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

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  •  Question: (4+ / 0-)
    Recommended by:
    jwinIL14, sawgrass727, Simplify, ER Doc

    I've always wondere why, for jets that have underwing engines, the engine is so far forward of the wing.  does this have something to do with balancing the aircraft's weight?  Or is there another reason?

    The road to Hell is paved with pragmatism.

    by TheOrchid on Tue May 07, 2013 at 05:19:14 PM PDT

    •  Great question (5+ / 0-)

      Wish I knew the answer.

      I'm purely guessing here, but it might be to keep the engines away from the flaps when the flaps are extended.

      I'm sure someone will come along that knows more about this than I do.

      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 Tue May 07, 2013 at 05:34:01 PM PDT

      [ Parent ]

      •  Major, is there an exception to that longer than (1+ / 0-)
        Recommended by:
        ER Doc

        it is wide rule in the form of that solar-powered airplane currently crossing the country?

        Pics make me think it's Buff-sized wingspan, maybe plus, on a Spirit of St. Louis or smaller fuselage.

        LBJ, Lady Bird, Van Cliburn, Ike, Ann Richards, Barbara Jordan, Molly Ivins, Sully Sullenburger, Drew Brees: Texas is NO Bush League!

        by BlackSheep1 on Tue May 07, 2013 at 07:05:33 PM PDT

        [ Parent ]

        •  What "rule" is this? (2+ / 0-)
          Recommended by:
          northsylvania, BlackSheep1

          There are many planes that are much wider than they are long. (E.g., the ASH-25 in the video I posted above. It has an 83' wingspan, with a 29.5' fuselage.)

          They fly fine, but but roll response is much slower than pitch response. There can also be some inter-axis couplings, but AFAIK that tends to be more of a problem with short wings (e.g., F-104).

          •  well, I was extrapolating (0+ / 0-)
            haven't talked about lateral (side to side) balance, just fore and aft. It's not a big deal with the passengers or cargo. The plane is much longer than it is wide, so there's not a lot of side to side moment. If the all chickens move to one side of the plane we're not going to do a barrel roll.

            LBJ, Lady Bird, Van Cliburn, Ike, Ann Richards, Barbara Jordan, Molly Ivins, Sully Sullenburger, Drew Brees: Texas is NO Bush League!

            by BlackSheep1 on Wed May 08, 2013 at 07:45:38 AM PDT

            [ Parent ]

            •  Some care... (1+ / 0-)
              Recommended by:
              BlackSheep1

              has to be taken in larger aircraft with relation to lateral w/b.

              As Major Kong noted, the maximum allowable fuel imbalance in his aircraft-of-choice has a limit of 1950 lbs. It also has interactions with aircraft structure -- wings usually or often have multiple tanks to stop or limit fuel movement between the wing root and wing tip. Fuel sloshing from one end to the other could make even an aircraft with identical amounts of fuel in each wing uncontrollable if most of the weight in one wing sloshes to the root and the other to the tip.

              This is also a problem in the longitudinal case -- there were fatal P-39 airacobra accidents in WW-II due to longitudinal fuel sloshing at landing time. (I don't know of any references to this, but heard it from a Lt. Col. who was tasked with investigating after ferry-flight crashes on successive days at the downwind-base turn.)

              •  wasn't one of those close to the end of (0+ / 0-)

                the service life of that aircraft?

                LBJ, Lady Bird, Van Cliburn, Ike, Ann Richards, Barbara Jordan, Molly Ivins, Sully Sullenburger, Drew Brees: Texas is NO Bush League!

                by BlackSheep1 on Wed May 08, 2013 at 10:25:25 AM PDT

                [ Parent ]

                •  Not AFAIK... (1+ / 0-)
                  Recommended by:
                  BlackSheep1

                  These were being ferried to AK at the time, so they were likely destined for the Soviet Union. I don't know any reason service life should have entered into it, but apparently it was fairly sensitive to the total amount of fuel in the tank, and the ferry leg to Alaska was just the right distance if started w/ full tanks.

                  •  oh, no, I think I was thinking when they (0+ / 0-)

                    were being "retired" from US service as an aircraft, on the whole.

                    LBJ, Lady Bird, Van Cliburn, Ike, Ann Richards, Barbara Jordan, Molly Ivins, Sully Sullenburger, Drew Brees: Texas is NO Bush League!

                    by BlackSheep1 on Wed May 08, 2013 at 09:33:16 PM PDT

                    [ Parent ]

      •  At least partly... (5+ / 0-)
        Recommended by:
        exatc, VTCC73, Simplify, ER Doc, kurt

        because there tends to be significant interference drag between the engine nacelles and the wings. So the engine pylons need to be fairly long, and that works much better for pylons under tension.

        Improved aero computer codes have relaxed that a bit; newer 737s managed to get larger high-bypass engines working efficiently, but it took a lot of work and a lot of fussing about airflow around the nacelles to do it and still keep ground clearance. (You can see a lot of small air diverters around the wing and nacelle to help with the drag.)

        •  I don't know why… (2+ / 0-)
          Recommended by:
          eyesoars, ER Doc

          But after years of seeing the 737s with the original "cigar tube" engines, the first time (and just about ever since) I saw one with the high-bypass engines and the "flat tire" nacelle bottom, I laughed out loud…or at least snorted.

        •  aside from drag (2+ / 0-)
          Recommended by:
          eyesoars, Major Kong

          Locating the engine, especially the mount in the forward aspect of the wing...

          Recall from Major Kong's piece stating that "center of gravity must be forward of center of pressure".

          Between those two load factors, the entire weight of the airplane is in the forward third of the wing (approximate value).

          Conveniently, that is where the wing is thickest and built like the proverbial brick man-cave...
          The height of the wing in that region is tallest and most resistant to all loads and distortions.
          To quickly understand this, imagine if a wing had to bear the strength at the thin trailing edge; developing a structural strength on a razor's edge would be impossible.

          Locating the engine mid-wing would be aerodynamically preferred.
          However, doing a "fix", "on the ramp", in a buried engine rather than in a stand-off nacelle enclosed engine has to be considered.

          Enagaged activism wins elections. 100 million words on liberal/progressive websites gets beat by one new GOP voter casting their vote.

          by Nebraska68847Dem on Wed May 08, 2013 at 05:59:07 AM PDT

          [ Parent ]

    •  Torque on the wing (9+ / 0-)

      Long, thin wings, especially swept wings, tend to twist under load.  My impression is that those engine pylons are intended to let the engines pull the wing rather than twist it.  

      So why to they put the engines under the wing instead of embedding it inside?  Two reasons--the less crap on the wing, the more efficient it is, and those embedded engines were a fire hazard, because most modern aircraft have "wet" wings--fuel tanks inside the wing.  

      The KC-135's I flew, and presumably most other modern jets, use spoilers for roll control because it reduces the twisting forces on the wing.  Rather than making one wing rise and the other drop, like ailerons, the spoilers make one wing have more drag and less lift, causing the same result with less structural stress.  

      How many wrongs does it take to make a right?

      by pdknz on Tue May 07, 2013 at 06:08:24 PM PDT

      [ Parent ]

    •  I did some reading and found 2 other answers... (1+ / 0-)
      Recommended by:
      eyesoars

      ...in addition to the helpful ones above:

      1. Pushing the engine forward of the wing unexpectedly reduces drag between the engine and wing, increasing efficiency.

      2. Pushing the engine forward of the wing also means that, if one of the engines' rotors fail, the flying parts exploding from the engine casing do NOT go directly into the wing fuel tanks (and thereby does not create a fireball).  I find this strangely comforting, given the scenario...

      The road to Hell is paved with pragmatism.

      by TheOrchid on Wed May 08, 2013 at 09:30:30 AM PDT

      [ Parent ]

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