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  • Interesting talk...However, as Gabe did (or didn't) say isn't trim runaway trim runaway (with more than one mode in which it can happen), and isn't there plenty of time to manage it and isn't it Boeing that has a big conspicuous wheel that spins (and I'd hope a fairly obvious and intuitive trim position indicator.)

    Sometime long ago, in aviation typists (or maybe it was Dummy Pilot on a Parlour talk forum that crashed (I'm 100% serious on this))…reported the trim quit working and that his plane was pointing pretty nose up...so much so that they reduced power...but then the trim started working again.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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    • Originally posted by 3WE View Post
      Interesting talk...However, as Gabe did (or didn't) say isn't trim runaway trim runaway (with more than one mode in which it can happen), and isn't there plenty of time to manage it and isn't it Boeing that has a big conspicuous wheel that spins (and I'd hope a fairly obvious and intuitive trim position indicator.)

      Sometime long ago, in aviation typists (or maybe it was Dummy Pilot on a Parlour talk forum that crashed (I'm 100% serious on this))…reported that his plane was pointing pretty nose up...so much so that they reduced power...but then the trim started working again.
      It is both Boeing and Airbus that have a trim wheel on each side of the pedestal, and both are mechanically linked. We know almost nothing about the new Boeing MCAS system, but apparently, stopping the trim wheel (or using the pickle switches) will only momentarily stop the autotrim, and it will resume after a few seconds if the cutoff switches are not used. But at this point we even don't know if the MCAS system is just a pitch trim function.

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      • Originally posted by Evan View Post
        system certify 737NG more relocation more forward increased new engines new power/geometry more pronounced further compensate replacement [acronym] more pronounced.
        JMO...maybe it's time to design a new airplane to replace the 757 (and the 737)?

        Duct tape and bailing wire is cheap, but how about wings and tails and engines designed to meet the needs instead of compensating systems.

        /Parlour pontification.
        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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        • Originally posted by 3WE View Post
          JMO...maybe it's time to design a new airplane to replace the 757 (and the 737)?

          Duct tape and bailing wire is cheap, but how about wings and tails and engines designed to meet the needs instead of compensating systems.

          /Parlour pontification.
          Concur.

          (Word is that Boeing is drawing up something for the 2030's)

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          • Originally posted by Evan View Post
            (Word is that Boeing is drawing up something for the 2030's)
            The NMA? There's a lot of discussion whether or not it's a niche market.

            I think Boeing was caught off-guard by the success of the A320 NEO, so they had to respond quickly with the MAX.

            Your explanation about the engines and pitch-up makes sense to me.

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            • Oh for the good old days, fly with the side windows open.Click image for larger version

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              • Originally posted by Evan View Post
                So now tell us about the physics...
                We have 2 parts.
                The underslung-engines pitch coupling is quite obvious. Add thrust and there is a nose-up pitching moment (engines pushing the airplane from under the GC), reduce thrust and there is a nose-down pitching moment.

                The farther down the CG the greater the effect, the higher the thrust the greater the effect. Bigger 737s obviously have more thrust, but they are also longer so they have the stabilizer and elevator farther back which should compensate and, if not, you can always increase the size of these surfaces (as Boeing did with the fin).

                I don't know what role did the ever-increasing fan diameter had on the vertical location of the thrust vector.

                There is a second effect which is less obvious. Let's start with the basic. In the beginning all was darkness then came Newton and showed that if you take a mass of air and accelerate it (that requires making a force on it), the air will do you a force that is equal in magnitude and opposite in direction. That's the 3rd Newton's law of motion and that's how thrust works in a jet engine, in a turbofan engine, in a propeller engine, in a rocket engine (except you have another gas not air). By the way, this is also how lift works.

                So if you take a mass of that is coming from the left, put it through a pipe, compress it, heat it up so it increases its volume and hence it's speed through the pipe, and let it go the other side, you are accelerating the mass of air to the right and the air will push on you to the left. If you put the pipe at a 45 degrees angle and have the air come for the top left corner of the screen and do the same, the air will accelerate towards the bottom right and will push on you to the top left. That is thrust 101.

                Now figure a plane flying in a horizontal trajectory but at a somehow high angle of attack of let's say 10 degrees (i.e pitch 10 degrees nose up since the path has 0 degrees). The air is coming from the left but when it leaves the engine (no matter how much it increases the speed inside the engine) it will have a "new" downwards component (since the air is forced to align with the engine). So the plane had zero vertical component before the engine but a downward component after the engine, we've just accelerated the air down (again even if it leaves the engine at the same velocity that in entered) so we have an upwards force made by the air on the engine. Of course if you increase the speed of the air inside the engine, then you will accelerate it more both horizontally and vertically, so this 90 degrees force (relative to the airspeed vector) will also increase. And the higher the AoA the more you deflect the air so again more 90 degrees force. For practical reasons this 90 degrees force is split in 2: A force that is always there for deflecting the air even if you are not increasing its speed (a force that would still be there if instead of the engine you had just an empty pipe, that acts like a sort of aerodynamic surface), which is proportional to the speed squared and the AoA (i.e. the intensity of the deflection), plus an additional force that is proportional to the thrust (even if the thrust is negative) and again to the AoA.

                We can treat the first part of this 90-degree force as an aerodynamic force that is just pat of the normal longitudinal stability equation (like if you had a small lifting surface where the engines are) and then for the second force, let's imagine that we have constant thrust just to simplify a bit, you have a force that is proportional to the AoA. Ok, force proportional to the AoA and that acts at 90 degrees of the airspeed vector... if it is acting at a distance from the CG it creates a pitching moment... This is another longitudinal stability force! If the engines are aft of the CG, it is stabilizing (a higher AoA will produce a higher vertical upwards force that will push the nose down) and if it's ahead of the CG it will disstabilizing (an higher AoA will create a higher force ahead of the CG that will push the nose further up).

                Now, this effect is very well understood, easily taken into account, and managed satisfactory in airplanes that had a very powerful engines (compared with the airplane size and weight) that were WAY ahead of the CG. Think about any propeller airplane (yes, the prop has the same effect). Think of the King Air with 2 propellers spinning ahead of the pilots' feet, or the Pilatus PT-12. And not even mention something like a P-51 Mustang or any of the Red Bull racers.

                As bad as this situation was and is, this should be an easy fix for Boeing. Possibly they will issue special procedures as containment action and then tweak some software code in the logic. More or less like the AT and RAlt issue (Turkish).

                --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                --- Defend what you say with arguments, not by imposing your credentials ---

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                • Originally posted by Gabriel View Post
                  We have 2 parts.
                  The underslung-engines pitch coupling is quite obvious....this should be an easy fix for Boeing. Possibly they will issue special procedures as containment action and then tweak some software code in the logic.
                  Stimulatingly Gabrielian. Thank you.

                  Q: How does the location of the thrusting force above or below the CoG effect pitch-coupling?

                  Q: How does the reduction of horizontal stabilizer area (for reduced drag and fuel efficiency) effect pitch-coupling?

                  More or less like the AT and RAlt issue (Turkish).
                  They replaced the entire unit with a more reliable one. They did this, not after a dozen or so reported incidents had revealed the danger, but after a fatal crash had brought it to the public's attention. And it was a recommended upgrade on existing aircraft, not a requirement.

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                  • Originally posted by Evan View Post
                    Q: How does the location of the thrusting force above or below the CoG effect pitch-coupling?
                    The farther the thrust line is from the CG, the greater the effect will be.

                    Try this: place a CD (or DVD) on a table. Try to slide it across the table by pushing on its side. If you push directly in the middle you will probably succeed. If you push off-center, the disc will want to rotate rather than slide.

                    The above is technically not quite the same as in an aircraft since it's based on the difference between the thrust line and the disc's center of drag rather than its center of gravity, but the effect is similar.

                    Originally posted by Evan View Post
                    Q: How does the reduction of horizontal stabilizer area (for reduced drag and fuel efficiency) effect pitch-coupling?
                    Gabe can probably speak to this better than I, but I'm going to guess to a first order there's no difference, as pitch coupling is mostly related to the relation between the thrust line and the aircraft's center of gravity. However a smaller horizontal stabilizer could have two secondary effects: first it will allow the aircraft's pitch to change more rapidly, and a smaller stabilizer is likely to have a smaller elevator which would reduce the pilot's ability to compensate for unwanted pitch changes.
                    Be alert! America needs more lerts.

                    Eric Law

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                    • Originally posted by BoeingBobby View Post
                      Oh for the good old days, fly with the side windows open.[ATTACH=CONFIG]19864[/ATTACH]
                      And if it's raining, bring a sump pump for the cockpit!

                      OT: I actually rode on a DC-3 a few months ago! I was at an airshow and they were selling rides. I told my wife that of all my flights on airliners, it's the only one I wished had lasted longer.
                      Be alert! America needs more lerts.

                      Eric Law

                      Comment


                      • Originally posted by elaw View Post
                        Gabe can probably speak to this better than I, but I'm going to guess to a first order there's no difference, as pitch coupling is mostly related to the relation between the thrust line and the aircraft's center of gravity. However a smaller horizontal stabilizer could have two secondary effects: first it will allow the aircraft's pitch to change more rapidly, and a smaller stabilizer is likely to have a smaller elevator which would reduce the pilot's ability to compensate for unwanted pitch changes.
                        The size of the horizontal tail doesn't affect the how much the thrust-generated pitching moment changes when the thrust changes, but it will affect how much the angle of attack and pitch rate changes as a result of the change in thrust. In the obvious inverse way: the bigger the horizontal tail, the smaller the effect in the pitch dynamic caused by any given pitching moment.

                        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                        --- Defend what you say with arguments, not by imposing your credentials ---

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                        • Push forward, houses get bigger. Pull back houses get smaller. Keep pulling back and houses get bigger again.

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                          • Originally posted by BoeingBobby View Post
                            Push forward, houses get bigger. Pull back houses get smaller. Keep pulling back and houses get bigger again.
                            There we have it...what Airline pilots are taught and what we always suspected: not enough emphasis on stall prevention!

                            This is in strong conflict with the Parlour Talker's procedure: Push forward, houses get bigger. Pull back houses get smaller. BUT DO NOT PULL BACK RELENTLESSLY OR they get bigger again followed by getting smaller as they burn to the ground after you crash into them.
                            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                            Comment


                            • Originally posted by 3WE View Post
                              There we have it...what Airline pilots are taught and what we always suspected: not enough emphasis on stall prevention!

                              This is in strong conflict with the Parlour Talker's procedure: Push forward, houses get bigger. Pull back houses get smaller. BUT DO NOT PULL BACK RELENTLESSLY OR they get bigger again followed by getting smaller as they burn to the ground after you crash into them.
                              My point exactly, and not in blue font!

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                              • Originally posted by BoeingBobby View Post
                                My point exactly, and not in blue font!
                                So- a serious question:

                                Is it crappy training

                                OR

                                Is it that even with the best efforts, you can't weed out all the goofuses.

                                OR

                                Do people simply brainfart sometimes.

                                (I know the absolute "OR" may not be valid).
                                Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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