Understanding Fixed Pitch Propellers: Exploring Angle of Attack and Propeller Terminology

A fixed pitch propeller is a key component in aircraft that cannot be adjusted in flight to change its pitch. However, the angle of attack can change, and understanding this concept is crucial for pilots and mechanics.

The Basics of Fixed Pitch Propellers

When a metal aluminum propeller is overhauled, it's possible to alter the pitch by bending the metal or selectively removing material. However, there are manufacturer-specified limits within which these changes can be made, allowing for adjustments that either reduce takeoff distance or increase cruise speed. These modifications are limited to specific ranges to ensure airworthiness and safety.

Angle of Attack and its Impact

When discussing propeller pitch, it's important to distinguish between the fixed pitch of a propeller and the angle of attack. The pitch of a fixed pitch propeller, once set, cannot be changed in flight; however, the angle of attack can vary depending on the aircraft's speed and maneuvering.

The angle of attack is the angle between the propeller blade and the relative airflow. In a slow climb, the angle of attack might be different compared to a fast dive. As the aircraft moves faster, the angle of attack changes, affecting the performance of the propeller. If the angle of attack is not optimal, the propeller’s efficiency is reduced, potentially leading to decreased thrust and optimal performance.

Efficiency and Limitations

At the core of the fixed pitch propeller’s design is the fixed angle and pitch. This design is inherently limited in terms of adaptability. Unlike variable pitch propellers which can be adjusted by oil pressure on a piston in the prop hub, fixed pitch propellers rely on other methods to control thrust and speed.

The only practical way to vary the thrust from a fixed pitch propeller (FPP) is through RPM (Revolutions Per Minute) control. By adjusting the RPM, pilots can control the speed of the propeller blades and thus the thrust. However, this method is limited by the aircraft’s engine limitations and other factors.

Another approach involves manipulating the aircraft’s attitude. By changing the nose-up or nose-down attitude, pilots can cause the upward blade to lose thrust and the downward blade to approach stall. This method, while theoretically possible, is not very effective due to the limited thrust and stability it provides.

Conclusion

In summary, a fixed pitch propeller's pitch remains constant, but the angle of attack can change based on the aircraft's speed and maneuvering. This angle of attack significantly impacts the propeller's efficiency and thrust. The limitations of fixed pitch propellers highlight the importance of variable pitch propellers in modern aircraft for better performance and control.