Propeller blade aerodynamics
A propeller blade is a rotating wing: airfoil, cambered on one face, flat on the other. Converts engine power into thrust (PHAK ch.5).
Blade angle vs pitch (PHAK ch.5):
| Term | Meaning |
|---|---|
| Blade angle | Angle between blade chord and plane of rotation, measured along blade |
| Pitch | Inches propeller screws through air per revolution, no slip |
- Distinct, but often used interchangeably: more of one usually means more of the other.
Blade AOA (PHAK ch.5):
- Each blade section moves two ways at once: rotating about hub, forward with aircraft. Relative wind = vector sum of both.
- Relative wind to chord angle = angle of attack (AOA), distinct from blade angle.
Thrust (PHAK ch.5):
- Same physics as wing lift: camber and AOA deflect airflow.
- Pressure rises on engine side, drops ahead.
- Reaction acts forward, not up: plane of rotation is vertical, not horizontal.
- Newton's Third Law: propeller accelerates air mass backward, equal and opposite reaction pulls aircraft forward (TP 1102).
Blade angle and efficiency:
- Blade angle sets size of air bite per revolution: loads propeller, controls RPM.
- Propeller efficiency = thrust HP / brake HP.
- Efficiency set by slip: gap between geometric pitch (theoretical no-slip advance per revolution) and effective pitch (actual advance).
- Efficiency lowest at rest or start of take-off roll: AOA yields little thrust for power spent.
Twist (PHAK ch.5):
- Blade angle decreases root to tip: tip travels faster than root at same rotation rate.
- Without twist: root could sit at negative AOA while tip stalled at cruise.
- Twist keeps AOA, and thrust, even along blade.
Trap: treating blade angle and AOA as the same. Blade angle = fixed geometric setting; AOA = measured to relative wind, changes with airspeed while blade angle does not.
References
- PHAK ch.5 (FAA-H-8083-25)
- TP 1102 (Basic Principles of Flight)