5 ms·

>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 o

• erokar 8y ago
  It happens when thrust is added abruptly, in e.g. a go-around.
  • subhro 8y ago
    Still does not explain the relationship between forward shifting CG and abrupt thrust.
    • CydeWeys 8y ago
      The center of thrust is shifted forward when the engines are shifted forward.
    • salawat 8y ago
      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.
• cjbprime 8y ago
  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.
• aphextron 8y ago
  >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.
  • gsnedders 8y ago
    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.)
  • teleclimber 8y ago
    The center of thrust moving forward would not have any effect on the pitching moment.
    • aphextron 8y ago
      >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-and-thrust-be-on-a-jet 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.
      • teleclimber 8y ago
        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
• gilbetron 8y ago
  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".
  • subhro 8y ago
    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.
    • TimTheTinker 8y ago
      It’s also that the engine nacelles themselves begin to produce lift (and a consequent pitch-up moment) at high AoA.
      • subhro 8y ago
        Oh well, I did not know that. Do you have any citation that explains this further?
• sangnoir 8y ago
  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
  • Gibbon1 8y ago
    They should have put spoilers on the nacelles instead of trying to fix the planes bad aerodynamics in software.
    • iamaelephant 8y ago
      Yeah you would know.
      • Gibbon1 8y ago
        How do you know that I don't know. Please explain.
  • subhro 8y ago
    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.
    • tostr 8y ago
      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.
      • subhro 8y ago
• olivil 8y ago
  This is all in relation to the wing as the point of rotation.