How To Choose The Perfect Motor For An RC Plane


How To Choose The Perfect Motor For An RC Plane

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Once the exciting world of RC planes grabs your attention, you’ll be hooked. If you’ve recently discovered the enjoyment both you and your loved ones can experience together with RC planes, then you’ll probably want to learn a bit more about improving the performance of your RC plane. If that’s the case, you’ve come to the right place!

So, how do you choose the perfect motor for an RC plane?  Unfortunately there is no one-size-fits-all answer when it comes to finding the best engine for all RC planes. Instead, the best motor for your aircraft can vary depending on the type of RC plane that you’ve purchased.

Since there isn’t a lot of information available on the Internet today that covers how to pick the best RC motor for one’s RC plane, we’ve created this guide to help you out. Below we’ll give you a simplified overview of what you can expect from the many motor components and motors that you’ll find available for purchase on the market today. 

RC Plane Motors as Power Systems

Nowadays, figuring out a high-quality power system for your RC plane’s motor is more complicated than what it sounds like overall. That’s because it requires a certain amount of educated guesswork when you are trying to figure out the best RC plane motor for your RC plane.

While you could take the time to calculate each bit of information you’d need to figure out the best power system for an RC plane model, it’s almost more comfortable to use the approach of an educated guess. 

To help you understand how you can go about making your educated guess concerning your RC plane’s motor, you’ll have to learn some new knowledge. That’s what our guide will focus on below.

We’ll attempt to give you the experience necessary so that you can make an educated guess about the motor that’s right for your RC plane. 

Since different types of RC planes perform better with different types of motors, there is no one-size-fits-all answer to this question. Instead, you’ll need to educate yourself about the requirements you’ll need for your RC plane’s motor, and use the information we provide you with below to help make an educated decision about the type of RC plane motor you should purchase.

Accounting for Wing Load and Stall Speed

When trying to make an educated decision about what RC plane motor is right for your RC plane, you’ll need to account for both the wing load and the stall speed of your RC plane. To start, a wing load refers to the aircraft’s loaded weight divided by the wing’s area.

Knowing your RC planes’ wing load capacity means you’ll have a general understanding of your plane’s lift-to-mass ratio, and that affects how fast your plane climbs, how it turns, and how much of a load the aircraft can carry. 

Some RC plane enthusiasts feel that the best way to approach an RC plane is to make their RC plane as lightweight as possible when it comes to motor components. While this may initially seem like an odd approach, there’s an educated reason why these RC plane enthusiasts want a lighter plane.

If you have a lighter aircraft, then you’ll have a light wing load that makes the airplane more comfortable to fly. That’s because the lighter plane decreases the likelihood that the aircraft will stall.

When an RC plane makes a turn, it feels a gravitational force that then makes its weight increase. This process is similar to the weight on the end of a string.

When you spin around the end of the string like a lasso, it will get heavier on one end. With an RC plane, the heavier the wing loading is during a turn, then the higher the stall speed becomes. 

Other things can affect an RC plane’s rate of stall speed as well. The aerofoil shape, as well as the plane’s aspect ratio, are both things that can affect an RC plane’s rate of stall speed. If you have a heavy RC plane model and you want to lighten its wing loading capability, then you’ll need to make the size of the wings on your RC plane larger. So, models that have high side loading capabilities will also offer you higher stall speeds. 

The calculations we usually use for wing loading are oz/ft² or gr/dm², one dm² = 15.50003 inch². If you want some help calculating the wing load on your RC plane, you can go here

Accounting for Volts, Amps, and Watts

Once you can consider for your plane’s rate of stall speed, the next thing you’ll need to figure out when choosing the right RC plane motor for your RC plane is volts, amps, and watts. To better break down how these items work and affect your RC plane’s engine, it’s a good idea to use the hydraulic analogy. The hydraulic analogy compares electric circuits to water-filled pipes since voltage resembles water pressure. Voltage helps figure out how quickly the electrons can pass through the channel.

Next, you’ll need to know about how current works using the same analogy. Current is usually measured in amperes and means a measure of the water’s volume that moves beyond a certain point. The rate of the movement determines the voltage, and the total amount of the voltage’s output is measured in watts. So, the equation that’s typically used to figure out all of these components is volts x amperes = watts. 

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You’ll notice as we continue discussing power with RC planes how the size and effectiveness of a motor work with the load that the propeller puts on the aircraft, which can all affect the volts and amps your RC plane uses. So, you’ll want to make sure that you are picking a motor, battery, and a propeller combination that will make your model fly the way you want it to operate within the component specifications. 

To help you understand a little more about how all of this works, we’ll move onto discussing how you can select a motor below.

Selecting a Motor for Your RC Plane

Before selecting a motor it is important to know the differences between each type of motor that is on the market. The video below gives some clear explanations to help you understand the different motors better.

When it comes to choosing a motor for your RC plane, you’ll need to consider a few things. Those things include weight power and dimensions, in-runner or out-runner engines, the kv, gearboxes, electronic speed control, cut off voltage, and power selection. We’ll cover these items in more detail below.

RC Plane Weight Power and Dimensions

An essential thing you’ll need to factor in before you pick a motor for your RC plane is the plane’s weight and dimensions. For example, you don’t want to have to add more weight to your RC plane to ensure that it has the correct center of gravity. Instead of doing that, it’s better to have an RC plane that’s larger, heavier, and that features a better motor rather than purchasing a smaller, less powerful engine.

Depending on your budget, we understand that you won’t always necessarily have a choice in the matter. However, when you do, you don’t want to forget about the dynamic between your motor’s weight and your RC plane’s center of gravity. The dimensions of your RC plane’s engine will be very important, and you need to avoid purchasing a motor that you know won’t fit into your model in the first place.

So, when searching for the right RC motor for your RC plane, you’ll need to find an engine that gives the type of performance for your current RC model plane. If you have a 3D model, for example, you’ll need to get a motor that has a thrust that is larger than one to one. On the other hand, if your RC model plane is a scale WWI biplane, you won’t need as much thrust for your model.

Keep in mind that when you keep your motor above its maximum efficiency, the watts per pound rule won’t be as accurate as it usually would. That’s because you’ll have a higher percentage of Watts being used to make heat instead of creating power. Below we provide some ideas in Watts per pound for your RC plane. 

  • 70-90 watts per pound: Training and slow-moving aerobatic models.
  • 90-110 watts per pound: Sport aerobatic and fast-moving models.
  •  110-130 watts per pound: More advanced user aerobatic and faster models
     
  • 130-150 watts per pound: Lightly loaded 3D models as well as ducted fan models.
  • 150-200+ watts per pound: Unlimited 3D model planes.

Now that we’ve discussed a bit about the weight and power you’ll need for your RC plane’s motor, we’ll move onto talking about what type of engine would be best for your RC plane, an in-runner motor or an out-runner motor.

RC Plane Motors: In-Runner and Out-Runner

Now it’s time for us to discuss what type of RC plane motor would be best for your RC plane. When talking about both in-runner and out-runner engines, you’ll learn that they each have their pros and cons. You’ll need to pick the motor depending on the type of RC model plane that you are using and consider what is best for the RC plane that you currently own.

In-Runner RC Plane Motors

First, we’ll cover the basics behind in-runner motors. With in-runners, this type of engine is usually made with magnets that are attached right on the shaft. The copper windings then encircle that shaft. With the magnets being located so close to the shaft, the shaft winds up, spinning around at a fast rate. So, these types of motors create a high speed of RPMs, but they have low torque. You can convert the high RPMs into torque through a gearbox if you’d like.

When compared to out-runners, in-runners are traditionally considered the more powerful and efficient choice of the two. However, in-runners do need to feature a gearbox if they are being used to move large propellers. In-runners also create higher revs per volt when you compare them to out-runners. If you have an RC plane model that needs a small prop running at a fast speed, with pylon racers and ducted fans, then the most popular choice is usually an in-runner with a gearbox.

Once you put a gearbox into your in-runner, however, you’ll wind up with a few new pros and cons. One thing you’ll have to keep in mind about gearboxes is that they tend to cost some money, and they don’t come cheap. Also, gearboxes will require more maintenance, and they tend to be noisier. However, with a gearbox, you’ll get the best level of efficiency and power, featuring a geared in-running that can spin a giant prop.

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Now that you understand how in-runner motors work, we’ll move onto to cover how out-runners work.

Out-Runner RC Plane Motors

Out-runner RC plane motors are typically manufactured with copper windings on the inside of the engine. On an out-runner, the shaft area will be combined with a “bell,” which could also be a casing with the magnets. The magnets and the casing allow the motor to spin around the copper windings. Since an out-runner engine has the extra weight from the bell on it and magnets that are spread farther out from the shaft, this motor works like a flywheel. 

An out-runner motor will create a lower RPM and a higher torque when compared to in-runners. So, an out-runner engine is capable of spinning a giant prop without forcing you to add a gearbox. Since out-runners don’t need a gearbox, you’ll wind up with less maintenance, a quieter motor, and a cheaper engine overall because you won’t need to buy the expensive gearbox. 

For some, the combination of benefits you get with an out-runner that doesn’t need a gearbox is challenging to pass up. So, the quiet operation and more affordable price, for some RC plane lovers, will outweigh the better efficiency and power that you’d get with an in-runner.

What is KV?

When it comes to knowing how to pick the right motor for your RC plane, you’ll also need to understand what KV means. KV refers to the number of revolutions per minute (RPMs) that an electric motor can spin at per volt when there is no load. So, what’s the difference between having a high KV and having a low KV? Think of these differences as being similar to the difference between a high-performance racing two-stroke motorcycle engine and a standard Harley Davidson’s four-stroke motorcycle engine.

While both the two-stroke and four-stroke motorcycle motors will use the same amount of horsepower, a two-stroke motor can do this at 11,000 RPMs. On the other hand, the four-stroke can do this at 3,000 RPMs. This comparison resembles how high and low KV electric motors also function. 

If both motors use the same type of voltage, then a high KV in-runner motor that utilizes a small diameter propeller would work well in a high-speed model like a racer. On the other hand, a low KV motor that features a larger diameter propeller would work better for power, like flying a sailplane to its altitude. 

You can figure out the KV of a motor by determining the number of winds or turns made by the propeller. The number of winds or turns means the number of times the copper wire goes around the stator pole. The more winds, the lower the KV. The less the winds, the higher the KV. 

Lower KV or Higher KV?

So, what are some of the differences between lower KV and higher KV? We did mention a bit about this above, but we’d like to talk about this in more detail below. When you have a high KV motor, it will spin at a faster rate compared to a lower KV motor that is operating at the same voltage. So, if you are using a smaller voltage battery back, you might want to consider using a high KV motor. This concept is especially true if you like to compete and race, in which case a higher KV motor might help you.

However, if you don’t have to worry about the limitations of a specific voltage, then you might want to instead use a lower KV motor at a higher RPM by initiating a higher voltage. If you tend to fly large out-runners that have a KV of 200 to 300, then you’ve got an excellent example of a good performing low KV motor. You’ll also need to take a close look at the voltage limit when you are thinking about buying a new engine.

Choosing an In-Runner and a Gearbox

If you are opting to go with an in-runner motor, then you’ll need to know a bit about how you can select and in-runner and an engine. While the process can be a bit challenging, if you take the time to educate yourself about how to make a good purchase, then you won’t run into any issues. Selecting an in-runner and a gearbox is similar to picking an out-runner, but with an out-runner, you don’t have to worry about the gearbox.

If you know you’ll need to purchase an in-runner motor; you’ll usually have two different options from which to pick. We’ll use Feigao as an example. On the Feigao website, Feigao.com, you’ll see all of their motors in a list and a full set of numbers for every engine. You’ll even notice that Feigao includes different suggestions for setting up the motors. Feigao’s smaller diameter motors are called “380s,” and their larger ones are called “540s.” 

Both the Feigao 380 and the Feigao 540 come in three sizes, small, large, and extra-large. Most of the Feigos work like copies of Hacker in-runner motors, with 380 translating into the B40 format and 540 translating into the B50 size. So, you can find a successful set-up if you want to copy it and do some Internet research. You’ll also discover that the ratios for both Hacker and Feigao gearboxes are similar at 4:1:1 for the 380 sizes and 6:7:1: for the scale 540.

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However, what does the ratio 6:7:1 mean? To figure that out, you’ll need to know the KV of the prop shaft with a gearbox. To figure that out, you’ll need to divide the motor’s KV by the gearbox’s ratio. If you need some help figuring all of this out, you can try the Peak Efficiency website. On that site, you can play around to figure out how your potential motor set-up will work with your RC plane. This website is helpful because it will give you an idea of what might happen as you play around with different options. 

RC Plane Motors and Electronic Speed Controls

Now that we’ve given you more background information on both in-runner and out-runner motors, we’ll talk about electronic speed controls, also known as ESC. In the RC model plane world, you’ll discover that there are two types of ESC in existence, brushed or brushless motors. When it comes to this, you’ll need to be aware of the fact that you won’t be able to use a brushed ESC with a brushless motor, nor will the rule apply the other way around.

Because of this, you’ll want to start by considering the different features you’ll need for your RC plane’s motor, like a decent brake or a soft start. You’ll want to factor in a brake on your RC plane if you use a folding prop and a soft start. It’s also a good idea to have a brake on your RC plane if you are also using a gearbox and a switch for on and off with the throttle. The types of features we just described are commonly found on RC sailplanes.

Match Your ESC to Your Motor

You’ll want to make sure that you match your ESC to the motor you wish to when you select your ESC. If you’re able to use an ESC that has a higher amperage than what you think you’ll be using your motor at, you’ll get some protection against stressing your ESC and damaging your RC plane. When it comes to an ESC, you’ll see a burst rating most of the time, and that tells you how you can run your ESC at its maximum amperage for a short amount of time.

If you exceed that limit, you’ll be doing damage to your RC plane. Most practiced RC plane enthusiasts like having an ESC that has ten to twenty percent more amperage than what they ever planned on using. You’ll have to get a meter to help you measure the amps and volts your power system can generate. That will help you to ensure you aren’t over-stressing your ESC, the battery, or your motor.

To help you better understand this concept, we’ll describe what a BEC is in the RC plane world. A BEC is an acronym that stands for “battery eliminating circuit.” So, a BEC is a device you can use to get power for the servos you have in your RC plane. Often, an ESC will have a BEC that can only work well over a specific number of servos and a particular voltage. The more you increase the voltage, then the fewer servos you’ll be able to use.

On the other hand, if you use too many servos on your BEC, your ESC will wind up overheating and eventually totally failing. That can be terrible if your BEC fails in flight while your plane is in motion. So, purchasing an external BEC for your flight battery pack is an excellent way to ensure you’ve got what your plane can handle.

RC Plane Propeller Selection

Remember, the propeller does put a load on your power system. So, if you purchase the wrong type of propeller, you can harm your battery while also damaging your motor and ESC. Remember, props work much like a car’s gear system. With some props, you’ll have a first gear that operates at a high RPM to make the motor go slowly. At that rate, you’ll have quite a bit of power but not a lot of speed.

If you want to go fast instead of having a lot of power, then you’ll wish to a propeller that works more like a car’s top gear. That means once you speed up your plane, it can continue at that rate for some time. If this is what you want, you should get a prop that has a diameter of about ten inches, and a total pitch of four.

Final Thoughts

When considering the perfect motor for your RC plane, there will be a few things you’ll need to think about overall. Since different RC plane models function better with different types of motors, you’ll need to educate yourself about the kind of engine that is best for your RC plane. Once you know that, you can use some of the calculations we provided above to help you figure out the best motor for your RC plane.

Once you’ve figured out the perfect motor for your RC plane, you’ll be able to get hours of enjoyment flying your aircraft with friends and family!

Matt Robbs

I love to spend time with my wife and 3 kids. There is no better way to get them off the couch and outside than for us to grab the RC cars or boat and enjoy the sunshine!

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