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The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has been added to help with this - to help the crew push the nose down in such situation by using the stabilizer trim.

The forward-backward position of the engines have nothing to do with their resultand pitch torque. What matters is that they are below the CG.

The engines being forward matters because it shifts the center of pressure forward, giving an inherent pitch-up tendency that becomes more pronounced at high angles of attack.

And like you say, all planes with underslung engines have this issue to some degree. Maybe the issue with the MAX is severe enough to cause certification problems but the stall recovery procedures for the NG apparently already call for reducing pitch angle before going to full power or you may not be able to get it down.

The fore/aft position has a lot to do with the AofA-dependent effect, as it results from the approximately cylindrical engine nacelles producing some lift when their axis is at a positive angle of attack (fuselages have a similar effect [1].) On airplanes with rear-mounted engines, this effect on their nacelles increases the longitudinal stability [2].

The high/low position matters for the power-dependent effect, and furthermore, what matters in steady-state is the pitch moment created by misalignment of thrust and drag (when accelerating, the moment around the CofG also matters.) A modest tendency to pitch down when the power is reduced is preferable to the opposite.

The issue MCAS was supposed to solve was due to the AofA-dependent effect. That's why it has AofA as an input, and not any measurement or proxy for thrust.

[1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil...

[2] https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stabil...

Isn't that exactly what I said? Edit: not quite, good point about center of drag in steady state conditions.

"The forward-backward position of the engines have nothing to do with their resultand pitch torque. What matters is that they are below the CG.

"The engines being forward matters because..."

Janoc repeated a mistake that has been made several times, attributing the problematic pitch-up to the thrust of low-slung engines, and also made some mistakes about how that effect works. I am sure you did not intend it, but by combining corrections to both the former and the latter, and not differentiating between the two, you ended up with something quite confusing, so I felt some clarification would help. The key points are that the engines do produce lift at high AofA, and it is this, not thrust, that creates the problem MCAS was intended to fix.

The key points are that the engines do produce lift at high AofA, and it is this, not thrust, that creates the problem MCAS was intended to fix.

Is that really true? I agree that the pitch-up torque from engine thrust does not vary based on AoA, but the elevator deflection needed to counteract said torque definitely depends on airspeed and hence on AoA. Even if there was no aerodynamic effect of the engines at all, the pilot would be required to trim nose-down as airspeed goes down relative to an airplane with centerline thrust.

If the thrust gets high enough, at some point this pitch torque would overwhelm the natural tendency of the airplane to pitch down as airspeed goes down and result in negative longitudinal stability.

My impression is that MCAS was intended to counteract the decreasing stick force with decreasing airspeed. Both effects, offcenter thrust and aerodynamics, combine to produce the same effect.

First you said I was repeating what you said, now you are saying the opposite?

Various good sources [1],[2] state that the issue is lift, not thrust. The article I linked to from my earlier post shows that this lift is not unique to 737 Maxes, and that it contributes to the stability of the DC-9, on account of being behind the CofG on that airplane.

The LEAP-1B engines of the Max variants are not significantly more powerful than those on NGs (they are more efficient and somewhat quieter.) As the whole problem arises from the fact that they cannot be mounted lower, it seems unlikely that an increased thrust moment is what is making the Max handling unacceptable.

> If the thrust gets high enough, at some point this pitch torque would overwhelm the natural tendency of the airplane to pitch down as airspeed goes down and result in negative longitudinal stability.

There seems to be some confusion here over what longitudinal static stability is: it a matter of the rate of change of pitching moment with respect to AofA (take a look at the links in my first post for data from real airliners, where a negative slope indicates stability.) Thrust is not a function of AofA, and consequently is not factor in the relationship that gives longitudinal stability of ordinary airplanes - and, as I said before, neither thrust nor any proxy for it is an input to MCAS (while AofA is.)

[1] http://www.b737.org.uk/mcas.htm

[2] https://leehamnews.com/2018/11/14/boeings-automatic-trim-for...

Sorry for making unclear references. My question was referring to "it is [lift], not thrust, that creates the problem MCAS was intended to fix", which I never addressed before.

The article you linked has some nice cross-sectional illustrations that makes clear that the thrust line of the MAX engine actually is higher than on the NG. I hadn't realized this but it makes sense given their larger diameter and needed ground clearance. That would indicate that the thrust dependent pitch up is likely smaller on the MAX than the NG so, if the MAX behaves worse, it stands to reason that the aerodynamic effect must dominate.

I guess the issue is that lift overall is just so much larger that a small shift in the center of lift has a large effect on pitch moment?

As for your comment about longitudinal stability, I agree that the pitch moment due to thrust is independent of AoA and hence can't change the shape of the pitch moment curve. I gotta think about this some more.

Yes, I should have been clearer about that as well. There are indeed two issues there - the lift of the nacelles and the thrust pitch-up effect.

Thanks. Your post is the first time I've read a physically plausible explanation of this.

This is completely wrong. An Airbus is not made conformant with FAR 25 airworthiness standard by software protections. It conforms to all of FAR 25 requirements without any of the safeguards in place, whereby stick inputs are directly translated to control surfaces. The airworthiness standards are predicated on aerodynamics, not software abstraction of an otherwise unairworthy design.

The idea there are switches in the airplane I can use to make the airplane unairworthy is absurd. And it is equally absurd that the aircraft flight manual and an emergency airworthiness directive would so direct a pilot to make the plane less airworthy than it already is, let alone unairworthy, as a corrective action. It's just absurd. You're welcome to provide a citation, however.

I submit FAR 25. I've read the entire thing and there's nothing in there that permits software to paper over an otherwise unairthworthy aircraft design.

> The engines on the MAX are forward of the CoG

It doesn't make much difference, most of what you've said is accurate to the best of my knowledge, however the thrust can be as far forwards of the CoG as it likes, that doesn't increase the pitch up moment. Having them lower may increase that moment and them having more power too. But the main issue on the Max is airodynamic lift generated by the nacelles at high AoA.

> the main issue on the Max is airodynamic lift generated by the nacelles at high AoA.

This simplifies the physics so much that it's wrong. It is true that there are texts attributing everything just to the "nacelles", but the engineering doesn't work the way you argue:

First, you can't put the engines on the plane without some casing for them (you have to have nacelles).

Second, if you'd try to modify the casing of the engines (the said nacelles) to have other aerodynamic properties, you'd interfere with other design goals (e.g. you can increase the fuel consumption which was the most important design goal for the MAX).

Third, the position of the engines definitely does matter for the behavior like "nose up" as long as the rest of the plane has some already determined form, which in this case is what has to be kept to have the plane being 737 and not something completely different.

In short: the 737 MAX is Boeing's equivalent of VW's diesel cars. Its behavior was determined by the prescribed design goals (by those who sold the plane): Boeing simply directly lied, selling something that can't be achieved given the "sold" design characteristics.

Just like the diesel cars can reduce nitrogen oxides production, but then they are less "efficient" than as they were sold (they need more refills, or use more fuel, i.e. cost more than advertised). Likewise, the 737 MAX-like plane could have been safer, but then Boeing could have not sold it as a plane for which there's "no need for pilot's re-certification and retraining."

At the end, Feynman conclusion of his Shuttle investigation report holds:

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled."

Note also: https://www.theguardian.com/business/2019/apr/15/former-head...

("Former head of Volkswagen could face 10 years in prison -- Martin Winterkorn and other executives charged with fraud over emissions test cheating")

I'm quite sure Boeing's "heads" won't even be charged. And no, the "nacelles" aren't the main issue. It's the fraudulent selling of the planes which can't deliver what they were sold for. Fraud.

(P.S. It's "aerodynamic")

I wonder if thrust vectoring could be of use here? I know it's primarily a military technology now, but maybe the MAX can be saved by having engines that slightly divert their thrust in very specific situations to offset this undesirable effect?

For this to be stable it would seem very elegant if you could design this in such a way that it occurred in equilibrium with any undesirable lift generated by the nascelles.

As supposed to an active measure controlled by electronics based on a separate AoA sensor, if somehow the lift caused the engine the thrust vector in opposition to the lift as a matter of physics.

I mean, in theory there’s nothing to say inherently such a design would be more reliable than triple redundant sensors and a bunch of code, but that would be pretty neat.

>But the main issue on the Max is airodynamic lift generated by the nacelles at high AoA.

I agree.

I did not find the original article I read about it but e.g. this article ( https://moneymaven.io/mishtalk/economics/boeing-737-max-unsa... ) contains a statement similar to what I read - a bit more aggressive and I did not actually read the article and I do not know the website (so yeah, it might be a bad and unconfirmed source). Anyway, just focusing only on the nacelles and AoA here is the statement:

>>because the engine nacelles were so far in front of the wing and so large, a power increase will cause them to actually produce lift, particularly at high angles of attack. So the nacelles make a bad problem worse.

>>I’ll say it again: In the 737 Max, the engine nacelles themselves can, at high angles of attack, work as a wing and produce lift. And the lift they produce is well ahead of the wing’s center of lift, meaning the nacelles will cause the 737 Max at a high angle of attack to go to a higher angle of attack.

Summarized, I understand this as a looping chain reaction: the higher the AoA the higher the lift produced by the nacelles, which in turn (without auto-trim by MCAS or active pilot correction) will increase the AoA which in turn will increase the lift, etc... .

I admit of not being able to understand this:

>>>because the engine nacelles were so far in front of the wing and so large, a power increase will cause them to actually produce lift,

I thought that it would make the plane "tilt" upwards, but the quote mentions "lift".

Does this happen because the engines, being in front of the wing and as well very near to it are "sucking" air that would otherwise flow as well >>over<< the wing, therefore creating a lower-than-usual pressure above the wing, therefore making the airplane climb/generate lift?

It has nothing to do with power. Even cylindrical bodies produce some lift when their axis is not aligned with the airflow, and all engine nacelles do this. If they are forward of the CofG, this produces a pitch-up moment that increases with increasing angle of attack, and therefore reduces stability. The further forward they are, the more the leverage and the stronger the effect. With rear mounted engines, the effect produces a nose-down moment, which contributes to the longitudinal stability. See the links in my other post: https://news.ycombinator.com/item?id=19904685

A good way to visualize it, I've found, is imagine the low pressure area created by the wing as a bubble attached to the aircraft, "sucking" the aircraft up. Imagine the air going under the wing as analogous to the surface of a lake.

The more the plane diverges from parallel to the airstream, the stronger the tendency for the bubble to suck through the "shadow" created in the airstream, and the plane to "skip" on the surface (the air getting pushed under the airfoil).

In reality, these are two different ways of illustrating the same force. This visualization though, let's you pick out two different aspects.

Air is a fluid. Just like water will follow the edge of a pan when you pour it, the curvature of an airfoil does the same thing with air flowing by.

Those nacelles basically enhance the surface area across which that pressure differential acts through. In short, generating extra lift, "pulling" the plane up more, (or pushing, if you prefer looking at it from the skipping stone aspect) creating more lift, all the way into a stall condition.

Thx!

The Boeing whistleblower in the recent 60 Minutes Australia exposé said that's exactly why it was designed. Not only was a new type rating unacceptable, engineers were not permitted to make any design changes which would require simulator training.

> That's BS. By that logic no Airbus or any fly-by-wire plane could ever be certified - they actually require the computer augmentations to fly.

I would imagine that, to be certified, the Airbus planes had to demonstrate that:

a) The computer augmentation was unlikely to fail,

b) when failed, the plane was still flyable enough, and

c) the pilots were trained to fly the plane when the augmentation failed. (I believe this is called “direct law”, and I would be utterly shocked if pilots are not trained to fly the plane in direct law mode.)

They are so trained, and the airplane is still considered airworthy in that mode, including in particular as it related to this conversation, the FAR 25 positive static stability and stall behavior requirements.

Is the 737 MAX’s proposed behavior with one AoA sensor broken also compliant?

Fly by wire systems are classified as safety critical and require much more redundancy. The Boeing 777 has three flight control computers, four sets of low-level controllers, and a fault-tolerant air data sensor system.

Bad air data (airspeed, altitude, angle of attack) has been a big problem in the past. Each of those involves a sensor that sticks out through the skin, and can potentially be damaged or plugged. Remember Air France 447, where the airspeed sensors iced up. Those things need redundancy.

The Boeing 777 has three flight control computers

And still only two alpha vanes.

>The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has been added to help with this - to help the crew push the nose down in such situation by using the stabilizer trim.

And the pilots canot push the nose down in such a situation by actuating the stabilizers?

I've never flown an airplane, but as far as I know, MCAS doesn't have unique access to controls that the pilot does not. (eg, it's different from ABS in a car where the driver does NOT have 4 individual brake levers).

It seems to me if the pilots are trained and certified on the Max, they'll do a better job flying it than a poorly designed system.

OTOH, the system is also fixable, but it is becoming a matter of trust.