Multi-rotor design – Building a DIY Drone

Choosing to design your own drone, means choosing motors, ESC's, flight control and frame. In this page we're just going to look at brushless motors and what they will support.

Thrust

Most online hobby retailers will provide a thrust table for each of their motors. I've listed an example table below, along with a commonly used motor.

Example thrust table:
2212ThrustTable

Example Specifications:
KV ----------------------------------------------- 920
Configuration -------------------------------- 12N14P
Shaft Diameter ------------------------------- 3mm
Motor Dimension(Dia.*Len) -------------- Φ28х24
Weight(g) --------------------------------------- 48g
ldle current(10)@10v(A) ------------------- 0.4A
No.of Cells(Lipo) ----------------------------- 3-6S
Max Continuous current(A)180S ------- 20A
Max Continuous Power(W)180S -------- 400W
Max. efficiency current --------------------- (3-10A)>85%
internal resistance --------------------------- 62mΩ

The 2212/920KV motor is classically found in DJI Phantoms I and II. The only part of this information that is not of use is the shaft diameter, because the prop adapter is part of the bell.

The thrust from a quad copter's motors is best considered at a minimum of half of the weight of the aircraft, including batteries. 7.2A is about half the maximum current this motors will take, I wouldn't go to 20A. So the aircraft could weigh 2.6Kg and fly OK.

Weight

2.6 kg is rather heavy and 6 minutes is not really enough flight time. The battery will be operating at 10 C in a hover. That's where the six minutes comes from. Even though we have chosen a 20C battery the battery won't last long used like this. So the aircraft would have to be significantly lighter to obtain a reasonable flight time. A 1 kg craft, would be a much better option.The DJ I Phantom is about this weight. But it doesn't include a camera and gimbal.That's why DJI chose this motor for the phantom.

Weight accessment
1. motors 48g x 4
2. ESCs 30g x 4
3. Propellers 15g x 4
4. 3000mAh 3S battery 240g

Total=612g

The frame, flight control and receiver, will make up the rest of the weight. We had about 400g to work with.This is a fairly tight ask, but do-able.

The camera and gimbal will constitute a payload of ~300g. A reasonable quality gimbal will be 160g – 210g and GoPro Hero 4 about +75g, depending on the model.

In total have a weight of around about 1300 g. This is a about half of that calculated maximum weight. This should take our flight time to about 12 minutes with a camera and gimbal. My experience is, that you can usually take this up to about 15 minutes. As the battery ages and this will decrease again.(note: I don't own a Phantom)

Agian, you can see in this description, why DJI chose this motor for its DJI Phantom I. Many copycat manufacturers, have use this model to produce their own.If you wish you can use this model to produce your own DIY version.But it is more important to understand the process by which you calculate what you require in a motor, propellers etc.

The most important factors here are thrust versus weight of motors and supporting hardware. Air time is not just dependent on the capacity of the battery.The batteries weight increases with its capacity. This is counter-productive.

Propellers

Larger propellers are often more efficient. Large propellers require a large talk from the motor.Larger talk motors usually have a larger diameter. This is because the magnets have leverage over the driveshaft. This leverage converts to talk. Low torque motors produce heat if they can't cope with the talk requied by the propellers. So just applying larger propellers will not usually solve your problem. You need to match the propellers to the motors or vice versa. I'll deal with this calculation in our later article.

The thrust table provided by the supplier is so valuable that it would have to effect choice of motor in the design process. You can make your own but this could be very time consuming.

Conclusion

Please note that the above scenario is one of a few possible scenarios, based on the thrust table. The whole idea is you can build anything you can imagine, to know if it will fly you need to apply the principles above. Good luck and be persistent.

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