PT6 Prop Governor Operational Principles

Understanding the PT6 Propeller System

Introduction to Propeller Systems

• The speaker introduces a review of the operational principles of the PT6 turboprop engine's propeller system, emphasizing its common use in the industry.

General Principles of Propellers

• The discussion begins with general principles applicable to all propeller systems before delving into specifics about the PT6 design.

• Arc velocity is introduced as a key concept; as different parts of the blade spin, those further from the hub travel longer distances and achieve higher airspeeds, which is crucial for lift production.

Lift Production and Blade Design

• The force generated by arc velocity can cause bending in the airfoil. Modern propellers mitigate this through blade twist, but longer airfoils face challenges in maintaining effectiveness.

• The angle of attack varies along the blade length; lower RPM results in a higher angle of attack. Maintaining this between zero AOA and critical AOA (approximately 16°) is essential for lift.

Thrust Bending and Airfoil Length

• When airfoils exceed certain lengths, they must be designed to withstand thrust bending forces rather than relying solely on blade twist.

• Coning refers to rotor blades bending upwards during operation. Each rotation should ideally move forward based on geometric pitch, but actual movement often differs due to speed variations.

Effective vs. Geometric Pitch

• Effective pitch is defined as how far an airfoil moves forward per rotation compared to geometric pitch; discrepancies lead to "slippage," which represents wasted energy.

• Fixed pitch propellers are effective at specific speeds: climb props work well at low speeds while cruise props are optimized for faster speeds.

Variable Pitch Propellers

• Variable pitch or constant speed propellers adjust their pitch during flight, allowing them to function effectively across a broader range of speeds.

Aerodynamic Twisting Effects

• Aerodynamic twisting occurs due to wind effects on rotating blades; wind ahead increases pitch while wind behind decreases it.

• Counterweights are used near the hub to counteract aerodynamic twisting by ensuring a forward center of gravity (CG), balancing forces during operation.

Centrifugal Force and Counterweights

• As centrifugal force increases with speed, it twists blades towards coarse pitch, counteracting aerodynamic twisting effectively when calibrated correctly.

• There’s a common misconception among pilots that counterweights move blades towards feathered positions; however, they primarily balance aerodynamic forces without moving the blade significantly.

Conclusion on Propeller Mechanics

Understanding Propeller Mechanics

Propeller Piston Functionality

• The propeller piston translates linear motion into rotational force, allowing multi-engine aircraft to adjust blade pitch. Single-engine aircraft utilize empty slots for fine pitch adjustments.

• A hub spring assists the propeller piston by pushing it rearward; some designs may use a nitrogen-charged pressure vessel for added support.

• Beta ranges in propellers include multiple beta rods that provide adjustable fine pitch stops, actuating a slip ring used for engine-mounted components.

Propeller Operation Ranges

• Propeller operation is categorized based on flight phases; single-engine aircraft typically do not allow full coarse or feather positions.

• The reverse beta range requires specific pilot actions and should not be engaged during flight; movement through various pitch settings is crucial for performance.

Oil Pressure Control Mechanism

• Oil pressure controls the prop piston movement, with oil supplied from the engine sump and increased by the primary governor to approximately 375 PSI.

• An over-speed condition occurs when RPM exceeds set values, causing flyweights to vent oil back to the engine sump, resulting in coarser blade angles.

Managing RPM Conditions

• In an underspeed condition, speeder spring force allows high-pressure oil into the prop hub, moving the piston forward and reducing blade angle to increase RPM.

Understanding Propeller Overspeed Protection Systems

Overview of RPM Control and Overspeed Risks

• The reduced drag from a finer propeller angle increases RPM, prompting the governor to return to an on-speed condition. If systems fail, uncontrolled RPM increase can lead to catastrophic failure.

Functions of the Overspeed Governor

• The overspeed governor is a separate unit with two main functions: electronic RPM control via a solenoid and physical overspeed protection through the governor itself. It closes at 104% of maximum RPM.

Mechanism of the Physical Governor

• At 104% RPM, centrifugal force extends flyweights, blocking oil supply to the primary governor. This locks the propeller setting until repairs are made.

Solenoid Valve Operations

• The solenoid valve allows testing of the overspeed governor during run-up by engaging at 92% max RPM, preventing full climb to 100%. This reduces tension on the speeder spring.

• The second function engages the auto feather system by venting pressure from the prop hub, moving it to feathered position regardless of primary governor settings.

Auto Feather System Engagement Steps

• To engage auto feather, pilots must arm it using a cockpit switch; activation occurs when power lever reaches approximately 90%, also triggering pilot notifications.

• If torque drops below 400 ft-lb while in test mode, auto feather deactivates for opposite engine ensuring both engines do not feather simultaneously.

Fuel Topping Governor Functionality

• The fuel topping governor is inaccurately named as it regulates fuel flow rather than acting as a traditional governor. It activates at 106% set RPM by venting bleed air from fuel control units.

• When engaged, it reduces engine power regardless of current prop pitch or oil pressure. Its functionality relies on primary governor operation; if that fails, management defaults solely to overspeed protection.

Reversing Function via Beta System

• The beta system prevents props from entering beta range during low power settings. Oil pressure pushes props into beta range where beta valve cuts off oil supply and vents back to sump.

• During ground operations with power levers pulled over second gate, three functions occur: pivoting beta lever rearward allows movement without adjusting slippering; maintaining oil pressure in beta range; and commanding fuel control unit for reversing thrust production.

Important Considerations for Reverse Thrust Selection

• Before moving over gates for reverse thrust selection, ensure prop lever is fully forward to maximize speeder spring tension and maintain constant speed unit in underspeed condition.

PT6 Propeller Control System Overview

Understanding Propeller Response and Safety Features

• The propeller reacts to venting pressure by shifting from a negative angle of attack to a finer pitch, which paradoxically increases propeller speed instead of decreasing it.

• In multi-engine aircraft, this can lead to uncontrollable asymmetrical thrust and significantly increase landing distances in single-engine scenarios.

• To mitigate these risks, the PT6 design incorporates additional safety features that prevent overspeed situations.

Fuel Topping Governor Functionality

• When the power lever is positioned over the second gate, it lowers the fuel topping governor setting to 95%, which helps manage engine performance during critical phases.

• If an overspeed condition arises, the fuel topping governor vents air and reduces engine power before transitioning into alpha range.

Multi-Engine Applications: Cinco Phaser

• The Cinco phaser (or prop sync) is crucial for multi-engine applications as it addresses noise and vibration issues caused by resonant frequencies in propellers.

• It utilizes an electronic tachometer to monitor RPM and adjusts signal strength accordingly to harmonize propeller speeds without pilot intervention.

Pilot Intervention in Prop Harmonization

• If the system fails or cannot adjust sufficiently, pilot intervention may be required using the prop sync indicator (PSI).

• Pilots can harmonize props by adjusting them in the direction indicated by PSI; smaller adjustments are needed when RPM differences are minimal.

Conclusion on PT6 Constant Speed Propeller System

• Once harmonization is achieved, no further adjustments are necessary. Continuous rotation of the PSI indicates ongoing adjustment needs rather than successful harmonization.