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Old 04-20-2010, 01:56 AM
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Sandmann_AU
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Default RE: How big a problem is to much engine

How much of a problem is too much engine? That really depends on how much too much engine you have, and what sort of plane it's going to go into. You can generally go "one size up" without much of an issue as long as it'll physically fit into the airframe, but going from a 26cc gasser to a 75cc is a whole 'nother kettle of fish! You're talking about an engine that's 3 times the recommended size there which will have a bunch of implications. Here's a few...

Weight. You've already mentioned this as refers to CG but if goes further than that. The extra lead you're going to have to add to the tail to get the plane balanced is really going to add a lot of weight to the plane. This in turn will push up the wing loading (ratio of lift generated by the wings to weight of the plane) quite a bit. You're considering putting it onto a warbird and these planes already have wing loadings that're right up near the edge of the envelope to get good manoeuvrability. I think this would make the plane very prone to tip stalls and be fairly un-flyable.

Ground clearance. As soon as you go up an engine size, you also have to go up a prop size to absorb the extra power or the bigger engine will just spin itself to pieces. This is why Spitfires, Corsairs etc "grew extra blades" with later versions as the engines became more powerful. Generally going up "one size" isn't a big problem so long as you're careful with your landings, however I think you'll find that the prop the 75cc needs will be so big you won't even get the plane level on its wheels before you're digging holes in the ground. You can reduce your overall prop size by adding blades (eg: going to a 3, 4, or more blade prop) but you then lose efficiency from the extra air friction & weight of those extra blades. You could always dig a trench up the middle of your airstrip for the prop to fit into but that's a little unwieldy.

Fuel Supply. A larger engine is going to be a thirstier engine, however there's only so much tank you can fit into a model. Often ARFs come with tanks (and mounting points for them) that are already on the small side for the recommended engine and you end up choosing between carving out balsa (eg: structure) to fit a bigger (and heavier - see wing loading) tank, or only having flight times of a few minutes.Your engine has three times the volume of the recommended one and will require a tank that's about three times the size of that plane's standard tank to get about the same flight time... that's going to be hard (or even impossible) to fit into the fuselage.

Airframe Design. Our models are designed with a given engine range in mind and it's assumed you'll use one of the recommended prop sizes for that engine which will deliver a known amount of power and vibration to the airframe. The designers allow for this in their design and use structures that will absorb that power & vibration for the expected lifespan of the plane, using no more material than they need to in the trade-off of strength & weight. This is why no-one designs a model to tolerate a 300% increase in power or vibration - it would simply be too heavy to fly and too costly to build & sell to the intended market.

Now... I'm all for overpowered planes - all bar one of mine have bigger engines than recommended, in fact I once shoe-horned a twin cylinder 160 4 stroke into a biplane intended for a 90 4 stroke! A little extra power's a useful thing and there's always a throttle if you're going too fast, however in this case I think you're exceeding the envelope of sanity and getting yourself some really expensive trouble with this combination. I suggest you get yourself an engine to match the plane you want, or buy a bigger plane to match the engine you have.