How can I find the transfer function of a quadcopter's motor?
Operation dynamic rms current, I (t) is proportional to torque of rotor blade profile which has lift and drag.

V is proportional to no load RPM but with rotor blade obviously is affected by RPM reduction from load. Where the HP of the motor ought to be matched to the optimum energy efficiency for desired lift at some RPM as efficiency drops with rising lift forces.

Start surge current is Vrms/R (coil DC resistance which is often 100x idle current at low RPM or 10x max HP current .

The dynamic transfer function depends on which variable you wish to control.

{RPM, lift force, rate of change of lift ("jerk") and velocity of roter blade tip}
- pick one then ask again.


then include all options for rotor blades, rotor efficiency and battery ESR, motor pole design, K/V and BDC controller budget, which may or may not adversely affect transfer function then specify 
what transfer function do you expect ? vector state, Torque , power etc, spectral (s) , also what functions do you understand?


? If you can create a sensor to measure output of any parameter, and measure input ( Vrm, I, P) then you can measure the transfer function by ratio analysis of the data.  Do you know how?