RPM- counts in all of this - I tried to show that it was part of the formula , in my previous post.
To really fix the problem of operating in thin air -the really good fixes are : A larger prop with more pitch and a more powerful engine , to turn this prop.
The big models turn slower but the tip speeds are still fairly high- due to diameter.
My personal approach tho --
rather than build a model and try to power it correctly, I find the power to weight engine I like best ----then --- build a model to fit .
Old timers may remember, that is the approach used by the best fighter aircraft designers in WW 2.
A classical example is the Japenese fighter ,called the Zero.
The designer had a specific engine for use.
His job was to make a craft based on that engine.
As Bob the Builder says "we can do it!"

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