Blog 7: Pardon Our Dust
Going subsonic: Why drones have their distinctive hum and how propeller design can help fix it.
Blog 7: Pardon Our Dust
This week, I had no time for project-specific tasks, though I spent the most time working of any week so far. As I alluded to in my last blog, the Pilot Institute was moving offices over my Spring Break and this week. I ended up helping a lot, loading the trailer and van for moving, bringing boxes to their designated places, building furniture, and tearing sets apart for shipping. I was at an office an average of five hours per day, every weekday, a total of around 27 hours, half spent doing pretty heavy labor. Needless to say, I’ve had a physically exhausting week and have been separated from my work computer and printers, meaning no tangible progress on my project.
What I did accomplish on my own was shopping for the drone parts I need. I showed off the drone frame I purchased, but that’s only one of the parts, as well as one of the cheapest. I will need many more parts, including:
- Battery
- Four Motors
- Four Speed Controllers
- Main Flight Controller (and Sensors)
- Radio receiver and Remote Control
I will be borrowing the Remote from Pilot Institute because they cost upwards of 300 dollars minimum and much more for a high quality one. The receiver, too, will come from Pilot Institute since I will thus be able to avoid figuring out connectivity issues. Most of the remaining parts are mass-produced or “commoditized,” meaning that they are widely available and thus indistinguishable between brands. For this reason, as well as advice from PI’s drone experts, I will look for the cheapest possible parts for each specification. Motors and Batteries are cheap from the supplier, as Temu offers them.
Batteries have a few specifications to balance. The first, logically, is the energy capacity, measured in mAh, which affects how long the drone can keep flying. Storage is the biggest driver of weight as well, so balancing battery weight and flight time is important. I am looking at around 1300mAh batteries because they are recommended for light drones like mine.
The next measurement is the charge/discharge rate, or C-Rate. It is measured in C, which represents the number of times the battery can be discharged per hour. Very abstract, I know. To explain with an example, a 2C rating means the battery can fully discharge in 30 minutes safely (2 times per hour). The reason for the C unit is that hours are also found in the mAh rating. mAh * Charges per hour = mA. If a battery needs to give a lot of power, like mine will, a high C value is required. The battery I’ve used to test so far has been 80/160C, though that is unnecessary for non-kinetic (not crashing a lot) uses.
Finally, Voltage indicates how much force can be given to the Motor, more on that later. I’ll be looking for around 12V, though many batteries work fine with less. My test battery is 11.1V, simply due to the standard cell it uses. For more information about that, look up “3S1P,” the battery layout I will probably be using.
Motors also have a few measurements. They are stator diameter, stator height, and kv rating. The diameter and height of the stator affect its performance and its responsiveness. A wide motor has better cooling, making it run better, and may have better bearings due to the better size. A tall stator will provide more torque thanks to containing more and larger coils. Both of those must not be too large, though, to avoid poor responsiveness. The other measurement is kv rating, which means rpm of the motor per Volt applied. A high kv rating means high speed/low torque, so it must be balanced for the propeller size and thrust needs. For a small propeller like what I’ve made a 2300kv motor has performed fine. Also, though it looks like kV, or kiloVolt, kv actually stands for konstant-velocity in German. Fun fact!
Most of the batteries I found are around 20 dollars or less, and Motors run about 6 dollars to 10 dollars each at the Temu price point. Race motors, which are rated for higher speeds and voltages, are around 40 dollars each, but I do not need–and cannot afford–high performance and durability for the size of my drone. Speed controllers are needed for each motor, so I will buy four of the same brand for around 7 dollars each. The flight controller is the easiest to shop for, as you only need one per drone (think of a motherboard). They cost around 20 dollars each, so I’m happy only to need one. In total, the pieces I am buying will be around $120, not including the controller and receiver. Despite technological development, drones are still expensive for my budget.
My parts should arrive within two weeks, within which I will mostly be finishing my propeller testing. Last blog’s testing was much scarier than I described, which has motivated me to build a more purposeful rig that I don’t need to be nearby for testing. I’ve fully moved into the new office now, so progress will continue as usual next week.
Propeller spreadsheet (Unchanged): https://docs.google.com/spreadsheets/d/1QnVORPgaP6eOXAQWGW8OxpOkJtlu77fsbD5rLiyG7hs/edit?usp=sharing.
An amazing page about the specifics of motor performance that I did not discuss here: https://oscarliang.com/motors/#Motor-Size-Explained.
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