5 ms·

> 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

> 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.

acqq 7y ago
> 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-of-volkswagen-could-face-10-years-in-prison 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")
- JudgeWapner 7y ago
  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?
  - zaroth 7y ago
    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.
zepearl 7y ago
>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-unsafe-to-fly-new-scathing-report-by-pilot-and-software-designer-ed5jwi2s8kCuGTAgFXR0GA/ 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... .
- zepearl 7y ago
  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?
  - mannykannot 7y ago
    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 https://news.ycombinator.com/item?id=19904685

acqq 7y ago

> 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-of-volkswagen-could-face-10-years-in-prison 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")

JudgeWapner 7y ago
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?
- zaroth 7y ago
  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.

zepearl 7y ago

>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-unsafe-to-fly-new-scathing-report-by-pilot-and-software-designer-ed5jwi2s8kCuGTAgFXR0GA/ 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... .

zepearl 7y ago
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?
- mannykannot 7y ago
  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 https://news.ycombinator.com/item?id=19904685