Chris, thank you for your answer.
If 203 km/h at weight 518 kg, ie 5080 N,
375 rrpm means blades pitch in flight = + 4.3° aerod. (more than the pitch setting because underbalancing), then according my spreadsheet: rotor drag = 454 N (less profile loss), and disk A.o.A = +5.1° Longitudinal flapping a1 = 4.2°
With 100 HP, propeller thrust = 1050 N
Difference= parasitical drag = 596 N
Because Drag = ½ ρ V² S Cd, we have S.Cd= 2 D/ ρ V² = 0.32 which is in m²
unrelated to the drag of a flat plate, but proximity gives a good mental picture.
Note: standard air density at 400 feet above see level is 1.17 kg/m3 .
If 190 km/h, then rotor drag = 485 N according my spreadsheet.
Disk A.o.A = +5.45° a1 = 3.85° Blades pitch in flight = + 4.12° aerod.
With 100 HP, propeller thrust = 1110 N
Difference= parasitical drag = 625 N
=> S.Cd = 0.38 m²
Main uncertainty is the airspeed.
Other point.
In my post # 26, I mentioned that my calculation is for a 8.6 x 0.2 m rotor, without being contradicted. Yet the autorotation limit would be reached before 210 km/h.
Since you said reach 220 km/h, this means an inconsistency.
Are you sure on the cord of 0.2 m? Not more ?
This hardly changes the calculated parasitic drag, but I am curious.