Re: "even if you have a tach the true test of a prop is when the plane is in the air." < this is true, but you soon learn how fast different length props need to be turning in order to get the plane off the ground. For instance I found out that for a 40 size plane on our grass field while using a 10" or 11" prop it better be spinning around 12,000 rpm for a decent take off speed. If its up around 14,200 like I get with my OS FX engines then the plane will have lots of power for takeoff. If its only around 10,000 then you will be lucky to get the plane off the ground! Also this low rpm means that either you have a prop with too much pitch on it or the needle valves need some serious readjusting. The same 12k to 14k rpms will also work well for the larger 52 sized engines. [8D]