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Supersonic flight

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  • Supersonic flight


    I don't really get why at supersonic flights, a straight wing has a lower CD than a swept wing.
    "Swept wings reduce CD at transonic flights but the reverse in supersonic flights" (q 811251)

    Is it in relation with shock waves at Mcrit ? meaning that above Mcrit, wave drag will be more pronounced in this supersonic region hence generating more drag with a swept wing than a straight wing which generate this phenomenon earlier in the transonic range ?
    Hence swept wing (which retards Mcrit) will be more efficient in the transonic range than in the supersonic range compared to a straight wing ?

    Thank you

  • #2
    Note that the question is about Supersonic flight above Mdet, about Mach 1.3. It is not about the slower transonic region. We are beyond thinking about Mcrit and drag divergence Mach Number
    At these speeds all wings will have oblique shockwaves attached to the leading edge and trailing edge. None will remain on the wing surfaces.

    Here are a few thoughts.
    1. The Learning Objectives simply require us to know the effect of wing sweep on drag , not to understand a pretty complex issue!
    2. Note the red comments in the explanation to QID 811251, which casts doubt on the validity and wording of the reported question.

    But the answer given is true, and if you want some understanding of why, I recommend you search YouTube for Shell High Speed Flight and you will get a few films on the whole topic made in the 1950s . These give clear explanations, despite their age.
    Supersonic flight is covered in the 38 minute version entitled High Speed Flight Parts 2 and 3. Supersonic flight starts at about minute 20.

    Enjoy! Amongst other things, you may discover that:
    1. Supersonic airflows behave very differently from subsonic flows, and
    2. Straight wings designed for supersonic flight will probably require a double wedge aerofoil section, whereas swept wings used for supersonic flight are likely to use a more conventional aerofoil section to ensure acceptable handling at low speed.