Model Airplane Propeller - Choosing, installing, and balancing RC airplane props

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Most rc airplanes move through the air by a model airplane propeller, but the importance of selecting, balancing and caring for these props is often overlooked. If you are a beginner, you might be thinking that the propeller isn’t very important and they are all alike.

When you start flying, you will soon realize none of that is true. The tips and information shown here will not only make your model airplane fly better, but you will also have a safer and more enjoyable flight.

Choosing the right propeller

Often, many people think you have to pick out your model airplane propeller according to the engine size, but really you need to buy the propeller that is appropriate for your planes airframe. This is why engine manufacturers often recommend a range of props with various diameters and pitches.

So what model airplane propeller is correct for your airplane? Well first you need to know that some props mess up the engines operation, others are inadequate to fly the model, and some have both problems.

If a propeller is too large, it has too much diameter and pitch. Changes in diameter effect engine load the most. Oversized props force the engine to operate to slowly, this will limit the horsepower needed to fly and may induce overheating.

If a model airplane propeller is too small, it will have excessive pitch and marginal low speed thrust production and probably will not be able to take-off. After taking off into the wind, propellers with insufficient pitch may not maintain the minimum flight speed required to avoid a stall spin. In summary, to small of a propeller will allow over-speeding, increase fuel consumption and reduce engine life.

Most rc airplane pilots learn about prop requirements by trial and error, they copy a friend or they read the airplanes manual for recommendations. Many desire improved performance though, which is basically better climbing ability, top-speed, and take-off acceleration. Unless you increase the engine horsepower, you won’t be able to do those things with just a new model airplane propeller.

So you want a propeller that will increase your engines RPM’s? Here’s how to find the right propeller for your engine. Propeller load and engine RPM are inversely related. As load increases, RPM decrease and vice versa.

Load is represented by the propeller, change the propeller size and the load is changed. By using the Propeller Load Factor formula (PLF), incremental propeller load changes can be determined. The formula is PropLoad=D / 2 (P). If you don’t understand it, it is the propellers diameter squared, times the propellers pitch, which equals the prop load.

How do you do this equation yourself? Well, when you are buying a model airplane propeller, they come in different sizes, and are labeled like “9x7”. The first number is the diameter, and the second is the pitch the propeller will have. So in this case, the formula would look like this: 9x9x7=567. It is 9x9 because the diameter is squared, which means you times it by itself. Then you times that by the pitch, and you have your propellers load. The lower the load number, the more RPM your engine will have (and vice versa).

To low of number may make your plane faster, but you might be sacrificing something else you want.

Propeller balancing

Properly balancing your propeller is one of the most important things a model airplane pilot can do. If you are a beginner or just a spare time flyer, then this really won’t apply to you. But if you are an advanced flyer, or you want to be, you need to know how to balance your propeller. First, you need to make sure you get a proper prop for your propeller (say that 3 times fast!).

That basically means a good prop stand that will hold your propeller while you check the balance. If you go to your local hobby shop, you can find a propeller balancer. They range from very simple wooden balancers, to high tech magnetic balancers that are precise. Whatever you choose to buy, it will work.

Propeller balancing is very simple. You just put it on the balancer, and spin it. If the propeller stops horizontal with the same edge of the blades facing downward every time, then those edges are too heavy. A balanced propeller should stop and remain anywhere on the circle without any tendency to rotate at all.

If the bottom edges are heavy, then you need to sand their faces. Sand a little, then test, then sand again, and then test again. You will eventually have it perfectly balanced when your propeller stops at random positions. If you are having trouble deciding if your propeller is stopping in random positions, then look at the picture below.

In that picture, you see that a model airplane propeller can be split into 4 different sections. Mark those sections on the blades with a pencil, and then spin it.

Every time your model airplane propeller stops, write down what position it is in and what sides are facing downward. Keep doing that, and after about 10 or 15 times, look at your results.

If they are random and have no pattern, then your propeller is properly balanced. If they do have a pattern and are not random, then sand the faces of whatever blade points downward the most.