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Comparing two bodies with the same streamlined longitudinal shape, and the same cross-sectional area, one with a circular cross-sectional shape, the other with a quadratic cross-sectional shape, which one of these two bodies would create more aerodynamic drag?

My prediction would be that it is the one with quadratic cross-sectional shape. As around the sharp edges where the two straight surfaces meet, flow interference would occur, which increases the total aerodynamic drag.

Note that these two bodies have 0 angle of attack relative to the incoming flow.

Related question: Why is the fuselage on an airliner circular-shaped?

Frank
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  • Related: https://arc.aiaa.org/doi/pdf/10.2514/1.C036032 and https://apps.dtic.mil/sti/pdfs/ADA389409.pdf – Frank Jun 30 '23 at 13:00

1 Answers1

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The quadratic body has more surface area compared to the round body, so this alone will guarantee a higher friction drag.

If the angles of attack and sideslip were always zero and no lift is created, the edges would not cause trouble. But as soon as a lift-producing wing is attached, the interference will create crossflow which will result in a higher drag of the rectangular fuselage.

Peter Kämpf
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    @MichaelHall Yes, a circle has the minimum perimeter for a given area. Any non-circular shape (with the same cross sectional area) will therefore have more wetted area than a circular shape. – Rob McDonald Jun 30 '23 at 16:31
  • I had a momentary doubt, but mathed it out to confirm for myself. Thanks! – Michael Hall Jun 30 '23 at 18:22
  • Lengthwise, I was surprised to learn that the optimum fineness ratio was surprisingly stubby; 3:1 or something? The Questair Venture, the stubby little high perf kit plane, was close to egg shaped and it is supposed to have close to the ideal min drag profile. – John K Jun 30 '23 at 20:26
  • Hi. Can you please explain how the "crossflow" would increase drag? I'm having a hard time visualizing it, thanks. – Frank Jan 22 '24 at 15:02
  • @Frank With sideslip the flow has a sideways component that must now negotiate the sharp corners of the rectangular fuselage. This will lead to separated flow on the leeward side and lower local pressure, which has a rearward component, adding to drag. – Peter Kämpf Jan 22 '24 at 15:18
  • @PeterKämpf I understand how this may happen in a sideslip. But in the answer you said the attachment of a lift-producing wing will cause interference that creates crossflow and add to drag. Does wing-fuselage interference have association with crossflow and drag? If so, how? – Frank Jan 22 '24 at 15:46
  • @Frank The wing produces upwash ahead and downwash aft of it which locally acts much like the crossflow in a sideslip. At zero angle of attack this does not cause much drag, but when the fuselage is inclined, this adds a bit of drag, too. – Peter Kämpf Jan 22 '24 at 19:13
  • @PeterKämpf Oh, so upwash and downwash don’t simply end at the wingroot, but extend a bit inboard, and have some effect on the fuselage. Is that correct? I believe the additional drag by this effect would be fairly small then. – Frank Jan 23 '24 at 04:15
  • @Frank correct. – Peter Kämpf Jan 23 '24 at 08:28