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Hello Instructables!
We are a team of engineering students that want to get started in the world of multirotors. Our objective is to build a long endurance/high payload quadcopter for industrial applications, and we plan to innovate in the power system area.
So, without further ado here's our first problem. I've searched the internet for a while now and I've struggled to find information on how to choose an appropiate motor-propeller combination. I know the total maximun thrust should be a bit more than two times the the total weight but motor and propeller manufacturers rarely list the "thrust per rpm" of their props. Our initial estimation for the quadcopter's total weight is 3,3 kg. considering 200 g for each motor.
I did manage to find this static thrust calculator but I have no idea how reliable it is since it doesn't say how the thrust is calculated. (http://personal.osi.hu/fuzesisz/strc_eng/index.htm)
So that's about it. Any help would be greatly apreciated, no matter how small. It can be advice, information sources, or even builds that are similar to what we want to make and have already been tested.
Thank you very much for your time!

PS: Excuse me if my english sounds a bit weird, it just isn't my native language.

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I understand what you say, thrust calculations involve a lot science, mainly fluid mechanics, and are out of reach for hobbyists. But then, how do you decide what motors and propellers to use when building a custom quadcoter?

I think it depends on the hobbyist.
A hobbyist building a custom drone from the ground up typically has some goal in mind. Optimizing for that design starts off at the very beginning I think and continues until the end product is produced. Rotors are very generally designed so finding one that matches concepts, specifications, and such isn't impossible.

Taken a bit further, someone only interested in building a drone kits doesn't really care about what the effects to tweaks, especially small tweaks, to individual components have on their builds. After all, deviating from the kit is deviation from the kit, but if they're looking for a high powered trick drone, that's going to look a bit different.

For me, I actually have some distant aspiration to make, design, and/or pilot drones that can operate at great distances performing light industrial tasks, so I can kinda see what you're going for.

Also... one thing i kinda glossed over, the torque limit should be calculated by the forces being applied to the most distant part of the rotor from the central point. lever actions mean that forces applied near the center are multiplied as they get further out away from the central pivot. That's why CD's being spun at thousands of RPMs explode from the outside in. The force being applied on the outside edge due to drag forces is more than the plastic can tolerate.

Yeah... so there's a good reason you can't find that statistic.
Thrust, as generated by a motor and propeller combination, incorporates values from both components.

The first thing you need to look at is the overall RPM of the motor. Motors also place a constraint on torque, but, usually rotors are more limiting for torque, so that's typically where that limit is drawn from.
As far as the motor is concerned, High RPM is better. Maximize the RPMs to the weight of the engine as much as possible. Slip or slippage is not necessarily good, but it is not bad either, as it can act as a fail safe if the aircraft becomes over burdened. You need to weigh the amount of slip with the application and figure out what's correct.

Now, on to rotors... and it starts getting hairy pretty quick.

First, you're constrained by the material that the rotor is made out of. More than likely, you're going to be looking at a composite material. The bulk material properties will impose the first constraint on how much air, and thus how much force, the rotor can with stand. If you missed it, this is the absolute most ideal torque limit of the craft.

The next determinant property of the rotor is the geometry of the rotor and this is where I really can't help. The cross sectional area of the rotor and the total surface area contacting air are the next two bits, but I don't have the tools to tell you anything beyond that.
The cross sectional area that needs to be looked at is the thickest point on a propeller that looks like a tear drop, if the prop is sliced in half. This is because that determines the angle of attack and the angle of convergence of the air currents put off by the prop. There is software for this, which i do not own. This determines the amount of air moved per rotation. If memory serves, my friend who was doing a masters thesis on this stuff, said something like "Thinner is better for higher rotation, but too thin and you're unnecessarily limiting the lift generated.".

Anywho.... hope it helped.