Text and Pictures by Mark Deseck
Simple Multi-Prop Balancer for 2 or 3-piece Props
While there are a couple of methods for balancing a one-piece prop, I was in need of an inexpensive, easy to use, and accurate balancer for a 2 or 3-piece (bladed) prop (such as that found on my original motor, the DK GT). So, with the idea gleaned from other sources, I built a cheap (less than $10 in material costs), simple, yet effective prop balancer (fitfully called the Simple Multi-Prop Balancer).
The basic idea behind the Simple Multi-Prop Balancer is to place the hub end of the prop in a static position and then measure the weight at the tip of the prop blade (or, more accurately, the moment or torque created at the hub). To do this, we position the base of the prop blade near a fulcrum (or pivot) point, use a counterweight to offset the moment created (the critical balance point), and then mark the location of the counterweight. Each remaining prop blade is then measured. Using the blade that provides the greatest moment as the benchmark, either paint or tape could be added to the other prop(s) to match the same moment as the benchmark prop. Thus, a complete set of prop blades that are identically balanced to one another can be obtained.
40" length 1x4" (wood)
3" length of dowel rod (wood) - Diameter should be determined by your prop mounting holes - most likely 5/16"
Constructing the Simple Multi-Prop Balancer
The balancer consists of a fulcrum bar (a 1x4" with a couple of dowel pins installed to hold a prop blade) and a counterweight block (a 1x4x4" block). (See Picture 1.)
Picture 1: Picture of fulcrum bar and counterweight block
Cut a length of the 1x4" approximately 36" long for use as your fulcrum bar and another piece 4" long to be used as your counterweight block. The fulcrum bar length will eventually be cut down to accommodate the weight of your own props (the length of the bar that I works well with my DK GT 39" C8 props is 20 inches). For now, leave the length at 36" as this should allow plenty of excess counterweight to accept the largest of prop blades.
Next, install a couple of dowel pins into the fulcrum bar. Again, use a dowel diameter that will fit snugly in your prop holes (most likely 5/16"). Carefully drill a couple of holes through the fulcrum bar (about a 1/4 inch from one end) that match the pattern of the holes of your prop. Keep the holes as perpendicular as possible to the surface of the bar (use a drill press if you have one) and parallel to the end of the bar. Then, cut a couple of dowel pins approximately an inch and a half in length and insert a pin with a dab of glue into each hole about a half inch deep so that an inch of pin protrudes from the top of the bar. (See Picture 2.) Now, draw a perpendicular line across the top of the fulcrum bar 3 inches from the end. This line (called the `Pivot Line') will be used to designate where the pivot point of the fulcrum bar needs to be when we balance.
Picture 2: Installed dowel pins on fulcrum bar and pivot line
Fully insert a prop unto the pins, with the tip facing away from the fulcrum bar (See Picture 3.) The prop should fit so that no slop exists, but it should be fairly easy to get on and off. Make adjustments to the pins, if required, in order to obtain a proper fit.
Picture 3: Fulcrum bar with prop installed
Now it's time to adjust the length of the fulcrum bar. With a non-damaged prop blade installed on the bar, place the bar onto a table with the edge of the table even with the pivot line on the bar (thus, the bar will be perpendicular to the table edge). (Refer to Picture 4.) I use a thin plate of aluminum (mounted to the table with a clamp) to provide a nice `clean' edge for the fulcrum bar to pivot on.
Picture 4: Align pivot line with edge of table (prop not shown)
After aligning the line to the table edge (with the blade installed on fulcrum bar), take your hands off the assembly. If the blade tip does not drop down, then proceed to cut one inch off the opposite end of the fulcrum bar and repeat the process. Continue removing material from the fulcrum bar until the tip dips, and then finally cut two more inches off the bar for good measure.
Construction of your Simple Multi-Prop Balancer is now complete!
Using the Balancer
Place the fulcrum bar onto a flat, clean table with the balance line aligned to the edge of the table. The surface of the tabletop doesn't have to be perfectly level; what's important for getting consistent results is that the fulcrum bar should always remain in the same position on the table when balancing each prop. Now, carefully place a prop unto the pins of the fulcrum bar. (If you were to let go, the prop tip should fall towards the floor if you cut the fulcrum bar length correctly during the construction phase.) Take the 1x4x4" counterweight block and place it on top of the fulcrum bar. Slide the counterweight back and forth along the length of the bar until the blade tip no longer tends to pitch down on its own. This is the critical balance point (See Picture 5.)
Picture 5: Finding the critical balance point with counterweight
Once you've located the critical balance point, mark the edge of the counterweight block unto the fulcrum bar. It doesn't matter which edge you mark off of, just be consistent. Use a lead pencil so it can be erased afterwards. (See Picture 6.)
Picture 6: Marking critical balance line on fulcrum bar
Carefully remove the blade, keeping the fulcrum bar in place with one hand, and repeat the procedure with your remaining blades. When complete, you should have a set of lines marked on the fulcrum bar (one for each blade) - just keep in mind to remember which blade goes with which line. (See Picture 7.)
Picture 7: Three marks for three measured blades
The prop that creates the line farthest from the dowel pins represents the "heaviest" prop (thus, use this line as the benchmark line to measure the other props against). Add weight to the other props (tape, paint, etc) and rebalance them until their 'mark' is the same as the benchmark line.
Wala, you're done! For best results, rebalance the entire set again to check to verify results.
Do's and Don'ts When Prop Balancing
DO keep the fulcrum bar in the same position while balancing each prop blade as this will provide the most consistent results.
DO make sure that the prop blade is mounted fully unto the pins of the fulcrum bar.
DO make sure that your props are repaired and balanced correctly before using them to prevent prop disintegration and possible personal injury or property damage.
DO balance your props often, especially after any prop strikes of any degree.
DON'T measure in windy conditions as this can cause inaccuracies (measure indoors for best results).
Alternative Balancer #1
An alternative multi-piece prop balancer can be devised if you have a small scale available (such as a letter scale). The scale should have a fairly high resolution for best results. The hub end of the prop blade would be placed unto a block of wood (or better yet, into a notched section of the block to provide locating consistency), and the tip would be placed unto the scale. A small block of wood might be placed under the tip to improve accuracy. (See Diagram 1.)
Diagram 1. Alternative 1 - Using scale to determine moment
The scale would be used to read the moment about the hub. Each prop could be measured in this fashion and the results compared to one another. (All the components of the balancer assembly should not be moved during the balancing phase in order to keep your results as accurate as possible.) The blade that creates the greatest moment (highest value on the scale) would be the benchmark for the remaining blades. Add weight to the lighter blades and rebalance until you get the same results shown on the scale.
Alternative Balancer #2
This balancer design uses an existing balancer already on the market (Dave Beres popular bubble balancer). Fabricate a hub to mount the prop blades to and to allow the bubble balancer to be mounted. Diagrams 2a and 2b show a 2-piece prop balancer. For a 3-piece balancer, the hub could be fabricated to span the blades 120 degrees from one another.
Diagram 2a: 2 piece prop balancer using David Beres Bubble Prop Balancer (Side View)
Diagram 2b: 2 piece prop balancer using David Beres Bubble Prop Balancer (Top View)
Although I believe the previous designs are relatively accurate methods for prop balancing, I have not had the opportunity to determine the total variation resulting from using these methods. If anyone has the will to do so, I'd be very interested in knowing the results!
Please feel free to contact me if you have any questions or improvements to these designs.