Assembly: Propeller Balancing

Assembly: Propeller Balancing

In an aircraft as small as the crazyflie, the biggest enemy of great performance is vibration. The engineers at bitcraze realized this and thus gave some optimization tips for maximizing performance in the tips and tricks section of their wiki. It is with this in mind that I began the assembly of my crazyflie.

The first order of business was to get organized. My plan was to use one of the five motors to test the balance of the propellers and group them accordingly. Naturally, I needed to find the motor with the least vibration and then the propellers (both clockwise and counterclockwise since there is a distinction) with the best balance. I needed a AA battery (an Apple NiMH rechargeable fit the bill), some paper, a pen, and a little bit of tape. I drew a grid on some paper to help me organize the task.

paper grid for propeller balancing
Determining which motor had the least vibration of the five was a little tough but I think I choose well. It was simply a matter of taping the negative lead to the battery anode (negative or bottom) and then touching the positive lead to battery cathode (positive or top end). If I held the motor ever so loosely between my thumb and forefinger, I could just barely feel the vibrations as it spun. I did this three or four times with each motor until I was convinced I’d found the one with the least amount of vibration.
selecting a motor

Once I had my “test” motor, it was time to organize the propellers. There are five of each propeller. Five counter-clockwise and five clockwise. This makes two complete sets and one backup for each direction. My clockwise propellers were clearly marked with an ‘A’ near the center, so it was easy for me to tell them apart. The bitcraze assembly wiki page mentioned the propellers can also be differentiated by the curvature of their leading edges. I quickly group the propellers into the counterclockwise (CCW) and clockwise (CW) sections of my paper grid.
counterclockwise propellersclockwise propellers

The direction differentiation complete, it was time to start testing for vibration. On the bitcraze forum, one member (orcinus) posted a rather hardcore tutorial on precisely balancing the props. I didn’t have the equipment he used readily available and I decided I didn’t need such perfection, so I just used the “balance by feel” method I used to find the motor with the least vibration. I held the motor loosely between my thumb and forefinger and felt for vibration and listened for motor noise. I found it surprisingly easy to tell which propellers were better balanced than others, so I organized them into ‘A’, ‘B’, and ‘C’ string groupings with ‘A’ being the most balanced (least vibrating) and ‘C’ being the worst balance (most vibrating). I used the bottom half of my paper grid to help group the props.
propellers grouped into categories

After the work to get my propellers categorized according to balance, I decided I’d go ahead and use the ‘B’ string props to start with, since I figure I’ll have a number of crashes before I become a well-practiced pilot. There’s no reason to beat up my best propellers when I could be saving them for later, right?

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