# Propeller Basics: Torque to Thrust

> How a propeller converts rotational energy into thrust, pitch definitions, and efficiency.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-07 — Propellers
- **Topic code:** 081-07-01
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/propellers/conversion-of-engine-torque-to-thrust/

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## Conversion
The propeller acts as a rotating wing.
*   **Resultant Force**: Has a forward component (**Thrust**) and a rotational component (**Torque Drag**).
*   **Engine Torque**: Must overcome the Torque Drag to keep the propeller spinning.

## Pitch Definitions
1.  **Geometric Pitch**: The distance the propeller should move forward in one revolution if it were a screw moving through a solid (theoretical).
2.  **Effective Pitch**: The actual distance it moves forward in one revolution (in air).
3.  **Slip**: The difference between Geometric and Effective Pitch. $Slip = Geometric - Effective$.
    *   **Note**: Slip is necessary to produce Thrust (creates Angle of Attack). Zero slip = Zero thrust.

## Blade Twist (Washout)
The blade angle decreases from the root to the tip.
*   **Reason**: The rotational velocity ($V_r$) increases towards the tip ($V = \omega r$). To maintain a constant, efficient Angle of Attack (AoA) along the entire blade, the pitch must be coarser at the root and finer at the tip.

## Efficiency ($\eta$)
Ratio of Power Output (Thrust Power) to Power Input (Brake Power).
*   $\eta = \frac{Thrust \times TAS}{Brake Power}$
*   Optimal efficiency occurs at a specific Advance Ratio ($J$).

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