Hi sigrun, that engine data I gave you is from OS instructions and from Peter Chinn's engine articles on sceptreflight.net. The Schneurles are more powerful and thus one can get away with a smaller displacement engine. There are some oddities though. I found that the Enya .15-III TV put out the same amount of thrust as the OS Max .15FP-S if propped right. The Enya with a 8x6 Masters put out the same lap speeds as the .15FP with an 8x4 Masters. The .15-III will do wet-2 to fast-2 to wet-2 like it was made for it. The Enya's torque is at a lower RPM around 12,000 whereas the OS is around 15,000. Thus putting larger diameter or steeper pitched props will take advantage of the lower RPM torque curve.
I suppose one could put a 1966 OS Max .10 or about same vintage Enya .09-III on the Akro and fly on 42 foot lines. The later Schneurle .10 ABC / ABN's put out about the same horses as the better .15's of the day plus aren't as critical on fuel oil content. At least from an RC perspective, the 1966 OS Max .10R/C was no slouch. At only 3.1 ounces, it was a definite upgrade for a half-A engine. My single channel with quick blip throttle 39 inch wingspan Top Flite Schoolmaster would climb out at 45 degree angle on full throttle. Could idle the engine for long periods of time, then climb on full throttle without faltering. At full throttle it was the same loudness as a Cox .049 reedy, at half throttle or lower it sounded muffled with mufflerless exhaust baffle plate. I got 3 of those engines now. Those were the days.
Just ordered a Brodak Oriental profile kit for $54, which is a steal, Brodak is clearing out his die cut kits from what I heard. It will be my first one larger than my 42 inch span 400 square inch Ringmaster S-1. At 55 inch wingspan and 555 square inches, think I'll put my Testors Series 21 McCoy .40 Black Head in it. May have to shorten the nose a touch to get it to balance. I feel I'm in tall cotton ....
Sorry to deviate a little ...