So much so much of aviation relates to power to weight ratio, it is important to know how to prop glow engines. Generally much of what has been published by engine manufacturers is useless because they are looking at minimizing noise which mean lugging the engine down to much lower rpm's. Lower rpm also means lower power and worse yet a much more critical needle setting. A lightly loaded engine is very easy to needle with a broad range, while a heavily loaded engine has a narrow range that varies greatly with slight changs in temperature and humidity. For sport engines and sport airplane's including trainers, having a reliable engine run makes it much more likely to have a less stressful flight with an overall better outcome.

OS does publish the rpm where each engine makes it's peak power, and that includes the muffler it is sold with. Now an airplane running wide open on the ground is a loaded rpm, and as the airplane takes off, the engine/prop unloads. While it is possible with today's telemetry systems to take measurements, ground rpm and knowledge of unload gets you close enough. For 14,000 in air, you would shoot for about 12,000 on the ground. A muffler equipped sport engine is not very peaky, the power band is usually fairly broad, over a range of a thousand either way. So you are just about as good at 11,000 to 13,000 and this is best determined by flying. These numbers are generic 40-45 type numbers, so 60's might be slightly lower, etc.

It is unfortunate if people assume the thrust line on any model is a well defined position for a specific design. Manufacturing tolerance, weight, balance, engine and prop selection make a difference. Getting one setup correctly makes the flying characteristics of any airplane better. Even a change from a 2 cycle to 4 cycle engine will generally make a difference in the prop size which means that one setup does not work the same for either engine selected. Generally the larger diameter requires a change to the right thrust, and errors in down thrust causes greater difficulties in pitch control for landings. A handful of small washers is about all it takes to change the thrust line of 99% of models.

Not all the information directly applies to a very basic trainer, but I have noticed that some pilots are already looking at there second or third models, so they need to be aware of more advanced subjects long before they actually get there. Virtually all of the information I talk about here is nothing new, I read about it in the mid-60's in the pages of RCM, Flying Models, American Aircraft Modeler and other magazines of that era, which was two and three years before my first RC model. As a result I was able to build the airplane, break in the engine, and solo'ed on the third flight at the age of 16. I even landed the model on the first flight and did the take off on the second. And that was the extent of my formal training, the rest was just hard knocks. However this was after flying control line for 6 years.