If you are a watcher of the world of drones, or multirotors, you may have a fixed idea of what one of these aircraft looks like in your mind. There will be a central pod containing batteries and avionics, with a set of arms radiating from it, each of which will have a motor and a propeller on its end. You are almost certainly picturing a four-rotor design, such as the extremely popular DJI Phantom series of craft.
Of course, four-rotor designs are just one of many possible configurations of a multirotor. You will commonly see octocopters, but sometimes we’ve brought you craft that really put the “multi” in “multirotor”. If the computer can physically control a given even number of motors, within reason, it can be flown.
There is one type of multirotor you don’t see very often though, the trirotor. Three propellers on a drone is a rare sight, and it’s something we find surprising because it’s a configuration that can have some surprising benefits. To think about why, it’s worth taking a look at some of the characteristics of a three-rotor machine’s flight.
If you think for a moment about a typical small helicopter, it will have a tail rotor. The tail rotor is there to provide a sideways force to counteract the natural tendency for the fuselage to spin in reaction to the rotation of the main rotor. With a twin-rotor helicopter such as the famous Chinook military aircraft, the tail rotor is superfluous because the tendency to rotate imparted by the front rotor is counteracted by an equal but opposite force from the contra-rotating rear rotor. If you take the counteracting forces in a Chinook as analogous to those on a multirotor, you will then understand that equal numbers of contra-rotating propellers cancel any tendency for the craft to rotate, and allow the craft to be flown .
Now imagine the same interplay of forces in a trirotor, and you will instantly appreciate that it has an odd number of propellers, and thus it will have an excess of rotational force that is not counteracted by another motor. A simple three-rotor design will therefore naturally spin in flight, and be next-to-useless as an aircraft without some means of counteracting that rotation. In the simplest of flyable trirotor designs one of the rotors is simply mounted at an angle to vector some of the force sideways in an analogous manner to the tail rotor on a small helicopter. This has the desired effect of creating a flyable machine but offers no advantages, and a few disadvantages, over a four-rotor design.
Where three-rotor craft come into their own though is when instead of being mounted at a fixed angle, one propeller is mounted at a variable angle. When the force counteracting the rotation is under variable control, this ability for the rear propeller to yaw gives the craft some of the characteristics a fixed wing aircraft gains from its tailplane. The result is that tri-rotor craft can deliver a more stable and more agile flight than their multi-rotor cousins, at the expense of an extra servo with its control circuitry and software. The additional benefits of the layout are that with fewer drive motors it can be more energy-efficient, and with a larger gap between rotors it can have larger propellers and provide a more unobstructed view for a camera.
So given these advantages, why do we rarely see a trirotor? There are probably several factors behind the proliferation of quadrotors as the most common configuration. The first is simply market inertia: fashion, if you will. The majority of customers now expect a quadrotor, so the manufacturers make the machines they want. Then there is the cost of fitting that yaw servo. It’s a slightly complex mechanism which probably requires a few precision parts, so if you are a factory in China making millions of them you are likely to pick a design that saves you a few cents. If the extra motor on a quadrotor is cheaper than a servo and linkage, then you’ll make quadrotors. And finally, there is a reliability angle. If you crash a quadrotor, as you inevitably will, all its arms are equally strong. The yaw servo is an expensive weak point on a trirotor, so in the event of a crash there is a good chance that it will be first to go.
All is not lost for the trirotor though. If you fancy owning one then you can build one yourself by following this Instructable. We’d suggest becoming proficient at flying a quadrotor first though, otherwise the shop selling yaw servos is going to like you a lot.
Thanks [Jared] for the impetus for this piece.
Header image: Toglefritz [CC BY-NC-SA]