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What Motor Do I Use for my New RC Airplane?

How to select the right electric motor for radio control flight

Early gas model airplane engine

Early gas model airplane engine


Electric motors have revolutionized radio control model airplane flight. Long gone are the days when an RC pilot could spend the entire day fiddling with a balky gas model airplane engine, trying to get it started and running correctly.

With today’s brushless electric RC motors, all the hassles associated with fuel powered engines are eliminated. Just flip a switch and an electric motor is up and running, providing silent and vibration free power for RC flight.

Modern electric motors can equip any model airplane from a quarter scale World War I fighter to the tiniest indoor micro flyer. It truly is amazing the impact that electric power has had on the RC model airplane hobby.

Brushed electric motor

Older technology brushed electric motor

But the question has always remained, “What motor do I use for my new RC airplane?” The issue of motor selection comes up when either adapting an existing gas engine powered design to electric flight, or if designing an original model airplane. What motor do I use? And how can I select from the hundreds of electric motors available? Read on, and I will provide an easy method to choose the correct motor. Preview hint: Use the model weight.

Brushless electric motor

Brushless motor and mount

Selection background

First, some background behind the difficulty with answering the question of what motor do I use for an RC model plane. In the not so distant past, say around the year 2000 or so, RC modelers relied for the most part on brushed electric motors. Brushed motors were inexpensive and widely available, but did not provide much power due to the commutator brushes that were part of the motor’s technology.

During this period, RC design modelers had to be exceptionally careful with electric motor selection and gearing/prop arrangements to have any hope of successful fight. Various websites and publications had extensive charts and reference tables to try and answer the question of what electric motor to use for a given design. If you did not get this choice right, many times the resulting electric powered model could not sustain level flight.

 

With the advent of today’s precisely engineered brushless electric motors, combined with lightweight lipo in-flight batteries, there is essentially no concern regarding the need for a precise match up of an electric motor and the gearing arrangement. Outrunner brushless motors provide plenty of power for their size. Outrunners typically work best connected directly to the propeller, thus no need for gearing as with the older brushed motors.

Rimfire motor


Tower Hobbies Rimfire motor page - note tech notes and accessories to the left. Note also all brushless motors have three wires. Brushed motors use two wires.

To outfit a standard RC aircraft, you will need an electric motor, electronic speed control and a battery. But the question remains: What electric motor should you pick for a given plane?

Motor dimensions

Motor dimensions from manufacturer website

Based on my experience designing six radio control model aircraft, I recommend using aircraft weight to select the proper engine. If designing a plane from a clean sheet, make an educated guess at the final weight.

Use design history

I typically evolve my designs, and use this historical experience to estimate final aircraft weight. For example, I constructed the Demoiselle from a Sig kit, thus I knew the plane’s weight. I took the basic layout of the Demoiselle to sketch out the Chickadee’s dimensions. Correlating the Chickadee and Demoiselle weights I made my engine selection. The Blackburn progressed from the Chickadee. As the dimensions of the two aircraft were similar, I used the same engine, battery and ESC arrangement in both flyers.

Electrifly ESC

ElectriFly electronic speed control. Note that you can download the manual.

Once you have the aircraft weight, the next step is to select the electric motor. This is where the internet and manufacturer websites come into play. I’ll use Tower Hobbies as an example, as they have superb technical support notes for all of their products.

For this exercise, I’ll use the Great Planes Rimfire 400 28-30-950 Outrunner Brushless electric motor. Just go to the Tower Hobbies website, and search for Rimfire motors. You’ll get a long list of the various sizes of these motors.

The important part of the Rimfire web page is the Tech Notes. The numbers associated with the Rimfire motor usually stand for metric dimensions and power output of the motor. These are of little help with deciding what motor to use for a model.

Other items for Rimfire

Other Items of Interest from the Tower Hobbies website

The key tech notes information item you should look for is the model airplane weight that a motor is designed for. Note Tower Hobbies mentions that the Rimfire 400 28-30-950 is designed for sport airplanes up to 32 ounces and 3D (acrobatic) airplanes up to 20 ounces. This is the important data point you require for motor selection. Simply match an electric motor to the weight of your model. If in doubt, just chose a slightly larger motor. The very slight difference in weight with the bigger engine will not be significant.

 

Tech notes

Technical notes showing suggested model aircraft weights for the Rimfire motor


Once a motor is designated, the Tower Hobbies web page offers information on suitable ESCs, propellers, batteries and prop collet adapters. All of today’s motor accessories used for electric flight have similar light weights and sizes. I typically go with whatever is recommended.

Rimfire mounting

Free download of a Rimfire manual shows mounting options


Motor test flights

Once you install the motor and get a few test flights done, make note of how well the engine worked with that size airplane. I get a few motor/battery/ESC combinations that I am comfortable with, and oftentimes will design the airplane around the power package. Note that I used the same electric power setup for the Yard Ace, Chickadee and Blackburn.

In summary, when determining what motor do I use for an RC model, I figure out the model weight and select an electric motor suitable for that aircraft weight, based on information from the manufacturer’s website and a downloaded owner manual. I go with manufacturer recommendations for the ESC, propeller and battery. I test fly how that works out, and use this experience to adapt as needed for future design projects.

Author: Gordon McKay



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