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View Diary: How Airliners Work - Weight and Balance (160 comments)

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  •  Question: Since a plane's weight decreases (0+ / 0-)

    continuously during flight, how is lift continuously decreased to maintain a constant altitude?

    •  Good question (1+ / 0-)
      Recommended by:
      AnnArborDem

      I had to think about that for a minute.

      Assuming we're at a constant airspeed, we can maintain the same amount of lift with less angle of attack and use less thrust as a result.

      This was very noticeable in the B-52. At very heavy weights we would fly slightly nose high. At lighter weights we would have a very noticeable nose down pitch.

      If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

      by Major Kong on Thu May 09, 2013 at 01:32:26 PM PDT

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      •  I assume, then, that the computer systems of (1+ / 0-)
        Recommended by:
        RiveroftheWest

        modern jetliners continuously adjust the thrust and angle of attack to maintain a constant altitude.  Otherwise it seems that the aircraft would accelerate upward until the reduced air pressure reduced the lift to equal the plane's weight.

        •  You are correct (1+ / 0-)
          Recommended by:
          RiveroftheWest

          At cruise we're normally on autopilot and have the auto-throttles engaged.

          We dial in what speed and altitude we want it to hold and it does the rest.

          On long flights we will sometimes "step climb" up to a higher altitude after we burn some fuel off.

          Going higher gets you better fuel economy, once you're light enough to get up there.

          I plan on devoting a diary to the auto flight systems  sometime in the future.

          If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

          by Major Kong on Fri May 10, 2013 at 04:50:14 PM PDT

          [ Parent ]

    •  With Concorde it was relatively simple.... (2+ / 0-)
      Recommended by:
      AnnArborDem, RiveroftheWest

      you simply pumped fuel from the rear to the front to alter the trim :) On G-BOAF, I watched this happen!

      'Per Ardua Ad Astra'

      by shortfinals on Thu May 09, 2013 at 08:15:52 PM PDT

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      •  They had to build it that way (2+ / 0-)
        Recommended by:
        AnnArborDem, RiveroftheWest

        When you go supersonic the center of lift changes noticeably and the center of gravity has to change with it.

        Not significant on a fighter-sized airplane but very much so on something like Concorde.

        I've talked to B-1 guys and they said they didn't even really know where the fuel was at any given moment because the plane was automatically shifting it around.

        If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

        by Major Kong on Thu May 09, 2013 at 10:53:44 PM PDT

        [ Parent ]

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