When we have a propeller main features (diameter, CT and CP curves), the pitch angle (for a fixed pitch angle propeller) remains to be set. It can be long and tiresome.
A very convenient way that I use to do this is using a "variable pitch" propeller file. I added such a file in the c172p "Engines" folder.
1 - From the c172p, copy the prop_75in_2f_NACA_15-30deg.xml (done from the NACA curves for a two-blade propeller). Paste it in the J3Cub "Engines" folder.
2 - Change the diameter
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<diameter unit="IN"> 74.0 </diameter>
(and possibly the file name)
3 - In the FDM, add this propeller, replace the ususal one for this filename
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<!-- <thruster file="CM7445 MCCauley"> -->
<thruster file="prop_74in_2f_NACA_15-30deg">
4 - in the sim, you can set the blade angle from 15 to 30 deg in /fdm/jsbsim/propulsion/engine/blade-angle = .....
IMO, a good start is 18 to 20 deg.
18 deg gives a climbing at 2000 RPM, power-hp = 65 hp (best use of the engine power).
Unfortunately, the climbing rate (Gross Weight 1205 lbs) is about 700 RPM, greater than you think it should be.
However, I currently don't see a true fix to noticeably decrease this climbing rate, other than increasing the aircraft drag. The consumed power is due to the aircraft drag and the power needed for climbing at this vertical velocity. This power (plus some lost due to energy efficiency) corresponds to the one given by the engine. And this balance is only weakly (slowly) dependent on the propeller settings. The propeller settings just make that the engine is (or is not) used at its max power.
Or make a "bad" propeller !!