The Wonderful World of
A Quick and Dirty Tour
It turns out there are many ways in which to strap propellers to something and make it fly. Most people are now familiar with the quadcopter; you can buy one for $50 at just about any toy store. Now drones with six and eight rotors are also becoming more prominent, especially in the professional video and photography world.
There are, however, more configurations of multirotor craft than you may think. In this article I’ll go over both the familiar and unfamiliar, ordered by how few rotors they have.
First I’d like to clear up some confusion between helicopters and multirotor craft. While most of us recognize there’s a difference between multirotors and helicopters, helicopters can have more than one main rotor. There are tandem helicopters such as the Chinook and coaxial (multiple rotors on the same shaft axis) helicopters such as the Ka-50 Black Shark.
In general these are not considered “multirotor” aircraft. I’m just pointing it out here because some multirotor aircraft only have two rotors, but are not helicopters in the traditional sense.
Bi-rotor or “Bicopter” Craft
This design uses two rotors on either side of the craft’s main body just like those small aircraft in the Avatar film. It’s probably that movie’s fault that people are even interested in the design in the first place. The closest full-size craft design is probably the V-22 Osprey tiltrotor, which has some limited vertical takeoff and landing ability, but faces the rotors forward during normal flight.
Several people have managed to build working bicopter drones, but they’re pretty unstable and very tricky to get working at all, so this design is fairly rare. The two rotors counter-rotate to cancel out each other’s torque.
Basically a flying tripod, providing the minimum of three stability points. Each rotor is usually 120-degrees from the next, but there are flying t-shaped drones as well. Compared to a quadcopter, the tricopter is a less-able aircraft in general, except that it is much better at yawing (or turning) at speed since it uses a servo on the rear rotor to turn even faster. This makes it popular with some pilots that participate in drone racing with tight and twisty tracks. Most tricopters that you’ll see are nothing but the bare bones needed to fly, as they try to be as light as possible. The rear rotor usually rotates counter to the front two.
Quadcopters (X4 and Cross/Plus)
By far quadcopters are the most popular drone design. Four arms, usually 90-degrees apart, each with a rotor. Two rotors counter-rotate relative to the other two. A rotor will counter-rotate with its neighbor. Quads can use an X or cross configuration. With the cross configuration one rotor is clearly the “nose” of the quadcopter.
The Y4 Design
This is a tricopter frame, but the rear motor has a counter-rotating coaxial rotor driven by a separate motor. This design does not use a rear servo at all and has more lifting power for almost no weight penalty. This is still predominantly meant for racing.
Take the Y4 and make the two rear rotors sit on the same boom tilted away from each other, both facing up. Now you have a V-Tail. No one really uses this, to be honest.
Five rotors. Not much more to say really. Good luck actually seeing one of these.
This is a configuration that’s becoming more and more popular. Usually you’ll see hexacopters towards the serious, professional end of camera drones, but these days even nano multirotors can come in a hex design. Hexacopters are usually great fliers and have better lifting ability than quads. They can usually also land safely if one rotor fails. Not so for a quad copter.
Professional hexacopters are huge, however, and need complicated folding frames for transport purposes. The larger ones are also hella expensive.
Remember how the Y4 had two coaxial rotors on the tail? Do that for all three arms and you get a Y6 design. Presto, you get almost as much lift as a hex with the space-saving of a tricopter. Coaxial rotors are less efficient though, so power usage is less than optimal.
Add two more arms to a hexacopter and you have an octocopter. More lift, more size. You get the idea.
It’s like the Y6, but with an extra arm. Same issues apply as with the Y6.
So Many Choices
Which configuration should you go for? Well that depends on what you want to do. The hex and octo configurations are the most stable, efficient, and powerful; they are also huge and expensive. Using a Y6 or X8 coaxial setup can deal with the space issue, but diminishes the other two factors a bit. So really, it comes down to whether you are comfortable carrying the big drone designs around or not.
This isn’t the end of the road either. People are constantly coming up with more wacky ways to strap propellers to things, so I’m sure I’ll be updating this article sooner rather than later.
Whatever design ends up taking your fancy, just make sure it will get the job done. There’s nothing worse than having spinning propellers in all the wrong places.