Propellers and engine/propeller interface (governors, vibration)
Propellers and engine/propeller interface
This section covers propeller types, governors and vibration where the propeller loads the engine. The exam tests how a constant-speed governor controls blade angle and how propeller faults appear.
Core rules and concepts
- A fixed-pitch propeller has a blade angle chosen for one compromise; a constant-speed propeller changes blade angle to hold selected RPM as power or airspeed changes.
- The governor senses RPM with flyweights opposed by a speeder spring. If RPM rises above the setting, the flyweights move out and the pilot valve directs oil to increase blade angle, increasing load and slowing the engine.
- Feathering turns blades edge-on to reduce drag after engine failure; reverse pitch is used on some installations for ground control. Both need specific governor and propeller features.
- Propeller imbalance, track error, blade damage or a loose spinner can produce vibration. Dynamic balancing reduces vibration but does not repair damage.
- The engine/propeller interface includes the crankshaft flange, dowels, bolts and safetying; correct torque and runout are essential.
Worked example
On a governor test, RPM overspeeds when the throttle is advanced. The governor should increase oil pressure to the propeller to raise blade angle. If RPM continues to rise, suspect the pilot valve, oil supply or propeller linkage-not the magnetos.
Traps
Do not say the governor sets manifold pressure; it controls RPM. Do not confuse feathering with coarse pitch; and do not accept vibration as normal until balance, track and blade condition are checked.