021 Airframe, Systems and Powerplant topic guide
Constant-Speed Propellers
A fixed-pitch propeller is efficient at only one combination of airspeed and RPM, which is fine for a simple trainer but wasteful for anything flown across a wide speed range. A constant-speed propeller solves this by letting blade angle vary continuously, so the propeller can present the ideal angle of attack to the airflow whether the aircraft is climbing at low speed or cruising fast, while the engine itself is held at a chosen RPM.
The governor is what actually makes this work, and almost every exam question on the subject is testing whether the reader understands the governor's three-state logic rather than the mechanics of the blades themselves. Get the governor logic solid and the rest of the topic, feathering, windmilling, lever behaviour, follows naturally from it.
How the governor holds RPM
The constant-speed unit compares actual propeller RPM against the RPM the pilot has selected with the propeller lever. If RPM matches the selection, the unit is said to be onspeed and it holds blade angle steady. If RPM tries to rise above the selection, for instance because the aircraft has entered a descent, the unit is in an overspeed condition, so the governor moves the blades towards a coarser (higher) angle, which increases the load the airflow presents to the blades and pulls RPM back down.
When the propeller tends to underspeed, for example the aircraft pitches up and the airflow slows, the governor moves the blades towards a finer (lower) angle, reducing the load on the blades so the engine can maintain the selected RPM against less resistance. This constant small adjustment, coarsening against a rising tendency and fining against a falling one, is what lets the pilot select one RPM and have it held nearly constant across a wide range of flight conditions.
Fine and coarse pitch in practice
Fine pitch, a low blade angle, presents less resistance to the airflow for a given RPM and is what the propeller adopts for take-off and initial climb, where high RPM and high power are wanted from a relatively slow-moving aircraft. Coarse pitch, a higher blade angle, is what the propeller settles into for cruise, where the aircraft is already moving quickly and a shallower angle would let RPM run away.
Between the two extremes sits the normal governing range, but there is also a mechanical fine-pitch stop that prevents the blades going finer than needed even for take-off, and a feathered position at the extreme coarse end used only when the engine is deliberately stopped.
Feathering and windmilling
If an engine fails or is shut down in flight, an unfeathered propeller keeps being driven by the airflow, a state called windmilling, which is aerodynamically very draggy and can also motor the failed engine in a way that is undesirable. Feathering rotates the blades to an edge-on angle relative to the airflow, minimising drag and stopping the windmilling rotation.
The propeller lever position and its associated indications are what the exam checks students can read correctly: moving the lever towards the coarse end commands a coarser angle for a given power setting, and feathering is a distinct, further action beyond the normal coarse end of the governing range, not simply the coarsest point of normal cruise adjustment.
Worked example
Worked example: reading a governor response
Not quite. The correct answer is B.
Correct answer: B
- A. This describes what would happen with no governor at all, or with the unit failed, since a working governor actively resists the rise rather than allowing it.
- B. Correct. The higher airspeed tends to speed the propeller up, so the governor moves the blades to a coarser angle to increase resistance and holds RPM close to the 2400 selected.
- C. This pairs the wrong direction of blade movement with the correct outcome. A fining movement would add to the overspeed tendency rather than opposing it.
- D. A constant-speed unit works by continuously adjusting blade angle. Treating blade angle as fixed describes a fixed-pitch propeller, not the constant-speed system in the question.
Step by step
- Identify the disturbance: increasing airspeed in the descent tends to drive the propeller towards a higher RPM than selected.
- Recall the governor's job: hold the selected RPM by opposing any tendency to move away from it.
- To oppose a rising tendency, the governor coarsens the blade angle, increasing the load the airflow presents.
- The net result is blade angle increasing while RPM is held close to the 2400 selected, not allowed to rise.
Common mistakes
Getting fine and coarse pitch backwards
Fine pitch is the low blade angle used for take-off and is associated with high RPM at low forward speed; coarse pitch is the higher angle used in cruise. Swapping the two flips the answer on every question that asks which pitch belongs to which phase of flight.
Assuming the governor allows RPM to change with power or speed changes
The entire point of a constant-speed unit is that RPM stays close to the selected value across changing conditions; it is blade angle, not RPM, that does the adjusting. Answers that describe RPM drifting freely under normal governing describe a failed system, not a normal one.
Confusing feathering with simply reaching the coarse end of normal governing
Feathering is a distinct, deliberate action that takes the blades beyond the normal governing range to an edge-on angle, used to stop a failed engine windmilling. Describing it as just very coarse cruise pitch misses why it minimises drag on a stopped engine.
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Last reviewed July 2026