Automating RC Motor Efficiency Testing

Small brushless motors and LiPo batteries are one of the most impressive bits of technology popularized in recent years. Just a few years ago, RC aircraft were powered by either anemic brushed motors or gas. Quadcopters were rare. Now, with brushless motors, flying has never been easier, building electric longboards is simple, and electric bicycles are common.

Of course, if you’re going to make anything fly with a brushless motor, you’ll probably want to know the efficiency of your motor and prop setup. That’s the idea behind [Michal]’s Automated RC Motor Efficiency Tester, his entry to the 2016 Hackaday Prize.

[Michal]’s project is not a dynamometer, the device you should use if you’re measuring the torque or power of a motor. That’s not really what you want if you’re testing brushless motors and prop configurations, anyway; similarly sized props can have very different thrust profiles. Instead of building a dyno for a brushless motor, [Michal] is simply testing the thrust of a motor and prop combination.

The device is very similar to a device sold at Hobby King, and includes a motor mount, microcontroller and display, and a force sensor to graph the thrust generated by a motor and prop. Data can be saved to an SD card, and the device can be connected to a computer for automatic generation of pretty graphs.

Brushless motors are finding a lot of uses in everything from RC planes and quadcopters, to robotics and personal transportation devices. You usually don’t get much of a data sheet with these motors, so any device that can test these motors will be very useful.

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9 thoughts on “Automating RC Motor Efficiency Testing

  1. Yea but that setup measures only static thrust, which is quite different to what you get when the prop is in fast-moving air, i.e. low-slip, lower-thrust, higher-speed operation where differences in propeller aerofoil design become quite important for efficiency.

    I guess this is still pretty useful for quadcopters, but I think it’d be quite misleading for a fixed-wing engine.

    1. The whole “moving-air” stuff is relevant for quadcopters aswell. Well, at least the small racequads because those tend to move very fast quite often. But if you’re rolling a large flying machine to carry a camera and hold it as steadily as possible, then yes, knowing static thrust should be enough.

    2. Having built similar testing rigs I can assure you that what you learn about motors and props from one of these rigs is useful. When I find a good motor/prop/voltage level using my static rig its a very good indicator of how that combo will behave in real use. Inefficient combos tend to always do poorly in real use. Efficient combos tend to always perform well in real use. Granted, “perform well” is subjective. In my case it’s “the plane feels responsive and doesn’t feel like I’m fighting to keep it in the air.”

      In other words, I’ve never found myself saying “ah, this motor looked so good in testing! If only I’d been able to test it in a low-slip situation!” I’m not saying that those factors don’t matter, they just don’t matter very much.

      The only “real” way to test a motor and prop combo for a fixed wing airplane is to build a sophisticated wind tunnel. You’d need to be able to repeatedly and accurately test motor/prop/voltage combos in different air pressures, wind speeds, temperatures, humidity levels, and whatever else comes into play atmospherically speaking. You’d also need simulate the interaction between the human, transmitter, radio waves, and receiver. Maybe even simulate what happens when people are talking to the pilot of a plane. The goal would be to truly know the efficiency of a motor and prop right? So you’d want as much “real” data as possible so you could know if the motor is garbage or if the motor might be influenced by other factors.

      Consider the difference of complexity and cost between the most accurate measurement setup and a simple static thrust jig. Is the data that the costly and complex system provides worth the cost and complexity used to obtain that data? No. Not when we’re talking about radio controlled aircraft.

  2. I’m building something similar to this:
    but rather than reinventing the wheel, mine is just a swing arm that pulls horizontally on the guts I took out of a cheap keyring “fish scale” type luggage scale.
    it’s not super accurate (within 10g) but is good enough for giving an comparative indication of the lift performance of various prop / motor / speed controller / battery combinations

  3. I’ve used the exact scale sensor in a project before. It’s actually a very clever design. The two overlapping holes actually create a rigid parallelogram. On one of the sides a strain gauge is glued. The parallelogram construction makes sure you’re only sensing the force perpendicular to the surface. (Maybe somebody else can explain better) This is also how a scale works. It does not matter *where* you put the weight on the scale. By using this sensor, Michal can get rid of the lever construction.

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