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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, 2019 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, 2019 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, 2019 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, 2019 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, 2019 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, 2019 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, 2019 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, 2019 at 12:21
  • $\begingroup$ Being above the CoG makes no difference. It's ahead of the CoG that makes the difference. Think of it like a see-saw with the fulcrum at the CoG. Any upward force applied to one side of the see-saw will cause a pitch-up moment on that side, regardless of whether that side is currently above or below the fulcrum point. The bottoms of the engine nacelles will normally be way below the CoG of an airliner. $\endgroup$
    – reirab
    Nov 11, 2019 at 19:00
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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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  • $\begingroup$ I did not vote you down. However, what evidence do you have that MAX suffers from a pitch stability problem? $\endgroup$
    – JZYL
    Jul 28, 2019 at 3:23
  • $\begingroup$ I'm waiting to hear from Boeing. The consensus here seems to be, under certain conditions of AOA and airspeed, the larger, forward set engine (at full thrust) degrades pitch stability. Video of Lion final moments very moving as well. My feeling is that blast from higher set engine may be "Coandaing" the empennage (pulling it into the airstream). Down wash affecting the empennage may be a issue as well. The monster twin concept seems to be working fine on the 777, and the 757 is still in production as a freighter, so they may have some way of fixing it, or other options to pursue. $\endgroup$
    – Robert DiGiovanni
    Jul 28, 2019 at 9:04
  • $\begingroup$ The aerodynamic data are proprietary, so unless you are a Boeing aerodynamicist who has worked on this program, everything is a conjecture. I didn't see any reliable reports on the stall characteristics being worse with max thrust. The nonlinear effects at high incidence are complex, especially with shocks at high speed, I don't see why you can boil it down to "Boeing should've used larger tail". $\endgroup$
    – JZYL
    Jul 28, 2019 at 16:42
  • $\begingroup$ @Jimmy actually I like the 727 a lot better. May also be a question of where to put the tail (and the engines). But the finished product, especially for passenger service, should be as simple and easy to fly as possible. My first curiosity would be to evaluate the airflow at high AOA. I would look to a larger tail as a possible solution, perhaps with the same leading edge width but greater depth (lower aspect). I hope the company can "boil it down", keeping the distillate, not the bottoms! $\endgroup$
    – Robert DiGiovanni
    Jul 28, 2019 at 17:14
  • $\begingroup$ I'm sorry but increasing tail size to help with Fs/g at transonic Mach and stall ID seems pretty far fetched. You'd be over designing for pretty much all of the envelope. I don't understand your comment about LE. Are you talking about hstab LE? Why would that matter, at all? $\endgroup$
    – JZYL
    Jul 28, 2019 at 18:13
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Everyone acts like the lift produced by the engine nacelles only acts to increase the amount of down force required of the horizontal tail and pilot effort to cause this increased down force.

What is not being talked about is that the lift produced by the engine nacelles is forward of the lift produced by the wing resulting in the total lift vector of the airplane to move forward. As AOA is increased the lift of the nacelles increases in a non-linear fashion (greater than one would expect. I believe that this causes the total airplane center of lift to move forward of the center of gravity. Under this condition an increase in AOA results in a greater pitching moment which can very rapidly result in a runaway condition and stall.

The 737 MAX problem is aerodynamic and cannot be fixed by the MCAS which was originally intended to be a longitudinal stability enhancement.

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  • $\begingroup$ The MCAS is an auto-trim feature only triggered under some conditions, which is a little different than longitudinal stability enhancement which can be thought as device acting in the whole flight envelop. When not triggered, the 737 MAX seems to have no aerodynamic problems. $\endgroup$
    – Manu H
    Jul 27, 2019 at 19:39
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    $\begingroup$ MCAS is not a longitudinal stability enhancement. It was originally devised as a maneuvering stability enhancement. $\endgroup$
    – JZYL
    Jul 28, 2019 at 3:00

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