23. MODULE 17A (B1) — PROPELLER

Synchrophasing allows the pilot to adjust the angular phase relationship between the rotating propellers so that blade pressure pulses interact in the most acoustically favourable way.

The total aerodynamic force on each blade resolves into thrust (perpendicular to the disc plane, propelling the aircraft forward) and torque (tangential to rotation, opposing the engine’s turning effort).

Diving increases airspeed and reduces the blade’s effective pitch angle, causing the propeller to accelerate beyond the governed RPM into an overspeed condition.

Higher airspeed increases the apparent pitch of each blade, tending to load the engine and reduce RPM; the CSU responds by increasing blade angle (coarsening) to maintain the selected RPM.

With the tail into wind, the relative airflow over the propeller is from the front but reduced in total; the prop blades experience a higher effective angle of attack from the reversed wind component, increasing resistance and causing RPM to decrease.

Large aircraft de-icing systems may use either AC or DC power depending on the aircraft design; both supply types are encountered in service.

A single-acting governor uses oil pressure to fine the blades against a return spring; if the pump fails and oil pressure drops, the spring returns the blades to coarse pitch.

If the propeller retaining nut is insufficiently torqued, the cones can micro-move under load, causing fretting and galling damage to the seating surfaces.

The manufacturer’s maintenance instructions must be consulted; some preservation oils are compatible with normal operation and may be left in place, while others require draining.

Maximum RPM is set by adjusting the tension of the governor speeder spring; increasing spring compression raises the RPM at which the governor ports oil to coarsen pitch.

Coarser pitch increases blade load, slowing the propeller (RPM decreases); the engine now has to work harder against the increased pitch, raising manifold pressure.

On a fixed-pitch propeller, blade angle is set and does not change with RPM; on a constant-speed propeller, blade angle is actively changed to maintain RPM. The statement that blade angle changes automatically with RPM (implying it is a passive/direct relationship) is incorrect.

When engine torque falls below a threshold (indicating engine failure), the low-torque sensor signals the system to initiate auto-feathering to reduce drag.

The inner portion of the blade from the root to approximately 50% of the radius generates the greatest thrust because the chord is widest and the section camber is most effective in that region.

Temporary storage is typically defined as a period of up to 90 days of non-use; beyond this, long-term preservation procedures must be applied.

These propellers use governor oil pressure directed into the hub cylinder to push the piston and rotate the blades to a higher (coarser) pitch angle.

The duct accelerates airflow through the disc and reduces tip losses, allowing a ducted propeller to generate more thrust than an unducted propeller of the same diameter at low RPM.

Only non-damaging methods may be used; masking tape and non-permanent marker are acceptable because they can be removed without leaving marks or stress concentrations on the blade surface.

Each flight fine pitch lock has an associated indicator; failure of one lock causes its indicator to show unlocked (light extinguished) while the other remains lit — one light on and one out.

The hydraulic pitch lock blocks the fine pitch oil supply path during an overspeed, preventing the blades from reducing pitch further and worsening the overspeed.

In the beta range, the power lever directly controls blade angle; advancing the lever (increasing fuel) simultaneously commands a higher (coarser) blade angle to absorb the additional power.

By continuously varying the blade angle to maintain the selected RPM, the governor keeps each blade section operating near its optimum angle of attack, maximising aerodynamic efficiency.

Power absorbed by a propeller blade is proportional to the total aerodynamic work done; in practice the blade absorbs equivalent power across all blade angles because pitch and thrust trade off equally.

FADEC systems on modern twin-engine turboprops typically implement both RPM synchronisation and phase angle synchrophasing simultaneously to minimise noise and vibration.

Once feathering is initiated, the holding coil keeps the relay closed to maintain electrical power to the feathering pump motor until the blades reach the fully feathered position and the pressure cut-out opens.

An epicyclic (planetary) reduction gearbox is used in turboprop engines because it provides a high reduction ratio in a compact, lightweight package.

Centrifugal forces from an out-of-balance mass increase with the square of rotational speed, so an unbalanced propeller causes proportionally greater vibration at high RPM.

The hydraulic safety stop blocks fine pitch movement; only movement toward coarse pitch remains possible, preventing the propeller from going to an uncontrolled fine pitch.

The magneto check is performed with the propeller control at full increase (fine pitch), which presents maximum load so the RPM drop during the check is accurately measurable.

Blade angle is defined as the angle between the blade chord line and the plane of rotation of the propeller disc.

The pitch lock prevents the blades from moving toward fine pitch (fining off) in the event of a governing or pitch change mechanism failure, avoiding an uncontrolled runaway condition.

Engine power is determined by reading the torquemeter oil pressure and comparing it against a reference chart that correlates torque pressure with shaft power output.