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I have seen many tiny brushed drones, about an inch from motor to motor, and was wondering whether it would be possible to create a brushless drone that small. Are there any brushless motors available that are extremely small, ones that could fit on the size of drone below? And is there anything else to consider if making a drone so small?
(I realize that you could put oversized motors on a small drone but I am asking about motors that would be proportionally normal)
Theoretically, I don’t see why we couldn’t make current motors even smaller than they are today - we already have brushless motors with stators less than 10mm across (for example these 0802 motors), and we could change the format by using inrunner motors instead. However there are certain factors that we need to consider.
The first of these is that we can’t just have the motors - we also need the ESC boards. This is either done with a 4-in-1 ESC or 4 individual ESCs. This adds complexity, weight, size and heat buildup.
We must also consider whether or not these negatives outweigh the positives. This is up to personal opinion, and in some instances it may well be worth it, however with miniaturisation comes increases manufacturing complexity with smaller tolerances.
These are all important factors to consider: is it possible and is it worth it.
I believe it is possible, however finding enough demand may be a limiting factor.
For toy drones, almost the entire thing is the power system. So a smaller drone needs less battery, smaller speed controllers and a smaller frame. The only part that doesn't change is the receiver and flight controller, which are already tiny - so the drone can be shrunk until you reach the limits of motor miniaturisation or the Rx/FC become a significant part of the weight.
Brushless motors tend to be used on FPV quads, where the extra power is useful. They have to carry a camera and video transmitter (and more complex brushless ESCs). There's no point making the motors smaller because they need to be large enough to carry the payload.
It is also worth mentioning that the smaller the motor is the less convenient it is to change propellers. If the motors are so tiny that threaded holes won't fit the only option is to press fit the props. Due to the construction of brushless motors it can be challenging to make a shaft that is both light and can withstand the force of push. If you can't fit the propellers properly they can slip and make the craft unflyable, but it's also likely that the shaft gets pushed through the motor.
At the top of this page (sorry no direct link to the product) they have a 0.15g brushless motor. Unfortunately it only produces 0.4g of thrust, so it can barely lift itself. To maintain a thrust to weight ratio of 1.5:1 you would need to find propellers, battery, flight controller, and radio electronics that weigh 0.4666g. There may be more tiny brushless motors, but it seems that at least with this one, the benefits of a brushless motor come down with size.
