Originally Posted by
FCeng84
Let me give two examples:
On 737 the maximum elevator displacement with full column input is either full travel if the elevator actuators are physically capable of pushing the elevators hard enough to get there or where ever the aerodynamic hinge moment balances out against full elevator actuator force capability. In both cases, the control system is trying to move the elevator as far as is physically possible but in high dynamic pressure case it has reached an actuator force limit.
On 787 the maximum elevator displacement with full column input is limited by a control system command limit schedule that is a function of flight condition. (I am simplifying here by ignoring for this discussion any envelope protection functions.) The elevator actuators may be capable of generating more force than required to get the surfaces to the command limits, but they are not allowed to. At low speeds the command limits are such that full elevator travel is achievable. At higher speeds control law limits are chosen to balance between (on the low end) elevator displacement needed to achieve pitch control power to meet maneuver requirements and (on the high end) deflections that would generate higher structural loads than necessary. This scheme allows the elevator limits to be tailored to provide the pitch control power needed, but no more so that the structure can be optimized for minimum weight. This is one of the ways in which FBW control is able to deliver improved performance.
The story is always a little more complicated than one might at first think.
Also, 787 do not need any MCAS
Instead of a 737 MAX Boeing should have built a 787 MIN
