Say... if the motor/ESC can handle 30 amps, the on-board micro would measure the current and adjust the ESC and pitch until the desired amount of current is achieved (throttle sets max current from 0 to 30 amps). Got to do some head scratching to map which gets adjusted first... more ESC "throttle" or more aggressive blade pitch. But it sounds like a nice juicy science project.

As I understand it, the fundamental goal of constant-speed props is to tune the pitch of the prop to maximize its efficiency at the particular airspeed the airplane is flying. Measuring ESC input current and voltage will tell you how much power you're putting in, but not what you're getting out, and you need to know both to get efficiency. Using the current-based control is flawed: a valid solution for static thrust at lower power settings would be to increase pitch as much as possible and stall the blades. That loads up the motor but doesn't produce meaningful thrust - or, at least, much less than could be achieved with a finer pitch at a lower amp draw.

"Advance Ratio" *J* (the x axis on the chart) is directly proportional to airspeed

*V*, inversely proportional to propeller angular speed

*n,* and inversely proportional to prop diameter

*D*. Beta is the blade angle, which can be set based on the measured advance ratio to maximize efficiency. Of course, maximum efficiency does not produce maximum thrust.

There's a ton of theoretical stuff out there about variable-pitch and constant-speed props but I don't think it's necessary to test one. The following is how I would figure out how a variable-pitch prop performs:

1. Build a flying testbed with the following sensors:

-Load cell between the motor and airframe to directly measure thrust in-flight

-Pitot tube

-Current and voltage sensor between the battery and ESC

Both the motor and pitot tube should be in aerodynamically undisturbed places as to get as close as reasonable to freestream conditions

Note: strictly speaking, current and voltage measurements are only necessary for the efficiency measurement, though they can also be used to enforce ESC current limits / motor power limits

2. Flight-test the testbed to collect data

-Devise a test matrix of various ESC PWM settings and propeller pitches, with multiple trials for each. Multiply the number of PWM settings by number of pitches by number of trials, this is the total amount of data collection passes you will have to do. This can get big quick so a good idea is to start with coarse steps to get the general trend then fill in the gaps with finer steps if you have time.

-Use a known safe constant-pitch setting for normal flight

-For each point in the test matrix, bring the testbed airplane close to stall speed and engage the desired ESC PWM setting and propeller pitch. Log thrust and ESC input power as the airspeed builds or falls, and allow it to reach a steady speed if possible. Do the same from the maximum level flight speed.

3. Process data

-Output power is equal to thrust times airspeed. Input power is equal to current times battery voltage. Output over input is equal to efficiency, a product of ESC/Motor/Prop.

-Make multiple 3D plots of thrust vs. ESC PWM and airspeed for different prop pitches, efficiency vs. ESC PWM and airspeed for different prop pitches.

-Fit 3D surfaces to each thrust and efficiency plot for each prop pitch setting, get their equations

4. Devise a controller

There are many ways to implement this. The first that comes to mind for me is "Proportional Thrust Control" - i.e. the throttle stick controls the percentage of the maximum possible thrust that is available at the measured airspeed. This would be achieved in practice by:

-Controller knows the equation of the 3D surfaces and the current airspeed

-Controller looks at the thrust 3D surfaces and finds the maximum thrust possible at that airspeed

-Controller knows throttle stick position and interprets it linearly from 0 to 100%

-Throttle stick position serves as a multiplier on the maximum thrust possible to determine the desired thrust

-Controller uses thrust 3D surface to calculate possible combinations of ESC PWMs and prop pitches that satisfy the desired thrust. It then cross-references those with the efficiency surfaces, and picks the combination of ESC PWM and prop pitch (interpolating on the latter) to produce maximum efficiency. This could also be done by finding minimum input power among the acceptable combinations. (Note: This could all technically be done with a single 5D hyper-surface-data-fit-object and its system of 5 equations, but It's easier to visually tune and verify numerous 3D surfaces and interpolate the remaining variables.)

Further additions could include checks to make sure ESC PWM is at minimum (motor stopped) when the throttle stick position is at 0%, adding filters on the airspeed and/or ESC PWM output to damp noise in the system, a kill switch to guarantee motor shutdown, a switch to disable active control and revert to a known good pitch setting, possibly a switch to feather the prop, and enforcing current/power limits.

I have done something similar to this in the past, measuring thrust in-flight with an EDF and a variable outlet area nozzle instead of a motor and prop combo. I ended up stopping after the data collection phase due to the effects being small compared to the noise of the load cell. I'd be willing to donate a flight controller, GPS, compass, and pitot tube of your choosing to the cause if you're interested, as

@CampRobber graciously did for my exploration of the variable are fan nozzle. I feel the need to pay it forward, and think there's potential here for interesting data to be collected.