title_body: How useful is knowing the torque generated by a motor prop combination? I'm designing a series of small drones for use in a swarm configuration in combination with terrestrial / marine / submarine robots.  As part of the process, I've designed a test rig to measure the thrust performance of various motor / propeller combinations.  The general layout of the rig is shown in the Sketchup diagram below:
  
   (click to enlarge)
  
  The motor mount is attached to a plate, which is bolted to a MGN12C linear bearing block mounted on a 100mm rail. (If anyone is interested, the details of the MG series linear bearings can be downloaded here).  This particular bearing was just one I happened to have available.
  
  At the other end of the plate, a load cell protrudes through a slot, and this is used to measure the thrust force generated by the motor / propeller combination.  The mount is designed to allow the load cell to be easily changed (I have quite a few 1kg, 3kg, 5kg, and 10kg load cells).  
  
  The data, together with temperature and vibration measurements from an accelerometer, will be gathered using an Arduino (or possibly a Raspberry Pi if I decide to integrate the rig with other test gear).
  
  The length of the plate means there is ample room to mount the ESC between the motor mount and the load cell.
  
  
  
  It occurs to me that it would be fairly straightforward to modify the rig to allow the twisting force, or torque, generated by the motor / propeller combination to be measured.  Probably the simplest way with the parts I have available now would be a pair of load cells fitted between a modified motor mount and the plate attached to the linear bearing as shown here:
  
   (Click to enlarge)
  
  Minimal modification to the electronics would be required to add this to the design, and I have more than enough load cells available to add this to the rig.
  
  
  
  We had to re-design arms in a previous model when we found the forces generated by the motors caused them to deform while testing the unit on a static rig, which is why this modification appeals to me.  However, I wonder whether anyone else has any experience of using this kind of data in drone design.
  
  So, how useful is it to know the torque generated by a motor prop combination when designing multi-rotor drones?
upvoted_answer: Not very useful.
  
  If you're designing at the absolute limit of what a frame can handle, such that it's as light as possible, then maybe you'll want to know the maximum torque, but you should be designing for orders of magnitude more strength than that (It's not hard, small BLDC motors aren't very torquey).
  
  There are control algorithms that take into account frame torque and how the forces are applied. I forget off-hand, but it was a matrix-solving control algorithm, as opposed to PID. Unless you're writing your own flight control software, that's not an issue.
  
  When changing motors, it may give minor insight into tuning the yaw axis (these motors have more torque, so less P-Gain may be needed), but yaw is done by applying torque to a propeller, so again unless the motor can barely spin the propeller (this should not be the case in a design), then this isn't a concern.
  
  Where you may see an advantage is when computing efficiency - ie how much torque do I have to apply to propeller x to get y grams of thrust? However, this is generally done by using current figures since those are more relevant to power system design.
  
  Another possibility is how much braking/accelerating torque a given ESC can provide - high performance machines needs rapid changes in motor RPM, so a given ESC may be better (due to FET latency, resistance) at control algorithms than others. This would be interesting data to see, but I don't think any ESC is necessarily better than another, since braking is just "FETS on" and accelerating is just pushing the control algorithm to its max. Noisiness is much more important than acceleration torque in practice.