5 ms·

>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

>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
- salawat 7y ago
  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.
  - zepearl 7y ago
    Thx!

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
salawat 7y ago
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.
- zepearl 7y ago
  Thx!