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I'm asking about general aerodynamics for under-wing twin-jets – I'm not after information/speculation on the Max 8 crashes/systems, but I mention the crash below because of what I see as an unclear (to me at least) explanation:

In response to Lion Air Flight 610, leehamnews.com wrote:

All objects on an aircraft placed ahead of the Center of Gravity will contribute to destabilize the aircraft in pitch.

  1. But as one commentor wittingly pointed out, wouldn't the fwd engine also move the CG fwd.

And then:

[G]enerating an angle of attack close to the stall angle of around 14°, the previously neutral engine nacelle generates lift. A lift which is felt by the aircraft as a pitch up moment (as its ahead of the CG line), now stronger than on the 737NG. This destabilizes the MAX in pitch at higher Angles Of Attack (AOA).

  1. I read this as insufficient pitch authority, not pitch instability. Going by the text above, I would also imagine the destabilizing force to be a drag from the larger nacelle's bottom, not lift. For example, the relaxed pitch stability of the MD-11 warranted an LSAS system, but not a special anti-stall system.

I tried to look for an official explanation for why MCAS was added to understand the general aerodynamics, but the prelim report doesn't mention it.

I've presented what has me confused, but the question is the one in the title, wrt general aerodynamics, not 737 Max or MCAS.

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  • $\begingroup$ "the destabilizing force to be a drag from the larger nacelle's bottom, not lift" the component perpendicular to the airflow is lift by definition. and given the aircraft attitude there will probably be a bit of both $\endgroup$ – Federico Mar 13 at 7:33
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    $\begingroup$ @Charles Bretena moving CG forward requires more downtrim force on tail. The issue with correcting weight imbalance aerodynamicly is that it only works at one speed, there for requires constant monitoring with computer. Unchanged, the higher trim setting will cause stronger nose up as speed increases. Once AOA increases to the point the nacelles add to pitch up, the stability formula changes now requiring less down force on tail. 3 pitch variables, changing at different AOA and speed. $\endgroup$ – Robert DiGiovanni Apr 11 at 7:11
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    $\begingroup$ Regarding horizontal stabilizer trim, OK Ralph J, we'll move the entire Hstab assembly to set pitch at cruise and have a trim tab for flaps? (and an elevator too). It seems they put too many eggs in one basket burdening an already undersized Hstab with the task of saving the plane from excessive pitch up. I had proposed a larger elevator with dual rates to do this, keeping it under pilot control. $\endgroup$ – Robert DiGiovanni Apr 11 at 7:24
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    $\begingroup$ But we may be getting somewhere with pitch change with flaps setting (from increased downwash on tail), as lengthening the fuse may not only reduce this, but also give the tail a greater torque moment. $\endgroup$ – Robert DiGiovanni Apr 11 at 7:29
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    $\begingroup$ A larger Hstab to compensate for nacelle area pitch up would leave only weight imbalance to correct, and guess what? A larger Hstab weighs more! (and also gives greater pitch stability). Notice, with a Cri-Cri style engine mount, thrust would provide pitch down. Some 1950s Air Force designs had forward fuse jet engine mounts, but brass liked 'em on the wings! Now you can move wing forward, increasing tail torque authority even more. Given the crash record (more than 70) of this model, some redesign seems in order. $\endgroup$ – Robert DiGiovanni Apr 11 at 7:50
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This is an "I think" rather than "I know" answer so if you disagree please feel free to explain why.

In normal straight and level flight it shouldn't have a significant impact. But at a high AoA, at least a significant portion of the engines are ahead of and crucially, above the CG. The nacelle will be producing both lift and drag the resultant effect of both forces, being above the CG, will be a pitch up moment.

Even if this extra force by itself isn't enough to make the aircraft unstable in pitch, it will require the pilots to make a stronger nose-down command, but when you're so close to a stall you want that to be as easy as possible.

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  • $\begingroup$ As the nacelles are hung below the (low-mounted) wing, they might actually be below the extension of the flight path vector that goes through th3e CG. $\endgroup$ – Charles Bretana Apr 11 at 0:37
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    $\begingroup$ This is my understanding, too. MCAS isn’t actually intended to prevent a stall, it’s to make pitch control feel more linear and prevent a step change in column force/pitch rate relation when nacelle lift „onset“ suddenly provides an extra upward „kick“ during a pitch-up manoeuvre. $\endgroup$ – Cpt Reynolds Apr 11 at 7:58
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    $\begingroup$ Nacelle uplift kick, swept wings, too small horizontal stabilizer, low thrust line, faulty sensor? Isn't it time to stop masking poor engineering with software? No need to risk walking further and further towards instability with civilian transports. We cannot ignore what makes things fly stably, going back to the spear and the arrow. Check out the Boeing YC-14, and the B-52. $\endgroup$ – Robert DiGiovanni Apr 11 at 12:21
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Placing a bigger (high by pass jet) engine slightly more forward and higher (on the wing) impacts the plane in pitch 2 ways.

  1. Moving engine forward moves center of gravity forward, requiring more down force on tail (unless cargo/fuel placement is adjusted). Generally forward CG improves directional stability but can only be trimmed for one speed.

  2. Moving engine forward creates more area ahead of the center of lift as viewed from directly underneath the aircraft, which affects pitch stability at higher angles of attack. The solution to improve pitch stability is to add area behind the center of lift, generally on the horizontal stabilizer. Another solution is to endcap the Hstab (like the B-24 Liberator tail) to make it more effective.

Understanding general aerodynamics is key here. A very pitch stable aircraft (where Hstab only allows very slow change in AOA) with a very gentle elevator (ridiculously safe for civilian transports) is what may be far more desirable for this application.

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  • $\begingroup$ Please leave a comment if you have a issue with the answer. I honestly don't care about the vote. It is more about getting some understanding here. My respect for Boeing and what they have done in this field is beyond words. Getting past this issue and continuing to improve is where I hope to contribute. $\endgroup$ – Robert DiGiovanni Apr 13 at 2:22

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