| Spin Balancing After you have finished static balancing, you need to make sure your prop will stay balanced when running at top speed. This is a lot more difficult to do, and it’s where I sometimes cheat myself. If you’ve taken the time to balance your prop as described above, you’re already doing better than 95% of the rest of the guys out there. Here’s how to get that extra 5%. Spin balancing is what they are doing to your car tires when you have them balanced at a garage. The tire is spun, and a computer inside the tire balancer tells the operator where to place a weight and how big it should be. He will spin the tire multiple times, until the computer tells him it’s done. Since there aren’t any such tools available for us, we need to make do with what we can. This is another one of the other reasons I like my Top Flight balancer. Basically, we will be spinning the propeller while using the stroboscope to make it appear as if it isn’t moving. If it isn’t running true, we remove weight from the heavy blade until it is balanced. Here’s how to do it: First, make a small mark with a permanent marker on the tip of one blade. Next, you spin the prop on the balancer shaft. While it spins, shine the strobe on the prop, watching carefully to see if the shaft is rotating perfectly. By watching carefully, you will be able to see if the shaft is running true or moving in a circle. The ink mark will allow you to identify which blade is heavier (the heavy blade will appear to be on the outside of the wobble). Lightly oil sand the heavy blade to adjust it. Repeat this until the prop is moving in a perfect circle, with no movement of the shaft. If you have done a perfect job of this, your prop will still be perfectly static balanced. If it isn’t, you will have some vibration at certain speeds but not at others. The goal is to make it perfect at all speeds, but if you can’t do that, just make sure it’s perfect at operational speed. The ideal spin balancer would spin the prop at the speed you would be turning it on the water. I don’t have a tool which would allow me to do that, so I just spin the shaft with my fingertips. If I ever figure out how to get 30K RPM on the balancer, it will be a good day at the races! Mount the prop on the driveshaft in your boat, and run the motor up to the expected operational speed, using the optical tach to find that speed. It will probably be between 50% and 75% of the maximum speed your motor turns, so you can work it from that angle as well. (This is where stick radios with the spring return taken out can be very handy.) Check to make sure the prop runs true at operational speed. If it doesn’t, you can find the heavy blade as you did on the balancer, but it might not be the prop that’s out of balance! Try making sure your driveshaft is balanced as well – it should run true at even full RPM. This is a very fine detail, but it will help keep you from wasting power. One little hint – always mount the prop the same way on the driveshaft. I like to make a very tiny notch which identifies how the prop mounts to the drive dog. This will help you keep your drive system balanced, not just the prop. Step 5: Polishing the Prop All my props, when I finish working them, are polished to a high shine. The shine, I believe, gives the propeller a smoother surface for slicing into the water. Since most polishes use a wax binder, you are also waxing the surfaces, making them slippery. To polish your prop, mount it firmly in the holder. Put a polishing wheel in your Dremel, and put on your safety glasses. For polishing compound I use a four-grit set of sticks that I bought at Sears. I find the variety of grits much more useful than what Dremel has to offer. Use standard polishing techniques to put a bright finish on your prop. I recommend that you do common sense things like always run the wheel so the prop does not dig into it, and try to keep from holding the prop in a way that it could fly into your face or body if it comes loose. Whatever you do, don’t use a full-size benchtop grinder for polishing. These props are just too tiny for that! You do not need to concern yourself with the possibility of taking a prop out of balance by polishing it. The amount of metal removed and of wax added is not of sufficient consequence. Storing Props After you have your prop balanced and polished, the next step is to accurately measure it and store it. I purchased a small plastic box with a dozen or more compartments to hold my props. Each compartment has a small piece of soft foam which fits snugly in the bottom, and another which covers the prop from the top. All the props are arranged in order by diameter as measured by a vernier caliper, with higher pitch props of the same diameter closer to the next size larger in diameter. A piece of card stock fits in the lid of the prop box to be used as a key to the prop sizes. On race day, I can pull exactly the prop I need for any boat. I also mark certain props which I have found particularly good on a hull, and never loan those out. If you have followed these steps, your prop should be about as balanced as is humanly possible. It may take a couple hours for each one at the start, but you will immediately hear and see the difference it makes. After you’ve been working your props for a while, you’ll probably get down to the point where it only takes about an hour each. If one of your propellers is damaged (minor!) in some way, don’t throw it out! Remove the damaged area with a file, then follow the steps to rebalance it, starting Step 1. That damaged prop might become the new star performer if it is reshaped and balanced! Author, Andy Kunz. e-mail Andy at: andy@rc-hydros.com Website: rc-hydros.com |
