What kind of drone should I build?
It sounds obvious, but believe it or not, a lot of people venture into a project like this with one thing in mind: "big!". This is completely the wrong approach to building a multicopter. Big is expensive, big is also less stable, and moreover, when something goes wrong, big causes more damage and is harder to repair. Ask yourself what your purpose is. Is it for photography? Videography? Fun and hobby interest? What will it carry? The example in this book is based on a standard videography drone that carries a camera between 2 and 7 pounds in weight. This covers everything from a camcorder to a small DSLR camera.
How many rotors should it have?
There are many configurations, but three of these rotor counts are the most common: four, six, and eight (quad, hexa, and octo-copters). The knee-jerk response of most people is again "big". It's about balancing stability and battery life. Although eight rotors do offer more stability, it also decreases flight time because it increases the strain on batteries. In fact, the number of rotors in relation to flight time is exponential and not linear. Having a big platform is completely useless if the batteries only last two or three minutes.
Redundancy versus stability
Once you get into hexacopter and octocopters, there are two basic configurations of the rotors: redundant and independent. In an independent (or flat) configuration, the rotors are arranged in a circular pattern, equidistant from the center of the platform with each rotor (as you go around) turning in an opposite direction from the one before it. These look a lot like a pie with many slices. In a redundant configuration, the number of spars (poles from the center of the platform) is cut in half, and each has a rotor on the top as well as underneath. Usually, all the rotors on the top spin in one direction, and all rotors at the bottom spin in the opposite direction. The following image shows a redundant hexacopter (left) and an independent hexacopter (right):
The advantage of redundancy is apparent. If a rotor should break or fail, the motor underneath it can spin up to keep the craft in the air. However, with less points of lift, stress on the airframe is greater, and stability is not quite as good. If you use the right guidance system, a flat configuration can overcome a failed rotor as well. For this reason (and for battery efficiency), we're going with a flat-six (independent hexacopter) configuration over the redundant, or octocopter configurations.
The calculations you'll need
There is an exorbitant amount of math involved in calculating just how you're going to make your multicopter fly. An entire book can be written on these calculations alone. However, the work has been done for you! There is a calculator available online at eCalc (http://www.ecalc.ch/xcoptercalc.php?ecalc&lang=en) to calculate how well your multicopter will function and for how long, based on the components you choose. The following screenshot shows the eCalc interface:
Overwhelmed? Don't be. That's why this chapter is so long. We're going to walk you through these choices step by step (as well as how to adjust choices based on the results).
