I ran a quick test indoors playing with the audio spectrum analyser for my iPad. Quite a cool piece of kit, though I did pay for a decent one.
The plot shows peaks at about ~65Hz, 126Hz and multiples of ~65Hz thereafter suggesting an RPM of about 3780. Kv for my motors is 980 rpm / V which suggests 3.86 volts.
The PWM was set up with 1400us pulses – given the range is between 1000us and 2000us, this suggests 40% power from a freshly charged (12.2V) battery => 4.88V
To be honest, it doesn’t surprise me these two values don’t match. In fact, I think Kv is as good as irrelevant for use with brushless motors, and in fact provides just another obfuscating factor to confuse the user:
Brushless DC motors work much like stepper motors. They are fed a 3 phase signal at a given frequency, and depending on the number of coils / magnets on the stator / rotor, the motor spins at a fraction of that 3 phase signal frequency. The amount of voltage driving the current they draw simply provides the power level required to drive the motors with a load (i.e. propellers) at the frequency defined by the 3-phase signal.
These days, the ESC uses a microcontroller to interpret the input PWM, and uses that simply as a factor to scale the motor rate from 0 to 100% motor spin rate. The ESC drives the motors with 3-phase PWM-like digital signals. They use inductive feedback from the motors to track how much power (i.e. the width of the 3-phase PWM pulses) needs to be applied to the motors to make them spin at the desired rate, regardless of the load (i.e. propeller size / weight).
In the world of brushed motors where the magnets are stationary and the coils spin, I can see how Kv is relevant: the rate the motor spins is directly proportional to the voltage applied. But brushed motors generate huge amounts of electrical noise, and wear out quickly, and so simply aren’t suitable for UAV’s (Unmanned Autonomous Vehicles – i.e. RC boats, planes, cars) of any kind.
IMHO, in the world of brushless motors, the mathematical value of Kv is frankly utterly meaningless, and only provides an arbitrary measure of what kind of battery can be used to provide suitable spin speeds for a given set of motors / propellers:
Kv of the order of 700 – 1000 are suitable for 3 cell / 11.1V LiPo’s with small propellers (8 – 10″)
As the Kv value drops into the say 300 – 700 range, then more cells are required to provide the same RPM – typically though, motors with these Kv values are intended for use with very big blades, where the required rpm to produce the same force is much less.
Obviously all the above is based upon my opinion, so may be utter bollox – I’m more than happy to be proven wrong.
P.S. On the plus side, the single sharp peaks at 65Hz intervals does suggest that all of my motors are rotating at the same speed, and none are obviously duff, which annoying means I still have no idea why yaw compensation is still plaguing Phoebe’s flights.