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Thread: Can you believe a company stretched a fuselage, put bigger engines on it,

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    Senior Member 3WE's Avatar
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    Default Can you believe a company stretched a fuselage, put bigger engines on it,

    and had to rig something for stall recovery?!?!

    https://www.youtube.com/watch?v=vsRxWspTTJI
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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    That's really cool. I had no idea, thanks.

    So I guess their ancient (compared to the MAX) system employs double redundancy - it's controlled by two behind-the-yoke in-seat full authority control devices.

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    Quote Originally Posted by Black Ram View Post
    That's really cool. I had no idea, thanks.

    So I guess their ancient (compared to the MAX) system employs double redundancy - it's controlled by two behind-the-yoke in-seat full authority control devices.
    Nope. The pylon flaps are activated by pilot action when the control column is moved fully or almost fully forward.

    The DC-9 family has an aerodynamically powered elevator, meaning that the pilot actually moves small "control tabs" located at the trailing edge of the elevator and mechanically connected to the column and it is the aerodynamic force on these tab what moves the elevator (on the ground at no or very low speed, the movement of the control column moves the tabs but, lacking an aerodynamic effect, the elevator itself doesn't move, even further, it is no unusual to see that the left and right elevators are "split" when the plane is parked).

    In certain conditions, at very high angles of attack and when you need to reduce that angle of attack, like during a stall recovery, the authority of these control tabs can lack enough aerodynamic force to produce a high-enough nose-down elevator deflection (when you need it most). To aid in this situation, since the oldest DC-9, the elevator has been fitted with an hydraulic elevator boost that pushes the elevator itself (not the tabs) in the nose-down direction when the control column is moved fully or almost fully forward. I believe that the same signal triggers the deflection of the pylon flaps in the MD-90.

    Dummy?

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    In certain conditions, at very high angles of attack and when you need to reduce that angle of attack, like during a stall recovery, the authority of these control tabs can lack enough aerodynamic force to produce a high-enough nose-down elevator deflection (when you need it most).
    Isn't that a situation where even powered elevators can have insufficient authority? Isn't that where rudder becomes your best redundancy?

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Isn't that a situation where even powered elevators can have insufficient authority? Isn't that where rudder becomes your best redundancy?
    What I was saying in the part you quoted is that the control column / control tabs will not have enough authority to deflect the elevator down enough. We are not yet in the situation where the elevator full deflection (however powered) will not have enough authority.

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    Quote Originally Posted by Gabriel View Post
    Nope. The pylon flaps are activated by pilot action when the control column is moved fully or almost fully forward.

    The DC-9 family has an aerodynamically powered elevator, meaning that the pilot actually moves small "control tabs" located at the trailing edge of the elevator and mechanically connected to the column and it is the aerodynamic force on these tab what moves the elevator (on the ground at no or very low speed, the movement of the control column moves the tabs but, lacking an aerodynamic effect, the elevator itself doesn't move, even further, it is no unusual to see that the left and right elevators are "split" when the plane is parked).

    In certain conditions, at very high angles of attack and when you need to reduce that angle of attack, like during a stall recovery, the authority of these control tabs can lack enough aerodynamic force to produce a high-enough nose-down elevator deflection (when you need it most). To aid in this situation, since the oldest DC-9, the elevator has been fitted with an hydraulic elevator boost that pushes the elevator itself (not the tabs) in the nose-down direction when the control column is moved fully or almost fully forward. I believe that the same signal triggers the deflection of the pylon flaps in the MD-90.

    Dummy?

    Still the pilots are the ones who have control over those flaps. Does it matter that much if they are incorporated into the movement of the control column instead of having a separate control lever (I think it would be unnecessarily complicated)

    I din't know the DC-9 had elevator trim tabs. So thanks.

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    Quote Originally Posted by Black Ram View Post
    Does it matter that much if they are incorporated into the movement of the control column instead of having a separate control lever.
    If something matters, it's IF this makes it a special TYPE of plane which requires special stall training (which it might not).

    I think we tend to agree that it's a nice, simple, intuitive design that may be less likely to call for a severe dive if a single sensor goes bonkers...(Then again, what if the little switch at the front of the yoke shorts out?) (I'm guessing the flaps are NOT as powerful as an unlimited nose-over stabilator setting.)
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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    Senior Member Evan's Avatar
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    At least MD had the good sense to mount the engines for a low-slung aircraft on the fuselage. If Boeing had killed off the 737 and built the 7J7 (with conventional turbofans), it could have easily mounted the LEAP-1A. And there would have been no need for artificial stability workarounds.

    But... they didn't.

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Black Ram View Post
    Still the pilots are the ones who have control over those flaps. Does it matter that much if they are incorporated into the movement of the control column instead of having a separate control lever (I think it would be unnecessarily complicated)
    No, it would matter if the system was autonomous, actuating by itself without warning or annunciation to the pilots, and its actuation was based on a single and fallible sensor, and the system was not decried in the flight manual or informed to the pilots during training. THEN it would matter.

    I din't know the DC-9 had elevator trim tabs.
    And you were right. It doesn't. It has 3 kind of tabs: Control tabs, geared tabs and anti-float tabs. None of them is a trim tab.

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    Quote Originally Posted by Gabriel View Post
    And you were right. It doesn't. It has 3 kind of tabs: Control tabs, geared tabs and anti-float tabs. None of them is a trim tab.
    LOL. Thanks man Are those all a type of servo tabs?

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Black Ram View Post
    LOL. Thanks man Are those all a type of servo tabs?
    I intentionally avoided the term "servo tab" (that I used to use a lot in the past) because of 2 reasons: It is used for different types of tabs and the names that I used are the ones that MD used in their manuals.

    Each side of the elevator is independent from the other, so they can move independently and they do, especially on the ground where it is common to see a "split elevator". Under normal conditions, with enough air flowing, the air flow keep both elevators aligned at the same angle and they move in unison (just due to being subject to the same aerodynamic forces).

    Each side of the elevator has 3 tabs: A control tab, a geared tab, and an anti-float tab (and any of them may or may not be called servo tab, depending on how you define that term).

    The control tab is mechanically connected to the control column on the same side (as a side note, both the control columns are connected to each other with a spring-loaded torque tube that keeps both columns, and hence both control tabs, synchronized as long as a big-enough differential forces is not applied to the columns, this is a feature designed to be able to control the pitch with one column if the other one becomes jammed).
    The control tabs move independently of the elevator (except due to aerodynamic forces). For example, parked on the ground moving the control columns move the control tabs but not the elevator, and if you move the elevator by pushing directly on it, neither the control tabs will deflect nor the control columns will move. With enough airspeed, the control tabs generate an aerodynamic force that move the elevator. Yes, in the same way that the trim tab of a Cessna 172, except that the trim tab is moved the trim wheel and not by the yoke and the control tab is moved by the yoke and not by the trim system.

    The geared tab is mechanically linked to a fixed part of the structure, in such a way that they move in sync with the elevator but in the opposite direction (i.e. the geared tab end up moving in the same direction than the control tab, but is not controlled by the pilot: if the elevator doesn't move, the geared tab doesn't move). In this way, the displacement of the elevator is greater than what would be accomplished by the control tabs only or, another way to see it, the pilot can achieve a greater deflection of the elevator with less deflection of the control column and the control tab and less stick force than it would be achieved without the geared tab.

    With enough airflow, if you videotape a smooth operation of the elevator, what you would see is that 2 these tabs (control tab and geared tab) go down as the elevator goes up. But you cannot tell which tab is which. However, if it was possible to make a super-abrupt instantaneous input on the control column (say a pull up), what you would see is the control tab moving instantly down and THEN the elevator moving up while the geared tab moves down at the same time.

    Lastly, the anti-float tab, I don't know exactly how it is geared but it moves up when the elevator exceeds a certain deflection up (I think it was 10 degrees). Why? This elevator is a free-floating one. It's position depends on the position of the control tabs but also on the angle of attack. At high angles of attack, the elevator would float at a higher angle than it would at a smaller angle of attack, even with the same control column / control tab input, because the air is hitting the elevator more from below. This can make it difficult to recover from very high angles of attack (like a full-blown stall). The anti-float tab ensures that the elevator will not float so high, thus helping in the recovery of those situations.

    If all that fails, then applying full nose-down control column input will activate the hydraulic boost that will push the elevator full nose-down directly, no matter what the tabs are thinking or doing. In the MD-90, that same full control column input will trigger the deflection of the pylon flaps, with the same objective: help in pushing the nose down.

    And lastly (now I mean it), trim is done with a movable horizontal stabilizer controlled by a jackscrew with electric motors, pretty much like in most transport category jets.

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    Senior Member 3WE's Avatar
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    As long as we are talking cost cutting and smart engineering- how many cars have reverse gears with higher ratios than first gear.

    Backing is more difficult, has generally less visibility and is much more dangerous...shouldn't reverse be a REAL low gear to encourage safer, slower backing?
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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    Quote Originally Posted by Gabriel View Post
    I intentionally avoided the term "servo tab" (that I used to use a lot in the past) because of 2 reasons: It is used for different types of tabs and the names that I used are the ones that MD used in their manuals.
    Awesome info, thanks!

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    Quote Originally Posted by 3WE View Post
    As long as we are talking cost cutting and smart engineering- how many cars have reverse gears with higher ratios than first gear.

    Backing is more difficult, has generally less visibility and is much more dangerous...shouldn't reverse be a REAL low gear to encourage safer, slower backing?

    No way, man!

    https://youtu.be/14Pl4U8NZog?t=57

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