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Stratolaunch's engines hang from pylons, not under the wing, but entirely forward of the wing's leading edge. (Even farther forward than, say, the 747's.) What benefit does this have, to compensate for the heavier pylons?

view of Stratolaunch from directly abeam

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    $\begingroup$ Good question. I wonder if and how the center of gravity moves with the payload? $\endgroup$ – Peter Nov 26 '20 at 19:33
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    $\begingroup$ I bet the engine are placed as they are to achieve certain CoG. The CoG is the same regardless of payload. Otherwise this contraption would not fly either with, or without the spacethingy it carries. $\endgroup$ – Jpe61 Nov 26 '20 at 19:44
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    $\begingroup$ Jpe61's guess would explain why they didn't just move the wing forward to where the engines are. That would move the heavy spacethingy forward too. $\endgroup$ – Camille Goudeseune Nov 26 '20 at 20:17
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    $\begingroup$ Looks very similar to the B-52 engine placement to my naive eye! geaviation.com/military/engines/b-52 $\endgroup$ – user2705196 Nov 27 '20 at 14:58
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The forward location has two benefits:

  • Wing upwash is smaller, so the flow direction at the intake varies less with speed and altitude. This makes the intake more efficient and requires less robustness in its layout.
  • The forward location shifts the wing's center of gravity forward and gives it better flutter damping. How that works is explained here.

Wing torsion in combination with the two fuselages will produce a flutter mode where both fuselages pitch in opposite direction, torquing the center wing in the process. The slender wing in combination with the high pitch inertia of the fuselages will result in a very low flutter frequency. I suspect that damping this mode is of paramount importance for the Stratolaunch.

I also suspect that the high wing on a high fuselage allows more vertical distance between engine and wing, so the exhaust stream will pass below the lowered flaps. The high turbulence in the exhaust stream would put additional loads on the flaps; to avoid this, Boeing uses exhaust gates or a gap in its trailing edge flaps. Such a gap would not be needed with the lower engine position of the Stratolaunch. However, to place them lower does not require to shift them forward, too.

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  • $\begingroup$ Neat, I'd never thought of the engine as a mass balance. But these two benefits apply to many airplanes, while other airplanes rarely (ever? B-58 Hustler?) mount engines so far forward. What's special about Stratolaunch? $\endgroup$ – Camille Goudeseune Nov 26 '20 at 23:42
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    $\begingroup$ @CamilleGoudeseune I'm not familiar with the Stratolaunch, but I suspect the high aspect ratio gives it more torsional deformation / less torsional stiffness than what is usual on other airplanes. The slender mid-section in combination with the high inertia of the two fuselages will produce a very low antimetric torsion mode (where both fuselages pitch in opposite direction), and damping this would be helpful. I expanded the answer accordingly. $\endgroup$ – Peter Kämpf Nov 27 '20 at 4:33
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    $\begingroup$ Yikes, one nose up and the other down, increasing in amplitude... "you will not go to space today." $\endgroup$ – Camille Goudeseune Nov 27 '20 at 4:38
  • $\begingroup$ Another reason is to reduce the shearing force on the engine pylon. To understand this, consider that the engines are producing thrust, and the wings and fuselages are producing drag, so the engine pylon is pulled backwards at the wing mount-point and forwards at the engine mount point. If the pylon were vertical then these two forces would be shearing forces against the mount points. By making the pylon close to horizontal in angle, the two forces are more closely opposed, hence the pylon undergoes mostly tension forces instead, which creates much less stress on the pylon and mount points. $\endgroup$ – James Mc Nov 27 '20 at 6:43
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    $\begingroup$ @JamesMc: The forces don't change with the pylon angle, so the loads on the attachments points are unaffected. This concerns only the stresses inside the pylon structure. By using a properly designed truss, those will become tension and compression loads only. $\endgroup$ – Peter Kämpf Nov 27 '20 at 7:29

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