Motors And Propellers: Heavy Lifting
The right electronics are a first step toward creating flight, but a multi-rotor isn't going anywhere without good old-fashioned practical physics to pull it upwards. Your choice in motors plays a pivotal role in the success of a capable setup.
This is also the point when specifications start to get complicated. You'll want to do some research before settling on the right configuration for your quad. And motors are expensive, making it even more important to consider the options carefully.
Motors used these days are almost exclusively of the “brushless” variety. That equates to minimal friction. A cylindrical shell of magnets rotates on precision bearings around a core of tightly and neatly coiled wire. The propeller is fastened atop. Many Tom's Hardware readers already know the composition of an electrical motor, but for enthusiasts dabbling in multi-rotors, the inner workings are unimportant. So long as reasonable care is taken and dirt kept clear of the bearings, brushless motors are famously reliable.
Motors are assigned various notations, the most consequential being the Kv rating. Confusingly, Kv does not refer to kilovolts in this case. Rather, it's a motor velocity constant denoting the revolutions per minute (RPM) that a motor will turn when a 1 V potential difference is applied with zero load. This number is important, as it defines a multi-rotor's flight characteristics based on specifications like battery voltage and take-off weight.
Also vital are the propellers you choose. The variety of props is arguably greater than any other component we discuss; materials, dimensions, and price span a mind-bogglingly wide range. Generally, cheaper props are less precisely manufactured and more prone to creating vibration. This applies especially to the relatively larger end of the prop spectrum, with differences becoming less perceptible for smaller craft. Again, some vibration can be acceptable, bolstering the case for less expensive propellers. But if you're flying a quadcopter with the intention of producing well-shot footage, expect to spend more money on propellers.
It's worth pointing out that a majority of props are designed for airplane, though we're starting to see more optimized for multi-rotors. Graupner is a favorite brand amongst enthusiasts, and the company's E-Props, designed for electric power systems, are often cited as favorites. Other common options include Gemfan, APC, T-Motor, and RCTimer.
There are three simple measurements to keep in mind. The first is length, usually given in inches. The higher the Kv of your motors, the smaller your props need to be. Smaller props allow for greater speeds, but reduced efficiency. A larger prop setup (with correspondingly-low Kv motors) is easier to fly steadily, uses less current, and lifts more weight.
Frankly, the best way to gauge the right range for motors and props is referring to manufacturer recommendations if you're building an ARTF kit. Or you can simply compare the setups of more experienced builders.
The second measurement, prop pitch, is less important, but of interest to more vigorous hobbyists. Prop dimensions are quoted in the form 9x4.7”, as a numerical example. The first number refers to the already-discussed length. The second is pitch, defined as the distance a prop would be pulled forward through a solid in a single full revolution, as if a screw through wood. The greater the pitch, the higher the thrust and necessary motor output. Typically, multi-rotors use props with pitches in the range of 3 to 5”. Lower pitches are more efficient, but lend a more sedate flying style.
Finally, we have bore measurement, which is simply the size of the hole in the center of the prop. This must be matched to the shaft of your chosen motors. Adapters are available to downsize a prop’s bore. Alternatively, some props, such as those produced by T-Motor, use a direct mounting system whereby screws secure the props directly to the motor head.
