Ch 25 - Propellers

Question 1

How a propeller works

Answer 1

A rotating propeller converts power from the engine into thrust.

Newtons second and third law.

Works in the same way as a wing, creates a pressure differential which creates an aerodynamic force

Question 2

Structure

Answer 2

Hub - blades are attached to
Spinner - Usually streamlining over the hub

Blade Root - where the blade attaches to the hub
Blade tip - End of propeller

Propeller Disc - The area that the blades rotate through always rotating right from inside cockpit (EASA)

Plane of rotation - Side on vertical view of disc (line that the blades cut down through)
Axis of rotation - perpendicular to the plane of rotation
Blade Face - Faces the cockpit

Blade Back - faces fwd

Has blade twist (washout) reducing the camber at the tip as it travels faster - to ensure same lift produced across whole blade

Question 3

Chord line of Propeller

Answer 3

The imaginary straight line drawn from the middle of the leading edge to the middle of the trailing edge

Question 4

Blade Angle

Answer 4

75% along the blade from root to tip

The blade angle is the difference between the chord line and the plane of rotation

Blade Angle = Helix Angle + AoA

Helix angle is the angle between the plane of rotation and the RAF

Question 5

Blade Angle of Attack

Answer 5

The angle between the chord line and the relative airflow

4’ is most efficient

Question 6

Geometric Pitch

Answer 6

How far the blade progresses fwd with one full 360’ motion

Theoretical

Question 7

Effective Pitch

Answer 7

How far the blade actually travels in 1 full 360’ rotation

Geometric Pitch - The slippage

The more efficient the wing is, the less the slip.

Decreased blade angle = decreased effective pitch

Increased blade angle = Increased pitch

Question 8

Pitch of the blade

Answer 8

Fine Pitch = Small Blade Angle (slower)

Coarse Pitch = Large blade angle (faster)

Trying to maintain 4’ for efficiency

Question 9

Types of Propeller

Answer 9

Fixed Pitch

Two Pitch

Adjustable Pitch

Constant Speed Pitch - Adjusts blade angle according to speed

Question 10

Fixed Pitch Propeller

Answer 10

RAF depends on rotational speed of the propeller (Engine RPM) and the aircraft TAS

Faster speed/Less RPM = smaller AoA (can be negative)

Slower Speed/Higher RPM = bigger AoA

High RPM, Low SPD = Very large AoA

Blade twist keeps AoA same across the blade allowing for SPD differences

Question 11

Variable Pitch Propeller

Answer 11

Fast = Coarse

Slow = Fine

Tries to maintain 4’ (optimum angle)

2 types: Manual Adjustment (depending on type of flying), Constant RPM - blade sets itself to maintain optimum angle

TO should be fine, and then coarsened as TAS increases

Question 12

Torque

Answer 12

Opposes Rotation when being driven by the engine

Question 13

Engine Failure Impact on Propeller

Answer 13

Torque is now in the direction of the blade fighting against the engine. Produces a lot of drag

A ceased engine produces even more drag

If it is a variable pitch propeller, coarse it as much as possible to decreases the drag

Question 14

Feathering

Answer 14

Maximum coarseness so that it minimises the drag to the minimum possible (parasite only)

Much less drag than windmilling

No torque as the propeller will be stationary

Question 15

Reverse Thrust

Answer 15

Produced by creating a negative AoA which produces an aerodynamic force rearward

More effective at the initial part of the landing (jet even more so)

Question 16

Propeller Efficiency

Answer 16

Never 100%
70% is considered good

Fine pitch is better for slow speed
Coarse pitch better for high speed

A variable pitch propeller is trying to maintain 4’ (optimum angle) at varying speeds

Efficiency = output propulsion power generated by prop / input shaft power

Efficiency = thrust power / shaft power

(Transmission losses)