Comment by mannykannot | Hacker News Reader

Comment by mannykannot | Hacker News Reader

Parent Full thread

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-stabil...

[2] https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stabil...

Isn't that exactly what I said? Edit: not quite, good point about center of drag in steady state conditions.

"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.

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.

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

[2] https://leehamnews.com/2018/11/14/boeings-automatic-trim-for...

Sorry for making unclear references. My question was referring to "it is [lift], not thrust, that creates the problem MCAS was intended to fix", which I never addressed before.

The article you linked has some nice cross-sectional illustrations that makes clear that the thrust line of the MAX engine actually is higher than on the NG. I hadn't realized this but it makes sense given their larger diameter and needed ground clearance. That would indicate that the thrust dependent pitch up is likely smaller on the MAX than the NG so, if the MAX behaves worse, it stands to reason that the aerodynamic effect must dominate.

I guess the issue is that lift overall is just so much larger that a small shift in the center of lift has a large effect on pitch moment?

As for your comment about longitudinal stability, I agree that the pitch moment due to thrust is independent of AoA and hence can't change the shape of the pitch moment curve. I gotta think about this some more.

Yes, I should have been clearer about that as well. There are indeed two issues there - the lift of the nacelles and the thrust pitch-up effect.

Reply on news.ycombinator.com