Originally Posted by
Trim Stab
I have to say that I thought that FCS would be able to take care of any anomalies in the aero-design.
Based on my involvement (management not engineering) with some unstable designs I believe a flight control “system” is limited by the slowest element in that system. Looking at the inputs to the FBW system you need to be able to see the change first in the sensors. Then the system needs to react to the sensor inputs. The computers need speed, but the limiting factor in many cases is the flight control actuators. Hydraulic actuators are not capable of instantaneous movement and in the pitch axis that tends to be the limiting factor.
Another limiting factor is the airframe structural rigidity. The inertial sensors providing inputs to the FBW computers are ideally located at the aircraft CG, however, there will still be some input from the structure bending under load. If the FBW system has very high gains to keep up with large amounts of aerodynamic instabilities the structural response can be large enough to affect the sensor inputs. In the extreme this “feedback” can cause the system to diverge (not good). It is also very important that the actuator mounting and connection to the flight control surface be as rigid as practical as it can have a similar effect.
(Based on the subject of spelling and grammar in some threads on this forum I apologize in advance for the “American” spelling and any other grammatical errors others may find offensive.)