Getting a quadcopter stable in the air isn’t trivial. Stability of a quadcopter relies on the harmonious working of all of it’s parts.
An unbalanced propeller produces excessive vibration. This vibration travels through the entire airframe affecting the handling of the aircraft, produces inaccurate readings by the sensors, and creates premature failure of motor bearings and parts. A balanced propeller is paramount to a stable aircraft. A balanced propeller produces less vibration and draws less current, which results in greater stability and extended flight times. You should balance any propeller before installing it on your aircraft. Balancing a propeller requires the use of a special tool, you guessed it a propeller balancer. The propeller balancer that I use is the Top Flite Propeller balance. It is essentially a shaft held by two magnets. The magnets create a frictionless surface for the shaft to spin freely. Read More…
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.
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.
LiPo batteries (short for “Lithium Polymer”) are the latest and greatest when it comes to battery technology. When considering power to weight ratio, LiPos are far superior compared to NiCad (Nickel-Cadmium) or NiMH (Nickel-Metal Hydride) batteries.
There are several options when choosing a LiPo battery. The first is the voltage level that the propulsion system runs at. A single cell LiPo battery outputs 3.7 volts. To increase the overall voltage, LiPo cells are connected in serial. Connecting the LiPo cells in serial, cumulatively adds to the overall voltage. For example, two 3.7 volt LiPo cells will output 7.4 volts and a three LiPo cells will output 11.1 volts and so on. For Scout, the motors and Electronic Speed Controllers that I have chosen run at 11.1 volts and so I chose 11.1 volt battery.
The second option is capacity. Capacity is how much power the battery can hold measured in milliamp hours (mAh). For example, a LiPo battery rated at 1000 mAh will discharge in one hour with a 1000 milliamp load placed on it. If the same battery had a 4000 milliamp load placed on it, the battery will discharge in 15 minutes. The higher the capacity of a battery, the longer the flight time, but also the bigger and heavier the battery will be.
The third option is discharge rate. Discharge rate is how many amps can be discharged at a time. The discharge rate is multiple of the total capacity of the battery and represented as the “C” rating. For example, a battery with a capacity of 2000 mAh and a discharge rate of 10C is capable of withstanding 20,000 milliamp or 20 amp loads.