thumb that says, for big, lumbering WWI bombers, take the number of wings* total surface area multiplied by the sqare root of the aspect ratio blah blah blah, then a miracle happens and you have an approximate displacement for your model.

Also, should the three engines add up to the total displacement required or are three less or more efficient than one larger one?

Jeff
Old 01-05-2004, 02:50 PM
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Flypaper 2
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Default RE: Engine size rules of thumb?

The other factor is the weight. If you keep it light, let the flying wires do their work, three 91 four strokes should do the job. Three engines added up are more efficient than one engine because of the larger propeller disc area.

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