title_body: How useful is knowing the torque generated by a motor prop combination?
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.