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News items around the recent aviation accidents involving Boing 737 MAX 8 aircraft suggest, that bigger engine diameter demanded for a different (longer) nacelle strut for proper ground clearance which in the end had a negative effect on flight characteristics, necessitating an additional control system MCAS. This reasoning may be correct but it is not self evident IMHO.

One would think that a different engine mount point primarily shifts center of gravity, which can be statically counteracted by shifting other masses. The attack point of thrust, which also generates angular momentum if not coincident with the center of aerodynamic drag, should not be significantly affected by this geometry change.

So how does this geometry change adversely affect margins of inherent static stability and impede return from increased angle of attack to equilibrium?

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  • $\begingroup$ The consensus here is that the area of the larger cowlings ahead of CP causes the pitch up effect aerodynamicly rather than thrust/drag imbalance. One other factor to consider is the increased amount of thrust at low speeds a fan would provide compared to a lower bypass jet. Even if the torque arm is shorter, the pitch up may be greater. This is why a higher bypass fan would have better short field performance and again leads to a need for greater pitch control, and, better training for the pilots. It may not be "just another 737" in its handling characteristics. $\endgroup$
    – Robert DiGiovanni
    Commented Apr 6, 2019 at 17:05

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The problem appears to be mostly the effect of a larger cowling, moved more forward, that generates significant lift when operating at higher AOA and has an effect similar to moving the entire wing forward, or moving the center of gravity aft, partially unloading the horizontal tail in the flaps up low speed regime and making the pitch stability a bit neutralish.

Seaplanes suffer from a bit of a similar effect in yaw from adding floats. The floats add quite a lot of side area forward which reduces the fin's weathervaning tendency. This is why you usually see additional fins added to the tails of float planes to restore the original yaw stability.

In the case of the MAX it appears the pitch destabilizing effect of moving the cowls forward really caught them by surprise because the problems were only found during flight test. To restore the original relationship of the effective center of lift to CG would have required some pretty radical redesign, say like adding a plug to the forward fuse to move the operating CG forward, which would also require making the tail bigger... and off we go on the one-thing-effects-another design change merry-go-round without end. What a mess. They were really between a rock and a hard place, and an artificial stability system using the stab trim system to work around what was a fairly narrowly focused problem provided a relatively easy escape route.

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  • $\begingroup$ You say the problems were only found during flight test. Is there some information publicly available on how these tests worked out? I ask because I expect any docs to shed some light on theoretical foundations of aerodynamic stability. $\endgroup$
    – Andreas
    Commented Apr 6, 2019 at 19:29
  • $\begingroup$ Sorry I'm not aware of any public documentation available, but Google "stability augmentation using pitch trim" or similar word strings and you might stumble onto some publicly available papers. I understand that this type of system is nothing new for Boeing. $\endgroup$
    – John K
    Commented Apr 7, 2019 at 1:05

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