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When calculating the wing-span of a tandem-wing aircraft, like the QAC Quickie Q2, which set of wings do we use for that value? There are two sets of wings, and they both provide lift. In the class, they just teach me that b is for wingspan, but they didn't go into detail about non-conventional aircraft. I am using that value to calculate the wetted aspect ratio of my aircraft

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    $\begingroup$ Maybe it depends on what you're using the value for? Estimating lift area, buying a hangar door, ... $\endgroup$ Feb 7 at 0:26
  • $\begingroup$ I am using that value to calculate the wetted aspect ratio of an aircraft $\endgroup$
    – user39178
    Feb 7 at 2:50
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Because wetted aspect ratio $b^2/{S_w}$ merely improves on wing aspect ratio as an estimate of efficiency, for $b$ just choose whichever wingspan is largest. That's good enough to compare one tandem planform to another.

You could get fancy and use a weighted average of the wings' spans, weighted by how much lift each one produces, with those in turn weighted by time spent in climb, cruise, and descent... but that seems against the spirit of a quick estimate provided by two easily measured numbers.

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