ppl_06_e2
.pdf
ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CH AP T ER 8 : AER O ENG INE F U ELS AND F U EL
Great care must be taken to ensure that the priming pump plunger is locked in after use. If it inadvertently becomes unlocked, it may vibrate open. This will create a risk of fuel being sucked from the top of the fuel strainer into the inlet manifold of the engine, thereby making the mixture extremely rich and perhaps stopping the engine.
FUEL SYSTEM MANAGEMENT.
It is essential that a pilot understands the fuel system of the aircraft he is about to fly. A pilot should therefore study the Pilot’s Operating Handbook (POH) and follow the recommended fuel management procedures.
The basic principles of fuel system management are as follows:
•Ensure that the aircraft has sufficient fuel for the flight being undertaken, including all necessary reserves.
•If the aircraft requires refuelling, ensure that the correct grade of fuel is used. Check that fuel caps are replaced and are tightly closed. Fuel caps situated on the top of wing surfaces are in the low pressure area of the airflow. If a cap in this location becomes loose, fuel can be syphoned out of the tanks in flight.
•Before the first flight of the day, carry out fuel contamination checks.
•Ensure that there are no leaks in the fuel system during the pre-flight check.
•Switch on the electric fuel pump before switching tanks in flight. Switch tanks according to the procedure detailed in the POH.
R e f u e l l i n g .
•No one must remain in the aircraft during refuelling.
•The engine must be shut down.
•Ignition switches must be off.
•Appropriate extinguishers and fire-fighting equipment must be at hand.
•No one must smoke in the vicinity of an aircraft being refuelled.
•All earth wires fitted to the refuelling equipment must be employed in accordance with operation procedures to eliminate the risk of static electricity generating a spark which might ignite fuel vapour.
If the priming
pump is left unlocked after
use, it may
allow fuel to be sucked from the fuel strainer into the inlet manifold, causing an extremely rich mixture.
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CH AP T ER 8 : AER O ENG INE F U ELS AND F U EL SY ST EMS Q U EST IO NS
R e p r e s e n t a t i v e P P L - t y p e q u e s t i o n s k n o w l e d g e o f En g i n e F u e l s & F u e l Sy s t
1.Duringpreflightchecks,waterinthefuelsystemismonitored.Thisisbecause water will cause:
a.Icing
b.The fuel to freeze
c.Fuel system contamination resulting in the loss of engine power
d.Emulsification of the fuel in the fuel lines which could cause them to become blocked
2.The type of aeroplane which may legally be fuelled with MOGAS may be found in:
a.CAAAirworthiness Notices and Safety Sense leaflets
b.AICs
c.Notams.
d.Flight Information Handbook
3.The fuel and the tank labels of refuelling installations are colour coded. The colour for all labels relating to 100LL is....(i)...., and the colour of the fuel
itself |
is....(ii)....: |
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(i) |
(ii) |
a. |
black |
red |
b. |
red |
blue |
c. |
blue |
straw |
d. |
red |
straw |
4.The electrically driven auxiliary fuel pump on a piston engine is located:
a.At the lowest point of the fuel tank
b.Upstream of the tank selector valve
c.In the tank-to-tank fuel transfer line
d.Upstream of the engine driven pump
5.A fuel priming pump normally delivers fuel directly to:
a.The induction manifold or inlet valve port
b.The carburettor float chamber
c.The combustion chamber
d.The accelerator pump outlet
6.In the aircraft tanks, fuel is most likely to be contaminated by water from:
a.Poorly fitting fuel caps
b.Contamination during refuelling
c.Leaks in the tanks that have let in rain
d.Atmospheric air remaining in partially-filled tanks
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ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CH AP T ER 8 : AER O ENG INE F U ELS AND F U EL SY ST EMS
7.It is important to ensure that the priming pump is locked after use because:
a.It may cause a fuel leak, resulting in an increased fire risk
b.It may cause fuel to be sucked from the fuel tank into the carburettor, causing an extremely rich mixture
c.It may allow fuel to be sucked from the fuel strainer into the inlet manifold, causing an extremely rich mixture
d.If it vibrates closed, it will cause the engine to stop
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T h e a n s w e r s t o t h e s e q u e s t i o n s c a n b e f o u n d a t
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 9
PROPELLERS
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CH AP T ER 9 : P R O P ELLER S
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ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CH AP T ER 9 : P R O P EL
PURPOSE OF A PROPELLER.
Most general aviation aircraft are powered by propellers. The purpose of a propeller (Figure 9.1) is to convert the power delivered by an engine into propulsive thrust.
The detailed theory of how thrust is produced is complex, but expressed simply, there are two principles which explain the nature of thrust:
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The |
propeller |
accelerates a |
mass of |
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air rearwards and, in accordance with |
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Newton’s 3rd law, experiences a force |
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acting on itself in the opposite direction. |
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This force is called thrust. |
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The |
propeller |
blades are |
aerofoils |
Figure 9.1 A Propeller. |
which act like rotating wings causing a difference in static pressure across the blades.
Just as a wing generates a lifting force acting upwards, the propeller generates a forward horizontal force called thrust. Airflow over a propeller is more complex than over a wing because the propeller is not only rotating, but moving forwards. Some aerodynamicists believe that both of the above principles of propeller thrust are connected, and are explained by Newton’s Second Law, in the sense that rotating propeller blades impart a rate of change of momentum to the air flowing over the blades, thus applying a force to the air, changing its velocity and pressure distribution.
The ‘Principles of Flight’ volume of this series discusses propeller aerodynamics in detail. In this chapter we will deal primarily with the technical and mechanical aspects of propellers and their operation.
BLADE GEOMETRY.
The propeller consists of two or more aerodynamically shaped blades attached to a central hub. This hub is mounted onto a propeller shaft driven by the engine.
Figure 9.2 Propeller Nomenclature.
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CH AP T ER 9 : P R O P ELLER S
The whole assembly is rotated by the propeller shaft, rather like rotating wings.
Like a wing, a propeller blade has a root and a tip, a leading and trailing edge and a cambered cross-section whose chord line passes from the centre of the leading edge radius to the trailing edge. At the root area, where the section of the blade becomes round, is the blade shank. The base of the blade, where any pitch change mechanism would have to be attached, is called the blade ‘butt’.
Ch o r d . Li n e
The chord line of the propeller blade is a straight line joining the centres of curvature of the leading and trailing edges of the blade.
Ch o r d .
The chord of the propeller blade is the distance between its leading edge and its trailing edge, measured along the chord line.
Bl a d e An g l e O r P i
The blade angle or pitch is the angle between the blade chord line and the
plane of rotation. |
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Blade angle decreases from the root |
Figure 9.3 Propeller Definitions. |
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to the tip of the blade (see Figure 9.5) |
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because the rotational velocity of the |
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blade increases from root to tip. This |
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twist along the length of the blade |
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ensures an optimum angle of attack |
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throughout the blade length. For |
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reference purposes, the blade angle is |
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measured at a point 75% of the blade |
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length from the root. |
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Propeller blades are
twisted along their
length to ensure an optimum angle of attack throughout the whole length of the blade.
Bl a d e T w i s t .
Blade sections near the tip of the propeller are at a greater distance from the propeller shaft and travel through a greater distance for each rotation. Therefore, for any given engine speed (measured in revolutions per minute
or RPM), the rotational speed of the Figure 9.4 Blade Angle. tip of the propeller is greater than that
of blade elements near the hub.
The twist in a propeller
blade is designed
to reduce the blade angle towards the tip.
The blade angle must be decreased towards the tip across the whole length of the blade to ensure an optimum angle of attack. This aspect of a propeller’s operation is discussed fully in the Principles of Flight volume in this series.
The blade angle determines the geometric pitch of the propeller. A small blade angle is called fine pitch while a large blade angle is called coarse pitch.
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ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Figure 9.5 Blade Angle decreases from root to tip.
CH AP T ER 9 : P R O P EL
G e o m e t r i c P i t c h .
The geometric pitch, (Figure 9.6), is the distance the propeller would travel forward in one complete revolution if it were moving through the air at the blade angle, just as a wood screw advances through wood as it is twisted by the screwdriver.
Figure 9.6 Geometric Pitch.
Ef f e c t i v e P i t c h .
In flight, the propeller does not move through the air at the geometric pitch, because as air is a fluid, and not a solid medium, slippage always occurs
The distance which it actually moves forward in each revolution is called the ‘effective pitch’ or ‘advance per revolution’ (Figure 9.7, overleaf.)
P r o p e l l e r Sl i p .
The difference between the geometric pitch and the effective pitch is called propeller slip, this is shown in Figure 9.7, overleaf.
T h e H e l i x An g l e .
The helix angle is the angle that the actual path of the propeller makes to the plane of rotation as shown in Figure 9.8, overleaf.
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CH AP T ER 9 : P R O P ELLER S
Figure 9.7 Effective Pitch and Propeller Slip.
Figure 9.8 The Helix Angle.
Bl a d e As p e c t R a t
Blade aspect ratio is the ratio of blade length to the blade’s mean chord.
An g l e o f At t a c k .
The angle between the blade chord and the relative airflow during propeller rotation is the angle of attack, shown in the diagram as alpha (α) (Figure 9.9).
The angle of attack of a fixed-pitch propeller depends on the propeller’s RPM and aircraft’s forward speed.
Figure 9.9 Angle of Attack.
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