Originally Posted by
PEI_3721
gums, Detent, et al, caution with specific comparisons.
Following on from zzuf, good refs; - associating the A320 with feel and bob wts could be misleading.
Airbus chose the combination of electrical signalling, digital computation, and side stick as their FBW system. All of these challenged the certification status quo; this required adaptation, interpretation, and new regulations. Airbus designed the future, rewrote the regulations.
Conversely, Boeing, using the improved technologies where possible, chose to follow history. Moving sticks, trim based force-feel, fleet commonality.
The piloting differences matter little (avoiding endless opinionated debate); these aircraft meet the safety requirements, and are flown as any aircraft can be.
Airbus chose a manoeuvre demand algorithm C*, a combination of pitch rate and 'g', with rate dominating low speed, 'g' higher. The trim followup restores a stable condition and minimises drag. (Military systems may favour 'g' more than rate)
Boeing is similar except the trim followup had to maintain the speed-dependent force-feel; this is the basis of the C*U algorithm, where U is the speed aspect.
The 737 Max with MCAS extended previous trimming adjustments (STS), to compensate for aircraft weaknesses; in no way is this FBW (electrical signalling at best). Post modification, MCAS fails safe, no trim demand or incremental runaway (although separately, the stab still could).
Relating this to AF447; the A330 did exactly what the pilot demanded, for the situation, as the crew interpreted it - UAS - pitch/ power. The technical / icing cause had be been identified from previous incidents and modification was in hand. The safety lessons stem from why this one accident differed from several previous incidents with the same trigger - go to previous threads for debate.
In the 737 Max accidents the aircraft did its own thing; crews were in recovery mode without explanation or guidance.
Even after modification, MCAS and the revised checklists could still lead to an AF447 type of incident. The checklist for AoA disagree requires action for unreliable airspeed; this biases the crew's mind to UAS, requiring pitch/power, bypassing wider situational aspects such as cross referring standby instruments which could clarify the situation, not requiring any manoeuvre.
This suggests that Boeing's operational approach to MCAS, alerting and drills, are still out of sync; perhaps not really appreciating if (why) MCAS is required - certification or training.
Old aircraft, even with new mods still require old style situational awareness to help crews understand, without ambiguity, and not presuming any failure, cause, or action. It is difficult to impose new technical philosophies on an old aircraft; similarly for operational aspects.
C* works nicely however it leads to the atrophy of procedures in the event of a law change, and that has led to a few rapid dissemble events. It is nice to fly though when working rite/write/right... reversion needs practice.
C*U makes you appreciate C*.
The aerodynamic issues of MAX (incidentally a popular name for dogs, and cats) are able to be resolved aerodynamically. The fact that there is a lifty surface poking out before the wing is not a problem, the non-linearity in what it does though is. At the same time, the bits that are added to the BRT to cure previous non-linearities are kept.... and no one got out an angle grinder? On NTRS, there's about 1000 papers related to high alpha lifting body flow control, and, surprise, the bits on the pained side of the cowl work on those factors, and got rid of the exact opposite issue that existed which led to the implementation of the sticky-outy stuff, which ended up assisting the non-linearity that was then the needy bit for the guys to redo the "lets run the trim nose down" bit of MCAS... whew. one breff.
Getting out the hacksaw or angle grinder would affect one thing, however, it would increase VS1g by a bit. But putting a LET on the mid-span TE Flap would do more than compensate for that, and even the inner flap of the Yehudi area would function to increase CL, and drag the CP aft mitigating what otherwise would be a Cm reducing shift of spanwise lift distribution inwards and therefore forwards... it's a swept wing.
There are a few pretty neat papers with NTRS and with AIAA ARC, DLR, DTIC, Delft,
TsAGI, RAeS etc on the matter of low aspect VGs on nacelles and flow interaction with a wing, they are easy reading, and how and why are well described. Slender body flow control gets to the same point, and John C Lin, Carranto, Bruce Storms, Li, and the rest of the mob of researchers covered the issues pretty well, following on Bob Liebecks works, however, the subject has always been missing a simple point, one group of researchers looked at low-speed stuff and just that area. in a cell next door, literally, was a bunch of guys looking at high-speed effects of another device, and apparently, there wasn't any time for two to compare notes over coffee, as the flow structures are the same... the effects are different the mechanisms are the same. The fringe benefit is you also get the real world supercritical flow outcomes that do not exist except on paper due to the point that details matter.
Anyway, a hacksaw or angle grinder would have removed the issues of stick force gradient and would have been readily offset by simple mods to the TE of the flap that would have reduced VS1g, and therefore retained or improved V speeds, while nicely reducing drag in transonic flight. There is a modest increase in trim drag in cruise due to the wing actually doing what it is supposed to do, but that is around 1% of the magnitude of the drag reduction on the wing (which is still only a little of the total lift, etc... ) some 16M USD later, we did prove to non-believers that, surprise, lift and drag are orthogonal, because they are, well, orthoganol. The negligible trim change was recorded, it was measurably less than the trim change resulting from shifting 1 x 67kg SLF from 18C to 1B. For the 737 particularly, the flap track design hates vibration, and the mods to the flap reduce that considerably because the reason for the vibration, instability of the Kutta condition at the TE, is resolved.
just sayin' MCAS was a hammer fix for a typo by a builder.