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Here is a picture of the Honda HA-420 HondaJet with strange extended engine housings. Why are they required? What are they for?

I am not asking about the red covers, but the white engine housing features that I put a red circle around.

  • $\begingroup$ Yes, because this is the first time I saw such cover. I have never seen it in any other airplane. $\endgroup$ – AirCraft Lover Feb 22 '19 at 3:03
  • $\begingroup$ Are you asking about the red GSE plugs for the jet pipes or the fairings at the end of the engine nacelles? $\endgroup$ – Carlo Felicione Feb 22 '19 at 3:12
  • $\begingroup$ No, I am asking about the "cover" at 120 degree or 240 degree for another engine. $\endgroup$ – AirCraft Lover Feb 22 '19 at 3:18
  • $\begingroup$ @GregHewgill, please, I am not asking the red cover. I am asking the white thing I put circle. I am not sure what is the name, is it cover or not. $\endgroup$ – AirCraft Lover Feb 22 '19 at 3:21

They aren't covers; they are fairings to integrate the flow going past the lower part of the cowl with the exhaust stream coming out the tailpipe above and the pylon below. It's because the cowl extends below the engine some distance to house various parts like accessories, plumbing, ducting etc. You have to create a nice transition for the airflow or you would have a lot of unnecessary drag.

You could say that the fairing is filling in all the 3 dimensional space that the free stream and the exhaust plume can't fill in because the change in direction is too extreme, and would result in flow separation, causing turbulence/drag.

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    $\begingroup$ Why do they some parts only? $\endgroup$ – AirCraft Lover Feb 22 '19 at 10:20
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    $\begingroup$ @AirCraftLover because having them any larger than absolutely necessary adds wetted area and, therefore, drag. $\endgroup$ – FreeMan Feb 22 '19 at 21:14
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    $\begingroup$ OK, tell me more. Why the position is not simply at the bottom? That position is not left/right side, but a bit shifted from buttom. $\endgroup$ – AirCraft Lover Feb 23 '19 at 8:12
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    $\begingroup$ If you mean the scalloped or cupped shape of the fairing below the engine tail pipe, it's because it's accommodating the exhaust plume coming out the back of the engine. $\endgroup$ – John K Feb 23 '19 at 17:06

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