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On the 747-200 the engines were tilted up to "meet the air at a streamline angle": enter image description here

Source: Wikimedia Commons enter image description here Wikimedia Commons enter image description here Source: RcGroups.com

But in the 747-8, the engines are not tilted up: enter image description here

Source: Airways News

Does anybody know why the engines are not tilted up anymore?
Thanks!

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    $\begingroup$ The -400 has no tilt too: avionale.com/2010/04/… $\endgroup$ – SMS von der Tann Mar 12 '16 at 15:02
  • $\begingroup$ tilted up to "meet the air at a streamline angle" Where did you quote this from? $\endgroup$ – TomMcW Mar 12 '16 at 19:30
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    $\begingroup$ That question is relating to DC-9/MD-80 which have rear-mounted engines. The angle on those is related to them being behind the wing. I looked at a bunch of pics of 747-2 and they don't appear to be tilted upward. Are you sure it's not just the angle of your photo? Usually wing-mounted intakes are slanted slightly downward if anything like in the B737 $\endgroup$ – TomMcW Mar 12 '16 at 19:54
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    $\begingroup$ Cool. I'm hoping somebody answers this. I always thought you wanted thrust vector to be pretty much parallel to the chord and the intake slightly downward to maximize efficiency during the higher AoA during take off and climb. I could swear@PeterKämpf talked about this before in an answer but I can't find it $\endgroup$ – TomMcW Mar 12 '16 at 20:23
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    $\begingroup$ @TomMcW: Yes, I did, but that question asked why the intake plane is tilted downward. This is to balance the flow conditions between cruise and take-off, but the reason for the unusual tilt of the 747-200 engines is unknown to me. I guess this is an artifact of wing twist - when loaded, the swept wing will reduce its local incidence. $\endgroup$ – Peter Kämpf Mar 13 '16 at 2:07
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On the 747-8 the engines are heavier and mounted farther ahead of the wing. This results in a torsion moment which reduces the incidence of the wing. As a result, even the unloaded wing will have a lower incidence than that of the older 747-200.

Transaero Boeing 747-346

Boeing 747-346 of Transaero (picture source). Note that the engines are no longer tilted up (and consequently the wing tip is tilted leading edge down). The lift force twists the wing such that the incidence in flight is lower than when unloaded and on the ground, especially when the high-lift devices are extended.

Also please make sure to read @Lilibete's answer. I don't know who thought it appropriate to downvote it - it contains another reason for the change in inlet angles, namely the difference in avoidance strategies for foreign object damage.

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    $\begingroup$ @ymb1: Thank you for the hint, and next time you may very well do the change directly. It is a clear improvement. $\endgroup$ – Peter Kämpf Sep 22 '17 at 23:20
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Peter's answer explains one aspect of the appearance of the engine angle being the way it is between the -200 engine angle relative to the angle of attack on the ground versus in the air and between the -200 and the -8.

There are also the considerations of FOD (foreign object debris) entering into the engine and the account for accessories off of the engine that are concealed by the cowlings. FOD is handled very different today (using bleed air screens) than when the -200 was designed when the angle on the engine's inlet was used. As far as the accessories that are concealed under the cowlings, they are also vastly different.

Another thing that drives the shape and size of the fan cowls is the bypass ratio. For the -8, the bypass ratio at takeoff is 9.0/1, so 90% of the air going through the fan goes into the bypass section. For the -200, the bypass ratio is 5.72/1, so roughly 83% of the air is bypassed. While the doesn't seem like a large difference, the geometries are significantly different. Those differences mean that the risk of FOD going through the fan versus the core are different and yield different inlet angles.

Other factors that make the appearance of the overall angle being different between the two versions are going to be side slips inlet designs and just the camera angles taking the pictures. The overall angles of the engine itself may not actually be that different.

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  • $\begingroup$ David, Peter's answer explauns why the angle appears to change due to the aerodynamic loading and unloading of the wings. We could talk appearance in this thread, but we won't get the answer of an actual angle because that would hold trade secret information that is very specific to performance. $\endgroup$ – Lilibete Mar 27 '16 at 15:57
  • $\begingroup$ Your explanation of the bypass ratio is off. 100% of the air passes through the fan always. then 90% goes around the compressor and turbine stages to produce the thrust and the remaining ten goes through these stages to produce the power required to run the fan. Note this is for the 9:1 bypass ratio. $\endgroup$ – DJ319 Jan 2 '18 at 13:52

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