Feathering propeller and reversible pitch propeller

Summary

This script discusses the mechanics of propeller control units (PCUs) in aircraft, especially during engine failure. It explains how PCUs sense RPM drops and adjust pitch to prevent damage, leading to a windmilling propeller. The concept of feathering the propeller to minimize drag and prevent further damage is introduced, along with the different types of control units and their operation. The script also covers auto-feathering systems, alpha and beta pitch ranges, and the use of reverse pitch for braking after landing, providing a comprehensive look at propeller management in various flight conditions.

Takeaways

• 🔧 When an engine fails in flight, the propeller control unit senses a drop in RPM and moves to the under speed or fine pitch position to maintain the selected RPM.
• 🌪️ The immediate result of engine failure is a large asymmetric drag that can yaw the aircraft violently towards the failed engine.
• 🔥 A continued windmilling propeller could lead to complete mechanical breakdown or even an engine fire.
• 🪄 To minimize drag and prevent further damage, some propellers can be turned edge-on to the air, a process called feathering, which stops the propeller from rotating.
• 🛫 Feathering is particularly important on multi-engine aircraft to maintain control and balance in the event of an engine failure.
• 🔄 Propeller control units (PCUs) contain an oil pump and a hydro mechanical governor for RPM control, with an added feathering mechanism for emergency situations.
• ✈️ Pilots can manually feather propellers using separate engine power and propeller control levers, or a single lever arrangement depending on the aircraft.
• 🔧 The feathering process involves moving the propeller control lever to the feather position, which lifts a valve and allows oil to drain, moving the pitch change piston to the feathering stop.
• 🔄 Unfeathering a propeller requires releasing spring tension and reconnecting the reserve oil pressure line to the fine pitch side of the propeller.
• 🛑 Some propellers are equipped with an auto-feather system that activates during engine failure if the engine controls are set to high power, reducing pilot workload.
• ⚙️ Propeller control units may also include a flight fine pitch stop solenoid for additional control over propeller pitch, especially during takeoff and landing.

Q & A

What happens when an aircraft engine fails in flight?

Upon engine failure, the propeller control unit senses a drop in RPM and moves to the under speed or fine pitch position, causing the propeller to windmill and the aircraft to yaw violently towards the failed engine.

What is the purpose of feathering a propeller in an aircraft?

Feathering a propeller minimizes drag and prevents further damage to the engine and aircraft in case of engine failure. It also allows for the propeller blades to be turned edge-on to the air flow, ceasing rotation and reducing aerodynamic forces.

How does the propeller control unit (PCU) respond to an engine failure?

The PCU responds to engine failure by sensing a drop in RPM and adjusting the propeller pitch to the fine pitch stop, which puts the propeller into a windmilling situation to minimize drag.

What is the difference between a single lever and twin lever cockpit arrangement in propeller control?

A twin lever arrangement allows the pilot to have separate engine power and propeller control levers, while a single lever arrangement provides a power lever only, adjusting propeller pitch, RPM, and engine fuel flow simultaneously.

How can a pilot manually feather a propeller in a twin lever arrangement?

In a twin lever arrangement, a pilot can manually feather a propeller by moving the propeller control lever to the feather position or using a separate feather lever, if available.

What is the role of the centrifugal latch pins in a propeller system?

Centrifugal latch pins prevent the feathering spring from pushing the propeller blades all the way to the feathered position, which could cause high loading on the starter motor during startup.

How does the auto feather system in a propeller work?

The auto feather system is active during takeoff and cruise, and it automatically feathers the propeller in response to a low torque signal from the engine, provided the engine controls are set to high power.

What are the alpha and beta ranges in propeller control, and how are they used?

The alpha range is used for high-speed operations like takeoff and in-flight, while the beta range is used on the ground for reversing pitch or ground fine pitch during landing. The alpha range is selected by moving the engine power lever to a position at or above flight idle, and the beta range is selected by moving the lever below flight idle or using the flight fine pitch stop lever.

What is the function of the flight fine pitch stop (FFPS) in a propeller system?

The flight fine pitch stop (FFPS) prevents the propeller blades from moving to ground fine pitch while in flight. It disengages during the landing roll to allow the selection of the beta range for ground operations.

How does a propeller in reverse pitch contribute to slowing down an aircraft after landing?

A propeller in reverse pitch generates forces that actively decelerate the aircraft after landing by increasing the load on the engine and using engine power to create a braking effect.