An Electronic Speed Controller, or “ESC” controls the speed of the motor. ESCs will have a power limit. The more power an ESC can handle, the larger, heavier and more expensive the ESC will be. When choosing an ESC, it needs to match or exceed the motor’s peak amperage. If the peak amperage of the motor is 13 amps, then an ESC rated at 15 amps will be sufficient. An ESC with a lower rated amperage will overheat and possibly fail.
Some common features of an ESC are a low voltage cutoff. The low voltage cutoff will cut the power to the motors when the voltage drops to a specific level. This is a protection feature for LiPo batteries. If a LiPo battery’s voltage drops below its minimal voltage, it can permanently damage the battery. The low voltage cutoff protects the battery from dropping below its minimal voltage.
Some ESCs can be programmed to have different throttle responses, adjust the low voltage cutoff limit, reverse the motor’s direction and change the switch rate.
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.
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.
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.
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.
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.
Propeller choice is one of the most important decisions of your quadcopter. These are the footwear of your quadcopter. Propellers affect the agility, stability and efficiency of your quadcopter.
Propellers commonly come in 2, 3 and 4 blades. The more blades on the propeller, the less efficient they become. However, more blades produce less noise and are able to handle higher power requirements.
Propellers are specified by their diameter and pitch. The diameter is measured length of the propeller. The pitch is how far the propeller will advanced in one revolution. For example, a 10×4 propeller has 10 inch diameter and will travel 4 inches in one revolution.
The diameter of a propeller dictates how much thrust can be generated. The larger the propeller the more thrust can be generated and also the more energy is needed to spin the propeller.
Propellers come in two spinning directions: clockwise and counterclockwise. The spinning direction is also referred to as “tractor” (counterclockwise) and “pusher” (clockwise) propellers. Tractor propellers are more common than pusher propellers. A quadcopter needs a matched set of tractor and pusher propellers. Because pusher propellers are less common than tractor propellers, propeller choice will be dictated by which propellers are available in pusher configuration.
Importance of Propeller Pitch
I discovered that my initial propeller choice of a 3-blade 8×6 propeller was the root of all my frustration in trying to stabilize Scout’s flight. After weeks of tuning Scout’s stability, I began to hit a wall. Even with the best tuning, Scout would still drift and sway during flight. I could not get Scout to hover in one place. I began to track down why Scout was so unstable. I initially thought it was too much vibration that was overloading the sensors. I added more foam padding to the sensor board and balanced the propellers. The stability marginally improved, but not as much as I would like.
I then thought it was the ArduPirates code that was the problem so I switched to the ArduCopter code. Scout was still unstable. I then remembered I had bought a set of 2-blade 8×4 propellers. I decided to give them a try. Eureka! Scout’s performance was remarkable. Scout transformed into a different animal. Without changing the tuning settings from the previous propellers, Scout’s stability is as smooth as glass. I surmised that the issue was not the 3-blade to 2-blade choice but the pitch of 6 inches was creating choppy turbulent air and the quadcopter could not stabilize.
I recommend using APC propellers. They are both rugged and perfectly balanced from the factory.