ArduPilot Mega IMU board

For Scout to auto-stabilize, it needs sensors to find its orientation in space.  No one sensor can do this on its own.  It takes several different sensors working together to calculate Scout’s orientation.  The sensors used to calculate orientation are gyroscopes and accelerometers.  More recently, magnetometers are being incorporated to calculate direction.

Conveniently, there are boards that incorporate all of these sensors together.  These boards are called inertial measurement unit, or IMU.  An IMU uses accelerometers and gyroscopes to measure and report the aircrafts velocity, orientation and gravitational forces on multiples axis.

Scout uses the ArduPilot Mega IMU shield.  I chose this board because it was developed to work with the ArduPilot Mega board.

Wireless Communication

XBee 1mW Wire Antenna Series 1 Modules

The ArduPilot Mega supports serial communication between the quadcopter and a computer.  This serial communication is used to upload the software, change tuning settings and to update the flight path.  The ideal wireless solution is XBee.  XBee modules are available at different frequencies and power outputs.  I am using the XBee 2.4GHz 1mW with the wire antenna modules.  The XBee 2.4GHz modules are more than sufficient for initial setup, tuning and short range use.  The XBee 2.4GHz modules operate at the same frequency as microwave ovens, WiFi routers and just about any other wireless consumer device.  These modules have to contend with radio interference and this why they are not ideal as a robust long range solution.  For applications requiring longer range and robust communication, using the XBee Pro modules is a better solution.   The XBee Pro’s operates at 900MHz the lower frequency allowing for a longer transmission range for the same power output as the 2.4GHz modules and are less susceptible to interference.  The XBee Pro’s are offered in different power outputs. The higher the power output, the longer the transmission range.


There are several communities developing software for the ArduPilot Mega platform.  Currently, the communities supporting ArduPilot Mega are ArduCopter, ArduPirates and AeroQuad.

The Arducopter and ArduPirates share the same code base.  The ArduPirates community has branched off and is developing the ArduCopter software for their own needs.  I initially chose ArduPirates but recently switched to the ArduCopter code because the code is well commented, simpler and easier to understand.  Both code bases work well, but I find Arducopter code easier to modify.

The only reason why I did not chose AeroQuad is because I could not get wireless communication with Scout working as I could with both ArduPirates and ArduCopter.

Wireless communication allows the freedom for on-the-fly tuning and reading sensor data from Scout without having to connect a USB cable to it.  If I had GPS installed, wireless communication would also allow for real-time updating of the flight path without having to bring Scout back to the base station for programming.


When choosing a motor, there are several different types to choose from.  The first decision is to choose between a brushed motor or a brushless motor.

Brushed Motors

A brushed motor uses brushes that physically contact the rotating shaft of the motor.  This physical connection is a point for wear and inefficiency in a brushed motor.  Eventually the brushes will wear and the rotating shaft will gradually corrode.

Brushless Motors

The benefit of a brushless motor is that there is no physical connection between the electrical moving parts; this makes the brushless motor virtually maintenance free and very efficient.

Great Planes Rimfire .10 Brushless Motor

The future is brushless motors, the only negative is the cost, the initial cost of these motors are higher than brushed motors, but because they are brushless, there are no parts to wear out.  Therefore, they are virtually maintenance free and will out last any brushed motor.

The second decision is to choose between an outrunner or inrunner motor.  This refers to whether the rotation shaft is on the outside of the magnets (outrunner) or on the inside (inrunner).  Outrunner motors are designed for low rpm, high torque applications.   Inrunner motors are the opposite as they are designed for high rpm, low torque applications, such as electric ducted fans or small diameter propellers.

The third decision is motor size.  The motor size is based on the propeller size.  If the motor is too small for the propeller, then the motor will struggle to spin the propeller.  If the motor is too large, then the excess weight of the motor will contribute to the overall weight of the quadcopter.

I chose the Great Planes Rimfire .10 35-30-1250 outrunner brushless motors because they are well made motors.  They are capable of handling propeller sizes from 10×4.5 to 10×7.  These motors make for explosive acceleration and maximum torque, eliminating the need for a gearbox.  The faster acceleration is useful for faster stabilizing.  The housing is made out of aluminum, which is great for reducing overall weight.  They have double-shielded bearings, which make them virtually maintenance-free and efficient.

Assembled Turnigy 2213 Motor

The Rimfire motors are excellent motors.  I used them for Scout simply because I already had them from past projects.  A much cheaper and more than capable alternative is the Turnigy 2213.