📖 The lesson
Forces acting on a propeller
- Centrifugal force — the largest load. It tries to pull the blades out of the hub, putting the blade roots and hub in tension. A large propeller can generate tens of tonnes of centrifugal force per blade.
- Thrust bending force — the thrust load bends the blades forward at the tips. Thinner blades at the tip bend most. Centrifugal force partly opposes it by straightening the blade.
- Torque bending force — air resistance to rotation bends the blades backwards, opposite the direction of rotation.
- Centrifugal twisting moment (CTM) — the mass of the blade, spread either side of the pitch-change axis, tries to rotate into the plane of rotation, turning the blade towards fine (low) pitch. It is greater than the aerodynamic twisting moment. Some pitch-change systems use CTM to move the blades to fine pitch, and counterweights on the blade shanks are fitted to oppose it and drive the blades towards coarse pitch or feather.
- Aerodynamic twisting moment (ATM) — the centre of pressure of the blade lies ahead of its pitch-change axis, so lift tends to turn the blade towards coarse (high) pitch. Some systems use ATM to assist feathering. During windmilling, the centre of pressure may move behind the pitch axis and reverse ATM.
- Vibratory stresses (from engine firing impulses, airflow and damage) are superimposed on these steady loads, so any nick or scratch can start a fatigue crack.
Torque and torque reaction
- The engine applies torque to turn the propeller; the air resists with an equal propeller torque. In steady flight, engine torque = propeller (aerodynamic) torque, so the rpm is constant.
- By Newton's third law the aircraft tends to roll opposite the propeller's direction of rotation — torque reaction. For a propeller turning clockwise seen from the cockpit, the aircraft rolls left. Designers counter it with wing rigging (wash-in/wash-out), aileron trim or offset engine thrust lines.
- Contra-rotating propellers (two propellers on the same axis turning opposite ways) and counter-rotating engines on twins cancel torque reaction.
Other propeller effects
- Slipstream (spiral) effect — the propeller wash spirals around the fuselage and strikes one side of the fin, yawing the aircraft (left yaw for a clockwise propeller). Fins may be offset to compensate.
- P-factor (asymmetric blade effect) — at a high nose attitude, the descending blade meets the air at a greater angle of attack than the ascending blade, so it produces more thrust. The thrust line moves to the descending-blade side, causing yaw (left yaw for a clockwise propeller at high power and low speed).
- Gyroscopic precession — the spinning propeller is a gyroscope. A force applied to the disc acts 90° later in the direction of rotation. Pitching the nose down (as a taildragger raises its tail) yaws the aircraft.
- On twin-engine aircraft, the engine whose failure most affects controllability is the critical engine, because of the asymmetric thrust lines produced by P-factor.
Vibration and resonance
- Every blade has natural frequencies of vibration. Engine firing impulses, aerodynamic disturbances (airflow over wings and engine nacelles, angled airflow) and imbalance provide forcing frequencies.
- Resonance occurs when a forcing frequency matches a natural frequency. The amplitude grows sharply, raising stresses and risking fatigue failure, often near the tips or roots.
- Engine–propeller combinations are tested to find any critical rpm ranges. These appear as a red arc (restricted range) on the tachometer, and steady operation there is prohibited; pass through it quickly.
- Typical causes of propeller vibration in service: imbalance (static or dynamic), blades out of track, unequal blade angles, loose blade retention, damage, a loose spinner or engine mount problems. Diagnosis and correction are covered in the maintenance chapter.
- Changing the blade angle, the blade tip shape or the number of blades, or modifying the engine dampers, can move natural frequencies away from operating rpm.