Engine Power to aircraft weight ratio?

[flypassion]

I am a beginner and want to built an aircraft all of my own, i.e. from the scrap.

My aim is to built a single-seater, fly-baby style aircraft, the overall empty weight (including assumed engine weight of 100 lbs)should not exceed 500 lbs.

Now, the question is, to select an ENGINE. Is there any rule of thumb by which I can determine the engine power needed to fly a design of this weight?
Assuming my design is optimized to use 70-80% of engine power as compared to what some 'good' designs do.

Pls. assume other parameters to be similar to that of an ultralight.
Eg. cruise speed - 30mph-40 or above

Thanks in advance...
Anshul

 

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