The balance indicator is a simple design with a few features borrowed from a design by Jeff Jeter. It works on the principle that a balanced prop hung horizontally and supported at its center of gravity (CG) will assume an absolutely horizontal attitude, that is to say “it will be square with the world”. The challenge is how to hold the prop at its CG. The prop’s CG should be at the exact center of the prop, radially at exactly the center of the prop hub, and axially halfway through the thickness of the hub.

The balancer allows the user to set the point at which gravity’s pull on the prop is resolved out of the prop and into the supporting twine, assuming that half-way through the prop hub and along the axis of the center hole of the prop is the correct spot of the CG. Moving the pliable slug up or down inside the 3/8 tubing, relative to this center position in the prop, the balance indicator can be made more or less sensitive to register any out of balance condition. Moving the slug up renders the balance less sensitive and down more sensitive. Think of trying to balance an upright dowel in the palm of your hand vs. holding it loosely at the top with your fingers. The slug can slide on the twine and be moved up or down inside the 3/8” tube. Inserting a small diameter dowel from the top or bottom can accomplish this. I use a bamboo skewer, but anything that fits will do the job.

The state of a props balance or unbalance is indicated by the relative position of the twine at the top of the 3/8” tube and it’s inside diameter. A perfectly centered position of the twine is of course what one wants but that really is not required. If the string is not centered in the tube the user can judge the amount of any out of balance condition that exists in the prop by taking one or more 1/4 sheet(s) of 20 lb. printer paper and draping the paper over the tip of the props high blade(s). This should tend to draw the string into the center position of the tube. Notice how much the twine moves in the tube. The direction it moves points to the light blade(s), this is where more weight needs to be added to the prop or perhaps removed from the heavier blade(s). By experimenting with paper weights in this manner the user can quickly tell how close the prop is to being in perfect balance (Figure 4) or whether it is out of balance (Figure 5). If the twine touches or bends at the tube's edge the out of balance condition of the prop may be much more than if it hung mid-way between tube's inner edge and the center datum line of the tube. The visual reference and the weight of paper added could be helpful in determining how much weight must be added or removed to bring the prop into an acceptable balance condition. See Appendix A for how to do this calculation.

I weighed a 1/4 sheet of 20 lb. printer paper on an ammunition reloading powder scale. The scale registered 19.5 grains (7,000 grains = 1 lb.). If that paper was to be draped over the tip of a 60’ diameter prop (30” radius) and the twine moved off the centered position in the 3/8” tube but not touching the tube side that would be equivalent to 1.337 inch-ounces of unbalance (30 x (19.5/7000) x 16). Put another way; if it takes a 1/4 sheet to bring the twine to a centered position then the imbalance would be 1.337 inch-ounces. I have no idea as to what an unacceptable out of balance condition would be. What I can tell you is that a prop I dinged, repaired, re-varnished and balanced ran true and without vibration all the way up to an engine speed of 6500 RPM. On balancing my repaired prop the twine was very nearly but not perfectly centered and the effect of a single "paper weight" placed on the prop's tip could easily be seen.

When removal of weight from a blade is required in the process of balancing a prop never remove material from the bottom surface or the leading and/or trailing edges. Material removed from these surfaces is more likely to alter the prop's airfoil angle of attack and thereby its lift, drag and thrust characteristics... more so than if the weight were removed from the top surface. Should it be necessary to remove weight from a blade do it along the top surface in the area of max thickness and try to maintain the same curve.

Axial balance can be achieved by adding coins (stacked if necessary) along the outside edge of the prop hub aligned radially between the mounting bolt holes if possible until balance is achieved. These coins are then weighed and an equal amount of lead weight (fishing line weights work great) are matched and set aside. A hole is then drilled under the coins and the lead weight inserted and glued in place. Material from the drilled hole is then mixed with a little glue and filled in the hole above the lead weight. All this is done prior to adding the finish varnish coat to the prop. Final touches to the props balance are usually obtained by applying more varnish to the lighter blade(s). By applying varnish to appropriate areas of the prop, in multiple coats if necessary, one can “walk in” its balance quite nicely.

Figures 6 & 7 depict parts of the balance indicator.

A few final notes:

The key to this balancer is the internal slug and its placement. It should fit snugly inside the 3/8” tube. How the tube is centered and held in the prop’s bore is not all that important. Remember … there are many ways to skin a cat! Just take care that the tube is centered in the bore and be sure to check that it is perpendicular to the prop’s plane of rotation (hub face surface). Check this in two directions, 90 degrees apart, as this is very important! The balance is sensitive to air currents and your balancing efforts should take place in a sheltered area or indoors. More information on prop repair and re-balancing can be found at: Wood Prop Repair & Balancing