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777 from overhead
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.

Nikhil Murali
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1 Answers1

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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:

enter image description here
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]
enter image description here

and in this photo:

enter image description here

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.

Fiddlesticks
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    Related: Why would aircraft engines be mounted with “toe out”? –  Mar 20 '20 at 14:02
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    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? – ROIMaison Mar 20 '20 at 20:32
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    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 – Nij Mar 20 '20 at 22:22
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    @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 – slebetman Mar 21 '20 at 12:30
  • Wouldn't the wings flexing backwards slightly due to drag also be a factor? That would make the engines in flight be slightly more outwardly-rotated compared to when they're on the ground. – nick012000 Mar 23 '20 at 01:02
  • @Nij That doesn't sound like a good reason. 1. They're different mounting points on the wing regardless. 2. If it makes that big of a difference to toe in the interior engines by 2 degrees then surely the financial efficiency of adjusting the outer engines would be worth it if the physics demanded it. 3. This isn't like some trivial interior ammenity where they can pickup economies of scale cost savings. This is the engines. – Brad Mar 23 '20 at 14:37
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    But why is the entire engine canted, instead of just the inlet? For example the MD-80 engine has the inlet tilted a few degrees, but the exhaust is parallel to the body. – zymhan Mar 23 '20 at 17:58
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    @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. – Bret Mar 23 '20 at 07:37
  • @Bret Huh, I hadn't considered the forces the engines would put on the wings. Wouldn't the lift/induced drag forces be greater than the thrust force, though? I know that the engines don't have enough thrust to carry a big aircraft alone, so the overall lift/induced drag force would be a lot greater than the engine thrust. – nick012000 Mar 24 '20 at 04:00