I'm not sure engine HP can be calculated by prop diameter, pitch and rpm.

The typical equation for prop 'thrust horsepower' is HP = Thrust (pounds) x velocity (mph) / 375

It got me thinking. Maybe one could attach a digital scale (I have a digital fish scale) to the model on the ground and see what the static thrust is being generated by the engine. Then fly the model and calculate the tangential velocity based on lap speed and line length. Then put the results in the equation above and see the resultant HP.

But not sure of the overall accuracy. The static thrust on the ground will be a bit higher compared to level flight as I understand the engine unloads a bit once in level flight.

You cannot determine the shaft HP from the static thrust, even on the ground. I can tell you now what the static drag HP is without even a measurement, it is zero, because no work it being done on the airplane.

One thing that you seem to be muddling is shaft power and the airframe power/drag power that the airplane is absorbing.

The unload it not "a bit", it is a factor of 2 or more, and varies greatly depending on the prop efficiency in the actual position. The shaft HP is function of the torque and the RPM, as stated above. The power being absorbed by the airplane in a static condition is the drag x the speed.

The shaft HP relationship to the thrust power is a function of the prop efficiency, which is very optimistically 60-70% (large high pitch at low RPM) and very low, maybe in the 30% range or lower (small diameter, low pitch, high RPM) and exactly zero either on the ground - it's not moving so no power is transferred to the airplane - or when it if moving forward with enough speed that the thrust goes to zero - the airspeed equals the effective pitch x RPM.

While I think it is important to understand the basic relationships and in this case, the terminology, it's usually not terribly critical to know your torque curve, drag HP, etc, to more than in the ballpark values, since you select everything by how it functions rather than by analysis. We have done a fair bit of actual real testing over the years, with good precision, but more-or-less to confirm our observations rather than using it directly to design or trim anything.

Brett