“Switch” quadcopter motors?

[mrlewp87]

Short story; a while back the wife acquired a “Switch” quadcopter, little or no cost. Recently I had a renewed interest in. Somehow I managed to stick it onto a low power line to a utility light. As a result it now needs two motors.
Many hours of searching over a couple of weeks, and the only thing found was 1 Youtube on the exact model Switch as mine. The author stated that it was exact copy of the Syma X5UW. I thought had gotten lucky, and could now search for Syma motors. They aren’t the same motors.
I’m looking for a size 16 x 7 mm, but no clue as to anything else than the size. So the answer finally came to me. Replace all 4 motors. Same specs, 2 A’s, and 2 B’s. Alas, it’s not that simple. I’ve seen 16 x 7 mm motors ranging from 40k to 70 thousand RPM’S. Most are 50k rpm. Again, no clue what the originals are.
Advice on how to choose? It’s powered by a 3.7 V LiPo battery, and does carry a camera.

 

[wafflejock]

There are some methods for working out what the original kv was but need a few pieces of electronics test kit to figure that out:

Text/Image version

Tutorial: How to measure the KV of a brushless motor – fishpepper.de

I realize not everyone has a $500+ scope sitting around but even the relatively very affordable $35 or $45 ballpark "DSO oscilloscopes" can easily display a 1ms-2ms pulse at 50Hz (50 pulses /cycles per second) without a problem, so I think for measuring motor frequency would also probably be sufficient. Pretty sure most of them are rated up to 1MHz (maybe only good for say half or less than that but still 500KHz or 250KHz should be sufficiently fast to see the width of the pulses (peak to peak or trough to trough, from which you can derive the actual frequency/rpm)... As shown in the video a multimeter that supports frequency counting or any other kind of tachometer will work too but think handy having an oscilloscope to be able to probe high frequency low power stuff I'm usually tinkering with.

 

[wafflejock]

You can also pop over to ecalc.ch and put in whatever values you know or can measure, prop size rough prop pitch (might be matter of finding similar props online for getting those values though) and then see what it spits out regarding flight time expected and power to weight etc. and see if it makes any sense. In general lower kv motors have more torque and so are better at picking up speed or changing speed in general but sacrifice top end RPM for that extra torque, on the opposite side of the coin a high kv motor has high top end but sacrifices some torque/ability to accelerate/decelerate the props/motors.