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PHAK · PHAK Chapter 6

Propeller Systems

Master fixed-pitch and constant-speed propellers, governors, blade angle, manifold pressure, and RPM control for the FAA private and commercial knowledge tests.

CFI's Whiteboard Explanation

Think of a propeller as a spinning wing that pulls you forward. Fixed-pitch props are simple — one blade angle, throttle controls RPM, and that's it.

Constant-speed props let you pick an RPM with the blue knob, and a governor automatically twists the blades to hold it. Now you have two power gauges: manifold pressure (throttle) and RPM (prop).

The rule of thumb when changing power:

  • Going up: prop forward, then throttle.
  • Coming down: throttle back, then prop.

That keeps MP from ever bullying a low-RPM engine.

Handbook Reference
PHAK Ch 6

6.propeller-systems. Propeller Systems

The propeller is a rotating airfoil that converts engine power into thrust. Just like a wing, each propeller blade has a leading edge, trailing edge, cambered (curved) face, and flat back, and it produces lift — only this lift acts forward and is called thrust. Because a blade twists from hub to tip, every cross-section moves through the air at a different speed. The tip travels much faster than the root, so blades are designed with a higher blade angle (the angle between the chord line and the plane of rotation) at the root and a lower angle at the tip. This twist gives a roughly uniform angle of attack along the blade in cruise.

Two geometric concepts are central:

  • Geometric pitch — the theoretical distance the propeller would advance in one revolution if it were moving through a solid medium, like a screw in wood.
  • Effective pitch — the actual distance the airplane advances in one revolution. The difference between geometric and effective pitch is propeller slip, which exists because air is a fluid.

Propeller blade angle, often called pitch, is the primary factor determining how much thrust a given engine RPM and airspeed will produce.

Fixed-Pitch Propellers

A fixed-pitch propeller has its blade angle built in and cannot be changed by the pilot. It is the simplest, lightest, and least expensive design and is found on most training airplanes. Manufacturers select a pitch that optimizes the propeller for one phase of flight:

  • A climb propeller has a low blade angle (low pitch). It produces less thrust per revolution but allows the engine to develop higher RPM and more power, improving takeoff and climb performance at the cost of cruise speed.
  • A cruise propeller has a high blade angle (high pitch). It produces more thrust per revolution at cruise airspeed but limits engine RPM, reducing takeoff and climb performance.

With a fixed-pitch propeller, engine RPM is the only power indicator — the tachometer is the primary engine instrument. RPM varies with throttle setting, airspeed, and altitude.

Adjustable-Pitch and Constant-Speed Propellers

An adjustable-pitch propeller allows the pilot to change blade angle in flight. The most common implementation on general aviation airplanes is the constant-speed propeller, which is governed automatically: the pilot selects an RPM with the propeller control, and a governor adjusts blade angle to hold that RPM as airspeed, attitude, and power change.

A constant-speed installation requires two cockpit controls and two engine instruments:

  • Throttle — controls manifold pressure (MP), displayed on the manifold pressure gauge in inches of mercury ("Hg).
  • Propeller control — selects RPM, displayed on the tachometer.

Manifold pressure indicates the power being demanded; RPM indicates how fast the crankshaft is turning. Together they define power output. As a general rule, set MP and RPM in accordance with the POH and avoid operating with manifold pressure significantly higher than RPM (in inches vs. hundreds of RPM) unless the POH specifically allows it, to prevent excessive cylinder pressures.

When the pilot increases RPM (propeller control forward), the governor decreases blade angle so the blades take a smaller "bite" — engine RPM rises. When the pilot reduces RPM (propeller control aft), the governor increases blade angle, the blades take a bigger bite, and RPM falls. With the governor on speed, the propeller automatically maintains the selected RPM through climbs, descents, and airspeed changes.

Typical procedure when changing power:

  • Power increase: propeller (RPM) forward first, then throttle (MP).
  • Power decrease: throttle (MP) back first, then propeller (RPM).

This sequencing keeps MP from exceeding RPM during transitions.

Governor and Pitch-Change Mechanism

The governor uses engine oil pressure, boosted by a governor pump, to move a piston in the propeller hub. Flyweights in the governor sense RPM:

  • If RPM is too high (overspeed), flyweights tilt outward, oil flows to (or from, depending on design) the hub, and blade angle increases until RPM returns to the set value.
  • If RPM is too low (underspeed), flyweights tilt inward and oil flow reverses, decreasing blade angle.
  • On-speed: flyweights balanced against the speeder spring; oil flow stops and blade angle holds.

Most single-engine constant-speed propellers are oil-pressure-to-increase-pitch / spring-and-counterweight-to-decrease-pitch designs, so a loss of oil pressure drives the blades to low pitch / high RPM — the safe failure mode for a single. Most multi-engine propellers are the opposite: oil pressure decreases pitch, and counterweights plus a spring drive the blades toward feather (blades aligned with the relative wind, ~90° blade angle) on oil pressure loss, minimizing drag on the failed engine.

Preflight and Operating Considerations

  • During run-up, cycle the propeller per the POH to verify governor operation and circulate warm oil into the hub. Watch for an MP rise and an RPM drop, then RPM recovery.
  • Avoid prolonged ground operation at high RPM, which can damage the prop and engine.
  • Inspect blades for nicks, cracks, and erosion — small nicks can become stress concentrations and lead to blade failure if not dressed out.
  • Treat any propeller as if the magnetos were hot. Never move a propeller by hand without proper procedures.
Oral Exam Questions a DPE Might Ask
Q1What's the difference between a fixed-pitch and a constant-speed propeller, and how do you control engine power on each?
A fixed-pitch propeller has a built-in, unchangeable blade angle, and the pilot controls power with the throttle while monitoring RPM on the tachometer. A constant-speed propeller has a governor that varies blade angle to hold a pilot-selected RPM, so power is set by combining manifold pressure (throttle) with RPM (prop control).
Q2On a single-engine airplane with a constant-speed propeller, what happens to blade angle and RPM if the engine loses oil pressure?
Most single-engine props use oil pressure to increase pitch, with a spring and counterweights opposing it. If oil pressure is lost, the blades drive to low pitch, which results in high RPM — the safe failure mode that preserves thrust. Multi-engine props are opposite: they feather on oil-pressure loss to reduce drag.
Q3Why do we say 'prop forward first, throttle back first' when changing power?
To avoid running with manifold pressure significantly higher than RPM, which raises cylinder pressures and can damage the engine. When adding power you raise RPM before MP; when reducing power you lower MP before RPM, so MP and RPM stay within the POH-recommended relationship.
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Propeller Systems: PHAK Chapter 6 | GroundScholar