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Interference drag can start to appear when there’s imperfections or excrescences on an aircraft outer surface. For example the following image shows some of these typical imperfections: source:https://aerodyn.org/idrag/

Source of image:https://aerodyn.org/idrag/

These imperfections is sometimes unavoidable due to manufacturing restrictions. Under such circumstances, what is the best way to possibly reduce this interference drag produces by the imperfections? Are the smoother-looking “waves” or “bump” a better design than “roughness” and “cavity/gap”?

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Interference drag is defined as the additional drag that appears when two or more well defined geometric entities get together.

For example, wing alone has a certain drag $D_{wing}$; engine's pylon alone has a certain drag $D_{pylon}$; and engine's nacelle alone has a certain drag $D_{nacelle}$. But when these three bodies are joined together, the total drag is bigger than the sum of the single drags:

$D_{total}>D_{wing}+D_{pylon}+D_{nacelle}$.

And the difference is defined as interference drag.

Instead, the drag due to the imperfections visible in your picture is called... well, drag of surface imperfections.

Are the smoother-looking “waves” or “bump” a better design than “roughness” and “cavity/gap”?

As usual, to answer this kind of questions, the legendary Fluid-Dynamic Drag by Sighard F. Hoerner helps us.

The following pictures are taken from chapter 5 and give the additional drag due to the relevant imperfection:

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As a general rule, holes and cavities give a lower drag than protuberances and waves due to the the fact that the latter interact most with the boundary layer.

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  • $\begingroup$ Just another small question, how much does a SHARP wingtip contribute to total interference drag? $\endgroup$
    – Frank
    Jan 7, 2023 at 9:04
  • $\begingroup$ @Frank: chapter 7.4 at page 116 of the book I linked is what you're looking for. $\endgroup$
    – sophit
    Jan 7, 2023 at 10:00
  • $\begingroup$ Sorry, for my question I was focusing on the INTERFERENCE drag that the sharp wingtips would contribute to, in comparison with rounded wingtips. That page in the book was focusing on their effects on wingspan. $\endgroup$
    – Frank
    Jan 8, 2023 at 11:25
  • $\begingroup$ @Frank: interference in respect to what? Interference of the wingtip in respect to... $\endgroup$
    – sophit
    Jan 8, 2023 at 12:03
  • $\begingroup$ interference in respect to airstreams around the wingtip. Would the mixing of different airstreams around the wingtip cause interference drag? $\endgroup$
    – Frank
    Jan 8, 2023 at 14:04
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As noted by other answers, many sources of interference drag are difficult to reduce or eliminate.

However, when placing a protuberance on an aircraft -- say an antenna -- we often have the ability to choose where to place it. It is very reasonable to desire to place protuberance where it will cause the smallest increment in drag -- this amounts to minimizing the interference drag.

To do this, you want to place the protuberance where the local flow acceleration (the supervelocities) is small. For a transport fuselage, we can look at a simple flow solution...

Transport fuselage flow solution

Here we can see that after substantial acceleration around the curves of the nose, the flow settles down to nearly constant velocity (at essentially freestream) along the cylindrical portion of the fuselage.

Where would you place antennae and other protuberances?

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One way to reduce the interference drag is the "area rule". Basically, the net cross-sectional area should change as smoothly as possible - across the longitudinal axis. The area is bound to increase in the region of wing and tail, but by reducing the fuselage section in these regions, the change in area can be minimized.

Without area rule graph With area rule graph

At transonic speeds, a large part of drag increase is due to interference drag, and so it is no surprise that area rule is extensively used on modern transonic aircraft, like in the region of the horizontal stabilizer of this Dreamliner.

Picture of a Virgin Atlantic Boeing 787-9 tail, showing the area rule being used at the horizontal stabilizer mounting region

Picture source - Boeing 787


Another way of reducing interference drag is of course - to make the surface transitions smooth! One way of achieving this is "filleting", where additional surfaces are added just to smoothen these transitions. For example, filleting is extensively used at the wing mounting region of the DC-3.

Picture of a DC-3

Picture source - DC-3

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    $\begingroup$ I think the question is actually more focused on the small local imperfections of the surfaces and not on the global surface $\endgroup$
    – sophit
    Jan 3, 2023 at 23:03
  • $\begingroup$ @sophit Perhaps that's the case, and your Answer gives a great answer for that :-) $\endgroup$ Jan 3, 2023 at 23:08
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    $\begingroup$ But I agree that the terminology and the picture in the question do not match. But I suppose that the question is about the picture and not the the terminology. Anyway you're answer is also good 🖖 $\endgroup$
    – sophit
    Jan 3, 2023 at 23:11
  • $\begingroup$ Btw I don't understand where the area rule is used on the tailplane of the dreamliner $\endgroup$
    – sophit
    Jan 4, 2023 at 0:13
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    $\begingroup$ @sophit If you look at the mounting area of the tail, there is a depression ("dent") in the fuselage. This is to offset the increased cross sectional area due to tail; that's the area rule. Another reason for that depression is of course for the variable incidence (trimming) mechanism. $\endgroup$ Jan 4, 2023 at 0:18

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