Why do I describe a propeller as a specialized wing?
Each blade is comprised of numerous airfoil cross-sections, just like a wing. But because the blade is spinning (unlike a wing) the velocity each blade section “sees” changes based on how far away from the center hub that section is.
This is why prop blades have substantially more angled cross-sections towards the root, while the sections close to the tip are almost flat in comparison. The tip sections are moving much faster than those at the root, so they only need a small angle of attack to generate the needed thrust. Having a higher load might break the blade anyway—a big part of propeller design is balancing the load distribution across the blade.
What makes propeller physics even more complex than “regular” wing physics is the fact that, while the propeller is spinning about its axis, it’s also moving forward in space. The combination of these speeds is what determines thrust output, as well as power consumption, and therefore your propeller efficiency.
To explain how all of this interconnects, let’s do a thought experiment. Imagine a regular three-bladed propeller that is somehow encased in a big volume of Jell-O. Great visual.
At first, the prop is spinning around the hub but not moving forward at all. It’s generating a lot of thrust and slurping up a lot of power. But because there’s no forward motion, there is no power output, and its efficiency is near zero.
The prop now starts moving forward, still spinning at the same rate. It’s taking little nibbles of Jell-O, and because it’s generating a small power output (i.e., thrust applied at a velocity) the efficiency goes up.
As forward speed increases, each rotation takes a bigger bite out of the Jell-O to move the propeller—the power input creates a bigger and bigger output, which means higher and higher efficiency.
Eventually the propeller will be moving forward at a speed where it’s almost perfectly corkscrewing through the Jell-O: every rotation moves the prop as far forward as it possibly can. This is the point of maximum efficiency.
But if you continue moving the prop faster, the spinning can’t keep up with the forward motion. Each blade starts to block the flow slightly, generating drag. In our Jell-O example, you’d see the propeller start to just mush through instead of leaving the clean corkscrew shape. And because it’s now generating less net thrust and outputting less power, efficiency drops too.
It’s this strong correlation of forward speed, rotational speed, and propeller blade geometry that makes modeling performance more complex. It’s not a straightforward higher RPM = more thrust relationship.