6. Propellers
Introduction
The propeller is a means of converting the power developed by the aircrafts engine into a propulsive force. A rotating propeller imparts a rearward motion to a mass of air, and the reaction to this is a forward force on the propeller blades
The basic cross sectional shape of the propeller blade is that of an aerofoil similar to the wing or other lift generating surface. The propeller is is driven by the aircraft engine either directly from the crankshaft, or via a gear box which will usually reduce the RPM of the propeller in relation to the RPM of the engine
6.3.1 Propeller nomenclature
Angle of advance (Helix angle)
Angle between the relative airflow and the rotational velocity
Angle of attack
Angle between the relative airflow and the chord
Refer to Figure 1 below
Blade angle
Angle between the blade face or chord at a point and the plane of rotation
These angles may be linked together
BLADE ANGLE= ANGLE OF ATTACK+ANGLE OF ADVANCE
Refer to Figure 1 below
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Blade Twist
FIG 2
Figure 1
Pusher propeller
A propeller mounted behind the engine
Tractor propeller
A propeller mounted in front of the engine
Ground clearance
The clearance that exists between the propeller tip and the ground with the aircraft in normal flying attitude
Geometric pitch
The Geometric pitch of a propeller is the distance which it should move forward in onerevolution without slip, much like a screw thread and how it will move into its hole in one revolution.The air however is not solid and the propeller does not achieve its geometric pitch as a certain amount of slip will occur.
Geometric pitch is theoretical
2?r
Figure 2
Fuselage clearance
The clearance between the propeller tip and the side of the fuselage, usually considered on multi-engine aircraft
6.3.2 Conversion of engine power to thrust
The propeller is designed to convert the turning effort of the engine into a direct push or pull along the line of flight. This push or pull is called thrust .The propeller obtains this thrust by screwing its way through the air, in much the same way as a ships propeller does through water.
6.3.3 Design and construction of fixed pitch propeller
The fixed pitch propeller is often molded in Kevlar, cast in solid aluminum or laminated in wood often with protective reinforcement. The fixed pitch propeller is essentially one piece with no moving parts. The blade angles over the length of the blade are fixed. There are some propellers that are made with separate blades that are bolted onto a central hub, the blade angle on these types can be set after manufacture to suit a particular aircraft, these propellers are known as ground adjustable propellers
6.3.4 Forces acting on the blade
Figure 3
6.3.5 Variation of rpm with change of airspeed
With a fixed pitch propeller a change in the pitch attitude of the aircraft will in the case of
A nose pitch up (climb attitude) result in a decreased RPM
A nose pitch down (dive attitude) result in an increased RPM
6.3.6 Thrust efficiency with a change of speed
The efficiency of any system can be measured from the ratio Power out/Power in
The effect of speed on a fixed pitch propeller
At high speed the angle of attack of the blade will be close to zero lift incidence and thrust will reduce to zero.
There will only be one speed at which the blade is operating at its most efficient angle of attack thus efficiency will be maximum.
At low speeds thrust increases with angle of attack and provided the blade is not stalled, the thrust is large, however speed is low and efficiency is low
Figure 4
6.3.7 Design and construction of variable pitch propeller
A variable pitch propeller is one in which the angle of the blades can be adjusted. It can be visually identified by having separate components namely blades that are attached to a central hub which in turn contains the electrical or hydraulics used to effect the change of blade angle.
6.3.8 &9 Constant speed unit operation
At low airspeed the blade angle needs to be small for the angle of attack to be optimum, this is known as fine pitch. As the forward speed increases the blade angle needs to increase, or coarsen for the angle of attack to remain optimum. The device used to achieve this is the constant speed unit. It contains a governor whose function is to regulate the propeller speed (RPM) to that selected by the pilot .It does that automatically adjusting the blade angle electrically or hydraulically so that RPM is maintained irrespective of the airspeed and power delivered by the engine. Thus the RPM setting determines the angle of attack.
High RPM is fine pitch and low RPM is coarse pitch. Fine pitch is used for take off and climb and coarse pitch is used for cruise.
The aim is to have the propeller close to the best angle of attack and the engine RPM set for maximum efficiency throughout the aircraft airspeed range
6.3.10 Wind-milling effect
A propeller is usually driven by the engine. There are occasions when the propeller drives the engine and this is known as wind-milling- perhaps a steed dive with the power off, a sudden reduction in power or an engine failure. This really important for (twin) engine operations.
When a propeller is wind-milling the aero-dynamic twisting moment acts in the direction as the centrifugal twisting moment and tends to flatten the pitch
6.4 Systems
6.4.1 Electrical system
6.4.1.1 Construction and operation of generators/alternators and related components
Alternator
An alternator is a mechanically driven generating device that produces alternating current. It is driven via a a belt drive from the engine.
Most small aircraft require electricity as direct current, so the alternating current generated by the alternator must be changed to Direct Current, This is achieved by the use of diodes that convert alternating current to direct current.
The alternator produces 14 or 28 Volt Direct Current to provide electrical power for lights and radios and other services and also recharges the battery
Generator
A generator is a device which produces direct current from a (stationary) mechanical commutator and rotating magnets.
Generators are less efficient than alternators and require a higher RPM to develop sufficient power to charge the aircrafts battery. they are generally much heavier than alternators and their output varies with engine RPM. One advantage of a generator is its independence from the battery system in that no exciter voltage is required to start the generation of direct Current.
Bus Bar
Is the main conductor and the distribution Centre in the electrical system. Electrical power from the alternator or generator is supplied to the bus bar which is essentially a large number of interconnected terminals.
Inverter
Most light aircraft require only Direct current power, however as new systems are fitted some Alternating Current may be required and an inverter is used.
Remote indicating compasses and high quality gyroscopes are examples of this requirement.
Voltage regulator
The voltage regulator senses the output line of the alternator or generator and acts to maintain the output voltage at a preset value
6.4.1.2 Direct current supply
Most light aircraft utilize Direct current for electrical power requirements, this Direct Current is generated as Alternating current in the alternator and rectified into Direct Current by the use of diodes
6.4.1.3 Batteries, construction capacity and charging
The battery provide the initial electrical power to rotate the engine with an electric starter motor. It also provides back up or emergency power when the alternator is not working or the electrical load is too large. During start the battery is require to provide the Direct Current to excite the alternator magnetic fields. Once the engine is running the alternator is self-sustaining and will no longer need power from the battery, in fact the alternator provide sufficient current to recharge the battery after the engine has been started. However a flight should not be undertaken with the battery bin a poor condition as it could result in having no electrical power during flight if the alternator fails. Typical charge rates are shown in 6.4.1.4 below.
Batteries are storage devices the common type being the lead acid type where dilute sulpheric acid interacts with lead plates in battery compartments called cells. Sulpheric acid is very corrosive and dangerous to the aircraft structure. Hydrogen gas is given off from a battery is extremely flammable when mixed with air and for these reasons batteries are fitted into a sealed and vented container.
Battery capacity
Batteries are rated on their capacity to supply a given current for a particular time. A 40 amp hour battery will supply 40 amps for I Hour or 20 amps for two hours.
Example 12 volts 40 amp hours
Batteries may be connected together in series
Example
Two 12 volt batteries connected in series will give 24 Volts the amp hours will remain 40 amp hours
Batteries may be connected in Parallel
Example
Two 12 Volt Batteries connected in Parrallel the Voltage remains 12 Volt and the amp hours increase to 80 Amp hours.
each cell of a battery produces 2 Volts i.e. a 12 volt battery will have 6 cells
Master switch
The master switch controls the aircraft electrical system.
The master switch needs to be switched on for the bus bar to receive power.
In aircraft with an alternator fitted the master switch can be off the split switch type one half for operating the battery the other for operating and energizing the alternator
6.4.1.4 Voltmeters and ammeters
Voltmeters
Some aircraft are fitted with a volt meter in place of an ammeter .The volt meter indicates the voltage of the electrical system. Volt meters may look similar to load type ammeter but are calibrated in VOLTS
A typical voltage of a 12 volt battery would be in the region of 13.8 to 14 Volts with the engine running. With the engine not running 12 Volts would be indicated.
The charge voltage for a 24 Volt battery system with the engine running would be in the order of 28 Volts
Ammeters
Used to measure the electrical current called amps or amperes.
There are two distinct types
Load type ammeter referred as a load meter.
Measures only the output of the alternator. It is graduated from zero amps on the left end of the scale and increases to the right end of the scale, it may also be shown as a percentage of the alternators rated load.
With the battery switch on and the engine not running or with the engine running and the alternator switch off the ammeter/load meter will indicate zero
If the engine is started and the alternator is switched on the ammeter will show the alternator output current.
During start the battery discharges electrical power so immediately after start up the ammeter indication will be quite high during the initial battery recharging. When the battery is fully charged the ammeter should show a reading slightly above zero if all other electrical circuits are switched off. As extra circuits are switched on (lights, radios etc) the ammeter reading will increase.
If the ammeter reading falls to zero in flight it means an alternator failure indicated by a red warning light. In this instance the electrical load must be reduced to a minimum as only the battery will be supplying electrical power. Land s soon as practicable to have the problem corrected.
Centre Zero Ammeter
Shows the current to and from the battery
Current into the battery is charge with the needle deflected to the right of Centre.
Current out of the battery is a discharge with the ammeter deflected left of Centre.
With the battery switch on and no generator output the ammeter will indicate a discharge from the battery, i.e. the battery is providing electrical current for the electrical circuits that are switched on. The ammeter needle is to the left (discharge side) of Centre Zero. With the alternator on and supplying electrical power if the electrical load required is less than the capability of the alternator the ammeter will show a charge.
If the alternator is on but incapable of supplying sufficient power to the electrical circuits the battery must make up the balance and there will be some flow of current from the battery. The ammeter will show a discharge. If this continues the battery could discharge completely
6.4.1.5 Circuit breakers and fuses
Fuses circuit breakers and overload switches are provided to protect the equipment from any electrical current overload. If there is an electrical overload or short circuit a fuse wire will melt or a circuit breaker will pop out and break the circuit so that no current can flow through it. It may prevent the circuit from overheating, smoking or catching fire. As most circuit breakers use bimetal thermal sensors it is recommended that that a 90 second waiting period is observed to allow the element to cool before attempting a reset. If the equipment causing the fault can be identified it should be turned off. A circuit breaker should only be rest once.
Overload switches are combined on/off switches with a built in circuit breaker. Overload switches will switch themselves off if they experience an electrical overload. The pilot can switch them back on like a resettable circuit breaker.
6.4.1.6 Electrically operated services and instruments.
Avionics
HSI
Radio Equipment VHF, HF, Transponder, TCAS, ACAS
Navigation Equipment VOR, ILS, NDB, GPS, GNSS, RNAV, DME
Radar Weather radar, Radar Altimeter. Emergency locator Beacon
Navigation Lights
Strobe lights
Taxi Light
Landing lights
Instrument panel lighting
Cockpit lighting
Cabin lighting
Signage
Instruments
HSI
Artificial Horizon
Direction Indicator
Fuel gauges
Turn Coordinator
Flight Controls
Flap operation
Undercarriage operation
Auto pilots and electric trim
Hydraulic system electrical
Pitot Heater
Windscreen heater
De ice and anti ice systems
6.4.1.7&8 Recognition of malfunctions and procedure in the event of malfunction
An electrical overload will normally cause a circuit breaker to pop. This protects the affected circuit. Allow 90 seconds to cool and if there are no indications of smoke, fire or burning reset the breaker. But reset ONCE only. If the circuit breaker pops again do NOT reset it a second time.
The ammeter should be checked when the engine is running to ensure that the alternator is supplying sufficient current for the electrical services and to recharge the battery. A Centre zero ammeter indicates the rate at which the current is flowing into the battery and recharging it.
With the engine running the ammeter can indicate two faults
Insufficient current to charge the battery and too much current.
With insufficient current from the alternator, or none at all, unnecessary or non essential equipment should be switched off to conserve the battery and thought should be given to making an early landing. An aeroplane battery cannot on its own supply all electrical equipment for a long period.
With an excessive charge rate, the battery could overheat and the electrolyte, which is usually dilute Sulpheric acid, could evaporate, possibly damaging the battery. The cause of the excessive current charging is likely to be a faulty voltage regulator and equipment such as radios could be adversely affected. Many aeroplanes have an overvoltage sensor that would in these circumstances automatically disconnect the alternator and illuminate a red warning light in the cockpit to alert the pilots.
If the alternator 9or generator fails) switch off non essential services such as unnecessary lights and radios.Consideration should be given to terminating the flight at the nearest suitable aerodrome while electrical power is still available
6.4.2 Vacuum system
The gyroscopes in the flight instruments may be spun electronically or by a stream of high speed air directed at buckets cut into the perimeter of the rotor. The vacuum system(which draws this high speed air into the gyro instrument case where it spins the gyro)needs explanation.
Some aircraft (especially older ones) have a vacuum system operated by a venture on the outside of the airframe. Most modern systems use an engine driven suction pump. This evacuates the cases of the gyroscopic instruments creating a vacuum9low pressure)
Component
6.4.2.1 Pump
An engine driven suction pump is utilized
6.4.2.2 Regulator and gauge
The required suction is 3-5 inches of mercury (less than Atmospheric)The vacuum relief valve (regulator) is used to maintain the required pressure
6.4.2.3 Filter system
Air is drawn into the vacuum system through a filter which removes impurities which if allowed to enter the system could damage or cause excessive wear to the gyroscopes.
Recognition of malfunction
If the filter blocks or the vacuum fails the reduced airflow may allow the gyroscopes to gradually run down and the air driven instruments will eventually indicate erratically and/or slowly. A lower suction will be indicated on the suction gauge.
If the vacuum pressure is too high the gyroscopes may spin to fast and suffer damage.
Procedure in the event of malfunction
Flight under IFR is prohibited if the required suction pressure is below 3-5 inches of mercury at the before take off power check. The inherent danger is that under IFR Flight a pilot may follow the erroneous indications of the instruments which could lead to a s (graveyard) spiral dive.
6.4.3 Hydraulic system
Introduction
Hydraulic systems using fluid under pressure are extensively used to operate major aircraft services on modern civil and military aircraft. Services such as undercarriage, flaps, air brakes, wheel brakes and primary flying control surfaces are operated by hydraulic systems
6.4.3.1 Components of a simple system
6.4.3.1a Reservoir
The design of hydraulic reservoirs varies considerably however they all perform the same basic functions.
1 To provide a reserve of fluid to compensate for minor leakages
2 To allow for jack ram displacement ie the variations in fluid volume being returned depending on whether the jack is extended or retracted
3 To allow for expansion of fluid due to increase of temperature.
4 To provide space for returning fluid
5 To provide a head of pressure to prevent cavitation at the engine driven pump.
6 In some cases to provide a reserve of fluid for emergency purposes.
Reservoir Construction
Consists of a basic tank with a filler assembly at the top.
A filter to remove any foreign objects is usually fitted in the filler neck. The filler cap may also be fitted with a dipstick
6.4.3.1b Pressure pump
6.4.3.1c Accumulator
Allows a non-compressible fluid to be stored under pressure. The accumulator has two compartments separated by a flexible or moveable partition: a diaphragm, a bladder, or a piston. One compartment contains compressed air, the other is connected into the hydraulic system pressure manifold. When hydraulic fluid is pumped into the accumulator, the partition moves over and increases the pressure of the air, this air pushing against the partition holds pressure on the fluid. The compressed air or Nitrogen stored in the accumulator is called the accumulator precharge.
6.4.3.1d Actuator
The actuator is essentially a cylinder into which hydraulic fluid is pumped from the reservoir. The fluid presses against the jack which is housed in the cylinder causing it to move out of or back into the cylinder
Hydraulic fluid depicted by the blue arrows can push the actuator (jack) in or out of the cylinder depending upon which side of the actuator the fluid is pressing against, this would be controlled by using the control valve
6.4.3.1ePressure relief and bypass valve
6.4.3.1f Filters
Filters tend to be divided into two groups
a) Low Pressure filter
b) High pressure filter
a) Low pressure filter
Often referred to as a suction filter which is the type most commonly fitted between the reservoir and the pump, both hand pump and engine driven pumps. They are often of simple design and normally minimize resistance to fluid flow .This is essential when fitted between the reservoir and pump in order to minimize cavitation in the pump.
b) High pressure filter.
The high pressure filter sometimes referred to as a Pressure filter requires the system fluid to be under pressure in order to force it through the filter element and thereby remove any foreign matter from the fluid. Because of this, such filters will normally be fitter after the pump or downstream of the pump. In some cases high pressure filters may be fitted immediately after the pump or at the end of the supply system. Additional high pressure filters may also be fitted to individual circuits where the slightest contamination of the fluid may lead to serious damage or component failure. Such a system, or circuit is the Powered Flying Control circuit which operate hydraulically the primary control surfaces such as ailerons, rudder, elevators and spoiler’s. Failure of such a circuit on most modern aircraft may lead to loss of control and so it is important to ensure that only clean fluid reaches the Powered Control Units (PFCUs)
The PFCUs are made to very fine or close tolerances and the slightest scratch internally may create an internal leak and so make the unit unserviceable. In such a system the pressure filter is fitted at the beginning of of the PCFU circuit as a back up to the main or supply pressure filter.
Important note
In both cases the high pressure or pressure filter is fitted after the engine driven pump.
The important features of the filter are
An element capable of removing very small particles for example 5 microns a further name for these filters is a Micronic filter.
A Tell-tale indicator typically a red button which protrudes when the filter element is blocked warning that the element must be changed.
If the element becomes blocked a relief valve opens in the filter allowing unfiltered fluid to bypass the element in such an emergency
6.4.3.1g Type of fluid
There are various types of hydraulic fluid used on current civil aircraft the following are the most commonly used
a) D.T.T 585
Probably the most widely used, it is RED in color, is mineral based and systems using it require synthetic rubber seals to be fitted.
b) Castor or Vegatable
This fluid is either castor or vegetable based, is YELLOW in color and requires systems to be fitted with natural rubber seals.
c) Skydrol
A Phosphate Ester based fluid which is BLUE(Purple) in color and requires special Butyl rubber seals when it is used in a system
Operation indication warning systems
6.4.3.1h Auxiliary (Secondary Services) systems
Sometimes referred to as non-essential services only in the context of hydraulic fluid supply
For example the lowering of the undercarriage prior to landing is an essential function, however in an emergency it can be lowered by employing other methods. The following is a list of methods employed to operate services in the event of total hydraulic supply failure
Undercarriage
Lowered in an emergency by use of compressed air or nitrogen
Flaps
In a similar manner to the undercarriage the flaps may be lowered for landing by the use of compressed air or nitrogen
Wheel brakes
Emergency brakes are provided with accumulators which store sufficient fluid under pressure for a complete landing run plus a reserve factor
Hydraulically operated doors
The majority of hydraulically operated doors and similar devices have override systems which allow them to be operated manually
Air or speed brakes
In an emergency such as total hydraulic failure these would only be used on landing and would be operated by compressed air or nitrogen
6.4.4 Fuel systems
The function of a fuel system is to store fuel and deliver it to the carburetor or fuel injection system in adequate quantities at the proper pressure. It must provide a continuous supply of fuel to the engine(s) under all flight conditions including a change of altitude or attitude or a sudden acceleration or deceleration.
6.4.4.1 Fuel tanks, structural components types and supply lines
6.4.4.2 A fuel tank
Can be a metal container, a rubber bladder or a sealed portion of the wing (called a wet wing) Selection of the fuel tank to be used is made through the fuel selector
6.4.4.3 Fuel lines carry the fuel from the tanks tog the selector valve and from there to the strainer, the fuel pump and then to the carburetor or fuel injection system. Fuel lines may be a metal tube or specialized rubber hose.
6.4.4.4 Fuel Strainer
The fuel strainer(s) are located at the lowest point of the fuel tanks and also at the lowest point of the fuel system. The purpose of the strainer is to trap any heavier than fuel contaminants. The strainer is also the point at which fuel sample’s , used to check for water, are taken .A very important preflight check.
6.4.4.5 Venting system
6.4.4.6 Fuel selector
The selector provides precise control of which tank is selected and whether the fuel is selected left tank, right tank, both tanks, or off. Normally the fuel selector is left on while the aircraft is on the ground. There is agate or détente through which the selector must be forced to turn the fuel off, this is to prevent inadvertent off selection while flying. When changing tanks during flight it is advisable and indeed good practice to switch on the fuel boost pump to guarantee fuel pressure to the carburetor as the tanks are changed.
In larger aircraft the fuel system can become quite complex with multiple tanks.ht twin engined aircraft have a cross feed system which allows the left tank to feed the right engine and vice versa. It is important to keep the aircraft in lateral balance should one engine have to be shut down. Some aircraft have a transfer system that permits fuel to be transferred between tanks for balance and trim reasons
6.4.4.7 Fuel pump
In most light aircraft two pumps are fitted. An engine driven pump and an auxiliary (boost pump). Most light aircraft have a diaphragm type pump similar to a car.
Mechanical and electrical pump. The subtle difference with an aircraft pump is that it has a fuel vent under the diaphragm so if the diaphragm breaks the fuel will leak overboard, while we would prefer not to waste fuel, this is a better option than allowing fuel to leak into the engine sump causing engine failure. The mechanical pump is capable of supplying fuel for all normal operations of the engine. Prior to engine start an auxiliary or boost pump is used to prime the fuel lines and purge any vapor from them. Once the engine is started the engine driven fuel pump will take over. Correct functioning of the pump can be monitored with a fuel pressure gauge. It is usual to have the boost pump on for critical maneuvers such as takeoff and landing
6.4.4.8 Gravity feed.
Gravity feed means that the fuel, in the wing tanks of a high wing aircraft will with the fuel selector in the left right or both position will automatically flow out and down from the tanks as a result of gravity
6.4.4.9 Fuel Gauges
Most light aircraft have electrically powered fuel gauges in the cockpit, in which case the master switch will have to be switched on for the gauges to register.
Always make a physical check of the contents in the tank during the preflight inspection by removing the fuel tank filler cap and dipping the tank with a measuring device (dipstick)
Do not rely on the indications of an electrically powered fuel gauge they are often inaccurate.
Always keep a record of time versus fuel contents. Keep in mind it is time you have in the tanks
6.4.4.10 Fuel primer
Sometime s fuel priming is achieved by using a hand operated pump in the cockpit. Separate small fuel lines carry the fuel directly to the inlet manifold. This type of primer pump must be locked during flight to prevent excess fuel being drawn into the engine which could stop the engine if the fuel air mixture became too rich, this is known as a rich mixture cut. Priming is commonly achieved by pumping the throttle moving the throttle lever fore and aft two or three times by making use of the accelerator pump in the carburetor. For effective priming the master switch and boost pump must be switched on
6.4.4.11 System management.
The throttle controls the amount of air being drawn into the inlet manifold
The mixture control , controls the amount of fuel being drawn in
While many light aircraft fuel systems are similar to one another it is of ABSOLUTE IMPORTANCE to follow the procedures outlined in the Pilots Operating Handbook and/or Aircraft Flight Manual
