Very good post by
Cland, and a lot of agreement from this old FBW pioneer.
I was particularly impressed by this snippet:
Let's get to basics of aircraft stability: conventional certification rules state that no passenger transport aircraft
will get certified unless she displays adequate static and
dynamic stability in pitch. Static stability means aeroplane,
when disturbed from trimmed speed, will return to it stick-
free. Dynamic stability means it will do so in acceptable number of convergent oscillations. Stability is not just a matter of aerodynamics; control forces, controls architecture and power all affect it so it can be (and mostly is) significantly modified by FBW. FBW Airbi can not demonstrate hands-off stability in normal and alternate laws as due specific settings of their FBW they will not make an attempt to
return to trimmed speed after the stick is released.
As for how Airbus FBW works, lets first start with how it
doesn't. It was said it maintains 1g. It was said it maintains pitch. It was said it maintains alpha. In normal operation, none of this is true. It might maintain all three of it but not because it pursues them actively but only as a consequence of trying to maintain constant flightpath when stick is pitch neutral.
I would only point out that "stability" in the FBW jets is still a basic aero function of cee gee, center of aero pressure, moment arms and capability of the control surfaces, and on and on and on....
As
Cland pointed out, what we see is "apparent" stability. The FBW system works to modify the inherent aero stability (or lack of) in order to provide the basic pilot a plane that he/she can fly. I had a chance to fly the AFTI sim ( modified Viper) and see various control laws that were enabled with the software. The body rate law was very foreign, and was quickly passed by. The attitude law was same same. The pure AoA-biased law felt really great for any old pilot. But the best law was one based upon a blended AoA-gee. Sheesh! It was close to the one that GD had come up with ten years before. The AoA limits helped to prevent stalls and such. The gee limits kept us from ripping the wings off.
Due to our mission requirements (
Doze will jump on this, heh heh) and our immense speed envelope, we did not require "speed protection". In fairness, we had one troop die when diving down at a speed beyond the placard value, but the airframe was solid and the motor exploded. Vne was 800 knots CAS or so, and low-speed warning was about 120 knots CAS, so see what I mean?
IMHO, the 'bus designers learned from us, and their overall concepts seem very reasonable and directly related to the mission of the jet. Only thing I would have balked about was the AoA role and the plethora of reversion sequences.
Cland's description of what the pilot sees and senses is spot on. We didn't have the auto-throttle feature, so our workload with the throttle was much higher than holding an attitude. We stabilized speed when the drag equaled the thrust.
PLZ remember that I abandoned the "deep stall" theory after a few documents were made available to me. Don't know how old
Cland is, but my high school aero and the stuff I could read at the library didn't approach this characteristic of some planes. Wasn't until the 727 came out that we saw a discussion. I then flew the VooDoo about 6 or 7 years later and we had "pitch-up", but it was complete LOC and the jet did not remain fully stalled in any fashion. Sucker tumbled in pitch with a super roll/yaw component.
So 15 years later I flew a jet that had the classic "deep stall". Funny, but some of the entry maneuver had FBW control system aspects. The maneuver was exactly the same as AF447. Climb at a pitch attitude and power setting until the jet ran outta the air molecules required to get the nose down with full elevator deflection ( no THS, as we had the all-moving horizontal stabilizers). Super directional stability like the 'bus seems to have, and no large pitch changes - just a reasonable ride while the altimeter is unwinding at 10,000 feet per minute. Fortunately, we had a positive pitch moment at that combination of cee gee and AoA. Unfortunately, HAL was commanding nose down and the jet couldn't get there. So our wizards gave us "direct law" in pitch via a switch that would not even work unless HAL senses AoA above 30 degrees. It worked and we didn't lose a lotta jets.
Thanks,
Cland, for a great post.