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Comment by subhro | Hacker News Reader

>The Max 8 is outfitted with bigger, more fuel-efficient engines than earlier 737s, and the weight and positioning of those engines shifted the plane's center of gravity forward, increased the potential for the nose to pitch up after take-off.

Umm, a little confused here, if the center of gravity shifted forward, why will the pitch up tendency increase?

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>Umm, a little confused here, if the center of gravity shifted forward, why will the pitch up tendency increase?

Because the center of thrust is also moved forward, and its effect is much greater than that offset by the center of mass. The 737 airframe was never designed to handle such powerful engines.

The center of thrust moving forward would not have any effect on the pitching moment.

>The center of thrust moving forward would not have any effect on the pitching moment.

Sure it would. Here's a better explanation than I care to make: https://www.quora.com/Where-should-the-center-of-mass-lift-a...

"If the Center of Thrust does not act through the Center of Gravity there will be a moment causing the aircraft to pitch, yaw or roll about the axes of the aircraft and the moment(s) must be counter balanced by design (including provision for trimming when the thrust is varied)."

Boeing chose to counteract that newly induced pitch moment with software, so that they could sell the MAX 8 to airlines as not requiring any new training or type ratings. And now nearly 400 people are dead because of a marketing decision.

If you move an engine forwards (or backwards) you are moving it along its thrust vector. Therefore the moment resulting from the distance between the thrust vector and the center of mass is unchanged.

Source: Physics 101

Right, but if you increase the power, you increase the moment.

It's also a vastly longer aircraft than originally designed: the -100/-200 are 29m/30.53m long. The MAX 8 is 39.47m long. (And the MAX 10 will be 43.8m.)

It's not the center of gravity that causes pitch-up - it's additional lift by engine nacelles at higher angles of attack; this lift increases the AoA further due to the position of the engines relative to the wing. This feedback loop leads to a stall when the angle gets high enough

Probably not completely true. An airfoil stalls when the critical angle of attack is exceeded. For most airfoils, this sits at around 17°. For all commercial aircrafts (I think 727 onwards) there is a mechanism called stick pusher that will not allow an airfoil to go over critical AoA. I am not sure if this has been overridden somwhere in the 737 Max 8 though.

I was describing the physical "bug" Boeing was solving in software: MCAS is a stick-pusher because it forces down the nose - which led to the Lion air crash because of a faulty AoA sensor made MCAS avoid an imaginary stall, forcing the nose down into a powered flight into terrain.

Are you saying 17deg from horizontal? Let's ignore wind and other influences. A normal plane can not climb more steeply than 17deg?

I'm not implying you're wrong, just very surprised to see this low a number. I would have guessed much closer to something like 45deg, maybe even more.

The Angle of Attack is defined as the angle between relative wind and chord of an airfoil. So if your engine has enough juice to climb at 45° (and many aerobatic aircrafts do), your relative wind is coming at 45° from ground, but almost at 0° with respect to the chord. So your wing is still flying.

Let me know if that makes sense.

They should have put spoilers on the nacelles instead of trying to fix the planes bad aerodynamics in software.

Yeah you would know.

How do you know that I don't know.

Please explain.

I think it is actually that shifting the gravity forward makes the flight control surfaces more effective because they are now further away from the CG, but that's just from years of developing flight sims for the Navy, and no real knowledge ;)

It also could be with the engines moved forward from the wings a bit, their thrust tends to rotate the air frame "up".

You are right. Airframes are designed to be nose heavy. The wings tends to fly up, whereas the elevators tend to fly down. In a dead stick condition, the airplane is designed to fly with a pitch down attitude so you don't lose airspeed and have enough airflow over the wings to continue generate enough lift to glide. If the CG shifts forwards, it is actually a good thing.

Source: I am a pilot.

It’s also that the engine nacelles themselves begin to produce lift (and a consequent pitch-up moment) at high AoA.

Oh well, I did not know that. Do you have any citation that explains this further?

The pitch up tendency is only increased at high angles of attack, at which point the engine nacelles themselves add extra lift, possibly inducing a stall when you're already near one.

This is all in relation to the wing as the point of rotation.

It happens when thrust is added abruptly, in e.g. a go-around.

Still does not explain the relationship between forward shifting CG and abrupt thrust.

It's hard to describe, but here goes.

The Center of thrust is offset down and somewhat in front of the CG, this means high thrust imparts a pitch up moment. Small, but present.

The bigger problem is the nacelles shape though, they create body lift at high AoA; imagine a vacuum bubble trying to suck the wing/nacelle backwards forming in the airstream, "pulling" them back.

Combine both of these characteristics, and you have an airframe demonstrating near stall positive pitch instability. I.e. you get a moment that if left uncountered will reinforce itself until the plane ends up stalling; behavior completely unprecedented in the airframe.

So in order to "counter" this behavior an automated system was introduced which supposedly restored the pilot relative flight characteristics in line with the old airframe's specs.

I'm probably oversimplifying, but it's the closest I can get to a visualization that makes some sense.

The center of thrust is shifted forward when the engines are shifted forward.

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