4 ms·

The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has

by lutorm 7y ago

The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has been added to help with this - to help the crew push the nose down in such situation by using the stabilizer trim. The forward-backward position of the engines have nothing to do with their resultand pitch torque. What matters is that they are below the CG. The engines being forward matters because it shifts the center of pressure forward, giving an inherent pitch-up tendency that becomes more pronounced at high angles of attack. And like you say, all planes with underslung engines have this issue to some degree. Maybe the issue with the MAX is severe enough to cause certification problems but the stall recovery procedures for the NG apparently already call for reducing pitch angle before going to full power or you may not be able to get it down.

mannykannot 7y ago
The fore/aft position has a lot to do with the AofA-dependent effect, as it results from the approximately cylindrical engine nacelles producing some lift when their axis is at a positive angle of attack (fuselages have a similar effect [1].) On airplanes with rear-mounted engines, this effect on their nacelles increases the longitudinal stability [2]. The high/low position matters for the power-dependent effect, and furthermore, what matters in steady-state is the pitch moment created by misalignment of thrust and drag (when accelerating, the moment around the CofG also matters.) A modest tendency to pitch down when the power is reduced is preferable to the opposite. The issue MCAS was supposed to solve was due to the AofA-dependent effect. That's why it has AofA as an input, and not any measurement or proxy for thrust. [1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stability-part-2/ https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil... [2] https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stability-part-3/ https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stabil...
- lutorm 7y ago
  Isn't that exactly what I said? Edit: not quite, good point about center of drag in steady state conditions.
  - mannykannot 7y ago
    "The forward-backward position of the engines have nothing to do with their resultand pitch torque. What matters is that they are below the CG. "The engines being forward matters because..." Janoc repeated a mistake that has been made several times, attributing the problematic pitch-up to the thrust of low-slung engines, and also made some mistakes about how that effect works. I am sure you did not intend it, but by combining corrections to both the former and the latter, and not differentiating between the two, you ended up with something quite confusing, so I felt some clarification would help. The key points are that the engines do produce lift at high AofA, and it is this, not thrust, that creates the problem MCAS was intended to fix.
    - lutorm 7y ago
      The key points are that the engines do produce lift at high AofA, and it is this, not thrust, that creates the problem MCAS was intended to fix. Is that really true? I agree that the pitch-up torque from engine thrust does not vary based on AoA, but the elevator deflection needed to counteract said torque definitely depends on airspeed and hence on AoA. Even if there was no aerodynamic effect of the engines at all, the pilot would be required to trim nose-down as airspeed goes down relative to an airplane with centerline thrust. If the thrust gets high enough, at some point this pitch torque would overwhelm the natural tendency of the airplane to pitch down as airspeed goes down and result in negative longitudinal stability. My impression is that MCAS was intended to counteract the decreasing stick force with decreasing airspeed. Both effects, offcenter thrust and aerodynamics, combine to produce the same effect.
      - mannykannot 7y ago
        First you said I was repeating what you said, now you are saying the opposite? Various good sources [1],[2] state that the issue is lift, not thrust. The article I linked to from my earlier post shows that this lift is not unique to 737 Maxes, and that it contributes to the stability of the DC-9, on account of being behind the CofG on that airplane. The LEAP-1B engines of the Max variants are not significantly more powerful than those on NGs (they are more efficient and somewhat quieter.) As the whole problem arises from the fact that they cannot be mounted lower, it seems unlikely that an increased thrust moment is what is making the Max handling unacceptable. > If the thrust gets high enough, at some point this pitch torque would overwhelm the natural tendency of the airplane to pitch down as airspeed goes down and result in negative longitudinal stability. There seems to be some confusion here over what longitudinal static stability is: it a matter of the rate of change of pitching moment with respect to AofA (take a look at the links in my first post for data from real airliners, where a negative slope indicates stability.) Thrust is not a function of AofA, and consequently is not factor in the relationship that gives longitudinal stability of ordinary airplanes - and, as I said before, neither thrust nor any proxy for it is an input to MCAS (while AofA is.) [1] http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/mcas.htm [2] https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/ https://leehamnews.com/2018/11/14/boeings-automatic-trim-for...
thisrod 7y ago
Thanks. Your post is the first time I've read a physically plausible explanation of this.

by lutorm 7y ago

mannykannot 7y ago

The fore/aft position has a lot to do with the AofA-dependent effect, as it results from the approximately cylindrical engine nacelles producing some lift when their axis is at a positive angle of attack (fuselages have a similar effect [1].) On airplanes with rear-mounted engines, this effect on their nacelles increases the longitudinal stability [2]. The high/low position matters for the power-dependent effect, and furthermore, what matters in steady-state is the pitch moment created by misalignment of thrust and drag (when accelerating, the moment around the CofG also matters.) A modest tendency to pitch down when the power is reduced is preferable to the opposite. The issue MCAS was supposed to solve was due to the AofA-dependent effect. That's why it has AofA as an input, and not any measurement or proxy for thrust. [1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stability-part-2/ https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil... [2] https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stability-part-3/ https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stabil...

lutorm 7y ago
Isn't that exactly what I said? Edit: not quite, good point about center of drag in steady state conditions.
- mannykannot 7y ago
  "The forward-backward position of the engines have nothing to do with their resultand pitch torque. What matters is that they are below the CG. "The engines being forward matters because..." Janoc repeated a mistake that has been made several times, attributing the problematic pitch-up to the thrust of low-slung engines, and also made some mistakes about how that effect works. I am sure you did not intend it, but by combining corrections to both the former and the latter, and not differentiating between the two, you ended up with something quite confusing, so I felt some clarification would help. The key points are that the engines do produce lift at high AofA, and it is this, not thrust, that creates the problem MCAS was intended to fix.
  - lutorm 7y ago
    The key points are that the engines do produce lift at high AofA, and it is this, not thrust, that creates the problem MCAS was intended to fix. Is that really true? I agree that the pitch-up torque from engine thrust does not vary based on AoA, but the elevator deflection needed to counteract said torque definitely depends on airspeed and hence on AoA. Even if there was no aerodynamic effect of the engines at all, the pilot would be required to trim nose-down as airspeed goes down relative to an airplane with centerline thrust. If the thrust gets high enough, at some point this pitch torque would overwhelm the natural tendency of the airplane to pitch down as airspeed goes down and result in negative longitudinal stability. My impression is that MCAS was intended to counteract the decreasing stick force with decreasing airspeed. Both effects, offcenter thrust and aerodynamics, combine to produce the same effect.
    - mannykannot 7y ago
      First you said I was repeating what you said, now you are saying the opposite? Various good sources [1],[2] state that the issue is lift, not thrust. The article I linked to from my earlier post shows that this lift is not unique to 737 Maxes, and that it contributes to the stability of the DC-9, on account of being behind the CofG on that airplane. The LEAP-1B engines of the Max variants are not significantly more powerful than those on NGs (they are more efficient and somewhat quieter.) As the whole problem arises from the fact that they cannot be mounted lower, it seems unlikely that an increased thrust moment is what is making the Max handling unacceptable. > If the thrust gets high enough, at some point this pitch torque would overwhelm the natural tendency of the airplane to pitch down as airspeed goes down and result in negative longitudinal stability. There seems to be some confusion here over what longitudinal static stability is: it a matter of the rate of change of pitching moment with respect to AofA (take a look at the links in my first post for data from real airliners, where a negative slope indicates stability.) Thrust is not a function of AofA, and consequently is not factor in the relationship that gives longitudinal stability of ordinary airplanes - and, as I said before, neither thrust nor any proxy for it is an input to MCAS (while AofA is.) [1] http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/mcas.htm [2] https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/ https://leehamnews.com/2018/11/14/boeings-automatic-trim-for...

thisrod 7y ago

Thanks. Your post is the first time I've read a physically plausible explanation of this.