If you’re like me, you often think about skinny drone propellers. Things like:
- Why are they so skinny?
- Why don’t they put some meat on those skinny propellers?
- Why do all drone manufacturers (besides Modovolo) have skinny propellers?
- Can you use a skinny propeller to shovel dirt?
All of these are important and compelling questions for sure, but let’s get serious here with some science and math…and Victorians.
And that means we need to first talk about the “Reynolds Number” – which has nothing to do with aluminum foil, but it has everything to do with a guy from Victorian England named Osborne Reynolds.
And like all people who lived in the 1800s in Victorian England, Osborne Reynolds had far too much free time on his hands and he used that free time to think about science things. Because that was really the only way to get a date back then.
You see, Victorian science nerds were a lot like rock stars today. You’d come up with some scientific thing (like a principle or law or a number) and you ended up with a lot of right swipes on Victorian Tinder.
Reynolds’s science thing was figuring out how and why fluids flowed the way they did at different velocities. And his “Number” is a nifty way to tell when the airflow changes from laminar to turbulent flow as you increase the velocity of the propeller. Now, why would anyone care about that?
Well, this is an oversimplification but:
Laminar Flow = Good, because laminar flow allows for the all-important pressure differential. You know, where the low pressure at the top of the prop and the high pressure at the bottom of the prop creates lift.
Turbulent Flow = Bad, because if you spin the prop too quickly, then laminar flow changes to turbulent flow creating eddies, separation bubbles, and many other things that disrupt the all-important low /high pressure differential.
And without a pressure differential, there is no lift. And the prop stops working. And the drone falls out of the sky. All bad things. Reynolds Number helps predict when all that will happen.
Well, you say, you should just spin the prop much more slowly. And that’s a good idea, but there is a problem with that approach. The electric motor driving the prop wants to go fast because electric motors are most efficient at high RPMs – like 20,000 RPMS. If the electric motor spins too slowly (like at 5,000 RPMs), it’s inefficient and uses too much power for a given amount of work.
Well, you say part 2, you should make a skinny prop. Because a skinny prop has a much smaller width so there is still (mostly) laminar flow over the prop even at higher RPMs but which is fast enough that the electric motor is somewhat efficient. Good point. And that’s just the compromise that all conventional drone manufacturers make.
And it is indeed a compromise. For a Mavic 3 drone, for example, its skinny propeller spins at approximately 4,000 RPM for hovering and 6,000 RPM for maneuvering. At those RPMs, even a skinny propeller begins to experience turbulent flow (and, therefore, loses efficiency dramatically) and its electric motor is also inefficient. And, because at those RPMS the electric motor is drawing way too much power from the battery to push that skinny propeller, all of this inefficiency translates to low flight time.
So how does a Beefy Propeller, like say, the Modovolo Lift’s propeller, solve this? Well, it can’t. Not if you use the same conventional thinking that leads to the skinny propeller.
You can pre-order your Modovolo Lift here. Only 200 pre-orders available.
