Going subsonic: Why drones have their distinctive hum and how propeller design can help fix it.

Blog 4: Bernoulli v. Newton

What makes drones fly?

There are many forces at play in a drone’s propeller, but two in particular are responsible for lift: Newton’s Third Law of Motion, and Bernoulli’s Principle. Newton’s law explains the most intuitive force; by “pushing” air down with the angled bottom of propeller blades, the air pushes back with an equal force, creating lift for the propeller. Bernoulli, however, describes pressure changes caused by differing speeds. As air flows over the wide, smooth top of a propeller blade, it must speed up to match the straight path of air underneath the propeller, thus creating a high-speed, low pressure volume above the propeller, effectively “pulling” the blade up into the low pressure zone.

These forces act on different parts of the propeller, so they are theoretically separate and can be isolated. Imagine a propeller with a flat bottom, for example. There is no “pushing” force, so Bernoulli’s principle alone could predict the lift potential. The same is true of a propeller with a completely flat top, which is only affected by Newton’s forces. This idea led me to design two propellers, modeled after a standard propeller I designed. One is flat on top, one is flat on bottom, thus I can test which force is more important for a propeller design.

This week, I took my Part 107 test and passed, meaning my “easy” avenues of accomplishment are over. For the proceeding weeks, I will be completing one experiment per week, starting off with “lift factor,” or “push-pull” testing. I spent a lot of time designing and printing propellers this week, and have figured out my test methodology. I have access to the Pilot Institute’s Drone Test stand, which I will most likely use for thrust tests, and I will buy a decibel meter to measure noise. Next week I will be designing toroidal propellers, which have a bizarre design that could hold opportunities for noise suppression. I will also be undertaking my first tests now that I know how to wire motors and have viable designs.

Propeller spreadsheet (A little more in depth but no data yet): https://docs.google.com/spreadsheets/d/1QnVORPgaP6eOXAQWGW8OxpOkJtlu77fsbD5rLiyG7hs/edit?usp=sharing.