propellors

Question 1

what are the two types of propellers?

Answer 1

fixed pitch and variable pitch.

Question 2

what is a propellor blade?

Answer 2

an aerofoil which generates an aerodynamic force in the same way the wings generate an aerodynamic force but instead of lift it produces thrust.

Question 3

as the propellor blade rotates what happens?

Answer 3

acceleration of the air over the front of the cambered surface causes a reduced static pressure ahead of the blade.

if the propellor blade has an AoA with the oncoming airflow, thrust is produced and this propels the aircraft forward.

Question 4

what is the construction of the typical propellor?

Answer 4

the blade back is more cambered and blade face is the flatter side. the angle between the chord line of the propellor and the plane of rotation is called the blade angle.

Question 5

the cambered shape of the blade propellor allows for what?

Answer 5

allows for reduced static air pressure to be produced ahead of the blade and there’s increased static pressure behind the blade creating a Total Reaction.

Question 6

if an aircraft is stationary the motion of the propellor is purely what?

Answer 6

rotational and the speed of the blade section depends on radius and RPM. propellor with a high RPM will have a greater rotational velocity.

Question 7

what is the forward velocity in a propellor?

Answer 7

as aircraft moves forward in flight there’s a forward velocity as well as a rotational velocity.

so the propellor experiences airflow from the direction of rotation but also from in the front meaning that the angle that the relative airflow strikes the propellor blade is made up from both the forward speed of the aircraft and the rotational speed of the propellor.

Question 8

what are the three main angles to consider in propellors?

Answer 8

AoA - the angle between chord lien and relative airflow
Helix angle-angle between relative airflow and plane of rotation
Blade angle-angle between chord line and plane of rotation.

Question 9

what is the helical path?

Answer 9

because propellor experiences a rotational velocity and a forward velocity, each propellor blade section follows a corkscrew path called a helix.

Question 10

the aircraft moving forward experiences what?

Answer 10

velocity in the direction of flight, the propeller’s rotational velocity combined with its forward velocity contribute to the overall relative airflow.

Question 11

what does relative airflow create?

Answer 11

a total reaction.

Question 12

the total reaction can be broken down to what?

Answer 12

two components:

• lift which acts 90 degrees to the relative airflow
• drag which acts parallel to the relative airflow.

since it’s a propellor its more appropriate to use thrust and propellor torque.

Question 13

on a propellor diagram how do thrust and torque work?

Answer 13

thrust acts perpendicular to the plane of rotation an propellor torque acts in the same direction as the plane of rotation.

Question 14

when considering a wing drag must be overcome to provide lift but what is it for a propellor?

Answer 14

propellor torque must be balance by an opposing force which is engine torque.

Question 15

how is engine torque increased?

Answer 15

opening the throttle and increasing engine power increases engine torque, causing the propellor to rotate faster up until a point where the propellor torque matches engine torque and the RPM then becomes constant.

Question 16

the center of the hub is not shaped like an aerofoil for what reason?

Answer 16

as it needs to be thick and strong to absorb all of the forces on the blade.

Question 17

having a fixed pitch propellor means what?

Answer 17

you cannot adjust the propellor pitch, so for a given RPM, this propellor will only operate at its most efficient AoA at one airspeed.

Question 18

what is the relationship between airspeed and fixed pitch propellers?

Answer 18

with a lower RPM set, the aircraft is travelling with a low airspeed. the relative airflow will approach from a little way in front of the plane of rotation.

Question 19

what happens if the aircraft has lowered the nose to increase their airspeed?

Answer 19

because of the increased airspeed, the relative airflow experienced by the propellor approaches from a more forward position reducing the AoA.

if lowered even more, it may reach such a high speed that the AoA of the blade will be so small that little to no thrust will be produced.

Question 20

what happens if the aircraft maintains its airspeed bt has different RPM settings?

Answer 20

high RPM with a high rotational velocity and the same airspeed, the relative airflow experienced by the blade produces a relatively high AoA.

if pilot were to instead have a lower RPM, but travel the same airspeed, the RAF experience by the blade causes a lower AoA.

Question 21

what is the difference in efficiency of different fixed pitch propellers across different airspeeds?

Answer 21

• fine pitch, propellor will achieve maximum efficiency at a relatively low airspeed.
• coarse pitch propellor, which achieves maximum efficiency at a relatively higher airspeed.
• note for a given RPM each of these propellers can only achieve maximum efficiency at one airspeed, indicated by the peak of the curves.

Question 22

what is the difference in efficiency of a fixed pitch propellor to a constant speed propellor to a coarse pitch propellor?

Answer 22

constant speed can vary the pitch of the propellor blades in flight. a pilot governs the pitch using a constant speed unit.

a fine pitch propellor will achieve maximum efficiency (most efficient AoA) at one airspeed, indicated at the peak of its curve.

coarse pitch propellor achieves its max efficiency at just one airspeed. a constant speed can be operated at its most efficient AoA over a much wider range of airspeeds and RPM.

Question 23

an engine with a constant speed system is referred as what?

Answer 23

as to having a constant speed unit (CSU)

Question 24

with a fixed-pitch propeller, you only need how many throttles?

Answer 24

only one to control engine power.

Question 25

with a constant speed propellor, you need how many controls?

Answer 25

2 controls, throttle and the pitch control which is coloured blue

Question 26

what is the function of the different throttles?

Answer 26

moving the pitch control adjusts the RPM at which the propellor will be governed.

throttle, the black control on the left, controls manifold pressure and therefore the power developed by the engine. moving throttle changes the amount of power the engine produces.

Question 27

what is the construction of the typical CSU governor?

Answer 27

uses flyweights that are rotated by the engine and so are sensitive to engine speed.

When they rotate they are subject to centrifugal force, causing them to fly outwards.

Question 28

in a CSU governor what prevents it from flying outwards?

Answer 28

the speeder spring, which has its tension set at a certain level by the pilot setting an rpm with the pitch control.

Question 29

what is the operation of a CSU governor?

Answer 29

when propellor rpm rises to whatever pilot selected, the force on the flyweights is balanced by the speeder spring tension.

a valve closes off the supply of oil to and from the propellor hub locking the pitch of the propellor.

Question 30

what would happen to the csu governor if the engine and rpm should drop?

Answer 30

in this condition the force on the flyweight reduces, the speeder springs open the pilot to allow high-pressure oil in which will cause the propellor blade angle to decrease.

Question 31

a reduced blade angle allows for what?

Answer 31

the RPM to rise again to the predetermined level.

once rpm is achieved, it returns to the on speed condition.

Question 32

what is the operation of a constant speed propellor to increase power?

Answer 32

to increase power, ensure the mixture is rich, then increase the RPM with the pitch control and then increase the manifold pressure to the desired value

Question 33

what is the operation of a constant speed propellor to decrease power?

Answer 33

reduce manifold pressure with throttle, then reduce rpm with the pitch control then set mixture.

Question 34

what is the order for setting power for a constant speed propellor?

Answer 34

increase = rich, pitch, power
decrease = power, pitch, rich.

with takeoff and landing done with a full fine propellor and during cruise the blade can be coarsened.

Question 35

what is the purpose of a reduction gearbox?

Answer 35

to reduce the speed of the propellor from that of the speed of the engine.

having engines able to produce a high rotational speed is a positive, a propellor is inefficient at the same higher RPM.

Question 36

what are the reasons why a propellor can never convert 100% of the engine power into useful thrust? (4)

Answer 36

• propellor aerodynamic losses
• friction
• airspeed and best thrust to torque ratio
• density altitude

Question 37

the most efficient a propellor can be is at what %?

Answer 37

80% which would be done on an AoA of 4 degrees.

Question 38

what is the advantage of a constant speed versus a fixed pitch propellor?

Answer 38

A Constant Speed Propeller is more efficient than Fixed Pitch Propellers because it allows for the engine to operate at the most efficient RPM for given conditions.