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Biplane wings interfere with each other, reducing the total lift often to about only 20% more than a mono wing of the same chord and span.

But at supersonic speeds - say, Mach 2, where Mach angle is 30 degrees, the wings (now provided with supersonic airfoils, before anyone asks) would have to be 0.5 chord-lengths apart for their shockwaves to interfere.

So, am I correct in saying that for a given chord-length separation, drag due to airfoil interference on vertically stacked multiplane wings interference decreases with Mach number?

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Amazingly, at supersonic speeds it can work the other way. In the Busemann biplane the shock waves interfere in such a way as to actually reduce drag and also to reduce the sonic boom. Wide enough separation to avoid interference would actually increase supersonic drag.

See for example:

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  • $\begingroup$ Yeah I knw about that one. I like it, though I heard it had shockwave-swallowing problems that needed variable geometry. $\endgroup$
    – Abdullah
    May 26, 2020 at 7:55
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    $\begingroup$ @Abdullah Not sure why you are asking if you already know the answer. perhaps you could clarify your question to be more precise in what you do want to know. $\endgroup$ May 26, 2020 at 8:33
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    $\begingroup$ Well, I wasn't asking about a Busemann plane. More about 2 diamond airfoils stacked on top of one another. $\endgroup$
    – Abdullah
    May 26, 2020 at 9:38
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    $\begingroup$ Then I would suggest that yo edit your question accordingly. $\endgroup$ May 26, 2020 at 12:07
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    $\begingroup$ Busemann biplane - No shockwave, but no lift. Well, there's yer problem! $\endgroup$
    – FreeMan
    May 26, 2020 at 13:57

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