17.6 propellor maintenance

Question 1

what are the three important types of balance on a propellor

Answer 1

Static Balancing
Dynamic Balancing
Aerodynamic Balancing

Question 2

how would you know a propellor is balanced?

Answer 2

If a body is in static balance, each time it is rotated it comes to a stop in a random position.

Question 3

how is static balance carried out?

Answer 3

The propeller is mounted on a mandrel which is placed across stable and perfectly levelled knife edges or rollers. The balance is checked in two planes, one horizontal and the other vertical.

Question 4

when is dynamic balancing performed?

Answer 4

while the engine is running

Question 5

what are the areas that dynamic balance is possible?

Answer 5

blades hub and spinner

Question 6

what is dynamic balance?

Answer 6

Balancing the propeller to reduce the moment of imbalance

Question 7

why are small propellors not typically dynamically balanced?

Answer 7

any shift of the centres of gravity is only small.

Question 8

what are the steps i dynamic balance?

Answer 8

-Obtain in-flight vibration information.
-Ensure vibration is greater than permitted limits.
-Calculate the mass and location to reduce vibration to an acceptable level.
-Install balance weight(s) and confirm vibration levels are within limits.

Question 9

what is aerodynamic imbalance?

Answer 9

No two blades are manufactured to 100% accuracy. This means that each blade can produce slight variations from the standard theoretical values for torque and thrust.

Question 10

if a blade is producing more torque what might it also be producing more of?

Answer 10

torque

Question 11

can we correct for torque and thrust variation?

Answer 11

it is difficult to manage both

Question 12

what do we do rather than correct for aerodynamic imbalance in both ways?

Answer 12

The blades are balanced for either thrust or torque. The choice will be marked on the blade as an Aerodynamic Correction Factor (ACF), preceded by the letter ‘T’ if the correction is for thrust, or ‘Q’ if it is for torque

Question 13

what is aerodynamic balance necessary for?

Answer 13

propellors with high performance

Question 14

what is done instead or aerodynamic balance for manufactured blades?

Answer 14

they are are compared individually with a master blade and receive, according to deviation from the zero lift angle, an aerodynamic correction factor in the form of a reference to the blade angle difference necessary to the basic setting.

Question 15

what is the universal blade protractor?

Answer 15

used to measure the propeller blade angle at a specific blade station to determine if the propeller is properly adjusted. The blade angle is referenced from the propeller plane of rotation, which is ninety degrees to the crankshaft centreline

Question 16

where is the reference station for the universal blade protractor?

Answer 16

This reference station is normally set at either the 30-inch, 36-inch, or 42-inch measurement on the propeller blade

Question 17

what is blade tracking?

Answer 17

Blade tracking is the ability of one blade to follow the other in the same plane of rotation

Question 18

what are the limits for blade tracking and reasons for it?

Answer 18

Tracking is held to reasonable limits to prevent roughness and vibration

Question 19

where is measurement usually taken from for blade tracking?

Answer 19

master station

Question 20

how is the propellor turned for blade tracking?

Answer 20

by hand

Question 21

what is flat bench tracking method?

Answer 21

This method requires the use of a smooth board placed just under the tip of the lower blade and paper taped to it.

On controllable props, move the tip fore and aft carefully through its small range of movement, making small pencil marks at each position.

Centre the blade between these marks and draw a line the full width of the blade

Question 22

what is the allowable distance between blades in blade tracking?

Answer 22

3mm or the AMM given figure

Question 23

how is block or pointer blade tracking performed?

Answer 23

Another method can be by means of a heavy block or pointer method.

As before, rotate one of the blades so it is pointing down.

Place a solid object (e.g., a heavy wooden block that is at least a couple of inches higher off the ground than the distance between the propeller tip and the ground) next to the propeller tip so that it just touches or attach a pointer/indicator to the cowling itself as shown in the image below.

Rotate the propeller slowly to determine if the next blade tracks through the same point (touches the block/ pointer)

Question 24

what is the primary inspection for early propellor failure?

Answer 24

visual whilst using touch and other senses

Question 25

what is tap testing typically used on?

Answer 25

composites for disbond and delamination

Question 26

what is penetrant inspection and which is recommended?

Answer 26

fluorescent dye to look for hairline cracks to be used over visual dyes

Question 27

what is ultrasonic inspection?

Answer 27

specialised equipment to send, receive, and process sound waves to detect flaws on or below the surface of the component being tested

Question 28

what is eddy current inspection?

Answer 28

specialised equipment to generate and measure an electric field that detects flaws at or slightly below the surface of the component

Question 29

what is magnetic particle inspection?

Answer 29

conducted at an appropriately approved maintenance facility. It is useful for finding cracks, inclusions, and imperfections at or near the surface of ferrous parts.

Question 30

where are most failure on propellors?

Answer 30

tip area

Question 31

what is erosion?

Answer 31

small particles wearing away the surface

Question 32

what do composite blades have fitted to protect against erosion?

Answer 32

erosion strips

Question 33

what is surface corrosion?

Answer 33

loss of surface metal due to chemical or electro-chemical action with visible oxidation products usually having a contrasting color and texture to the base metal

Question 34

what is pitting?

Answer 34

visible corrosion cavities extending inward from the metal surface. They can grow on the surface, under decals, or under improperly installed de-ice boots

Question 35

what is intergranular corrosion?

Answer 35

The presence of intergranular corrosion may be the result of the continued presence of moisture such as under a decal, in a fastener hole, or where the anodise and paint protective barriers have been lost. Exfoliation is sometimes visible as metal flaking and cracks on a blade leading edge

Question 36

what are woods susceptible to from impact damage that other materials are not?

Answer 36

internal damage from small stone strikes that can create delamination or microcracks and permit intrusion of moisture

Question 37

what should be inspected for in wooden blades?

Answer 37

glue lines for debonding; look for warp and loss of protective coating (paint or varnish)

Question 38

what is cracked paint in wooden blades a sign of?

Answer 38

flexural vibrations

Question 39

what are cracks at the beginning of and along the metal tipping in wooden blades a sign of?

Answer 39

differing expansion of the material

Question 40

what repairs can be performed to wooden blades?

Answer 40

Less significant damage on the trailing edge or on the blade can be filled with plastics. Indentations in the metal tipping can be filled by soldering. In this case the use of any significant heat is to be avoided and balance must be taken into consideration. Perforated tipping must be replaced

Question 41

how can surface damage on wooden propellors be repaired?

Answer 41

wood filler or a mix of sawdust and aero glue (casein)

Question 42

what cannot be repaired on wooden blades?

Answer 42

Cracks across the grain A splintered blade Delamination Missing material Cracks in the hub Enlargement of the hub shaft bore Elliptical bolt holes

Question 43

when preparing blade tips on metal blades what must be observed?

Answer 43

you cannot use heat treatment or to fill nicks with plastics. All types of cold working are also prohibited. On completion of the repair the surface is to be appropriately protected.

Question 44

what static checks can be performed?

Answer 44

Flight Fine Pitch Stop (FFPS) Auto-Coarsening Manual Feather Autofeather Unfeather

Question 45

what is a static check?

Answer 45

a check that can be done without running the engine

Question 46

when are static checks usually done?

Answer 46

prior to a engine ground run

Question 47

how would you do a static check?

Answer 47

using the electric feathering pump

Question 48

how are reciprocating engines further classified?

Answer 48

normally aspirated supercharged

Question 49

what are power ratings of reciprocating engines related to?

Answer 49

manifold air pressure

Question 50

how is reference RPM measured and what is this called?

Answer 50

-set propellor to fine pitch -push throttle until until manifold pressure reads zero -check RPM matches published figure a reference power check

Question 51

what is reference RPM?

Answer 51

RPM when it reaches zero boost

Question 52

what does tail wind do in terms of RPM?

Answer 52

reduces

Question 53

if there is visible moisture in the air a and temperature is below 10 degrees what is possible?

Answer 53

icing at inlet

Question 54

how is turbo prop engine power measured?

Answer 54

shaft horsepower

Question 55

what is the purpose of performance checks?

Answer 55

to ensure continued safe operation of propellor engine combinations

Question 56

what are the important parameters of performance checks?

Answer 56

fuel flow torque RPM EGT

Question 57

if there are visible moisture conditions and it is below 10 degrees what must be done for ground tests?

Answer 57

anti ice switched ON

Question 58

why must you run the engine at idle in engine ground test?

Answer 58

allow casing temps to stabilise