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Prop Balancing 101 by Rockin' Rick
I've seen a lot of vibrating and shaking airplanes on the ground while sitting at an idle. This is not normal and 9 times out of 10 this is due to improper balancing of the prop or not balancing the prop at all. I'll always ask the pilot "Did you balance your prop before mounting it?"" Yeah, yeah", is always the answer but there can't be any other cause to an excessively vibrating plane. Some of these pilots believe that the props come pre balanced from the distributors. Whoa, wait a minute here. What's worse is that this imbalance continues on through the mid and upper rpm range. This is a cause for real concern as damage to the prop could occur or even worse, damage to the engine is a real possibility. I'll show you what I consider to be an easy way to drill and balance a prop. I'll also take it one step further.
Sometimes it really annoys me when manufacturers make claims that just aren't true. Click on the the first photo and read. Simply Hogwash. This piece of written BS came with the prop that I'm going to balance and this prop was not in true balance directly out of the package as the manufacturer claimed. So don't believe it and treat every new prop like it has never been balanced before it came into your hands. You'll thank yourself later. The first thing you'll want to do is to get a decent balancer for large scale props. There are many out there on the market, but I have found the Du-Bro RC True Spin prop balancer to work the best for my applications. It's inexpensive and will last a lifetime with proper care. It will balance any size prop and is not just limited to tinker toy size like so many others. Next I made a template from a piece of 2 x 6 to hold the prop steady and secured the prop with a 10mm bolt while drilling it. It is imperative to drill out the prop using a drill press to achieve true bore holes. I guess you can buy the drill guide from the engine manufacturer if you want, but I think the drill press is a better tool than a hand held drill motor for this application. I used the prop retainer as a guide and had absolutely no trouble at all drilling the holes for the retainer bolts. They lined right up to the engine prop hub perfectly.
When drilling was completed, I mounted the prop onto the prop balancer for the initial testing. As you can see, the tip on the left side is heavy. Some people advocate sanding material off of the heavy side but I don't agree with that approach for CF props. I'll use some spray clear acrylic sealer to even the weight by applying it to the light side of the prop. Simply spray a few light coats near the outside tip of the prop and let dry. Then remount the prop to the balancer and check it for horizontal evenness. If it does not level out, apply a little bit more to the light side of the prop. When the prop will return to perfectly level from any position, the tips of the prop are balanced. Nice job... but don't think that we're done. This ain't no glow engine were working with, this is a gas engine with some real power and we've got another step to complete before we're done.
I promised that I'd take this balancing job one step further and here it is. Few pilots will take the time to balance the hub of the prop. Considering how easy this is to do, I'm surprised that so many pilots either skip this step or just don't know how to do it. People always are asking me how I get my planes to run so smoothly without vibration at idle and this is the clincher. A piece of self adhesive Velcro is applied to the light side of the prop hub. The light side of the hub is the one that is UP when the prop tips are balanced correctly. You can get this stuff from Lowes or Home Depot. Use the fuzzy side and stick it on the prop hub. Position the prop at the 1 O'clock position and then at the 11 O'clock position. The prop will return to the 9-3 o'clock position. (level) Add some thick CA glue to the fuzzy portion of the Velcro and perform the same operation. You will notice that the prop still tries to return to the 9-3 o'clock position but will do so more slowly. Add more CA glue to the Velcro until the prop will remain stationary in the 1 o'clock and 11 o'clock positions or in any other position without movement back towards the 9-3 o'clock position. Then and only then, is your prop completely balanced. RR
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