[mltony]

> The new engines were too big to fit in their traditional spot under the wings. To combat the problem, Boeing mounted them forward on the wings. Moving the engine position forward shifted the plane’s center of gravity, which altered the aerodynamics of the aircraft. The position of the new engines pulled the 737 tail down, pushed its nose up, and put it at risk of stalling.

Can someone explain to me how this is possible? In my understanding If you move the engines forward, it would move the center of gravity forward as well, and this would push the tail up and pull the nose down - exactly the opposite from what's written in the article. Does some fancy aerodynamic effect play some role here that leads to stalling?

[afterburner]

Most descriptions of the changes don't go into nearly enough detail. Even pilots have a very abbreviated understanding of everything that's going on, using rules of thumb that gloss over nuance in calculating the stability of an aircraft. Probably a lot of aircraft engineers don't have a full understanding in general unless they're forced to "do the math" on a particular plane.

Making the engine more powerful (as is the case here) means there will likely be more pitching up moment (turning about the aerodynamic centre or AC) due to the position of the engine relative to the AC. Speaking of which, there is an AC, an aerodynamic centre that is separate from the center of gravity. It represents the point about which the plane pitchs up/down when a force is exerted on it.

Not only is the engine more powerful, but the different position it was placed in (forward primarily) would also create a different force about the AC, and change the forces the other parts apply to the plane (by moving the AC and CG).

There might have been more to it but even a few months after really diving into this the details are getting fuzzy... plane stability dynamics can vary greatly.

[Obi_Juan_Kenobi]

The pitch-up moment caused by the engine placement only occurs at high AoA. Basically, as you start to pitch up more and more, eventually the engine nacelles 'catch the wind' forward of the wings and CoM, and try to flip the nose up.

For whatever reason, the basic facts of this whole debacle are consistently wrong in reporting and discussion. Common falsehoods:

* The 737MAX is inherently unstable. No, it is a stable aircraft as required by FAA regulation that all passenger aircraft be as such. Nothing about the airframe or design is even that remarkable, beyond the simple fact that it is different from the older models.

* MCAS is anti-stall. No, MCAS does exactly what it's named for, to modify and augment the 'feel' of the flight stick for the pilot. Because Boeing uses a 'pilot in the loop' design approach, the physical feedback from the flight stick is critical to safe flying, and cannot be faked.

What MCAS does is make is to that you have to apply a proportional amount of force on the flight stick to pitch up a given amount (regulations define how far from linear this relationship can be). With the new engine placement, it would not require as much force as you would expect from earlier planes, which could lead to pilots approaching stall conditions unknowingly without that feedback. There are still anti-stall measures to prevent an actual stall (e.g. stick shakers), but the intuitive 'feel' of flying would be lost.

The initial MCAS design was reasonable, but test pilots felt that it wasn't strong enough, so it was changed. It is at this point, with no critical review being triggered, that the failure took place. The stronger system put pilots in a catastrophic situation in the event of a malfunction, and two independent flight crews failed to overcome the error.

[salawat]

Basically, the new engine nacelle shape and placement contribute extra lift at high AoA. The additional lift in front of the CoG creates a pitching moment that drops the tail, and lifts the nose.

Think of it like attaching an extra conditional lifting surface to the wing. In normal, low AoA flight, no pressure differential is created. At high AoA though, that extra lift works through the lever arm of the nacelle assembly.

Control feelwise, this would result in a slackening of control force needed to yield more degrees AoAby the plane after a certain point. That's a regulatory no-no.

[alumowa]

Layman here. What I picture in my head is due to the engines being placed higher and more forward on the wing, it will want to rotate nose up when thrust is being produced.

[salawat]

This is correct, and normal behavior actually. That's already accomodated for in modern designs.

What isn't is the extra lift at high-AoA and longer lever arm for the lift force to operate through. MCAS was meant to counter that, and arguably would have been a perfectly reasonable fix if they'd have designed it to the proper degree of redundancy, and actually told pilots how it worked and that it existed.

[leetrout]

My armchair physics example would be a balance seesaw and let’s say the left side is the “front”. If you attached a rocket motor (or jet engine) in front of the the CG point would the the thrust push the left side of the seesaw up or down?

I believe the issue is precisely this - the engines were moved (and they are bigger so more powerful perhaps?) forward and the thrust probably pushes the nose up.

[salawat]

Seesaw isn't the best model really. A seesaw with a turbine engine strapped to it, (besides making a godawful mess and maybe just maybe making the HOA angry enough to worth the trouble of setting up) wouldn't do much since it's a fixed structure and not interacting in any way remotely consistent with a body in motion through a fluid.

Boat might be a better model, or even better, use your hand hanging out the window of a moving car.

If you taped a couple of projections under your hand, and slowly increase the Angle of Attack of your hand to get a feel for what the old design would do.

Then do the same thing but with those protrusions up in front of your hand.

You should notice that it becomes harder in the second arrangement to counter the tendency of your hand to continue pitching up after a point. Assuming your weird mini-not turbine construction stays together in the airstream.

[NeedMoreTea]

Not just forward, the 737MAX engines are mounted forward and higher.

If you move CofG forward, and up you move the balance point forward and up, reduce elevator authority, and with similar tail mass now a metre or two further behind the balance point, it will want to sit tail-heavy. So they tried to trim it out, and we are where we are.