Source: twitter.com
I saw a picture of a 777 from overhead and I was wondering why the engines point "in" to the fuselage, rather than being parallel to the fuselage.
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1$\begingroup$ related $\endgroup$– FreeManMar 20, 2020 at 16:51
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3$\begingroup$ they look like they are pointing out to me... but then, I guess I'm looking at it from the point of view of which way the exhaust is going. $\endgroup$– MichaelMar 23, 2020 at 2:36
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1 Answer
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It's called 'toe-in' and it's done basically to match the local airflow which is slightly divergent (heading outboard along the underside of the wing). If the engines were mounted exactly parallel to the fuselage, they would be moving slightly obliquely through the local airflow and therefore incurring extra unnecessary drag.
Another view of a 777 showing the engine toe-in:
Source: airliners.net
The Boeing 747 has a 2 degree toe-in on all four engines, as can be seen in the following plan:
[Key: BBL 0 = Body Buttock Line zero, the centreline of the fuselage; WBL = Wing Buttock Line, parallel to that centreline; NAC BL0 = Nacelle Buttock Line 0, the centreline of the nacelle body]
and in this photo:
For rear-engined aircraft, the opposite is true: the engines are mounted slightly' toe-out' to match the local airflow which is converging around the tail of the aircraft.
answered Mar 20, 2020 at 13:53
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7$\begingroup$ Related: Why would aircraft engines be mounted with “toe out”? $\endgroup$– user14897Mar 20, 2020 at 14:02
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9$\begingroup$ This is interesting, as I would expect the local airflow to be less inward when you go from the inboard engine to the outboard engine (as there is less influence of the body). This would lead to a smaller angle in the outboard engine. Anybody has an explanation for that? $\endgroup$ Mar 20, 2020 at 20:32
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3$\begingroup$ If it costs more to make them different than it does to make them inefficient, that's what will be picked, as with everything else in large-scale manufacturing and in aviation, likely the reason. @ROIMaison $\endgroup$– NijMar 20, 2020 at 22:22
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3$\begingroup$ @ROIMaison The swept wing is also a major contributor in outboard flowing airflow. If the aircraft uses a straight wing instead then we may see near zero spanwise flow on the outer part of the wings $\endgroup$ Mar 21, 2020 at 12:30
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1$\begingroup$ @nick012000 The forces on the wings would result in a forward deformation between the fuselage and the engines; the engines push forward and the fuselage is dragging behind. However, due to the length of the chord and the way the wings are laid up, they are VERY rigid in the fore/aft plane. Wings flex up and down, not fore and aft. As to the outboard engines, Slebetman is correct; swept wings induce a significant outboard flow at the leading edge. Where the engines are mounted the outboard flow is reduced but still present. $\endgroup$– BretMar 23, 2020 at 7:37