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The modern model propeller is a very important part of what we use to make most model aircraft fly. It requires some care and maintenance in order to provide you the best results and safe operation. While there are certain required tasks that you need to perform before and after you use a propeller, by far the most critical is proper balancing.
Before you use any propeller, it is a good idea to prepare it for safe operation and then check to see if it is balanced. After you use a propeller, it is a good practice to check the balance periodically. The propeller can sometimes become worn unevenly or damaged during landings or take-offs. A quick check to see if it is still “in balance” is well worth the effort.
What do we mean by being “in balance? It is very close to what happens if a car wheel is not in balance. Look closely at one of your car’s wheel rims. You will see small lead weights clipped to the outside of each metal rim. These weights are applied to your wheel/tire assembly so that they are in proper balance.
Sometimes, one of these weights falls off. The assembly becomes unbalanced, with one side now slightly heavier than the other. You don’t notice any change until the car reaches about speeds around 55 mph or so. Suddenly, the entire car starts to vibrate. If a front wheel is unbalanced, the steering wheel may also shake. The car’s suspension system takes a real beating, even when the vibration seems to disappear at speeds above 75 mph. It doesn’t really disappear, the vibrations are now so fast that they cannot be transmitted to the passenger compartment, but suspension damage still is occurring.
Model aircraft propellers work the same way. Since they rotate so quickly, up to 13,000 times per minute, balance is very critical. Usually, one blade is heavier than the other. Therefore, at certain speeds, or in this case revolutions per minute (rpm), the propeller will try to make the engine move up and down or side to side or any combination of both movements.
“Balancing the prop” is an expression that you will often hear at the field. Before you even go to the flying field, it is a wise move to balance the propeller. Balancing means making sure that the propeller will rotate without causing undue vibration to the airframe and all of the electronic components.
The destructive power of this vibration on our airframes and radio equipment cannot be underestimated. Problems with engine fuel mix can often be traced to fuel foaming in the tank because of vibration. Engine and muffler mounting bolts loosen regardless of the amount of “Loc-Tite” applied. The vibration causes the engine to lose top-end power as vibration disturbs fuel flow and gaseous mixture transfers. Glow plug elements break far more quickly than they should. The vibration may even cause engine bearings to wear prematurely.
It is important to ensure that both blades of a model propeller are the same weight over their span. While there are more sophisticated balancing issues, such as hub balancing, they really apply to much larger propellers than those that come with .40 to .60-size aircraft and engines. The balancing we will be looking at here is for those propellers in the 9-13” range.
In order to balance your model propellers, you will need a propeller balance measurement device that will show you if you have achieved a perfect balance. The weights involved are too light to use the old “nail in the wall” balancer normally used on lawn mower blades.
There are several commercially available products that will do the job. The one made by Master Airscrew suspends the propeller in a very low friction magnetic field. This product will more than adequately handle the small- to medium-sized propellers. Top Flite’s Power Point Precision Balancer (photo 1) works in a similar manner. Great Planes Fingertip Prop Balancer cost less than $5.00. The point being that there are many types of model propeller balancers. A quick visit to your local hobby shop will show you just what you need.
Photo 1
Other tools you might need are: sandpaper and/or files, magic marker, denatured alcohol, safety razor blades.
Photo 2
As photo 2 illustrates, there are many different types of propellers. Construction materials vary, as do shapes. Tip shapes are especially important. Each manufacturer tends to have a different tip design (photo 3). What is important to you—whatever you do to the propeller—is that it is essential to keep the shape of each tip, the same. One way to do this is to draw an outline around the tip before you work on the propeller.
Photo 3
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