17.2 Propeller Construction Flashcards
What are propellers categorised by ?
The material used for their blades
-wood
-composite
-metal ( aluminium or steel)
Where are wooden propellers most commonly used ?
Older fixed pitch propellers and some controllable pitch propellers fitted on vintage aircraft
What finish do most wooden propellers have ?
Natural wood finish
Other than a natural wood finish, what other finishes can wooden propellers have ?
Grey or black plastic coating
How are most wooden propellers constructed ?
Several layers of wood bonded with a waterproof glue (aero-glue)
What is the most commonly used wood for wooden propeller construction ?
Birch
Other than birch, what other woods can be used in construction?
Mahogany, cherry, ash, beach, oak and walnut
When 5 layers of wood are laid down and glued, what do they form ?
A blank
What is done with a blank ?
It is roughly shaped and left to rest for a week to allow the equal distribution of moisture. This is known as a white
Why is Compton fabric sheathing applied to the outer 12 to 15 inches of the tip?
To reinforce the thin sections of the tip
Once cotton fabric sheathing is applied, what must be applied to the fabric and why?
Dope is applied to protect it from the weather and the UV
Why us a protective metal tipping strip rebated into the leading edge?
To prevent damage from small stones during ground ops
What metal is used for the metal tipping strip?
Brass, stainless steel, Monel or terneplate
How is the protective metal tipping strip fastened in place ?
Using countersink screws and copper rivets near the tip
What is used to keep rivets and screws in place when securing the protective metal tipping strip ?
Solder
Why are small holes drilled in each tip of the propeller ?
To allow draining
Once holes are drilled to allow draining, what needs to be done?
Propeller needs to be rebalanced and finishing coats of varnish are applied
Some wooden propellers do not use doped cotton fabric, what else can they use?
Coated with plastic for protection and extra strength
Some types of propellers have a coating of polyurethane paint, what does this provide ?
Mechanical and UV ray protection
When a wooden propeller is constructed to gave controllable pitch, what is the difference In construction between these and fixed pitch propellers ?
Blade shank is secured into metal sleeves using lag screws or coach bolts
What ways could a wooden propeller be attached to the crankshaft?
-forged steel hub that fits a splined crankshaft
-connected to a tapered crankshaft by a tapered, forged steel hub
- bolted on a steel flange forged on the crankshaft
What must be adhered to when installing a wooden propeller ?
Strict torquing values laid down in the maintenance procedures
Why might a composite propeller be used instead of a metal propeller ?
They are lightweight and stronger than metal propellers
What is a major setback for composite propellers?
Difficult to detect damage
What are bonded to composite propellers leading edges to protect from impact damage?
Nickel or stainless steel erosion shields
Other than erosion shields, what else is incorporated into the composite propeller designed to limit damage ?
-stainless steel wire wire in the laminations to inhibit blade tip erosion
-metal foil mesh on the surface of the blade to limit lightning strike damage
How can composite propellers be de iced ?
External boots and internal heating elements
What is a spar made from in a composite blade ?
Titanium, aluminium alloy or stainless steel
Shat surrounds the spar until it meets the composite shell?
Low density foam or honeycomb
What materials are used for composite shells?
Fibreglass, Kevlar or carbon fibre shells
What forces do the spar absorb ?
Centrifugal and bending loads
What does the shell provide ?
The necessary torsional rigidity
Wheee are heating elements and metal guards placed on a composite propeller ?
Heating element on the inner surface of the leading edge and metal guards on the outer part
On composite blades, the surface is protected by a coat, what is thus coat and what does it do ?
Conducting polyurethane paint, which serves as protection against erosion and prevents the blades from becoming statically charged
What forms a spar when a composite blade is constructed without a metal spar ?
Two carbon fibre components
What is the centre of a composite blade without a metal spar filled with ?
Polyurethane foam
What is different about the construction of composite blades with a metal spar and ones without a metal spar?
In composite blades without a metal spar, the shell surrounds the carbon fibre components ( the spar ) leaving the centre hollow, whereas when a metal spar is used, the spar is centred, leaving the shell to contact the foam/ honeycomb
What is used to make minor balancing adjustments ?
Lead wool placed in a hollow balancing tube
What stops the lead wool from leaving the balancing tube ?
A rubber plug
Why might a metal propeller be used ?
If an aircraft has great engine power
What is the mist widely used propeller material ?
Aluminium
Why are aluminium alloy made blades better than wood blades ?
Thinner, more efficient aerofoil scan be constructed without sacrificing structural integrity. Better engine cooling is also achieved by carrying aerofoil shape nearer to the hub to direct more airflow over the engine ?
How are aluminium blades constructed ?
Made from extruded formers of aluminium and are finished to the desired shape by machine and until recently manual grinding by skilled technicians
Who or what will final grinding be carried out by ?
Used to be manually by skilled technicians, however it is now by CNC machines to produce more accurate, uniform results
What is done to aluminium blades once they have been finished and balanced ?
Feet are finished by plating, chemical etching, anodising and/or painting
What is the most common finishing process ?
Anodising
Are steel propellers common ?
No
On what aircraft could steel propellers be found ?
Older aircraft and some transport aircraft
What is done to steel propellers to keep weight to a minimum ?
They are hollow
How are steel propellers constructed ?
They are made from folded sheets welded over a ribbed structure which is foam filled in the outer section of the blade
Why is the hollow centre of steel blades filled with foam ?
To dampen vibrations and to ensure rage profile shape stays in tact
How do you get solid blades to their desired contours ?
By forging or machining
How do you get solid blades to their required pitch ?
By twisting
What is the blade divided into ?
Stations
What are stations ?
Divisions of the propellers radius taken from the centre of the hub from root to tip
Where should blade angle be measured ?
At the “master station” using a blade protractor
If the angle at the master station is correct, what can be assumed about the angles at all the other stations?
They are also correct
Why should you measure blade angle at the master station ?
As the angle of each station will be small from root to tip
Where is the master station usually located ?
- 75% from the centre of rotation on a fixed pitch propeller
- 50-75% on a variable pitch propeller
At what point along the propeller is the most thrust generated ?
At the master station
What is propeller blade angle ?
The angle between the blade chord line and the plane of rotation
What is the propeller face ?
The flat thrust producing side of a propeller blade. As the propeller rotates, the face strikes the air. On a single engine aircraft with a tractor propeller, this is the part of the blade facing the pilot.
What is the propeller blade root/shank ?
The thickened portion of the blade nearest to the hub. The propeller blade shank connects the blade root or butt to the aerofoil section of the propeller blade.
What is the propeller back ?
The curved side of the propeller blade facing the direction of flight. The back is cambered or curved, similar to the upper surface of a wing and generates a lower pneumatic pressure as the blade rotates.
What is the propeller hub ?
The propeller hub is the centre portion of a propeller to which the blades are attached and by which the propeller is attached to the engine.
What pitch is needed for steep climbing and towing ?
Fine pitch
What pitch is needed for a more gradual climb and cruise flight ?
Coarse pitch
What sort of workload do fixed pitch propellers give to the pilot ?
None, no input in needed
What flight condition is a fixed pitch Angle best suited for ?
Cruising
What are some disadvantages of fixed pitch propellers ?
- Maximum propeller efficiency is possible only at one predetermined RPM and airspeed.
- The power available at take off is reduced because of the lower RPM.
- The continual changes in RPM with every variation of altitude and airspeed is detrimental to the efficiency and life of the engine.
Why were variable pitch propellers invented ?
As a different pitch is needed at every variation or AC speed, attitude, altitude and airspeed
What two controls do aircraft with controllable- pitch propellers have ?
-A throttle control, which controls the power output of the engine which is registered on the manifold pressure gauge (black knob in image below).
-A propeller control, which regulates the engine RPM and in turn the propeller RPM. The RPM is registered on the tachometer (blue knob in image below).
What can the pilot set the throttle control and propeller control at any desired manifold pressure and RPM setting within ?
Engine operation limitations
What is the most common variable pitch propeller design ?
Beech-roby propeller system
How is pitch change initiated in the cockpit ?
Via a crank handle or a toggle switch
What are fitted at minimum and maximum pitch stops ?
Limit switches to shut off the electric motor
What is a constant speeding propeller ?
Designated to automatically change its blade pitch to allow it to maintain a constant RPM
How does a constant speeding propeller achieve its functions ?
Via a constant speed unit or a governor
What principle do constant speed units work on ?
Centrifugal force and they incorporate a speeder spring and a set of flywheights
When a finer pitch is needed , where does oil flow in the CSU ?
To the oil sump
Where does oil flow when a coarse pitch is needed in the CSU ?
Into the propeller
What is the benefits of using a constant speeding propeller ?
Allows the manufacturer or pilot to select the propeller speed appropriate to the situation and then automatically maintain that
What is the benefits of using a constant speeding propeller ?
Allows the manufacturer or pilot to select the propeller speed appropriate to the situation and then automatically maintain that RPM under varying conditions or altitude, airspeed,phase of flight and engine power
What are the most common shaft types ?
- tapered shaft
- splined shaft
- flanged shaft
Where are tapered shafts found ?
Smaller, older, low powered piston engines
How does the propeller have to be fitted in a tapered shaft configuration?
With a hub to adapt it to the engine
What shaft do high output radial engines generally use ?
Splined shaft
Why is there a threaded portion at the end of a splined shaft ?
For installation of the propeller hub nut
How may a splined shaft be coupled ?
Either via reduction gearbox, or directly as an extension of the engine crankshaft
How are sizes of splined shafts identified ?
Via an SAE number
What horsepower range is SAE 20?
Up to 200 hp
What horsepower range is SAE 30 ?
300 to 400 hp
What horsepower range is SAE 40 ?
500 to 600 hp
What horsepower range is SAE 50?
For up to 1000hp
What horsepower range is SAE 60 and 70?
Larger than 1000 hp
What is important to remember when fitting a propeller to a splined shaft ?
Grooves and lands are the same size, and one groove is missing or has a screw in it to form the master spline ensuring that the propeller is installed in only one orientation
What is the most common shaft type used on modern reciprocating or turboprop engines ?
Flanged shaft
Why should a propeller on a flanged shaft be installed in only the correct position ?
For the purpose of synchronising and or synchrophasing
What should be followed when fitting a propeller and what might this include ?
The appropriate maintenance manual, which might include the checks, lubrication instructions, torque loading and locking retaining parts
Why are spinners fitted ?
For better aero, mechanical protection for the hub and for aesthetic reasons
What are spinners usually manufactured from ?
From spun aluminium alloy or glass fibre composites, they are attached to the spinner backplate
What loads are extremely high on the spinner ?
Dynamic loads
What must be done if there is a crack on the spinner ?
It must be replaced
There must be no what on installation of the spinner ?
Wobble or cracks
