PROPELLER BALANCE:
What you should be looking for

By LLOYD SULLIVAN

Let’s talk about balancing propellers. There is more to it than just throwing the propeller up on any one of the many different balancers and sanding the heavy blade until it is level. This, of course, is better than not doing anything and assuming it is balanced from the factory, but in my 27 years in this hobby, I can only remember three instances where I did not have to do anything before using the propellers. As a matter of fact, just the other day I picked up a 28x10 Bolly propeller that was perfectly balanced, and I did not have to adjust it. This is very unusual. Typically the propellers and/or the hubs are out of balance.

What to check for
The following list (in order) shows what I check on every propeller I buy.

• Hole is in the center (most are)
• Hub faces are square (most are)
• Tip shapes are identical (usually not a problem)
• Lateral side-to-side balanced (usually needs attention)
• Propeller hub balanced (usually needs attention)

I know this sounds like a lot to check, but the destructive power of vibration on our airframes, radio equipment, and even engine fuel mix via fuel foaming are relentless.

How do I check all of this?
Checking all of this is not as involved as it may seem. If the hole is not in the center or the hub faces are not square, I usually just take or send the propeller back and get a replacement. If the hole is not in the center laterally, it’s usually not a problem to sand the long tip until it is the same length as the shorter one. If the hole is not centered in the hub on the vertical axis (with the blades horizontal), get a replacement. The bad news is checking that the hole is in the center and the hub faces are square are the hardest on the list to determine. The good news is they are the ones that are most likely to be right.

There are two things that are essential to accurately balance a propeller: the knowledge to properly check a propeller and a balancer that is accurate enough to allow you to achieve perfect balance.
I hope this article gives you the knowledge you need. The balancer is another issue. It needs to support the propeller hub perfectly centered on the balancing shaft, and the propeller needs to be able to swing through its entire arc friction free. If you cannot position the propeller blades vertically while on the balancer, you never will be able to finish this procedure.

Balancing a propeller laterally is only one small step toward achieving proper balance. I use a High Point Balancer, which is no longer made under that name. I think DuBro markets it now with a plastic base. It supports the propeller on a shaft with sliding cones and the shaft rests in the “V” formed by overlapping wheels about 2 inches in diameter. This is a friction-free and accurate balancer. There are others made by Master Airscrew and some which use magnets. The key here is friction-free. The less friction there is, the more accurate the results.
 

There is more to balancing propellers
than just throwing it up on a balancer
and sanding the heavy blade until it is level.

Procedure
1. Hole is in the center: This is rarely a problem and I usually don’t check it unless I am doing a lot of work to bring a propeller into proper balance. To check the hole on the propeller blade axis, I find a bolt that fits the center hole snugly. Then I measure from this bolt to the propeller tip on each side. It is important that these measurements are equal. If one blade is a little longer, I sand it to match the shorter side. To check the hole from the other directions, I use a digital caliper. The measuring device doesn’t have to be digital but it needs to measure in thousandths of an inch.
 
I usually draw three lines across the hub face. One line is straight across the hub and 90° to the blades. The other two are 45° to this line so the hub looks like it has a straight line and an “X.” It is important that each of these three lines go through the center of the hole. Measure the distance from the edge of the hole to the end of each of these line segments. All four measurements on the 45° lines should match if the hole is in the center. Both measurements of the straight line should be the same, but they may not be the same as the measurements of the 45° lines.

2. Hub faces are square: This check only requires a flat surface a little longer than the propeller and a good ruler. I prefer a metric ruler because it has greater resolution, yielding a more accurate measurement. Place the propeller hub face on the flat surface and measure the distance each tip is from the surface. If the hub is square, the tips should be the same distance from the surface.

Check both hub surfaces for square. Another way to check for square is to look at the tips while the engine is idling. If the hub faces are not square, the tips will be out of track. Checking this at idle is important because propeller flex under load will affect the observation. Don’t stand in line with the propeller arc above an idle for safety reasons.

3. Tip shapes are identical: Tip shapes should be identical as well. I just put the propeller on a piece of paper and trace the tip shape with a sharp pencil. Place the other tip on this tracing to check that both tips are the same. If they are not, shape the larger tip to match the smaller one. Although the two are usually very close and rarely need attention, it is something that should be checked.

4. Propeller hub and lateral balance: This problem is often overlooked. Hub balance is just as important as lateral balance. In fact, you cannot achieve proper lateral balance until the hub is balanced.

To check hub balance, position the propeller on the balancer so that the blades are vertical. If the propeller swings to one side, most likely the hub is heavy on that side. I usually test this two or three times to be sure the results are consistent.
 
To correct a heavy hub condition, you can sand the hub flat on the heavy side or take a 3/16 drill bit and drill shallow holes on the heavy side until you get as close as possible. If the propeller does not move when either tip is in the vertical top position, the hub is close enough. I also check the propeller in the 45° positions. Heavy hub and/or lateral balance will affect propeller movement here.

Position the propeller on the horizontal and check for a heavy blade. If the propeller balances horizontal, rotate it 180° and check it again. Heavy blades should be corrected by removing material from the front face of the propeller. Do not remove from the back side because the propeller will have a different pitch on one side. Be careful to preserve the airfoil shape while removing material. Go slowly. Sometimes a little goes a long way.

When the propeller balances horizontally, it is time to recheck the hub and fine tune if necessary. Place the propeller in both 45° positions. If it rotates consistently to a certain position, the hub needs attention at the low point. When the propeller is balanced, it will stay in any position you put it in on the balancer. Do not accept anything less. Do not try to correct an out-of-balance hub by removing material from a blade face.

Technical editor’s note: If you remove any material from a wooden propeller, you must reseal the wood and you must add the same amount of sealer to all blades so that balance is retained.

from Propwash
Propnuts Radio Control Model Airplane Club
Paul Shaffer, editor
Highlands TX