17.2 propellor construction Flashcards
where are wooden propellors typically used?
typically in older fixed propellor aircraft but sometimes in variable pitch planes
what is the construction of wooden propellors and typical woods used?
Wooden propellers consist of several layers of wood bonded with a waterproof resin glue. Birch is the most commonly used wood, At least 5 layers of wood are laid down and when glued together they form a blank
how long is a wooden blade left to cure and what does it form?
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
what is done to a ‘white’ after it is roughly formed?
The white is finished by shaving it into its correct form using templates and blade protractors to check the shape and pitch, ensuring it meets the exact aerofoil and pitch dimensions required for the propellers desired performance characteristics. At the same time the centre bore and bolt holes are drilled out.
what is applied to the outside of a wooden propellor after it is formed?
A cotton fabric sheathing is then applied to the outer 12 to 15 in of the tip to reinforce the strength of the thin sections. The fabric is doped to protect it from deterioration by the weather and the sun’s (UV) rays.
A protective metal tipping strip is rebated into the leading edge of the propeller to prevent damage from small stones during ground operations.
what is the metal used on wooden blades constructed from?
The metal used is brass, stainless steel, monel (nickel/copper alloy) or terneplate (steel coated with a lead/tin alloy).
how is the metal protection held onto a wooden blade?
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
what is the purpose of the small holes drilled in wooden propellors?
releasing any moisture and allowing the wood to breathe. The propeller is then balanced and the finishing coats of varnish are applied
what is sometimes used instead of cotton dope?
plastic before the metal tipping is applied. This provides protection and added strength.
what does polyurethane paint provide to a propellor?
mechanical and UV ray protection.
what type of screws are used for controllable pitch propellors?
lag screw or coach bolt
what are the benefits of a composite propellor?
lighter in weight and stronger than metal propellers. With new methods of lay-up, these propellers are taking over from metal blades
what is one downside to composite propellors?
damage can be difficult to detect apart from very minor scratches
what is used on composite blades to protect from impact damage?
Nickel or stainless steel erosion shields are bonded to the leading edges
what can be done to limit blade tip erosion in composite blades?
a stainless steel wire mesh is put into the laminations
what ca be done to limit lightning damage?
A metal foil mesh can be incorporated on the surface of the blade
what is used for de icing composite blades?
Most composite blades use an external boot for de-icing but some blades have been introduced with internal heating elements.
what is a spar typically made of in a propellor blade?
titanium
aluminium alloy
stainless steel
what does the central spar absorb in a propellor blade?
centrifugal forces and the bending loads
what does the shell absorb in a propellor with a metal spar?
torsional rigidity
what provides the heating function of the propellor blade with a metal spar and what is applied to the outside surface of the blade?
There is a heating element on the inner part of the surface of the leading edge
The surface is protected by a coat of conducting polyurethane paint
what is the purpose of conducting polyurethane paint in propellors with a metal spar?
protection against erosion and prevents the blade from becoming statically charged.
what is contained in the centre of a blade without a metal spar?
polyurethane foam
what forms the spar in a blade with no metal spar?
two carbon fibre components
what is used to allow for minor adjustments in non metal spar blades?
leads wool is placed in a hollow balancing tube and held in place with a rubber plug
what is the purpose of the metal pin on a non metal spar blade?
inserted into the pitch change mechanism
what are metal propellors typically constructed from in engines with greater operating power?
aluminium alloy and steel
what are the most widely used propellors in aviation?
aluminium alloy
what are the benefits of aluminium alloy blades?
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.
they require less maintenance which reduces their operating costs.
how are aluminium alloy blades constructed?
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.
what is used more modernly to grind aluminium alloy blades?
final grinding is usually carried out by CNC machines to produce more accurate, uniform results.
how are aluminium alloy blades finally balanced?
by hand
how are aluminium blades usually finsihed?
-plating
-chemical etching and/or painting
-Anodising is the most common
what are steel propellors used for and how common are they?
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
how are hollow steel blades usually constructed and why?
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.
how are blade stations measured?
divisions of the propeller’s radius taken from the centre of the hub to the blade tip
how are blade stations typically measured/ numbered?
from 1 onward in six-inch increments
what can the master reference point be assumed to be?
a reference point as to where most of the thrust of the blade is produced
what are the disadvantages of a fixed rotor propellor?
-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.
what can controllable pitch propellors variations in pitch be?
The number of pitch positions at which the propeller can be set can be limited, such as a two-position propeller with only high or low pitch available or
variable pitch and can be adjusted to any pitch angle between a minimum and maximum pitch setting.
what are fitted to controllable pitch propellors cockpits to control the propellors?
throttle control
propellor control
what is the most popular design for controllable pitch propellors?
Beech-Roby propeller system
what mechanisms are used to control pitch on a controllable pitch propellor?
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
what is the principle behind variable pitch propellors operation?
a series of gears turn a actuator with two arms which extend into the propellor hub and connect with actuating pins
what do most constant speed propellors operate on a principle of?
centrifugal force a set of speeder springs and fly weights
what is the purpose of a speeder spring in a Constant Speed Unit?
to balance the fly weights at a specific propellor RPM
what effect do the weights have in under and over speed conditions for a Constant Speed Unit?
overspeed force fly weights outwards
under speed forces the weights to swing inwards
what was the system in early Constant Speed Unit systems?
flyweights would mechanically change propellor pitch
what has changed in terms of pitch adjustment on a modern Constant Speed Unit?
oil is ported either from engines or independent supply to change propellor pitch
what are some manufacturers using instead of fly weights in Constant Speed Unit?
electronic governing mechanisms instead of fly weights
when is tapered shaft typically used to install propellors?
small older aircraft with low powered piston engines
where is a splined shaft typically used?
high output radial engines
where are flanged shafts usually used?
modern reciprocating engines
what is the purpose of a spinner?
aerodynamics, mechanical protection and aesthetics
what are spinners usually manufactured from?
one piece of spun aluminium or glass fibre composites
what are spinners mounted to?
spinner backplate
what is the purpose of then support at the front end of the spinner?
centring
what repairs are allowed on spinners due to high dynamic loading?
stop drilling only
what must be ensured when installing a spinner?
no wobble is visible
