Frames And Arms: A Foundation For Success
Just as important as good electronics, multi-rotors depend on sturdy, lightweight hulls for mounting components. There are countless frame designs of varying shapes, dimensions, and materials. Stiffer frames confer better flight characteristics, since less warping and bending can occur. If a frame is too brittle, though, your inevitable crashes will result in more frequent repair sessions. Frames need to be both strong and stiff, whilst being light enough to hop around in the sky with ease.
One of the most common materials for multi-rotor frames is carbon fiber. A great many of its physical properties are perfectly suited to the hobby. The only catch is that carbon fiber is known to block radio signals, which is obviously not ideal for a hobby that depends on multiple transmissions. It can be used though, and is often. Just be aware that blocked signals are a possibility.
Frames can also be built at home using aluminum or balsa sheet. But results will vary from manufactured frames, both aesthetically and in terms of flight attributes.
Common is the integration of power distribution circuits into the frame plates. Battery connections are soldered directly to pads built into the material, and power can be tapped at other solder points without the need for extensive, messy wiring. Although by no means essential, a clean setup is satisfying and less likely to fail, making power-distributing frames a worthy investment.
Arms, too, can be built at home using aluminium or hard plastic tubing. However, for many hobbyists, it's preferable to go with pre-built arms. These components are relatively cheap and easy to replace. Therefore, they're generally considered a pre-determined breaking point. In the event of a crash, the arms should be the first to give, preventing damage to motors or expensive electronics on the frame. You actually want them to be a bit brittle, particularly on a heavier, crash-prone quad.
Arms also play a vital role in the fight against vibrations, which can cause a number of different issues. Flight controllers, with their sensitive barometers and gyroscopes, do not generally react well to incessant shaking. Jostle them too much through a poor setup and you could see erratic behavior, sometimes bad enough to cause crashes. Vibrations are also the dread of anyone hoping to use a camera on a multi-rotor. The shaking ruins footage through an artifact referred to as "jello", wavy, headache-inducing distortion formed as a result of progressive scanning.
If the arms you're using have too much flex, they can reverberate and create harmonics that are transferred across the multi-rotor. On the other hand, arms that are too stiff directly pass on vibrations without any dampening, resulting in the same problems. There is a fine balance to be found.
The scale of a multi-rotor is often denoted by the horizontal width of the frame assembly, including its arms. The standard measurement is taken in millimeters from motor to motor through the center of the frame. If a model includes numbers in the title, they probably refer to this measurement. A DJI F450 is around 450 mm across, for example. The F330 is 330 mm, and so on.