What causes an increase in rev/min during the take off run at full throttle in a fixed pitch propeller?

An increase in rev/min during the take-off run at full throttle in an aircraft with a fixed pitch propeller is primarily due to a more efficient propeller blade angle of attack. At full throttle, the engine produces maximum power and torque, which drives the propeller. As the aircraft accelerates down the runway, the relative wind over the propeller blades changes, allowing the blades to develop a more favorable angle of attack.

In a fixed pitch propeller, this angle is predetermined and cannot be adjusted during flight. However, as the speed increases, the efficiency of the propeller improves up to a certain point, leading to more effective conversion of engine power into thrust. This increase in efficiency translates to greater rotational speed of the engine, reflected in an increase in rev/min.

Additionally, more efficient blade performance contributes to maximizing lift and thrust right off the ground during takeoff, which is critical for aircraft performance.

The dynamics of propeller operation are therefore tied closely to the angle of attack. While some other factors like blade slip or overspeeding may play roles in engine operations, the primary reason for increased rev/min correlates with the propeller's angle of attack increasing as the aircraft gains speed during the takeoff run.