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Lion Air 737-Max missing, presumed down in the sea near CGK (Jakarta)

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

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  • Gabriel
    replied
    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.

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  • elaw
    replied
    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.

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  • elaw
    replied
    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.

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  • Evan
    replied
    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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  • Gabriel
    replied
    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).

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

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  • Black Ram
    replied
    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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  • Evan
    replied
    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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  • 3WE
    replied
    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.

    Leave a comment:


  • Evan
    replied
    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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  • 3WE
    replied
    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.

    Leave a comment:


  • Evan
    replied
    Originally posted by Gabriel View Post
    Can you please provide more explanation of what phenomena you have in mind in reference to the parts I highlighted in bold font?
    The STS system was needed to certify the 737NG, due to more pronounced pitch characteristics in certain situations, such as with high thrust, light gross weight and aft CoG. As I understand it, this was attributed to the relocation of the engine mounts to a position more forward of the wing, and perhaps the increased thrust of the new engines. The new power/geometry would have more pronounced pitch-up effects in these situations. As I understand it, the MAX required Boeing to further compensate for this issue with a replacement for the STS that, as I understand it, has more pronounced behaviors.

    So now tell us about the physics...

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  • Gabriel
    replied
    Originally posted by Evan View Post
    This comes down to Boeing's decision to postpone the Y1 project, a clean-sheet replacement to the 737-757. The 737 was designed for turbojets and regional airports without jetways or large airstairs. Its achilles heel is its limited undercarriage: it sits too low to the ground to accommodate the kind of high-bypass turbofans that the modern age requires. Nevertheless, Boeing chose to crutch the 737 into the 21st-century by mounting the larger engines forward of the wing. This allowed them to squeeze in bigger fans, but at the cost of moving the center of thrust forward, where it would have a more pronounced pitch-coupling effect.

    The NG's therefore required the STS system to be certified. That worked out. There have been no upset incidents that I know of involving that system.

    But the MAX required even bigger fans, and, once again, Boeing responded by pushing the engines out further ahead of the wing. The MCAS system is apparently the STS replacement needed to certify the MAX. The concerns regarding the 737's pitch coupling potential only increased and perhaps something more extreme was required to give it the same flying characteristics.
    Can you please provide more explanation of what phenomena you have in mind in reference to the parts I highlighted in bold font?

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  • Evan
    replied
    Originally posted by Black Ram View Post
    There has been a lot of talk about this. Some have suggested Boeing needed the new system on the MAX to make it behave the same way as the NG, despite the heavier engines and altered center of gravity. This was needed for certification.
    This comes down to Boeing's decision to postpone the Y1 project, a clean-sheet replacement to the 737-757. The 737 was designed for turbojets and regional airports without jetways or large airstairs. Its achilles heel is its limited undercarriage: it sits too low to the ground to accommodate the kind of high-bypass turbofans that the modern age requires. Nevertheless, Boeing chose to crutch the 737 into the 21st-century by mounting the larger engines forward of the wing. This allowed them to squeeze in bigger fans, but at the cost of moving the center of thrust forward, where it would have a more pronounced pitch-coupling effect.

    The NG's therefore required the STS system to be certified. That worked out. There have been no upset incidents that I know of involving that system.

    But the MAX required even bigger fans, and, once again, Boeing responded by pushing the engines out further ahead of the wing. The MCAS system is apparently the STS replacement needed to certify the MAX. The concerns regarding the 737's pitch coupling potential only increased and perhaps something more extreme was required to give it the same flying characteristics.

    All this because the airframe, designed in the mid-1960's, wasn't suitable for modern turbofans.

    The other issue, which I think we will be hearing a lot more about, is how Boeing marketed the 737MAX on the merits of cockpit commonality with customers' existing 737NG fleets. This represented a large cost-saving advantage over the A320 to operators not already flying that type. Boeing was telling potential customers that the 737MAX would require only 16 hours of type-specific transitional training. Clearly, this training did not address the new pitch augmentation system.

    If all this proves true, then I think the fix will lie in adding at least one layer of redundancy to the MCAS system and more type-specific training, both on reacting to a malfunction and in flying the 737MAX more cautiously with the system inoperative. Because, without a properly-functioning MCAS, the 737MAX is apparently considered too unsafe to certify...

    Note: MCAS is even not listed in the 737MAX MMEL found online. There is a listing for "speed trim function", which indicates that two units are present and dispatch requires one to be confirmed operational, provided the faulty one is disconnected.

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