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Sensitive Static Prop Balancer

Designed by Jeff Jeter

By Paul Lipps

Photos and drawings by Pat Panzera

Prop balancer

Here’s the incredibly sensitive prop static balance device that Jeff Jeter designed and made for me. When correctly adjusted, it will show an out-of-balance indication when a No. 4 washer is placed at the tip of a 63-inch diameter prop.

I rebalanced my three-bladed ELIPPSE propeller after painting one blade red and another blade blue to go along with the white one. I just kept spraying color on the front side and flat black on the back side of a light blade until the balance was right on. Spray your way to balance! You can even watch the balance change as the paint dries! When I posted a comment on a Google chat group, I received several replies from people who wanted to learn how to balance their own; therefore, it’s only natural that I should share this technique with everyone.
 
The commercially produced propellers I have experience with have a 2-1/4-inch hole in the back that centers the prop on the crank flange of a Lycoming, but I’m not sure about Continentals. There’s generally a ¾-inch hole through the center, but not always (sometimes the 2-1/4-inch hole goes all the way through), and it shouldn’t be used as the reference center. That’s why the center tube is 5/8-inch outside diameter (OD) to clear it.

Jeff used Loctite to secure the tube into the base and then turned the base (on a lathe) so that everything is concentric. The 4-inch portion of the base disk bears against the bottom of the prop (replicating the prop flange) to support it so that the 2-¼-inch boss is used only for centering. A similar mechanism could be made that would have a ferrule inside the tube that’s adjustable up and down through a threaded mechanism. The ferrule would have a conical hole on the bottom of it that would fit over a pointed rod up through the center, rather than being supported by a cable. Of course that wouldn’t have the indicator disc. The real secret of the sensitivity of either form is to be able to adjust the support center up and down in the tube until it is just ever so slightly above the prop’s fore-aft center of gravity (CG). When it is there, just the slightest unbalance lets it hang at an angle. The cable-supported version has the big advantage of having that indicator disc that shows the magnitude and direction of the unbalance.

Prop balancer

When this setup is adjusted for maximum sensitivity, a comparable round bubble level would have to be way more sensitive than any of those you normally find. We had bubble levels that we used to level our Atlas guidance radar antenna that would show 1 arc-second. If you lit it with a flashlight, the pressure of the light would displace the bubble; true story! Since we could move the bubble back and forth, we had to shield it from any strong light source. We used two levels mounted at 90-degree angles with each other. We would read both of them every 30 degrees azimuth, then put the data through a simple graph that would show the magnitude and direction of level error. We had three adjustable support legs on the antenna’s base; one was never moved, and it was the reference. The graph would show which legs to adjust up or down. The legs had a vernier adjust mechanism that had a sensitivity of 10 clicks per second of level. The antenna level spec was 2.5 seconds P-P maximum. I have a bubble that is about 20 seconds/inch. Try holding that baby steady! But I digress…

Here are some photos of the balancer in operation. In the photo below you’ll see a prop out-of-balance. Note that the cap indicator disc shows not only long-axis unbalance, but also cross-axis.

Prop balancer

The long-axis is balanced by means of the nickel not too far out on the root as shown below. Finally, the addition of 2 quarters and one nickel gives the cross-axis balance. Very slight movement of the coins caused deflection in the cap disc indicator. It’s also important not to have even the slightest breath of air blowing on the prop. 

Prop balancer

Prop balancer
We were fortunate to have more than one prop to check with Paul’s balancer. In addition to the wood Sensenich prop previously shown, we checked a Warp Drive (above) and a Prince three-blade, shown below. With no visible signs of post-production balancing, we were stunned to see that the Warp was just about perfectly balanced.

Prop balancer

Prop balancer

Prop balancer
Larger view

Building Instructions
Start with a 7/8-inch by 4-1/2-inch aluminum disk. Drill and secure the 5/8-inch tube into the base using red Loctite, then turn the assembly on a lathe once the Loctite cures. This gets the base to run true and concentric with the tube. Also turn the ferrule and the two-piece cap. After turning the cap as a unit, slice off the end piece. Polish the mating surfaces of the cap and end piece for a friction-free fit. A little car wax certainly wouldn’t hurt.

Thread the 1/32-inch stainless steel cable through the two-piece cap and the ferrule. Cut a piece of a rubber pencil eraser (or similar material) and thread the cable through it while inserting it inside the ferrule. This supplies friction for holding the ferrule in place, midway inside the tube. The ferrule needs to be adjustable, as it will be positioned on the cable differently from one prop to another, depending on the thickness of the prop’s hub. Fasten a loop onto each end of the cable. Insert the base and the tube into the prop. Thread the ferrule in place and secure it in the tube in the approximate center of gravity of the prop being balanced. Place the cap on the end of the tube and hang the prop from the fixture, making sure that the wire retainer is centered in the bottom of the base. A little tape will help act as a third hand.

Start with the ferrule about midway down the tube and gradually push it down until the prop flops over and the tube rests on the cable. Move the ferrule back up slightly, and the sensitivity of the balancer should be set.

A plastic soda straw can be used from either end of the tube to properly set the ferrule just about the CG of the prop.

Drawing
Larger view - jpg

Larger view - PDF

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