Well..errrr...actually...as long as all the forces more or less balance....I nip into the break room and have a cuppa. I have enuff to ponder without worrying my alcohol ravenged brain (whats left of it) about which terms of reference my POF instructor used over thirty years ago. Where do you come up with these issues Z-man? You really need to examine an unclad female form or something at night....or take up Spider Solitaire on the four suit setting.
Could you be just a bit more specific in your frame of reference...which bit on the aircraft are we talking about....and in what context are the terms to be used...aerodynamics, physics, stress analysis...control dynamics....vibration resolution? We talking blades, bearings, spindles, rotor dynamics, performance?