ElectroFlightRC - Basics of Electric Flight

Electric motors come in many shapes and sizes. It can be a little confusing if you are new to the world of electric powered RC. I know I was confused when I first started but don’t worry. It’s not that complicated. There are only a few things you need to know to get started. The rest, you can learn as you get more experience in the hobby - if you even need to.

In-Runner and Out-Runner motors

In-Runner Motors

An in-runner motor is a motor where the case of the motor is stationary during operation. An in-runner motor has the permanent magnets positioned on the inside of the electromagnets.

In-Runners are a popular choice for Electric Ducted Fan (EDF) jets and for use with gear boxes. This is a picture of an in-runner motor:

Out-Runner Motors

An out-runner motor is a motor where the outer casing of the motor spins during operation. There is a stationary back-plate that is used to mount the motor to the plane’s firewall, and the rest of the motor spins. An out-runner motor has the permanent magnets positioned on the outside of the electromagnets (i.e. the permanent magnets are mounted on the inside of the canister:

Out-runners are a popular choice for direct drive applications. They can also be used with gear boxes but this is less common. Out-runners are also used in EDF applications, but typically these motors will look different than a normal out-runner motor.

This is a picture of an out-runner motor:

This is an out-runner used for EDF:

Mounting an out-runner motor to your plane

There are basically two ways you can do it. I remember this was a point of confusion for me when I got my first motor and tried to mount it to my new plane. So lets start by looking at the various parts that come with the motor (and/or those you may have to buy separately).

Typically, when you receive an out-runner motor, it comes with the following parts:

The motor:

Simply, this is the motor without any accessories or mounting hardware.

Just one little tip I can give.... You see the little set screw on the right side of the picture? It’s near the end of the motor shaft positioned on the inside of the motor. That set screw (could be more than one) is what holds the outter motor case (bell) onto the motor shaft. It is a good idea, when you get a new motor, to get into the habit of removing that screw and applying a little drop of loctite to it before putting it back in. Screw in the set screw tightly (not too tightly as to strip the set screw). Sometimes, the manufacturer does not tighten these set screws very well and that could lead to the prop flying off your plane with the bell of the motor still attached (if you mounted the motor in front of the firewall), or possibly the motor shaft (if you mounted the motor behind the firewall).

The X mounting plate

This is the mounting plate which allows you to fix the motor to the firewall of your plane. It is mounted to the motor using the inner holes shown in the picture and it is positioned on the side of the motor that has the motor shaft protruding (see above example motor image). There will be matching holes on the motor.

The threaded prop shaft

This item is not usually included with smaller motors because they use the motor shaft with a separate prop adaptor.

This is the threaded prop shaft you will use if you mount the motor in front of the firewall. It is mounted to the motor using the provided screws and it is mounted on the side of the motor opposite the protruding motor shaft (on the right side of the above example motor image).

The prop adapter

Usually, larger motors do not come with a separate prop adapter as it is only used if you are going to mount the motor behind the firewall and use the motor shaft (not threaded) that protrudes from the back of the motor. In most cases you must purchase this item separately and you must select the correct size that will work with the diameter of your motor shaft.

With smaller motors, a prop adapter will be provided.

There are 2 kinds of prop adapters (well that’s not completely true, but for the pupose of this example, we’ll say there are 2)

Collet Style Prop Adaptor

This type of prop adapter tightens onto the shaft as you tighten the nut on the threaded shaft to secure the propeller. As you can see in the above picture, the tapered “collet” will get compressed as the base is pressed down as you tighten the propeller down.

Make sure to put some loctite on the nut used to secure the propeller. The whole assembly will come off if it gets loose!

Set Screw Style Prop Adaptor

This type of prop adapter is secured to the motor shaft by tightening one or sometimes two little set screws. You can see them in the picture above. They are on the side of the adaptor beneath the bottom flange.

A good way to prevent the adapter from spining on the motor shaft as a result of torque, is to use a metal file or Dremmel tool to make a little flat spot on the motor shaft where the screw will come into contact with the motor shaft. Also, don’t forget to put a little bit of loctite on the set screw so it doesn’t get loose! Don’t put too much loctite or you may not be able to remove the set screw - or you may damage it while trying to remove it.

Mounting the motor to the firewall:

As mentioned earlier, there are typically two ways to mount the motor to your plane: in front of the firewall or behind the firewall. There is no particular advantage of one versus the other. The way you mount your motor to your plane depends on your planes firewall/motor-box and the mounting system you chose. More on this later.

For now, I will show you what you need to know about mounting the motor in front and behind the firewall (or mounting plate).

These paragraphs deal with mounting out-runner motors. In-runners can use some of the techniques and hardware mentioned here, but there are also other methods and hardware more specific for in-runners such as heat sincs. By and large, you can use the same techniques described here to mount in-runners.

Mounting in front of the firewall/mounting plate:

This is a pretty common scenario. You will have a firewall sunk behind the front of the cowl. To get the motor to the correct distance from the firewall and get the spinner back-plate to position just in front of the cowl, you will need some spacers/standoffs between the motor and the firewall. You will need the provided X-mount and the threaded prop shaft which you will mount to the other side of the motor using the provided screws.

The following is an image of the example described above:

As mentioned before, motors come in many shapes and sizes. Here is an example where the motor shaft protrudes from the opposite side from the X-mount. Actually, some motors have the option of “pushing” the motor shaft out from either end. Read the instructions that come with your particular motor to find out what you can do with it.

It looks like this one has a collet style prop adapter mounted on the motor shaft.

Mounting behind the firewall/mounting plate

Some planes come with a motor box that protrudes out so that the firewall (or motor mounting plate) is positioned perfectly for spinner / prop position. The one in the photo was custom built but some planes come this way out-of-the-box.

In the image above, it looks like the motor is mounted directly to the firewall, but you could also use the X mount. The prop adapter is then mounted to the motor shaft in front of the firewall.

Motor Mounting Accessories

There are many ways a motor can be mounted to a plane. The following is a list of the more common accessories that can be used to mount a motor to your plane:

Standoffs / Spacers:

There are a variety of standoffs you can find in any hobby shop. These come in varying lengths, outer-diameter, and inner-diameter. You can chose a set that will fit closely and use washers to add some extra length if they are a little short. You can also use more than one standoff. For example, if you need a 50mm standoff, you could use a 30mm plus a 20mm standoff.

Other than the off-the-shelf RC hobby standoffs, you can also use nylon spacers / nylon tubing commonly found in hardware stores. These can be cut to any custom length but be careful to make all four standoffs the same length or you will change the thrust angle of the motor - if that’s not your intention.

Alloy motor mounts:

There are a number of adjustable alloy motor mounts. These can typically have the motor mounted in front of the mounting plate or behind. Here is an example:

These mounts are a good choice for flexibility, but could add some wheight to your model.

Custom mounting:

One option is to do as shown in the picture illustrating how to mount a motor behind the firewall. You can build a motor box right up to the front of the cowl, and mount the motor behind the firewall.

One of my preferred techniques is to use 4 long bolts and use nuts to move a mounting platform to whatever distance works. Here is an example:

There are many many ways to mount your motor to your plane. Whichever way you pick is fine as long as it’s strong and doesn’t add too much weight to your plane. There are lots of resources to help you with this. Make sure to read the instructions that came with your plane. They will tell you how far the prop/spinner back-plate has to be from the firewall. Also, there are entire discussion threads dedicated to build logs for lots of planes on websites like rcgroups.com and rcuniverse.com. Do your research before starting your build. Often, you will find great tips that could help you avoid difficulties with your build.

Brushed versus Brushless Motors

The new style motors are Brushless. These motors have three wires and brushed motors have two wires coming out of them. A brushed motor has to be paired with a brushed speed controller (ESC) and a brushless motor with a brushless ESC.

A brushed motor uses brushes inside the motor to alternate the current to the poles which in turn keeps the motor turning. Click here for more information on how brushed motors work.

The big disadvantage of a brushed motor is the brushes. They get worn over time and the performance of the motor is affected. Also, brushed motors have a much lower power to weight ratio than a brushless motor.

A brushless motor (BL motor) has three wires instead of two, and instead of depending on brushes to control the current that makes the motor spin, it depends on the Electronic Speed Controller (ESC) to do that electronically. Click here for more information on how brushless motors work.

Motor ratings and power

Rated power (or Watts):

This is one of the most important ratings you should know about your motor. It specifies how much power the motor can generate. It is also a function of rated Volts and Amps where Watts = Volts x Amps. If the manufacturer does not specify rated Watts (max Watts), you can infer it from the Volt and Amp ratings.

However there is also a simple rule of thumb that will give you a rough approximation. Take the weight of the motor in grams x 3 - this assumes you are exercising some throttle control as excessive full-throttle usage for periods of one minute or more will reduce the power output. If you are going to use full-throttle in this fashion, do not exceed watts = motor weight x 2 as your calculation. Refer to the Motor Specification page of this presentation for the source of this rule.

KV:

This represents the motors rated RPM’s per input Volt. That means for every Volt applied to the motor, the motor will want to turn 1 x KV rating. For example, if a motor has a KV rating of 3200KV and you apply 11.1V (3s) to it, the motor will rotate at 11.1V x 3200KV = 35,520 RPM (no load). This is an important rating which you will use to make decisions about your power setup. More on this in the section on selecting you power setup.

Physical dimensions:

Diameter and Length of the motor.

Diameter and Length of the shaft.

The dimensions of the motor will help with figuring out if the motor will fit in the space you have available in your plane. And the dimensions of the shaft will tell you what size prop adapter you will need if the shaft is not threaded.

Input voltage range:

This represents the voltage range that the motor can operate in. It dictates the size of the batteries that can be used with the motor. For example, if a motor has a rated input voltage range for 7V to 12V, you will be able to use LiPo batteries ranging from 2s (7.4V) to 3s (11.1V). Sometimes, the manufacturer will specify a cell range instead of the voltage range. In this case, you can calculate the voltage range from the cell range by multiplying the cell count by 3.7V per cell. For example, if the range is specified as 2s-3s, the voltage range will be 7.4V (2sx3.7V) to 11.1V (3sx3.7V) -- assuming LiPo batteries.
On good quality motors, these numbers are conservative and you can sometimes exceed the recommended maximum voltage. You must be very careful when exceeding any recommended limit as you may cause damage to the motor and possibly cause an in-flight failure.

Input AMP range:

This represents the Amp range that the motor can operate in. It dictates the Amp output your ESC will have to be able to handle - and your batteries as well. More on this in the section on selecting your power setup.

There are more ratings related to motors, but the above covers the most important ones related to power setup selection..