Why are BLDC motors more commonly used in model aircraft engines rather than PMSM motors?

The core issue here is: the difference between PMSM and BLDC. The distinction between the two has been mentioned in my column before. To put it simply.

The permanent magnet synchronous motor (PMSM) can rotate at a constant speed that is synchronized with the power supply frequency, unaffected by load and line voltage. Therefore, PMSM is an ideal choice for high-precision constant-speed drive. The speed/torque characteristics of PMSM are highly suitable for direct driving of large horsepower, low-speed (rpm) loads.

The permanent magnets of the BLDC motor are magnetized and distributed on the rotor in a selected manner, enabling the waveform of the reverse electromotive force (the voltage induced in the stator winding due to the rotor's motion) to be trapezoidal. Thus, a DC voltage with a rectangular waveform can be used to generate a rotating magnetic field with low torque ripple. Since the rectangular voltage applied is easy to generate, for example, any MCU, or the C2000 series DSP, or even the 51 single-chip microcontroller can generate such a PWM wave, the control and drive of the motor become simple. However, it is necessary to know the position of the rotor at a certain angle to align the applied voltage with the reverse electromotive force. The alignment of the reverse electromotive force with the commutation action is very important. Only in this way can the motor operate as a DC motor with the highest efficiency. Therefore, the BLDC motor becomes the best choice for low-cost, high-efficiency applications.

Does PMSM control-wise rank lower than BLDC? Not really. You can also use FOC (vector control), DTC (direct torque control) to achieve control effects and performance similar to those of a DC motor, but the cost is really high and the algorithm is really complex.

Summary: Both are synchronous motors. BLDC is powered by DC, with a trapezoidal reverse electromotive force; PMSM is powered by sinusoidal current, with a sinusoidal reverse electromotive force.

So the question from the poster is: If I have two motors now, how can I determine which is PMSM and which is BLDC?

Can it be observed from the performance? It's not very accurate from the performance perspective. You can first look at the motor driver, whether the power supply is DC or three-phase AC. In one case, you can look at the waveform of the reverse electromotive force.