Constant-Speed Propeller: AFH Chapter 11

Handbook Reference

AFH Ch 11

11.constant-speed-propeller. Constant-Speed Propeller

A constant-speed propeller is a controllable-pitch propeller whose blade angle is automatically adjusted by a propeller governor to maintain a pilot-selected RPM. By varying blade pitch in flight, the propeller converts a high percentage of the engine's brake horsepower into thrust across a wide range of airspeeds and power settings, something a fixed-pitch propeller cannot do efficiently.

System Components

Propeller governor — driven by the engine; senses RPM through flyweights and uses pressurized engine oil to change blade angle.
Pitch-change mechanism — typically a hydraulic piston in the propeller hub. On most single-engine airplanes, oil pressure drives the blades to low pitch / high RPM, and a spring plus aerodynamic counterweights drive them toward high pitch / low RPM. (On most twins, the logic is reversed so the blades feather on loss of oil pressure.)
Propeller control — the blue knob in the cockpit, which repositions the governor's speeder spring to set the desired RPM.
Manifold pressure (MP) gauge — measures absolute pressure in the intake manifold (in. Hg) and indicates power output to the engine.
Tachometer — displays propeller (and crankshaft) RPM.

How the Governor Works

The governor is a speed-sensing device. Flyweights inside the governor are balanced against the speeder spring, which the pilot tensions with the propeller control.

On-speed — flyweight force equals speeder spring force; the pilot valve blocks oil flow and blade angle is held constant.
Overspeed — RPM exceeds the selected value; flyweights tilt outward, the pilot valve directs oil to the hub, blade angle increases, and RPM is brought back down.
Underspeed — RPM falls below the selected value; flyweights tilt inward, oil drains from the hub, blade angle decreases, and RPM is brought back up.

Within the governor's working range, RPM stays essentially constant even as airspeed, attitude, or manifold pressure changes.

Throttle and Propeller Control Together

The pilot now manages two parameters:

Throttle — sets manifold pressure (power).
Propeller control — sets RPM.

A useful mental model: MP is how hard the engine is working; RPM is how fast it is working. Power output is the product of the two. Always check the airplane's power settings table in the POH; arbitrary combinations can be inefficient or, in some engines, harmful (excessive cylinder pressures at low RPM and high MP).

Operating Procedure

A typical sequence on a normally aspirated single:

Run-up — at the recommended RPM (often 1,800–2,000), cycle the propeller control toward low RPM and back to high RPM two or three times. Watch for an MP rise, RPM drop, and the prop returning smoothly to the original setting. This circulates warm oil through the hub and verifies governor operation.
Takeoff — propeller full forward (high RPM, low pitch), throttle smoothly to full. Verify static RPM and MP are within limits.
Climb — reduce to climb power per the POH (for example, 25 in. Hg / 2,500 RPM). To reduce power, reduce MP first, then RPM.
Cruise — set cruise MP and RPM (e.g., 23 in. Hg / 2,300 RPM). Lean the mixture per POH.
Descent — anticipate the descent; reduce MP gradually to avoid shock cooling. RPM is typically left at the cruise setting.
Approach and Landing — propeller full forward on final or before entering the pattern, so full power is available for a go-around. To increase power, increase RPM first, then MP.

The MP/RPM Rule

A common rule of thumb: avoid setting manifold pressure (in. Hg) higher than RPM/100 unless the POH specifically allows it. For example, 23 in. Hg with 2,300 RPM is fine; 25 in. Hg with 2,000 RPM may overstress the engine. Always defer to the POH cruise power chart.

Why Two Controls Matter

A fixed-pitch propeller is a compromise: it is efficient at one airspeed only. The constant-speed propeller behaves like a multi-speed transmission:

Low pitch / high RPM — like a low gear; produces maximum thrust for takeoff and climb.
High pitch / low RPM — like a high gear; produces efficient cruise at lower fuel flow and reduced engine wear.

Understanding this gearing analogy makes the cockpit workflow intuitive: takeoff in low gear (prop forward), cruise in high gear (prop back), and shift back to low gear before landing in case of a go-around.

Common Errors

Reducing RPM before MP when reducing power (creates an oversquare condition).
Increasing MP before RPM when adding power (same problem in reverse).
Forgetting to push the prop control full forward before landing.
Chasing RPM with the throttle — RPM is now the prop control's job within the governing range.

Oral Exam Questions a DPE Might Ask

Q1How does a constant-speed propeller maintain a selected RPM?

An engine-driven governor uses flyweights balanced against a speeder spring to sense RPM, and routes engine oil pressure to a piston in the prop hub to change blade pitch. If RPM rises above the setting, blade angle increases to load the engine back down; if RPM drops, blade angle decreases.

Q2What is the correct order for changing power in a complex airplane, and why?

When increasing power, advance RPM (prop) first, then manifold pressure (throttle); when reducing, pull MP back first, then RPM. This avoids an oversquare condition where high cylinder pressures act on a slow-turning crankshaft, which can stress the engine.

Q3Why do we cycle the propeller during run-up?

Cycling the prop two or three times circulates warm engine oil through the hub and governor, replacing the cold, congealed oil, and verifies the governor responds correctly — you should see a momentary MP rise, an RPM drop, and a smooth return to the original setting.