How do safety cages around quadcopter rotors/blades affect lift capabilities?

Question

I am interested in building a quadcopter from scratch.

Because I like to err on the side of caution, I'm considering adding "safety cages" around each propeller/rotor, to hopefully prevent (at least minimize) the chance of the spinning rotor blades coming into contact with someone. Without knowing much about the physics behind how "lift" works, I would have to imagine that cages present two main problems for rotors:

1. They add weight to the copter making it harder to lift the same payload; and
2. They're sheer presence/surface area makes it harder for the spinning rotor to generate lift and push down away from the ground

The former problem should be obvious and self-evident. For the latter problem, what I mean by "surface area" is that I imagine that the more caging around a spinning rotor, the more difficult it will be to lift effectively. For instance, a spinning rotor might have the ability to generate enough power to lift, say, 2kg. But if we were to construct an entire box (not cage) around the entire rotors, with 6 sides and no openings, I would imagine its lift capability would drop to 0kg.

So obviously, what I'm interested in is a cage design that provides adequate safety but doesn't "box in" the rotor so much that it causes the rotor to be ineffective or incapable of providing lift. So I'm looking for that optimal tradeoff of safety (boxing/caging around the spinning rotor) and lift performance.

I would imagine calculating and designing this is a pretty huge undertaking with a lot of math behind it. I'm just wondering if anyone has already figured all this stuff out, or if anyone knows of a way to model this safety-vs-lift-performance trade off in some way.

Answer 1

To what degree are you looking to cage the fans? IP code gives some grades for fan protection, which may help you decide or even find some stock guards. Level 'A' protects against striking by the back of the hand, 'B' protects against striking a finger.

You are correct that the math is complicated; it's more than simply a reduction in diameter - flow around the grating will have a huge impact, especially at the flow rates for the fans.

I would suggest you decide what level of protection you want, purchase the guard for one fan, then setup a test rig where you can weight a fan assembly. Add weights until you can just keep the rig grounded. Add the guard, then remove weight until again, the rig is just grounded. This will give you a percentage reduction in thrust capacity.

I think you will find this method of empirical testing will be faster, easier, and more accurate than mathematical calculations, because you will find it difficult to get the model of the fan guard correct. Even if you could, assuming the guard is close to the blades, there will be an interaction between the fan blades and guard that is dependent on the blade shape.

This answer assumes you don't have access to ANSYS, LS-DYNA, or any other high power CFD tools, but even still empirical testing will be faster unless you already have experience with those tools and a library of blade and guard parts.

Finally, regarding the performance-safety tradeoff - safety is up to you to quantify. Performance impacts of adding guards can be quantified as I've described above. Once you have quantified how much safety matters to you (this is usually done with a failure mode analysis), then the tradeoff should become clear. Quantifying safety is usually one of the hardest problems for every engineering project.