Are you sure that all of the numbers that you have given are correct?
The useful power output is the propulsive power.
Propulsive power = Thrust x TAS
In straight and level constant TAS flight Thrust = Drag.
From your figures
An aircraft flies at 65 m/s. Drag is 4,22 kN,
We have propulsive power = 65 m/s x 4220 N = 274300 NM/s
That is 274.3 KW of propulsive power.
But you have stated that the power output is only 36 KW.
If this is the power going into the propeller then the propulsive efficiency is
Efficiency = THP/BHP = 100% x 274.3/36 = 762%.
