17.6 propellor maintenance Flashcards
what are the three important types of balance on a propellor
Static Balancing
Dynamic Balancing
Aerodynamic Balancing
how would you know a propellor is balanced?
If a body is in static balance, each time it is rotated it comes to a stop in a random position.
how is static balance carried out?
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.
when is dynamic balancing performed?
while the engine is running
what are the areas that dynamic balance is possible?
blades hub and spinner
what is dynamic balance?
Balancing the propeller to reduce the moment of imbalance
why are small propellors not typically dynamically balanced?
any shift of the centres of gravity is only small.
what are the steps i dynamic balance?
-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.
what is aerodynamic imbalance?
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.
if a blade is producing more torque what might it also be producing more of?
torque
can we correct for torque and thrust variation?
it is difficult to manage both
what do we do rather than correct for aerodynamic imbalance in both ways?
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
what is aerodynamic balance necessary for?
propellors with high performance
what is done instead or aerodynamic balance for manufactured blades?
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.
what is the universal blade protractor?
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
where is the reference station for the universal blade protractor?
This reference station is normally set at either the 30-inch, 36-inch, or 42-inch measurement on the propeller blade
what is blade tracking?
Blade tracking is the ability of one blade to follow the other in the same plane of rotation
what are the limits for blade tracking and reasons for it?
Tracking is held to reasonable limits to prevent roughness and vibration
where is measurement usually taken from for blade tracking?
master station
how is the propellor turned for blade tracking?
by hand
what is flat bench tracking method?
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
what is the allowable distance between blades in blade tracking?
3mm or the AMM given figure
how is block or pointer blade tracking performed?
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)
what is the primary inspection for early propellor failure?
visual whilst using touch and other senses
what is tap testing typically used on?
composites for disbond and delamination
what is penetrant inspection and which is recommended?
fluorescent dye to look for hairline cracks to be used over visual dyes
what is ultrasonic inspection?
specialised equipment to send, receive, and process sound waves to detect flaws on or below the surface of the component being tested
what is eddy current inspection?
specialised equipment to generate and measure an electric field that detects flaws at or slightly below the surface of the component
what is magnetic particle inspection?
conducted at an appropriately approved maintenance facility. It is useful for finding cracks, inclusions, and imperfections at or near the surface of ferrous parts.
where are most failure on propellors?
tip area
what is erosion?
small particles wearing away the surface
what do composite blades have fitted to protect against erosion?
erosion strips
what is surface corrosion?
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
what is pitting?
visible corrosion cavities extending inward from the metal surface. They can grow on the surface, under decals, or under improperly installed de-ice boots
what is intergranular corrosion?
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
what are woods susceptible to from impact damage that other materials are not?
internal damage from small stone strikes that can create delamination or microcracks and permit intrusion of moisture
what should be inspected for in wooden blades?
glue lines for debonding; look for warp and loss of protective coating (paint or varnish)
what is cracked paint in wooden blades a sign of?
flexural vibrations
what are cracks at the beginning of and along the metal tipping in wooden blades a sign of?
differing expansion of the material
what repairs can be performed to wooden blades?
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
how can surface damage on wooden propellors be repaired?
wood filler or a mix of sawdust and aero glue (casein)
what cannot be repaired on wooden blades?
Cracks across the grain
A splintered blade
Delamination
Missing material
Cracks in the hub
Enlargement of the hub shaft bore
Elliptical bolt holes
when preparing blade tips on metal blades what must be observed?
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.
what static checks can be performed?
Flight Fine Pitch Stop (FFPS)
Auto-Coarsening
Manual Feather
Autofeather
Unfeather
what is a static check?
a check that can be done without running the engine
when are static checks usually done?
prior to a engine ground run
how would you do a static check?
using the electric feathering pump
how are reciprocating engines further classified?
normally aspirated
supercharged
what are power ratings of reciprocating engines related to?
manifold air pressure
how is reference RPM measured and what is this called?
-set propellor to fine pitch
-push throttle until until manifold pressure reads zero
-check RPM matches published figure
a reference power check
what is reference RPM?
RPM when it reaches zero boost
what does tail wind do in terms of RPM?
reduces
if there is visible moisture in the air a and temperature is below 10 degrees what is possible?
icing at inlet
how is turbo prop engine power measured?
shaft horsepower
what is the purpose of performance checks?
to ensure continued safe operation of propellor engine combinations
what are the important parameters of performance checks?
fuel flow
torque
RPM
EGT
if there are visible moisture conditions and it is below 10 degrees what must be done for ground tests?
anti ice switched ON
why must you run the engine at idle in engine ground test?
allow casing temps to stabilise
