I am scratchbuilding an aerobatic plane for electric power (Axi 2826/10) with a 12/6 prop.
The wingspan is aprox. 51" and the length 40". the airfoil is fully symetrical - a bit like the the airfoil used in F3A, with 0.5 degree ind..
Is there a basic guideline how to decide the best down- and right thrust, or is 1.5 degree in both direction a good starting point?

Not sure if there's actually or formula or anything for it.... trial and error usually will get you close enough. I would try to make it adjustable, test with 0 side/down, and then add sidethrust and downthrust as necessary after you see how she flies. Sounds interesting, got any pictures?

Not yet.. I started drawing the plane 2 hours ago

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Try to estimate where the CG would be on the side view of the airplane, considering the weight of the locations of the battery, landing gear, wing, inverted engine or not, etc. Where you locate the engine on the nose will determine the amount of downthrust/upthrust you will need. You will need no downthrust or upthrust if you locate the engine so that the horizontal thrust line passes thru or slightly above the CG. If you locate the engine below the CG, then you should add downthrust until the thrustline passes thru or slightly above the CG. If you locate the engine above the CG, then you may need upthrust to have the thrust line pass thru the CG; however, because of other factors that are speed related, you can tolerate a larger displacement of the thrust line above the CG.
You are estimating where the CG will be, but you will never know until the airplane is completed, so a minor adjust might be required after flight testing.

feihu


I would not use any sidethrust on an aerobatic plane.
I have successfully designed and flown many planes in the past, and these are my opinions/suggestions.