After you build a sport plane, there are a couple things you need to do before you head off to the flying field. For your engine to operate properly you first have the break it in, and before doing that, it’s important to balance your propeller. Why? Most, if not all, propellers will be out of balance when you buy them. Some will be close to being balanced, others not so much. The advantage of flying with a properly balanced propeller is the elimination of or a great reduction in vibration. Like the wheels on your car, the effects at low rpm are almost unnoticeable, but as the rpm increases so too does vibration. This adds to the wear and tear on your engine’s internal parts and bearing as well as the airplane’s airframe.
In addition, your airplane will have better performance with a smooth-running engine and propeller. With glow engines, less vibration means more of its power is transmitted to the prop to produce more thrust. With electric airplanes, it has a similar effect, and you’ll also get more flight time from your battery packs.
What else is needed?
All you need to true up your propellers is the Du-Bro Products prop balancer; some sandpaper; some Zap CA glue and kicker; and a prop reamer. A sanding bar with 100-grit sandpaper is a great tool to use as it provides a very smooth surface finish.
Let’s Get Started
The first thing to do is to check the fit of the prop to the engine. The hole in the prop hub should be a precise fit with little to no slop or play. If the hole is too big, you can add a couple of layers of tape to the prop shaft until the propeller fits nicely. If the hole is too small, then you should use a prop reamer to precisely enlarge the hole diameter. With the O.S. .25 FX engine used in this article, I used a stepped metric reamer. Reamers are available at most hobby shops as well as online.
When using a reamer, hold it square to the prop hub and gently push it while twisting it counterclockwise between a quarter and a half turn at a time. Once you have reamed the full length of the hole, back it out while still twisting it in the same direction.
Level the Balancer
We’ve found that the Prop Balancer from Du-Bro is one of the very best and it does a great job. It is very easy to setup and has precision bearings for smooth and precise propeller support.
Now place the propeller on the balancer. With the Du-Bro balancer, the mandrel has a solid stop and a movable spring-loaded stop. All you have to do is remove the stop and spring, slide the propeller onto the mandrel, and replace the stop and spring along with the washer and the piece of silicone tubing that keeps the spring compressed against the stop. But before we can balance the propeller, you need to make sure the mandrel is level.
You want to set up the side supports so that the propeller can turn 360 degrees without touching the base. Adjust the balancer’s height with the four adjustment screws (two on each side) until the prop is free to rotate, then measure the ends of the mandrel, making sure they are the same height above your workbench. If the mandrel is not level, it will be difficult for you to balance the prop precisely.
By the Numbers
Place the prop and mandrel on the balancer to see which prop blade drops to the bottom. This is the heavy prop and you will have to remove some material from the blade using the sanding bar. You can remove some material directly from the very end of the tip, but you should not remove more than 1/16 inch. If this doesn’t bring the prop into balance, more work needs to be done.
Flip over the prop and the mandrel so that the prop is facing in the opposite direction. See if the same prop blade drops to the bottom. If it does, then you need to remove material from the front and back sides of the blade. It doesn’t take very much: Make three or four swipes with the sanding bar, clean off the sawdust, and replace the prop on the balancer; keep doing this until the propeller sits level on the balancer. Once it does, flip it over again so that it is facing the opposite direction and see if it still sits level. If it doesn’t, then the hub is slightly out of balance.
To determine which side of the hub is heavier, place the prop so that one blade is pointing straight up. When you release the blade, the heavy side of the hub will cause the tip to drop to that side. To bring the hub into balance, don’t sand the heavy side but, instead, add some weight to the lighter side.
Step 4. As with the blades, it doesn’t take a lot to balance the hub. Determine the lighter side of the hub, and while keeping the prop on the mandrel, apply a bead of glue about an inch long to the lighter side and add some kicker accelerator.
Let the glue dry and then place the prop back on the balancer. Keep applying a bead of glue at a time and apply the kicker until the propeller remains pointed vertically when you release it with the tip pointing straight up.
You’ll know that your propeller is perfectly balanced when you can place it in any position on the balancer and it remains there without a prop tip dropping. Once this is done, you can use the prop as is, or if you like, you can apply a couple of light-mist coats of clear paint to both blades to seal the surfaces and prevent moisture from getting into the wood grain. Once the paint dries, your prop is ready to use.
The same process can be used with plastic and composite propellers, with the exception of adding a coat of clear to seal it. Plastic props do not need to be sealed after balancing.
Now bolt your prop onto your airplane and head to the flying field. You’ll have improved engine performance and much less vibration than when using an unbalanced propeller.
For more information this and other Du-Bro products and accessories for your airplane, click here.