When you’re calculating aircraft performance, making sure you have a reasonable model of the aerodynamics is only half of the recipe. The other half is the propulsion system.
There are a handful of UAVs out there with jet engines as their powerplant, but the vast majority of UAVs are propeller-driven. Their power source might be an internal combustion engine, an electric motor, or both, but they’ve all got one or more spinning props.
Most test data for propellers is collected on a static stand. You attach the propeller and motor to a load cell, spin it at different throttle settings, and have some nice curves of RPM versus thrust, power, torque, etc.
It’s very tempting to take that static test data and use it directly in your models and calculations, with maybe a density altitude correction or something similar sprinkled on top. I’ve come across that instinctive choice a few times. And honestly, that might work okay for a very rough first-order estimate of thrust and power, especially if you’re trying to estimate performance without having made a final propeller selection.
But if you want a model that’s truly based in physics and reasonably accurate, you’ll need to use something different.
Why?
It’s because a propeller blade is fundamentally a very unique, specialized wing. And as such, if you want to calculate the forces and moments it produces (i.e., thrust and torque), you’ll need to take into account a whole variety of variables.
If you’ve read these emails for a while, you might guess where this is going. When we talk about aircraft aerodynamics, there’s a specific way we work with the forces and moments that strips out a number of those variables. This makes it much easier to evaluate the same airframe in different flight conditions.
We turn those forces and moments into coefficients—essentially dividing the actual force by the dynamic pressure and wing planform. These dimensionless numbers are simpler to work with in many kinds of analysis.
We can do the same thing with propellers. The calculations are a little different, but it’s the same concept. And it allows us to have a much more accurate representation of the propeller in our work.