17.6.1 Propeller Balancing

Question 1

What causes propeller roughness?

Answer 1

Bent blades, blades out of track, imbalance, loose mounting, out of tolerance blade angles, and spinner imbalance.

Question 2

What are the three types of propeller balancing?

Answer 2

Static Balancing, Dynamic Balancing, Aerodynamic Balancing.

Question 3

What is static balance?

Answer 3

A body is in static balance when its center of gravity lies on the axis of rotation.

Question 4

How is static balancing carried out?

Answer 4

It is done off the aircraft using a mandrel on stable knife edges or rollers, checked in horizontal and vertical planes.

Question 5

How can horizontal imbalance be corrected in fixed pitch wood propellers?

Answer 5

By applying varnish or paint to the light blade or adding solder to the metal tipping.

Question 6

How is vertical imbalance corrected in fixed pitch wood propellers?

Answer 6

By adding weights to the hub.

Question 7

How are fixed pitch metal propellers statically balanced?

Answer 7

By removing metal from the heavy side and refinishing with a protective coating.

Question 8

What is the general permissible tolerance for propellers of aircraft up to 5700 kg?

Answer 8

2 gm (0.070 oz), but manufacturer specifications are binding.

Question 9

How are metal and composite variable pitch propellers balanced?

Answer 9

By placing lead washers onto a balancing stud and in recesses of the hub.

Question 10

What is dynamic balancing?

Answer 10

Balancing the propeller to reduce the moment of imbalance while the engine is running.

Question 11

What are the four steps to dynamically balance a propeller?

Answer 11

1. Obtain in-flight vibration information. 2. Ensure vibration exceeds limits. 3. Calculate mass and location to reduce vibration. 4. Install balance weight(s) and confirm vibration levels.

Question 12

What is the function of a Propeller Balance Monitoring System (PBMS)?

Answer 12

To provide balance solutions based on data from a Magnetic Pickup Unit and Active Noise Control Unit.

Question 13

What causes aerodynamic imbalance in propellers?

Answer 13

Differing aerodynamic loads due to variations in blade angles.

Question 14

What is aerodynamic balance?

Answer 14

When each blade produces equal thrust, resulting in no periodic vibrations.

Question 15

What is the Aerodynamic Correction Factor (ACF)?

Answer 15

A reference to the blade angle difference necessary for balancing thrust or torque.

Question 16

What is blade indexing?

Answer 16

Adjusting the basic setting angle of each blade relative to others during assembly.

Question 17

How is the Universal Blade Protractor used?

Answer 17

To measure the propeller blade angle at a specific blade station.

Question 18

What must be determined before measuring the angle of a propeller blade?

Answer 18

The reference blade station from the manufacturer’s maintenance manual.

Question 19

How do you zero the Universal Blade Protractor?

Answer 19

Align the zeros on the disc and the ring, then engage the disc-to-ring lock.

Question 20

What should be done if the face of the blade is curved?

Answer 20

Attach a piece of 1/8 in rod to measure the angle with the protractor resting on the rods.