17.6

Question 1

What can cause propeller vibrations?

Answer 1

propeller roughness may be caused by bent blades, blades out of track due to improper mounting of the prop on the engine shaft, imbalance, a propeller loosely mounted on engine shaft, blade angles between blades out of tolerance with respect to each other and spinner imbalance due to improper mounting or to dirt, snow or ice inside the shell

Question 2

What are the three types of prop balancing?

Answer 2

• Static Balancing
• Dynamic Balancing
• Aerodynamic Balancing

Question 3

How is dynamic and static balancing best described?

Answer 3

Static and dynamic imbalance are caused by unequal mass distribution

Question 4

How is aerodynamic balancing best described?

Answer 4

aerodynamic balancing is to ensure each blade delivers an equal amount of thrust

Question 5

When is dynamic balancing typically needed?

Answer 5

Only with larger propellers or with fast running propellers may dynamic balancing be necessary

Question 6

Explain what it means if a prop is in static balance?

Answer 6

A body capable of rotating about a fixed point is said to be in static balance when its centre of gravity lies on the axis of rotation. If a body is in static balance, each time it is rotated it will come to a stop in a random position

Question 7

How is static balancing carried out?

Answer 7

The propeller is mounted on a mandrel which is placed across stable and perfectly levelled knife edges or rollers. The balance is checked in two planes, one horizontal and the other vertical

Question 8

How is horizontal imbalance corrected for in fixed pitch wooden props?

Answer 8

Horizontal imbalance can be corrected by applying small amounts of varnish or paint to the light blade or if larger amounts are required then solder can be
applied to the metal tipping of the lighter blade

Question 9

How is vertical imbalances corrected for in fixed pitch wooden props?

Answer 9

Vertical imbalance can be corrected by adding weights to the hub

Question 10

How are fixed pitch metal props statically balanced?

Answer 10

Fixed pitch metal propellers are statically balanced by removing some metal from the heavy side and then refinishing the propeller using a non-anodic
protective coating such as Alodine

Question 11

How are metal and composite variable pitch props balanced?

Answer 11

These types of propellers are balanced by placing lead washers onto a balancing stud within the hollow blade root/shank for horizontal balance while
lead washers are placed in recesses of the hub for vertical balancing. Minor adjustments to the propeller balance are corrected by placing lead wool
into the balancing tube located within the hollow root of the blade

Question 12

How are smaller metal and composite variable pitch props balanced?

Answer 12

On smaller propellers the lead wool can be placed in the counterbored area of the hub bolt heads. These bolts are known as Welch bolts because of the
Welch plug used to cap the bolt when lead wool is installed. Welch plugs should be in the bolt head regardless of whether or not lead wool is inserted
in the bolt. If a Welch plug is missing then the propeller is considered out of balance

Question 13

What are the four steps to dynamically balance a prop?

Answer 13

1. Obtain in-flight vibration information.
2. Ensure vibration is greater than permitted limits.
3. Calculate the mass and location to reduce vibration to an acceptable level.
4. Install balance weight(s) and confirm vibration levels are within limits.

Question 14

What is blade indexing?

Answer 14

Aerodynamic balancing is only necessary for propellers with high performance. Manufactured blades are compared individually with a master blade and receive, according to deviation from the zero lift angle, an
aerodynamic correction factor in the form of a reference to the blade angle difference necessary to the basic setting, and the whole process is also known
as blade indexing. The correction factor is usually marked on the blade root