If you have read The Physics of Quadcopter Flight, you know that one of the interesting issues with multirotor design and control is a phenomenon called induced yaw. Basically, when you have a bunch of propellers spinning while the multirotor is airborne, the friction between the spinning propellers and the air (called aerodynamic drag) has a tendency to make the multirotor rotate. In multirotors with an even number of propellers (basically everything except tricopters) we make half the propellers spin clockwise and half spin counterclockwise. This way, the induced yaw generated by each prop is canceled out by the induced yaw generated by props spinning in the opposite direction.

If we were to build a quadcopter with all the props spinning the in same direction, the whole craft would rotate in the air and it would be uncontrollable. So… →

This is why we build quadcopters with oppositely spinning motors. Each motor spins in the direction opposite its neighbors so that the quadcopter does not spin while in flight.

With tricopters though, we have an odd number of motors so we cannot use the strategy of fitting oppositely spinning motors to counteract induced yaw. We also cannot achieve yaw control the same way as we do for other types of multirotors, that is, by purposefully unbalancing the spin of the propellers.

Therefore, tricopters are unique in the multirotor kingdom because the rear motor on tricopters is equipped with a servo that allows the propeller to be tilted. By tilting the propeller, some of the thrust is directed sideways and yaw can be controlled.

Tricopters have a servo on the rear servo that allows the rear motor to be tilted side to side and thus allows the craft’s yaw to be controlled.

Every tricopter frame has some kind of servo mechanism for the rear motor and the quality of this mechanism is probably the most important consideration for choosing tricopter frames. On some tricopter frames, the rear servo mechanism is excellent, the servo mechanism discussed in this post, the Rotor Bits Servo Mount from HobbyKing, is quite a nice servo mechanism. On other frames though, like the X900 also from HobbyKing is terrible. So when you are shopping around for tricopter frames, just pay particular attention to the rear servo mechanism.

The X900 servo mount is extremely complex, difficult to assemble, and very fragile. This is not a good design.

Assembling the Rotor Bits Servo Mount

Anyway, let’s get on to the meat of this post, a tutorial on how to assemble the Rotor Bits Servo Mount from HobbyKing.

One of the things that is a bit frustrating about HobbyKing is that, even though most of their products are very nice, like the Rotor Bits servo mount set, they seem to forget that it’s necessary to include instructions with their more complicated products. The Rotor Bits servo mount set is a good example of this oversight. The kit is quite complex, there are a total of eighteen parts, and I have been unable to find any instructions, or even so much as some helpful tips, anywhere.

I hope this page helps others assemble the Rotor Bits servo mount without a great deal of confusion and fiddling.

Contents of the Rotor Bits servo mount set, plus a mini servo.

Step 1

Press the small metal gear onto the servo horn. Then, find the small brass bushing, and small screw. Insert the brass bushing into the metal gear. Finally, use the small screw to fasten the metal gear to the servo horn. Then set the servo with attached gear aside.

Press the small metal gear onto the servo horn.

Put the small brass bushing into the small metal gear and screw it on with the small screw.

The small metal gear attached to the servo.

Step 2

Find the largest plastic piece, the metal shaft, and one of the medium-length screws. Press the metal shaft into the plastic piece. You should see that the shape of the metal shaft matches the shape of the hole in the plastic piece. Then, insert the screw through the square opening in the plastic piece and thread it into the metal shaft.

Press the metal shaft into the large plastic piece and screw it down with the medium metal screw.

The metal shaft attached to the large plastic piece.

Step 3

Find the round motor mount, and small plastic cap. Simply insert the small plastic cap into the motor mount.

Find the round motor mount and small plastic cap.

Simply insert the metal cap into the motor mount.

Step 4

Now we will start putting everything together. Find the large plastic piece with attached metal shaft, the motor mount, the large gear shaft, and the long screw. Put the large gear shaft onto the smaller metal shaft, gear side first. Then put the motor mount onto the large gear shaft. Last, use the long screw to attach all the piece together; don’t tighten the screw too tight or the motor mount will be unable to move. It is probably a good idea to use a bit of thread lock to hold the screw in place.

Find the large plastic piece, large metal gear, motor mount, and long screw.

Step 5

Now take the servo with attached gear and place it into the rectangular tray on the bottom of the large plastic piece. Make sure you insert the servo so that the gears mesh. Use the zip-tie to hold the servo in place.

Place the servo in the Rotor Bits servo mount so the gears mesh.

Attach the servo using a zip-tie.

Top view of the attached servo.

Centering the Servo Horn

There is one last, slightly-tricky, step to getting the Rotor Bits servo mount working correctly. We need to make sure that the servo mount has the correct range of motion. When the servo is centered, we want the propeller to be level to the ground, and we want the motor to tilt 90 degrees in each direction.

The easiest way to adjust the servo mount’s range of motion is to:

1. lift the servo slightly so the gears disengage.
2. tilt the motor mount 90 degrees to one side
3. rotate the servo horn to one extreme of its range of motion to the same side
4. put the servo back down so the gears reengage

Test the range of motion by manually tilting the servo mount. You want the servo mount to tilt 90 degrees to each side.

The servo should rotate ninety degrees to the left

and ninety degrees to the right.

Once finished, you can mount your assembled Rotor Bits servo mount to the rear arm of your tricopter.

Mount the Rotor Bits to the rear arm of your tricopter.

Video Demonstration