- What are stick position stability and stick force stability?
- What are the difference between them?
- How will changing trim of an aircraft affect them?
Stick position stability.
In its broadest definition, stick position stability means: a larger aft stick deflection is required to maintain a lower constant airspeed. Or in an equation: $d\delta_e/dV > 0$, with $\delta_e$ being the elevator deflection, + = Trailing Edge Up
An aeroplane with stick position stability is easy and safe to control. If the pilot wishes to fly at a lower velocity, they only need to deflect the stick further aft in order to reach the larger Angle of Attack required to maintain lift. An aeroplane with stick position instability will react with an ever increasing nose-up deflection, requiring corrective opposite stick action.
Stick force stability.
An aircraft has stick force stability if deflecting the stick further away from trimmed position, requires a larger force: the stick feels as if it is loaded by a spring. Easy to achieve with irreversible flight controls, which are loaded by a mechanical spring. More difficult with reversible flight controls, where the airforces are directly fed back to the stick: at lower airspeeds the force gradient must be high enough to result in clear stick trim conditions (friction in the control loop plays a role here), at higher airspeeds the aircraft must still be controllable.
Stick position stability is a necessary condition for stick force stability, and stick force stability is an additional requirement for safe and easy control. And it is this stick force stability in a turn that required the implementation of MCAS in the B737MAX: stick force stability must be demonstrated in all flight states in order to obtain certification.