Propeller Basics

One area that seems to befuddle many new, and some experienced, fliers is selecting the right propeller.


The prop is the final piece of the electrical drive chain that starts with the battery and ends with the motor/prop combination. In general, it's a good practice to use a battery and ESC that's rated higher than the motor's spec – this gives you some "overhead" so that you're not driving the battery and ESC at their limits.

The prop's job is to accelerate air to keep the plane flying – the result is measured by thrust. We can estimate thrust for a given prop at a given rpm by the following formula:

You can see the large impact a prop's diameter has as its impact on the volume of air is squared. The power needed to turn a prop depends on its rpm, diameter and pitch as shown in the following equation:

A propeller's pitch is the theoretical distance it will travel along the axis of rotation in one complete revolution – the more pitch, the more the prop will travel, and consequently the faster the plane will travel – ie more speed. As the equation shows, a prop's diameter once again has more impact on required power than its rpm – replacing an 8 inch prop with a 9 inch will have a large impact on required power.

A simple formula to estimate a prop's load factor (PLF – credit to Lucien Miller for this) is:

Using this formula can help in determining equivalent props as shown below:

This can also give a good approximation on how the power requirement will change when switching to a different prop. It is certainly possible to trade off diameter and pitch to keep the power requirements about the same among various prop sizes.

There are basically two prop types in popular use:

• Slow Fly: Props designed to deliver high thrust at relatively low rpms (hence lower air speed) are designated as Slow Fly props. These props have a larger blade area compared to "E" props to move more air at lower rpms and the pitch will be finer – 10×4.7 is one example. Slow Fly props are more delicate than "E" props – although not all manufacturers give rpm limits, generally limiting these props to 65,000 rpm/diameter is a good practice – eg a 10" slow fly prop should not exceed 6,500 rpm.

  A slow fly prop is like a car's low gear – lots of power to start, climb hills etc, but driving on the highway at 70 mph in second gear is NOT recommended. Consequently slow fly props enable fast takeoffs, quick climbs, vertical acceleration, hovering and 3D type maneuvers better than "E" type props, but at the expense of a lower top speed. Overall high-drag planes, such as biplanes, are more suited to slow fly props.

   

• "E" Props: These props designed to enhance speed over thrust and have more pitch and lower diameter – a 9×7 is one example. These props look more "traditional" than slow fly props and will be faster in level flight than a slow fly prop – this if fifth gear, the "highway" gear for speed on level surfaces. E props are designed to handle higher rpms than slow fly props – although not all manufacturers give rpm limits, generally limiting these props to 190,000 rpm/diameter is a good practice – eg a 10" E prop should not exceed 19,000 rpm. Overall low-drag planes, such as flying wings, pylon racers etc, are more suited to E props.
  Props and Motors

  The prop used with the motor determines its load in amps – the volts are determined by the battery. Almost all motors specify some range of props to use with them – some more than others. E-flite, for example, typically recommends props this way:

   

  

  Not much to go on. Scorpion gives you a wealth of data:

   

  

  The Scorpion data is based on actual bench test data and enables you to fine-tune your setup. Other motor manufacturers supply varying levels of prop data; if you buy or have a motor that does not supply this data, then you must develop your own using a wattmeter to determine acceptable motor loads. Lacking any data on a motor places a real burden on the user – a start is to do a search for similar looking/size motors and use these specs as a starting point.

  Bottom Line: If you have a motor and can't find any specs, you need tools to make them up, a wattmeter at a minimum. A rule of thumb you can use is that each ounce of a brushless motor can handle about 100 watts of power; considering that you can't find any specs, this is likely a low-end motor and something like 80 watts may be more appropriate.

  There are modeling tools, such as MotoCalc, which can generate recommended propellers for a given configuration, but if you have no data for the motor you're using, this tool is not going to help much. In fact, any tool is a "GIGO" process – garbage in, garbage out; you need accurate data to approximate a real world result.