17.2 Flashcards
What materials can props be made from?
Wooden propellers. Composite propellers. Metal propellers. ◦ Aluminium Alloy. ◦ Steel
How is a wooden prop constructed?
Wooden propellers consist of several layers of wood bonded with a waterproof resin glue (aero-glue)
What type of wood is most commonly used and what other types of wood can be used?
Birch is the most commonly used wood, but mahogany, cherry, ash, beech, oak and walnut can also be used
How many layers of wood are typically used in wooden prop construction?
At least 5 layers of wood are laid down and when glued together they form a blank
What is a blank?
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 a white?
The white is finished to 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 used to reinforce the tip of a wooden blade?
A cotton fabric sheathing is then applied to the outer 12 to 15 inches (30 to 38 cm) of the tip to reinforce the strength of the thin sections at the tip
How long is the cotton sheath that reinforces the thin sections of the tip?
12 to 15 inches (30 to 38 cm)
How is the cotton sheath protected against the effects of uv?
The fabric is doped to protect it from deterioration by the weather and the sun’s ultraviolet (UV) rays
What is rebated into the leading edge of a wooden prop?
A protective metal tipping strip is rebated into the leading edge of the propeller to prevent damage from small stones during ground operations
What metals are used for the rebated protective strip on a wooden prop?
The metal used is brass, stainless steel, Monel (nickel/copper alloy) or terneplate (steel coated with a lead/tin alloy)
What secures the rebated tipping to the blade?
The 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
What is used to release moisture from the prop?
Three small #60 (0.04 inch or 1.016 mm) size holes, 3/16-inch 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
Instead of a cotton sheathing, what two methods of finishing is available?
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.
Some types of propeller have a final coating of polyurethane paint which
provides mechanical and UV ray protection
What is the difference when constructing a variable pitch prop compared to a fixed pitch prop?
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 - a heavy duty wood screw with a square or hexagonal head that is driven in with a wrench;
designed for metal to timber connections)
Composite blades are typically …?
Lighter and stronger than metal blades
What is used to protect the leading edge of a composite blade?
Nickel or stainless steel erosion shields are bonded to the leading edges to protect from impact damage
What can be used to inhibit blade tip erosion on a composite blade?
Some designs incorporate a stainless steel
wire mesh in the laminations to inhibit blade tip erosion
How is lighting strike protection added to a composite blade?
metal foil mesh may also be incorporated on the surface of the blade to limit lightning strike damage
What are the two methods of composite blade heating?
Most composite blades use an external boot for de-icing but some blades have been introduced with internal heating elements
Explain how a composite blade with a metal spar is constructed
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. 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 becoming statically charged
Explain the construction of composite blade that does have a metal spar
The two carbon fibre components in the blade form the spar and the hollow centre is filled with a polyurethane foam. A composite shell surrounds the spar.
At the root of the blade the spar is attached to the metal blade-root components. The operating pin is inserted into a pitch change mechanism
When are metal props typically used?
Metal propellers are used on aircraft with greater engine power and are constructed from either aluminium alloy or steel
Which type of metal prop is most widely used?
Aluminium Propellers
Why are aluminium props better than wooden ones?
Aluminium alloy is more durable than wood because thinner, more efficient aerofoils may 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
How are ally props typically made?
Typically extruded, finished to the aerofoil shape by means of machining and grinding, nowadays cnc machines finish the grinding
How is a blade finished?
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
Are steel props common?
No, used on old aircraft
What are the two types of steel prop?
Steel blades are normally hollow, but there are also blades which are solid. The primary advantage of hollow steel blades is their reduced weight
How are steel props made?
Hollow steel propellers are made from folded sheet 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
What are prop blade stations?
The designation of location of various points on a propeller blade. It is normally numbered from 1 onward or as the number of inches or centimetres from the
root of the blade toward its tip
What is the Propeller Master Reference Station?
A distance measured from the centre of rotation where all measurements are taken from. Normally 75% from the centre of rotation on a fixed pitch propeller
and can be 50% to 75% on a variable pitch propeller
What is the prop blade angle?
The angle between the blade chord line and the plane of rotation
What is the prop face?
The flat thrust producing side of a propeller blade
What is the prop blade root/shank?
The thickened portion of the blade nearest to the hub
What is the prop blade butt?
The base of the propeller blade where the root ends
What is the prop back?
The curved side of the propeller blade facing the direction of flight
What is the prop hub?
Central portion of a propeller which carries the blades
What is the dome assembly?
Encases the pitch change mechanism
What is the upper speed and power limit for a fixed pitch prop?
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)
What prop pitch is ideal for towing, steep climbing?
Fine
What prop pitch is ideal for gradual climb and cruising flight?
Coarse
Why are fixed pitch props favorable?
Production costs and maintenance costs
How are fixed pitch props mounted?
Typically directly to the engine with bolts, spacers can be used increase distance from the engine to create more favourable cowlings
How is a fixed pitch prop generally set up?
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
Explain fixed pitch props with regards to take-off speeds
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 in a longer take off run
Explain maximum forward speed for a fixed pitch prop
When the angle of attack is at its optimum of 4º then maximum RPM is available. 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
Explain a fixed pitch props power required to maintain constant rpm
The density of the air, and therefore its weight per unit volume, will have an effect on the amount of power that the propeller will absorb. If the density of the air increases, the greater weight of air will absorb more engine power causing the RPM to decrease. Therefore, to maintain a constant RPM the propeller will require more power (to double the RPM the power must be
quadrupled). With decreasing air density the propeller will require less power to maintain a constant RPM
Disadvantages of fixed pitch props
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
When adjusting ground adjustable propellers, what is the tolerance between the blade angles?
0.1 degrees
What is the typical angle range for ground adjustable props?
7 to 15 degrees
What is the most popular variable pitch system?
The beech-roby system
