Aircraft Maintenance Knowledge Levels

Knowledge Levels

• Categories A, B1, B2, and C in aircraft maintenance have basic knowledge indicated by levels 1, 2, or 3.
• Category C applicants must meet either the B1 or B2 basic knowledge levels.

Level 1 Knowledge

• Objectives: Familiarity with subject elements and ability to provide simple descriptions using common terms.
• Objectives: Ability to use typical terms.

Level 2 Knowledge

• General knowledge of theoretical and practical aspects, with the ability to apply that knowledge.
• Objectives: Understanding theoretical fundamentals, giving general descriptions with typical examples.
• Objectives: Using mathematical formulae with physical laws, reading sketches and schematics, and applying knowledge practically.

Level 3 Knowledge

• Detailed knowledge of theoretical and practical aspects, with the capacity to combine and apply knowledge comprehensively.
• Objectives: Knowing the theory and interrelationships, providing detailed descriptions using theoretical fundamentals and specific examples.
• Objectives: Using related mathematical formulae, preparing sketches and schematics, applying knowledge using manufacturer's instructions.
• Objectives: Interpreting results and applying corrective actions.

Propeller Fundamentals I (17.1) - Objectives

• The module will describe propeller blade element theory
• The module will explain propeller high and low blade angle
• The module will explain propeller reverse angle
• The module will explain propeller angle of attack
• The module will explain propeller rotational speed
• The module will explain propeller slip

Propeller Forces - Lift

• Lift is the aerodynamic force from airflow over an aerofoil.
• Aerofoil shape increases airflow velocity, decreasing pressure above, while higher pressure below results in upward force.
• Lift forms the basis of blade element theory in propellers, where a blade element is any randomly selected area.

Propeller Forces - Drag

• Drag is the force opposing thrust, due to air flow disruption on an aerofoil.

Propeller Forces - Thrust

• Thrust is the forward-acting force, the reaction to rearward acceleration of air mass, and forms the basis of momentum theory.
• Newton’s Third Law of Motion underlies momentum theory
• Momentum is the quantity of motion. It may be measured as a product of a moving boby's mass and velocity.

Total Reaction

• The cumulative reaction of a blade involves lift and drag and the thrust and torque.
• Vectors can derive total reaction and the propeller simultaneously experiences both pairs of forces as a rotating wing.
• Increased rotational speed increases these forces equally, but is limited to keep blade tip speed subsonic.

Effects on Propeller Thrust - Blade Angle

• The blade angle is the angle between chord line and plane of rotation, usually measured in degrees.
• Blade angle is measured with reference to a datum point, decreasing from root to tip.
• A datum point is measured from the propeller hub center to a position at 75% of the radius.
• The Aircraft Maintenance Manual AMM and Type Certificate provide definitive angles and positions to carry out this task.
• For a Sensenich M74DMS5-2-60 propeller, '74' indicates propeller diameter is 74 in. and '-60' indicates pitch of the propeller is 60 in. at the 75% station.

Angle of Attack

• The angle between chord line and relative wind/airflow is the angle of attack
• Best results are achieved between 2° to 4° to allow compression of the incoming air and expansion upon leaving, resulting in thrust .
• The angle of attack combines airflows from forward aircraft motion (True Air Speed, TAS) and propeller rotational speed (revolutions per minute, rpm).

Blade Twist

• The further away from the hub, the faster the blade travels.
• A blade twist is essential to ensure all sections of the propeller blade produce equal thrust.
• Gradual twist maintains a correct angle of attack at 2° to 4º along length of the blade.

Pitch

• Pitch is the distance moved forward by the propeller in one revolution, varied by different blade angles on variable pitch propellers.

Propeller Slip

• Slip equals the differece berween geometric and effective pitch.
• Geometric pitch describes a the distance a propeller should advance through a solid medium
• Effective pitch describes the actual distance a propeller actually advances through the air in one revolution
• If an aircraft moves forward 35 in. per revolution in the air with a geometric pitch of 50 in, the effective pitch is 35 in. and the
• The propeller is 70% efficient in moving through air
• Slip represents a 30% loss of efficiency with 15 in loss of efficiency, in practice, many propellers are 75% to 85% efficient.

Propeller Fundamentals II - Objectives

• Explain propeller aerodynamic forces
• Explain propeller centrifugal forces
• Explain propeller thrust forces
• Explain propeller torque
• Explain relative airflow effect on propeller blade angle of attack
• Explain propeller vibration and resonance

Effects on Aircraft Stability - Torque

• If a propeller is driven anti-clockwise, torque to drive the propeller will tend to roll rotate roll the aircraft clockwise.

Propeller Gyroscopic Effect and Slipstream

• Gyroscopic Effect and force applied at one place is delivered at at 90° in the direction inteded.
• Rotating mass of the propeller will resist change in its plane of rotation.
• Force applied at one place is delivered at a 90° angle to force application.
• A rotating propeller will impart a rotational motion to the slipstream in the same direction as the propeller with adverse effects.

Contra-Rotating Effect

• Contra-rotating propeller eliminates the effects of propeller torque, propeller slipstream and propeller gyroscopic effect.
• The second propeller straightens slipstream of the first, causing a straight high-speed flow of air.
• Propeller torque is cancelled because the propellers are spinning in opposite directions, therefore neutralising the gyroscopic effect.

Forces Acting on a Propeller

• Certain forces act upon a propeller when rotating
• They are: Centrifugal force, Centrifugal Twisting Moment (CTM), Aerodynamic Twisting Moment (ATM), Bending force, Thrust and drag.

Centrifugal Force

• Centrifugal force tends to throw rotating propeller blades away from the propeller hub and can amount to many thousands of newtons.

Centrifugal Twisting Moment

• Centrifugal Twisting Moment (CTM) tends to rotate propeller blades toward a fine blade angle on variable pitch propellers.
• CTM comes from the mass of the propeller trying to align with the plane of rotation
• CTM will always be a greater force than ATM.
• It is used by propeller manufacturers to alter blade angle from coarse to fine.

Aerodynamic Twisting Moment

• Both ATM and CTM (torsional stresses) are increased with an increase in RPM
• If the rpm is doubled, these stresses are quadrupled
• Aerodynamic Twisting Moment (ATM) moves the propeller blades to a coarser blade angle
• The Centre of Pressure is in front of the rotational axis of the blade
• This force tends to increase the blade angle and some designs use this force to aid in feathering.

Bending Forces

• The two bending force components are: Torque bending force (caused by drag), and thrust bending force (caused by thrust).
• Torque bending force comes from the load that air resistance (drag) places on the blades where it bends the propeller blades opposite to the direction of rotation.
• Thrust bending force bends the blades forward as the aircraft is pulled through the air, as exerted by the thrust propelling the aircraft.

Force Coupling

• Severe stresses are created because the coupling of centrifugal force and thrust is greater toward the hub
• The blade face is exposed to tension from centrifugal force and bending
• It should withstand the stresses that increase proportionally with rpm.

Angles of Attack

• Angle of attack or performance can be altered only if the viewer understands Vectors
• Vectors where a line is drawn to scale, show a velocity or force
• Length of vector indicates speed.
• Performance of a fixed-pitch propeller will vary with rotationsl velocity(rpm) and Airspeed (TAS in kt).
• A propeller designed to produce the correct angle of attack (2° to 4º) at say, 1500 rpm and 50 kt forward velocity will only act produce enough thrust until either rotational velocity or forward velocity adjust.

Increased Rotational and Forward Velocity

• Maintain the same speed, or knots 50kt forward. For a fixed-pitched prop aircraft you will need increase the engine power. This will mean an increase rotations per minute (RPM). This will increase the angle of attack.
• If forward velocity is increased to 120 Knots, then maintain rotation at 2000 RPM. The diagram shows that airflow has decreased angle of attack.

Blade Tip Speed Versus Efficiency

• Larger propellers are made to allow them absorb the enormous power that engines can develop
• Larger propellers that that increase in propeller diameter does not necessarily increase efficiency: larger propellers often lost performance through tip vibration or flutter.
• Flutter or vibration is caused by shock waves as the tip of the propeller approaches the speed of sound ( approximately 1200 ft/sec(or 660 kt) at sea level on a standard day of 15 degrees )
• It was necessary to keep the prop tips below the speed of sound to allow them to absorb the available engine power: one way it is achieved is by increasing the number of blades.

Blade Vibrations and Resonance

• The final force exerted on a spinning propeller is called blade virbrations that occur due to aerodynamic forces or pulses in a piston engine.
• Mechanical vibration more likely lead to metal fatigue and structural failure
• While concentrations of vibrational stress are detrimental at any point on a blade, the most critical location is about 6 in. from the blade tips. Most airframe-engine-propeller combinations have eliminated detrimental effects of vibrational stresses by careful design.
• The critical range can be indicated on the tachometer by a red arc and be limited to a brief passage from one rpm setting to another.

Propeller Construction l (17.2) Learning Objectives

• Explain materials and various construction methods for wooden, metal and composite propellers
• Describe terms like: blade station, blade face, shank, root and hub

Construction

• Propeller blades can be made from wood, steel, aluminum alloy, or composite.

Construction Materials: Wood, Steel and Aluminum Alloy

• Construction Materials: Propeller blades are usually made of wood, steel, aluminium alloy and composite (non metallic fibre).
• Materials include a high degree of wood glue to form high quality wooden propeller construction.

Laminating

• Selected Timber is layered with glue
• The selection are specially selected, and well-seasoned hardwood, and from imperfections such as holes, loose knots,decay. –Then the wood is placed in a kiln, where the pressure and the temperature are carefully controlled for a prescribed time and shaped using templates and protractors to ensure it meets design specifications.

Metallic Propellers

• Usually found on antiques or transport aircraft in hollow construction, composed of rib steel sheets attached to form to absorb virbrations and add rigid structure.
• Machined propellers use solid steel but early models used lightweight Duralumin which offered better cooler from the engine cooling at a higher pitch and are now made from Aluminum alloy
• Materials can include inner foam filling both blades, or fixed pitch steel propeller

Metalllic Propellers - Leading Edges

• When metal is used, the thick edge will strike the air that is in front of the aerofoil for rotation
• Blade back describes the area that is curved and joins to the aerofoil edge for rotation
• Blade face refers to the flat side of the propeller blade to describe to the chord line.

Aluminium Alloy Propellers

• A fixed-pitch aluminium propeller is usually manufactured by forging a single bar of aluminium alloy
• These propellers incorporate a centre bore to allow fitment of various steel hubs or adaptors
• .Due to the high strength and malleability of aluminium alloy, the aerofoil extends to the propeller
• The blade design will act as air cooling the engine that is located immediately behind this area.

Shot Peening

• The finishing treatments used in metal work and used in steel propellers -The shot (beads/glass/etc.) are thrown by force of air- pressure to certain areas
• Impacts causes deformations at depths of the surface all the way to thousands of an inch
• If work depth is needed, increase velocity or size of the shot
• Two more kinds of steel, or galss beads: It will be more effective
• For Steel- a great way for stress-cracking with small cracks

Materials - Composites

• Constructed using special plastic resins +glass, kevlar ,carbon, and boron to construct.
• One way is to use is to apply a spar or Aluminum alloy to the front
• Secondary to use a composit material shell to form shape.

Fiber Reinforced Plastic Moulding

• Made of kevlar and a foam core
• Kevar protects from flexing and buckling
• Shear webs are placed between the camber

Electrical

• Electric pitch changing enable aircraft with as little 25 horsepowers
• The settings are controlled via 3 settings to manage electric
• The mechanism is located on the ring for support and motor
• Electric pitch has microswitches to power down.
• Electrical sensors withing hubs manage including feathering.

Turboprops

• Used with a hydraulic medium and two levels of the cockpit/conditions in many forms like: two position propellers (bracket type), CTM, Centrifugal Force
• Hydromatic: pitch varies that of the piston
• Operation controlled by linear motion to the blade.

Single-Acting Governors

• Direct oil to action
• Use another set of means to move piston
• Springs, pressure or counterweights
• Includes a governor drive shaft,oil pump, valve, and spring
• Flyweights help regulate speeds on the shaft while the speeder spring affects pressure
• Pinion,worm, and levels change speed from the air craft.

Single Acting Governor Operation-Increase and Deacrease rpm

• Increase rmp: use speeder spring to lower pilot; oil from hub then prop will move

• Pilot lowers Pressure is decreased on

• Prop or oil pressure directed towards the hub, then will drop back to the coast

• This lowers back into the engine to equalize

• Force equals the speed

• With Fly Weight there is a pressure to the hub oil

• But towards the hubs coasts,

• In the future or in hubs,

• Prop increases -

Governor + Propeller Operating Conditions

• These variable-pitch propellers allow for setting flight conditions such that the angle could be optimized and be efficient
• On speed refers to how the propeller is doing what the control is set to
• Overspeed is when its set faster than the setting
• Underspeed is when its below the setting
• Feathering the process

Contra-rotating and AutoFeathering Systems

• A feathered prop is controllable with a pitch, one multi-engine, one turbine, and has must be used to allow the pilot to maintain single-engine to drag will use the prop.
• Props have a high pitch propeller that the rotation stops at the hubs that prevent from wind
• Auto-Feather: are armed, and can sense engine will detect if they over power and is low.

Reversible Propellers

• Add consistent speed capability in the variable-pitch
• Can use thrust reverser engines through blades at a negitive angle.
• They also will be in small aircraft, they could easily be told where to move

Hydraulic Components (Controllable Pitch)

• The design means for the blades to turn at the center to the sides.
• The for a variable-pitch, a removable bushing is fitted into a forging (taper bore) at the centre of the blade butt to provide a bearing surface for the blades to turn on when blade angle changes occur.
• Each blade is able to operate on a spider with an individual piston inside

Propeller Construction II (17.2) Learning Objectives

• Explain propel types including fixed pitch, and with different installations
• What is considered a factor for affection engines or aircraft.
• Where to locate flanged, or tapper installation
• Spinner installation

Propeller Maintenance (17.6) and Learning Objectives

• 1: What is balance from static to dynamic
• Two is the blade tracking
• Three is to address the blade with corrosion or damages
• Four explain with treatment and repair
• Five expalin engine.