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I was looking at images of the SkyCourier shown below

enter image description here and i have noticed the asymmetrical exhaust pipes in both engines (one pointing up and one is straight), doing my research on the internet, i found that it is used to take advantage of the Coanda effect and the flaps so that the gas moves from the top side of the wing and then gets deflected downward for a high lift effect in takeoff...

Yet i believe you can get the same effect if the gases were vectored under the wing and made hit the flaps, so why would they vector the gases above the wing, and why the second exhaust is not deflected upward ?

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  • $\begingroup$ I suspect the benefit is mostly theoretical. It would be interesting to see actual test data that showed a significant difference. One big problem with running exhaust over the wing like that is the heat plays havoc with the structure. DeHavilland Dash 7s had a problem with the top exhaust actually annealing the 7075 alloy upper skin planks, causing wrinkling, and required insulating doubler panels to be added in the field. $\endgroup$ – John K Mar 26 at 19:32
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    $\begingroup$ @JohnK I'm sure the designers of the AN-72/74, YC-14 and C-17 will not agree with "the benefit is mostly theoretical" $\endgroup$ – DeepSpace Mar 26 at 19:39
  • $\begingroup$ Those are turbo fans that are using primary thrust flow. A small turboprop has very little velocity energy coming from its exhaust. $\endgroup$ – John K Mar 26 at 20:49
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    $\begingroup$ The effect of the propeller wash would completely dominate any thrust related effects on lift. On TBPs with large propellers the variation in sink rate is instantaneous with power changes. The effect of the exhaust will be minor, and "lost in the noise" so to speak. The other issue is the carbon deposits from the exhaust are bad for the wing structure, carbon and alum being galvanically opposed, so it's not a good idea to have exhaust impinge directly on the wing. Operators will get a rude surprise during inspections 10000 hrs down the road. $\endgroup$ – John K Mar 27 at 0:14
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    $\begingroup$ @Super not "Canada", but "Coanda". The tendency of a fluid to follow a curved surface. en.wikipedia.org/wiki/Coand%C4%83_effect $\endgroup$ – DeepSpace Mar 27 at 9:24
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Assuming that both props are rotating clockwise as seem from the rear, the orientation of the exhaust pipes makes sense if the goal is to ensure that the exhaust gasses are kept clear of the wing.

The rotating propwash will tend to carry the exhaust from the right-side pipe (in rear view) of each engine well below the wing. If the left-side exhaust pipes were similarly positioned, their exhaust would be carried up by the propwash to impinge directly on the wing. The position we see in the photo helps ensure that the propwash carries the exhaust from the left-side exhaust pipe of each engine well above the wing.

A comment from another ASE member (John K) has stated that exhaust impinging directly on the wing can lead to galvanic corrosion due to carbon deposits.

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  • $\begingroup$ future edit : first sentence "seem" should be "seen" $\endgroup$ – quiet flyer Mar 27 at 23:47
  • $\begingroup$ So the purpose is not to take advantage of the coanda effect , instead it is totally the opposite , they are trying to deflect the exhaust gases away from the wing? $\endgroup$ – Ibrahim Jaafar Mar 29 at 5:19

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