Wood Prop Repair & Balancing
By: Jim Hayward
Everyone encounters prop nicks or dings at one time or another when using a wood prop. This tip comes from the model aircraft guys and has been used very successfully by ultralighters for many years now.
Swipe some of your wife's baking soda and buy some thin formula superglue. It would be wise to use "only" fresh superglue (less than 6 to 9 months old) for all fill and repair work. Scrape away any varnish or paint in the nick or clean the nick if it's dirty. Fill the nick with baking soda and apply the superglue. A reaction will take place immediately (complete with a bit of smoke) and be done in 3 or 4 seconds. The result is a rock-hard repair that may be shaped flush with the surrounding surface by using a fine-toothed file. Do NOT use sandpaper as this will usually cause sanding of adjacent areas which you don't want.... file only.
I have personally repaired numerous dings in my prop using this method and it works well. I've also repaired small (1 to 2 inches long) cracks along the wood grain on my prop twice using fresh superglue. I spread the crack apart and squirt the superglue in. Allow it to wick up into the crack then let the crack close up. The excess superglue will be squeezed out and should be wiped off quickly. If necessary, apply pressure to keep the crack closed up until the superglue has cured.
Some folks have repaired large areas the size of a quarter by saturating the area with superglue and sprinkling the baking soda on the area. As many layers as are needed can be applied with this technique to build it up to the surrounding surfaces. Tape off the surrounding area with masking or electrical tape to keep the repaired area where you want it. You should check the prop's balance after any repair.
Balancing is accomplished with, of course, a prop balancer. Vertical balance refers to balance along the length of the prop, tip to tip. Horizontal balance refers to balance across the prop, side to side. Don't try to use a lawnmower blade balancer as the prop sits too high on the balancer and the prop will simply tip off to one side... no need to ask how I know! Anyway, the tipping is due to the lawnmower balancer's "needle" not projecting up into the prop hub far enough which causes it's center of gravity to be above the needle (balance) point. It should be noted that for our relatively light wood props (less than 6 lbs.) vertical balance is all that's usually needed.
Balancers may be purchased for less than $25. California Power Systems or CPS sells one for $16.95 which has a string attached to a bubble tube. The string runs from one side of the prop hub's hole, up thru a shank the size of the prop hub hole (and inserted into that hole), and to a washer to hold or hang it from something. The string balancer is hung high enough so the bubble tube on the bottom of the prop hub is observable. The bubble will have to be changed in each direction for vertical and horizontal balance. As of Oct. '03, Aircraft Spruce & Specialty or AS&S sells the same balancer for $21.50.
I use one from Leading Edge Air Foils or LEAF for $16.95. It has a "needle" shank that is part of a three-legged, heavy wire stand and is placed on a table or counter for use. There is a (hollow) hub insert which fits up into the prop hub and this assembly then rests on the vertical "needle". A bubble disk is then placed on top, over the hub hole. This disk allows you to see where the balance point is. Weight is added to the light tip (for vertical balance) or hub (for horizontal balance) while watching the bubble disk. When the bubble centers completely in the disk, the prop is in balance. Either balancer type will balance a prop both vertically and horizontally. The weight of a 4" square piece of paper can be more than enough to bring a prop into balance so a coat or two of paint is often all that's needed.
Prior to doing any balance changes, be sure that the length of each blade is the same from the centerline. If the heavy side blade is also a tiny bit longer, sand down the end of that blade first before adding weight to the lighter side. If the original paint or varnish coat has been sanded down to bare wood, be sure to reseal it after sanding.
If a prop is out of balance vertically, the desired method to add weight is a coat of paint to the light end or remove material from the heavy end. The light end is sprayed with your choice of paint or varnish until the prop is brought into balance. The paint of choice these days is clear or colored Polyurethane paint. If a coat or two of paint is insufficient, removal of material may be done by sanding on the heavy end. This is usually done on the upper or curved side of the prop blade rather than the flat side as conventional wisdom may suggest. Try not to distort the airfoil (curved area) any more than necessary. The closer to the tip you can remove the material, the more effect that removal has on the prop's balance. However, you don't want to remove so much material that it makes the prop mechanically deficient in that area. Neither do you want a big gouge in the surface which would occur should too much material be removed from a small area. Spread the removal area out along the prop keeping it as close to the end as practical.
If horizontal balance is desired, weight may be added to the light side of the hub to bring the prop into balance or material may be sanded from the heavy side. If a prop is considerably out of balance, a combination of the two methods may be used. As mentioned above, horizontal balancing of our lightweight props is generally not necessary. Motorcycle shops have small rectangular weights with double-sided tape for adhesion to a wheel rim. These fit well on a hub for horizontal balancing.
The lead weight may be cut down easily with some side cutters if less weight is needed than the original rim weight provides. After determining how much weight is needed to bring the prop into horizontal balance, one should drill a hole thru the weight for a small #2x3/4" size brass wood screw. The weight is attached to the hub, a small pilot hole drilled into the prop hub, and the screw used to provide additional security for the weight. A dab of solder to the lead weight and brass screw will insure the screw won't back out. Be sure to incorporate the screw's weight with the lead weight when determining your balance. I've also seen one instance of some epoxy being used on the prop hub for balancing purposes although I'm not sure how the amount of epoxy to use was determined.
Propeller tracking causes more problems with our props than being out of balance. We should shoot for a maximum of 1/8" tracking error with 1/16" or less being great. Tracking checks are best done without the engine running. An easy way to check tracking is to set up some sort of stationary reference point for a blade then rotate the prop around by hand so the other blade is at the same measurement point. The difference between the two blades at the reference point is the tracking error. This check is assuming that the prop bolts have been tightened correctly in the first place.
To adjust the tracking, tighten the prop bolts on the hub, opposite of the blade you want to bring into alignment. Be aware of the 1torque requirements for the prop bolts and don't overtighten them. According to the late Tony Bingelis, author of 2"Firewall Forward" and a well known homebuilding guru, one can shim the prop and bring the "low" blade into track." A piece of paper or other suitable shim material may be used for adjusting the particular blade desired. The shim is placed between the backside of the propellor and the engine hub face. The prop bolts are then tightened and the tracking checked. Shims are added as necessary until the tracking is within limits.
One other thing... don't store your prop on an end such as leaning against a wall or something like that, since over time, this can cause some slight bending (tracking error) of a blade.
As can be seen, minor repair and balancing of propellers is not very difficult nor does it take much time. It cannot only save you the cost of a new prop or paying someone to repair it, but is relatively inexpensive to accomplish.
Rapid City, SD
1Tennessee Propeller Inc., - Wood Propeller Torque Spec's
2BOOK: "Firewall Forward: Engine Installation Methods", by Tony Bingelis, Paperback - 303 pg., Publisher: EAA Aviation Foundation; (June 1992), ISBN: 0940000938