17.1 Fundamentals Flashcards
What is a propeller?
A device, consisting of a rotating hub with two or more rotating blades; used to propel an aircraft
What must all propellers do?
It must absorb the power produced by the engine and transmit the power to the airflow passing through the propeller disk.
What is a blade?
Aerofoil section attached to the hub
What is the blade root/shank?
The thickened portion of the blade nearest to the hub
What is the blade station?
A distance measured from the centre of rotation, normally measured in inches or centimetres
What is the master reference section?
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 hub?
The central portion of the propeller which carries the blades
What is the dome assembly?
It encases the pitch change mechanism
What is the face?
The flat thrust producing side of a propeller blade.
What is the back?
The curved side of the propeller blade facing the direction of flight.
What is the blade angle?
The angle between the blade chord line and the plane of rotation
What is the plane of rotation?
The plane in which the propeller rotates. This is 90° to the engine centreline.
What is the pitch?
The distance advanced in one complete revolution.
What is the pitch change mechanism?
The device to alter propeller blade angle
What is Hi pitch?
(Coarse pitch) Large blade angle
What is Lo pitch?
(Fine pitch) Small blade angle
What is a tractor propeller?
A propeller mounted on the front of the leading edge of the wing or on the nose of the aircraft
What is a pusher propeller?
A propeller mounted on the trailing edge of the wing or at the rear of the fuselage.
What is reverse pitch?
Turning the propeller blades to a negative angle to produce a braking or reversing thrust
What is the spinner?
An aerodynamic fairing which covers the centre of the propeller
Where are all measurements for a blade taken from?
The centre of rotation
On what principle does a propeller work?
The reaction principle
What does a propeller convert engine torque into?
Thrust
A propeller accelerates a large mass airflow…
Slowly rearwards
What speed range is a propeller most efficient?
A propeller has a high propulsion efficiency at low to medium airspeeds (Mach 0.5 - 0.6)
How is the blade angle denoted?
Angle “β” (beta)
How is the Angle of Attack denoted?
Angle “α” (Alpha)
What is the equation for blade angle?
Blade angle = helix angle + angle of attack
What is the angle of attack?
The angle between the profile chord and the relative airflow towards it
What is RAF?
Relative air flow
How is the angle of Advance or Helix Angle denoted?
Angle “Ф” (Phi)
What is the helix angle?
The angle between the rotational plane of the blade and the RAF.
What happens to the helix angle (angle of advance) with increasing air speeds?
It increases
What are the advantages of a adjustable pitch propeller?
- Higher airspeeds can be achieved with the same engine power
- Good climb performance
- Towing
What is the disadvantage of a fixed pitch propeller?
It is only efficient with a given aircraft speed range
What is the sum of the propeller brake moment and engine torque at a constant rotational speed?
Zero
How can the angle of pitch be influenced at a constant rotational speed?
- changes in airspeed
- blade angle
What is the problem if the propeller exceeds its maximum possible rotational speed?
The blades can depart the hub or can be torn from the mountings
How many forces act on a propeller?
- 5 static
- 1 dynamic
What are the 5 static forces acting on a propeller?
- Centrifugal force
- Thrust bending force
- Torque bending force
- Aerodynamic twisting moment
- Centrifugal twisting moment
What two static forces always oppose each other?
Centrifugal twisting moment and aerodynamic twisting moment
CTM are always greater than ATM
What is the CTM?
The centrifugal twisting moment.
This is the mass of the blade being thrown out from the blades centre of rotation
What is the ATM?
The aerodynamic twisting moment.
This is the centre of pressure being forward of the blades centre of rotation which tries to turn the blade angle to a higher blade angle
What is the torque bending force?
Known as the “Braking moment”.
Tries to bend the blade against the direction of rotation which creates a resistance to the engine torque
Where are all static loads felt?
The blade root
Where can maintenance repair work not be carried out on a propeller?
Within the blade root area
Where on a propeller blade, do aerodynamic forces have a greater effect?
The tip of the blade
When will the maximum dynamic loading occur on a propeller blade?
Within its natural frequency range
How can vibrations from aerodynamic forces be reduced at the tip of the blade?
- tip design
- correct aerofoil shape
What will the natural frequency of the propeller blade depend on?
- length
- material
- blade root
- shape
What are the vibration frequency ranges for;
- Metal?
- Wood?
- Metal = 20Hz
- Wood = 60Hz
What part of the blade would have the point of maximum vibrations?
The outer nodal point (At around 80% of the length of the blade)
What region of a blade is most susceptible to failure?
At 80% of the blade length (outer normal point)
What section of the vibration graph shows the part to be avoided due to high vibration loads?
The red area
What is the maximum number of propeller blades on a propeller?
8
As engine power increases, what can be done to efficiently absorb the power?
- increase blade angle
- increase diameter of propeller disc
- increase RPM of propeller
- increase the camber of the blade
- increase the chord
- increase the number of blades
What is the minimum propeller tip clearance from:
- tip to fuselage?
- tip to nose wheel?
- tip to ground (nose wheel)?
- tip to ground (tail wheel in flight attitude)?
- tip to water?
- fuselage = 1 inch (25.4mm)
- nose = 0.5 inch (13.7mm)
- ground (nose) = 7 inches (17.78cm)
- ground (tail) = 9 inches (22.86cm)
- water = 18 inches (45.72cm)
What is the best option to increase efficiency of a propeller?
Increasing the number of blades
What does the amount of lift produced from a propeller blade depend on?
- shape
- RPM
- angle of attack of the blade sections
What is pitch distribution?
Where the angle of attack becomes smaller, with the distance from the root
What are root losses?
The thickened root area is less efficient as it is less aerodynamic due to having to withstand high stresses
What are tip losses?
Tip vortices and induced drag occur at higher RPM speeds
What is blade washout?
The leading edge of the blade is twisted downwards from the root to the tip
What is the geometric pitch of a propeller?
The theoretical, or design pitch that would occur if the propeller was 100% efficient (moving forwards with no other movement)
What is the effective pitch?
The actual helical path which the propeller advances through the air (one revolution of the propeller)
What is the slip?
The difference between geometric and effective pitch (efficiency losses of the propeller)
What can the efficiencies of slip be, for a propeller?
Between 50% and 90% depending on the amount of slip
How can propeller efficiency be calculated?
By dividing effective pitch by geometric pitch
What are the normal propeller efficiency ranges?
In the region of 0.8 to 0.9 (80% - 90%)
What is done to a wind-milling propeller to reduce drag caused by the blades?
It is put into the feathering position
- the leading edge is presented into the airflow
What can maximum achievable thrust with airspeed exceed?
The take-off thrust
Which Newton law of motion is associated with torque?
Newton’s 3rd Law
Every action has an equal and opposite reaction
How are modern propeller aircraft designed to reduce the torque effect?
The engine is offset
What can be used to compensate torque forces at different speeds on a propeller aircraft?
Aileron trim tabs and rudder trim
How is the yawing moment of the propeller engine corrected on the take-off roll?
The pilot uses rudder or rudder trim
What is done to the vertical stabiliser to compensate for the spiralling slipstream of air generated by the propeller?
It is mounted obliquely 1° or 2° to the aircrafts longitudinal axis
What is gyroscopic procession?
The resultant action when a force is applied to a gyroscope. This force acts 90° to its rotation
What type of aircraft is the gyroscopic effect mor prominent?
Tail wheel aircraft (usually during takeoff roll)
What is asymmetric loading?
(P-factor)
When one engine fails, the aircraft isn’t symmetrical and yaws to one side
What is rotation noise?
The rotating pressure field of the propeller produces a rotation noise at mach numbers of the blade tips between 0.5 and 0.85
What is vortex noise?
Noise caused by vortices leaving the blade tips
What is displacement noise?
Noise from the displacement of the air by the propeller blades. It is critical at higher Mach numbers
Above Mach 0.9 it becomes equal to the rotation noise
What is blade vibration noise?
This occurs with periodic stalls (usually heard with helicopter rotors)
If the power was doubled to the propeller, what would happen to the noise?
It would increase the noise level by approximately 5dB
What would happen to the noise if the propeller diameter was doubled?
If comparing at a constant peripheral speed, the noise would be reduced by 6dB
What would happen if the number of propeller blades increases from 2 to 3?
The noise is reduced by about 1.1dB per each added blade
How much heavier is a propeller with 4 blades instead of 3?
The 4 bladed propeller weighs 35% more
How much heavier is a propeller with 3 blades instead of 2?
50% heavier
What type of propeller produces the least noise?
A scimitar-shaped propeller
Straight tips = the most noise
Why are wooden or composite propellers more favourable?
They have better self-dampening functions which reduces noise
What can influence the propeller blade noise, in regards to the shape?
- tip shape = 3-6dB
- profile type = 2-3dB
- blade contour 1-2 dB
- blade twist = 1-2dB
- profile camber = 1-2 dB
- profile section ratio = 1-2dB
How much force can centrifugal force create in relation to the weight of a propeller blade?
7500 times
How much force can centrifugal force create in relation to the weight of a propeller blade?
7500 times
