17.2 Propeller Construction

Question 1

The requirements for the design, manufacture and testing of propellers is specified in Annex IX to ED Decision 2020/006/R, Certification Specifications and Acceptable Means of Compliance for Propellers (CS-P).

Answer 1

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Question 2

Propellers are categorised according to the material used for their blades. These are:

Answer 2

.Wooden propellers
.Composite propellers
.Metal propellers:
- Aluminium Alloy
- Steel

Question 3

Wood is often used on older fixed pitch propellers and some controllable pitch propellers fitted on vintage aircraft. Most have a natural wood finish while others use a grey or black plastic coating.

Answer 3

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Question 4

Wooden propellers consist of several layers of wood bonded with a waterproof resin glue (aero-glue). Birch is the most commonly used wood

Answer 4

At least 5 layers of wood are laid down and when glued together they form a blank.

Question 5

The blank is roughly shaped and allowed to rest for a week to allow the equal distribution of moisture through all the layers; after which it is known as a white.

Answer 5

. A cotton fabric sheathing is then applied to the outer 12 to 15 in (30 to 38 cm) of the tip to reinforce the strength of the thin sections at the tip. The fabric is doped to protect it from deterioration by the weather and the sun’s ultraviolet (UV) rays.

Question 6

The metal tipping is secured to the blade using countersink screws at the thick blade section and copper rivets near the tip, all are safetied in place with solder.

(WOODEN Propellers)

Answer 6

.Small holes (#60 (0.04 in or 1.016 mm) size, 3/16-in deep) are drilled in each tip for the purpose of releasing any moisture and allowing the wood to breathe. The propeller is then balanced and the finishing coats of varnish are applied.

Question 7

Some propellers do not use doped cotton fabric but are coated with plastic before the metal tipping is applied. This provides protection and added strength.

Answer 7

Some types of propellers have a final coating of polyurethane paint which provides mechanical and UV ray protection.

Question 8

Wood blades for controllable pitch propellers are constructed in a similar manner to fixed pitch propellers except that the blade shank is secured into metal sleeve using lag screws. (Lag screw or coach bolt

Answer 8

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Question 9

Composite propellers are lighter in weight and stronger than metal propellers. With new methods of lay-up, these propellers are taking over from metal blades. They can be constructed into complex aerodynamic shapes that improve performance and reduce drag

one problem is that

Answer 9

damage can be difficult to detect apart from very minor scratches. Composite blades cannot be repaired by anyone other than an approved repair organisation. Field repairs are not permitted.

Question 10

(Composite Propeller)

Nickel or stainless steel erosion shields are bonded to the leading edges to protect from impact damage. Some designs incorporate a stainless steel wire mesh in the laminations to inhibit blade tip erosion. A metal foil mesh can also be incorporated on the surface of the blade to limit lightning strike damage.

Answer 10

Most composite blades use an external boot for de-icing but some blades have been introduced with internal heating elements.

Question 11

The spar, made from titanium, aluminium alloy or stainless steel, runs through the centre of the blade, with low-density foam or honeycomb in front and behind as a filling material. The fibreglass, kevlar or carbon fibre shell is then constructed around these parts.

Answer 11

The spar absorbs the centrifugal forces and the bending loads whilst the shell gives the propeller the necessary torsional rigidity.

There is a heating element on the inner part of the surface of the leading edge and a metal guard on the outer part. The surface is protected by a coat of conducting polyurethane paint. This serves as protection against erosion and prevents the blade from becoming statically charged.

Question 12

Construction of Composite Blades without Metal Spar

Answer 12

Note that there are two carbon fibre spars running along the length of the blade. These combine at the root of the blade to form a hollow carbon fibre cylinder that gives added strength to the root. As the composite blade enters the metal root fitting the layers of carbon fibre are splayed out over fibreglass wedges trapping them between the inner and outer sleeves of the root fitting

the hollow centre is filled with polyurethane foam.

Question 13

To allow for minor adjustments to be made whilst the individual blade is balanced horizontally, lead wool is placed into a hollow balancing tube. The lead wool is retained in place by the use of a rubber plug.The operating pin is inserted into a pitch change mechanism.

Answer 13

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Question 14

Metal Propellers

Metal propellers are used on aircraft with greater engine power and are constructed from either aluminium alloy or steel.

Answer 14

Duralumin is the preferred alloy as it is hard and very strong in use whilst being easily forged and shaped when suitably heat-treated. Quite a lot of damage is easy to repair and can be carried out by operators.

Question 15

Hollow steel blades are used on some modern types but are gradually being replaced by composite blades. They are very strong and very resistant to damage. The disadvantage attached to steel blades is that

Answer 15

even minor damage is considered to be a major repair only to be carried out by the manufacture

Question 16

Aluminium Propellers

Answer 16

These propellers are the most widely used propellers in aviation. Aluminium alloy is more durable than wood because thinner, more efficient aerofoils can be constructed without sacrificing structural integrity. Better engine cooling is also achieved (reciprocating engines) by carrying the aerofoil shape nearer the hub to direct more airflow over the engine. Due to their durability, they require less maintenance which reduces their operating costs

Question 17

construction of Aluminium propellers

Answer 17

Aluminium propellers are made from extruded formers of aluminium alloy and are finished to the desired aerofoil shape by machine and, until recently, manual grinding by skilled technicians using a master blade as a template.

Today the final grinding is usually carried out by CNC machines to produce more accurate, uniform results.

Once the propeller has been ground to its final shape the blades are finally finished and balanced by hand. The blade surfaces are finished by plating, chemical etching and/or painting. Anodising is the most commonly used finishing process.

Question 18

Steel propellers are not in common use. They are found on some older aircraft or some transport aircraft. Steel propeller blades are typically hollow to keep weight to a minimum.

Answer 18

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Question 19

construction of steel propellers

Answer 19

Hollow steel propellers are made from folded sheets welded over a ribbed structure which is foam filled in the outer section of the blade. In this way, the profile shape remains intact and any blade vibrations can be dampened.

Solid blades are forged and machined to the desired contours and then twisted to achieve the required pitch.

Question 20

hese are divisions of the propeller’s radius taken from the centre of the hub to the blade tip. They are normally numbered from 1 onward in six-inch increments (15 cm) or the number of inches from the root of the blade toward its tip, as shown in the graphic below. For correct station identification, the propeller maintenance manual must always be referred to.

Answer 20

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Question 21

Propeller Master Reference Station

Answer 21

As the angle of each station reduces from the root to the tip the propeller blade angle is measured, using a blade protractor, at a nominated station called the ‘master station’. If the blade angle is correct at this point then it can be confidently assumed that all the blade angles at all other stations along the length of the blade are also correct. The ‘master station’ is identified in the propeller section of the maintenance manual and is normally 75% from the centre of rotation on a fixed pitch propeller and can be 50% to 75% on a variable pitch propeller. The position may be marked by a line drawn across the blade face, but this is not always the case.

The master station is also a reference point as to where most of the thrust of the blade is produced.

Question 22

Propeller Blade Angle

Answer 22

The angle between the blade chord line and the plane of rotation.

Question 23

Propeller Face

Answer 23

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.

Question 24

Propeller Blade Root/Shank

Answer 24

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. The shape of the shank ranges from circular or oval to a highly cambered form. The shank must be capable of absorbing the loads placed upon the propeller and transmitting the thrust to the hub.

Question 25

Propeller Back

Answer 25

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.

Question 26

Propeller Hub

Answer 26

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. The complexity is defined by the number of blades and whether the propeller is ‘fixed’, ‘ground adjustable’ or ‘variable’.

The propeller hub is designed to withstand all the forces experienced by the propeller during operation. The thrust generated by the propeller is transmitted to the engine and ultimately to the airframe through the propeller hub.

Question 27

fixed pitch propeller

Answer 27

This type of propeller is usually one piece and constructed of wood or aluminium. Fixed pitch propellers are used for up to about 200 kW (250 hp) performance and speeds in the region of 250 km/h (160 mph). The blade angle (pitch) cannot be changed and is determined in accordance with the purpose it is to be used

or steep climbing and towing low (fine) pitch is needed and for a more gradual climb and cruising flight a higher (coarse) pitch is preferred

Question 28

Fixed propellers are favourable with regard to production and maintenance costs.

Answer 28

to maintain a larger distance from the engine flange, which allows for a more favourable engine cowling, spacers are used, which are available in different thicknesses.

Question 29

(FIXED pitch propeller)

Blade Angle

Answer 29

With a fixed pitch propeller, the blade angle is set for one specified engine speed, airspeed and altitude only. This is generally the blade angle best suited for cruising conditions when the propeller is at its most efficient.

Question 30

Take Off Speed
At low forward airspeeds, maximum RPM is not obtainable because of the large angle of attack which also puts a large load on the engine. Higher engine RPM increases the angle of attack, resulting in increased resistance to rotation and increased load on the engine. An increased angle of attack also causes a loss of thrust. This means the propeller is inefficient resulting

Answer 30

in a longer take off run

Question 31

Maximum Forward Speed
During take-off and at low airspeeds a fine (low) blade angle is required, whereas when flight speed has been obtained a coarser (higher) blade angle is needed. When the angle of attack is at its optimum of 4º then maximum RPM is available.

Answer 31

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Question 32

If the density of the air increases, the greater weight of air absorbs more engine power causing the RPM to decrease. Therefore, to maintain a constant RPM the propeller requires more power (to double the RPM the power must be quadrupled).

Answer 32

With decreasing air density the propeller requires less power to maintain a constant RPM.

Question 33

disadvantages Of A Fixed Pitch Propeller

Answer 33

-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.

Question 34

An aircraft equipped with a controllable-pitch propeller has two controls:

Answer 34

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

Question 35

The pilot can set the throttle control and propeller control at any desired manifold pressure and RPM setting within the engine operating limitation.

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Answer 35

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Question 36

A low blade angle or decreased pitch, reduces the propeller drag and allows more engine power for take-offs

Answer 36

after airspeed is attained during cruising flight, the propeller blade is changed to a higher angle or increased pitch. Consequently, the blade takes a larger bite of air at a lower power setting and therefore increases the efficiency of the flight.

This process is similar to shifting gears in an automobile from low gear to high gear.

One of the most popular designs used today is the Beech-Roby propeller system.

Question 37

cockpit control of controllable pitch propellers

Answer 37

Pitch change is initiated in the cockpit by a crank handle which directly drives a pinion gear through a flexible cable. In more sophisticated designs a toggle switch is used to operate an electric motor to drive the pinion gear. Limit switches are fitted at the maximum and minimum pitch stops to shut off the electric motor.

Question 38

Constant Speeding Propeller

Answer 38

A constant speed propeller is a propeller that is designed to automatically change its blade pitch to allow it to maintain a constant RPM, irrespective of the amount of engine torque being produced or the airspeed or altitude at which the aircraft is flying. This is accomplished by means of a Constant Speed Unit, or governor, integrated into the propeller design.

Question 39

Constant Speed Unit

Answer 39

A constant speed unit, or propeller governor, is the mechanism which allows a constant speed propeller to work. Most constant speed units work on the principle of centrifugal force and incorporate a speeder spring and a set of fly weights

Question 40

If the propeller exceeds the preselected RPM, the fly weights are forced outward whereas a propeller under-speed would cause the fly weights to swing inward. In both cases, this changes the tension on the speeder spring.

Answer 40

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Question 41

Most engines produce their maximum power in a narrow speed band. This is especially true for a turboprop engine. .

Answer 41

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Question 42

the propeller has to be attached via the hub to a suitably designed propeller drive shaft. the most common ones in use are:

Answer 42

Tapered Shaft
Splined Shaft
Flanged Shaft

Question 43

tapered shaft

Answer 43

Tapered shaft drives are usually found on small, older, low-powered piston engines. The propeller has to be fitted with a hub to adapt it to the engine

Question 44

splined shaft

Answer 44

The propeller shaft of high-output radial engines is generally splined. It is threaded on one end for a propeller hub nut.

The propeller shaft may be connected by reduction gearing to the engine crankshaft, but in smaller engines the propeller shaft is simply an extension of the engine crankshaft.

The sizes of splined shafts are identified by an SAE (Society of Automotive Engineers) number. SAE 20 splines are used on engines in the 200-horse-powered range; SAE 30 splines are used in the 300- and 400-horse-powered range, and SAE 40 in the 500- and 600-horse-powered range. SAE 50 in the 1,000-horse-powered range and SAW (sic) 60 and 70 are used for larger engines.

The grooves and lands (the space between them) are the same sizes, and one groove is either missing or has a screw in it to form a master spline. This ensures that the propeller is installed in only one position relative to the shaft. This is done for synchronising and/or synchrophasing.

Question 45

Flanged Shaft

Answer 45

Flanged propeller shafts are used on most modern reciprocating and turboprop engines. One end of the shaft is flanged with drilled holes to accept the propeller mounting bolts. Some flanges have index pins in the propeller flange so the propeller can be installed in only one position relative to the shaft for the purpose of synchronising and/or synchrophasing. The installation may be a short shaft with internal threading to accept the distributor valve to be used with a controllable propeller.

Question 46

(Regarding propeller installation)

check the extent of the bluing of the cones. If the contact area is less than 80%, high spots can be removed by using an oil stone or, where permitted, by lapping on a suitable mandrel. Clean the cones and cone seating.

Answer 46

Turn the blades to the feathered angle and fit the pitch change mechanism. Care must be taken to ensure that the pitch change mechanism is in the correct blade angle position.

When the propeller is correctly torqued, the blade track should be checked to the tolerances specified by the manufacturer’s specifications.

Proper pitch change action and RPM parameters must be checked during run-up and the installation inspected for oil and grease leaks.

Question 47

Spinners are mounted for reasons of better aerodynamics, as mechanical protection for the hub and for aesthetic reasons.

Answer 47

They are usually manufactured in one piece from spun aluminium alloy or glass fibre composites and are attached to the spinner backplate (or spinner bulkhead). There is normally a support at the front end of the propeller hub for centring purposes.

The dynamic load on the spinner is extremely high. If there are cracks on the blade recesses or in the spinner mounting then the parts are to be replaced. Repairs are limited (mostly drilling to stop a crack is allowed).