3 BLADE Prop BALANCING
It doesn't matter what your prop is made of, Wood, Glass filled Nylon, Plastic, or Carbon
Fiber, the balancing technique is the same. Balancing a 3 blade propeller isn't difficult
if you go about it properly. It does take a lot longer to balance as compared to a 2
blade, since you have an additional element to contend with. In essence you ARE
balancing 2 blades at a time, and repeating it until all 3 blades are equal in
weight. So it is necessary to locate and identify the lightest
and heaviest blades.
You will need:
1. good quality prop balancer
2. SHARP Exacto knife or similar
3, 150 grit and 400 grit sand paper
4. A Sharpie or Magic Marker felt tip pen
PREPARE and IDENTIFY the prop:
Using 400 grit sand paper gently sand and remove the flash from the leading
edge (only) of each blade element. DO NOT sand the trailing edge. When this
is done, thoroughly clean the prop. Place the prop on your balancer and
let it settle in. The blade closest to 12:00 is to be marked with 1
DOT, on the hub. Now mark the second closest to 12:00 with 2
DOTS, and the heaviest blade with 3 DOTS. SEE: photo below. It is very important to mark the blades correctly.
Begin the balance process.
ROTATE the blade with 2 DOTS to
the 12:00 position. Scrape, and or sand the heavy blade 3 DOTS
until your "reference" blade (with 2 DOTS) stays at rest at
12:00. When this is done, ROTATE blade 3 (with 3 DOTS) to 12:00 and
repeat the balance process. When its done, ROTATE blade 1 (with 1 DOT)
to 12:00 and repeat the balance one last time.
These dots are your reference markers during your balancing procedure 1
DOT is your lightest, 3 DOTS is your
heaviest, and 2 DOTS is in between.. The object is to get
the "reference" blade to rest at dead-center or 12:00. You will scrape, and or
sand the leading edge of the heavy blade until the "reference"
blade rests at 12:00. When this is done, you will rotate the next blade to the 12:00
position and repeat the balancing process. You will do this until all 3
blades have been referenced at 12:00.
A well balanced prop will stop in random positions everytime its
rotated, with little or no backup.
You may wonder when to stop baancing... If the "reference" blade
is within 1/2 its width (or less), you are good to go for Higher RPM engines. If your
engine turns less than 8000 RPM a full blade width is acceptable but its not perfectly
NOTE: Balancing a 3 blade prop can be frustrating and
time consuming. The prop in the photo took me a good hour to balance but its smooth as
glass on the engine. It's running up to 9000 RPM and not a speck of prop vibration.
How to determine if your prop IS or IS NOT
Propeller vibration is directly proportional (in
amplitude) to engine RPM (frequency). In other words, the faster the engine runs
the worse the vibration gets.
As your engine RPM (frequency) goes UP, your overall aircraft vibration
(amplitide) will go down and you'll barely feel it at the wing tips and rudder.
As your engine RPM (frequency) goes up, the overall aircraft vibration
(amplitude) gets much more intense and you'll feel it throughout the airplane.