How calculate the thrust a motor

To calculate thrust, you must know as a minimum
* prop geometry
* airspeed

In addition, you must also know any one of the following sets of variables:
1) RPM
2) voltage, motor constants (Kv, Io, R)
3) current, motor constants
4) electrical power (voltage*current), motor efficiency.

For the static thrust case, one can also estimate the thrust from the power and prop diameter, using the formula:

T = [ (eta*P)^2 * 2 * pi *R^2 * rho ]^0.3333

T = thrust (in Newtons)
eta = prop hover efficiency = 0.7-0.8 is typical for unstalled low-pitch props
P = shaft power = voltage * current * motor efficiency (in Watts)
pi = 3.14159
R = prop radius (in meters)
rho = air density = 1.22 kg/m^3

Plugging in eta=1 will also give the absolute upper limit on the thrust obtainable for a given amount of power and a given prop radius.

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