Originally Posted by
Vessbot
Closed loop is like your car's cruise control: It compares the actual speed to the desired speed, and loops that difference back to the throttle control to undo whatever happened (surface roughness, low tire pressure, up/downslope, etc.) to cause the difference to be anything other than zero. Over and over again.
Open loop (meaning really, no loop) is if you set the throttle pedal in some position with a clamp, to set the speed you want. In a certain condition that speed will hold, but in any other condition (the surface roughness and other things) it will go faster or slower, and the car will neither detect nor do anything about that.
Airbus "direct law" is open loop. The elevator just goes in the position matching the stick, and that's it. It replicates cable and pulley.
C* and C*U are both closed loop (actually, it's a few loops, each handling different parameters, stacked in certain ways). C*U has everything that C* has, and in addition has a loop that continuously drives airspeed difference from trim speed to zero. Think of it as if with the autopilot off, FLCH or Open climb/descent is still on, and you're setting the speed with the trim switch (instead of MCP). The inner loop is running and killing attitude fluctuations, and the outer loop is simultaneously running on top of that, killing speed deviations.
Then in a way, would the pitch axis of the C*U be considered as kind of a semi open-looped system? Considering how it only maintains the reference speed, and sacrifices the aircraft pitch/attitude to maintain that.
Also, I thought the Airbus fcc still maintains path stability in direct law, right?