Considering both airplanes have 2 wings I wonder what are the pros and cons of each one and if a tandem design like the Quickie Q2 is more efficient than a biplane like the Steen SKYBOLT?
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Considering both airplanes have 2 wings
The premise of your question is not correct: the Q2 has one wing and one horizontal empennage while the Skybolt has... well, one wing and one horizontal empennage - they are actually the same 😉
In the Q2 (which is indeed a tandem design) the horizontal empennage has grown in size so much that it has become as big as the wing while in the Skybolt (which is indeed a biplane) the wing has been "divided" in two and put one half over the other.
Even if with two visually very different results, the core idea behind both designs is the same i.e. to split the total lift between two lifting surfaces in order to reduce the induced drag: being the induced drag proportional to $L^2$, if each lifting surface generates now $L/2$ then the relevant induced drag becomes $(L/2)^2=L^2/4$; having two lifting surface, the total induced drag is therefore $2 \times L^2/4=L^2/2$ i.e. half of what we get with only one wing, nice!
Obviously this is not the whole story.
To get each lifting surface half of the total area, their span and/or chord has to become smaller. But shorter wingspan means higher induced drag (which is the opposite of what we wanted to obtain in the first place) while shorter chord means smaller Reynolds number and again higher (viscous) drag.
In addition to that, each of the two designs has other compromises that further reduce the gain in induced drag:
In the tandem design, to maintain stability the CG must be closer to the forward lifting surface than the rearward; therefore the forward surface generates more lift than the rearward; this not 50-50 split of lift reduces the previously calculated ideal gain in the induced drag. Plus we have a rearward surface which is as big (and heavy) as the front one but that generates less aerodynamic forces: it is basically oversized. Furthermore, the downwash of the forward surface impinges on the rearward surface; this is not intrinsically a bad thing but makes the airplane unstable: let's say that, for whatever reason, the lift on the forward surface increases $\rightarrow$ the downwash increases $\rightarrow$ the rearward surface sees a lower AoA and generates less lift $\rightarrow$ we get higher lift forward and lower lift rearward $\rightarrow$ this creates a nose-up moment that further increases lift in the forward surface and reduces lift on the rearward aurface $\rightarrow$ the airplane keeps on pitching-up in an unstable way. Note that the Q2 partially solves this pitch-up problem setting the two surfaces at different heights.
In the biplane design, the two lifting surfaces are connected via bracing which increases drag. Aerodynamic interference among the upper and lower wing also increases drag.
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3$\begingroup$ If the Q2 isn't a tandem wing, then what is? $\endgroup$ Commented Jun 30 at 19:21
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1$\begingroup$ @CamilleGoudeseune: I was just stretching the definitions a bit 😉 $\endgroup$– sophitCommented Jun 30 at 19:39
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$\begingroup$ The smaller chord wing will become heavier while struts and bracing make the biplane wing much lighter. Both effects also influence induced drag, because they determine how much lift needs to be generated and must be taken into consideration. $\endgroup$ Commented Jul 2 at 1:27
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The glide ratio of both types is less than 10, with the "quickie" perhaps slightly better.
This all points towards fuselage and wingtip interference. The glide ratios are also similar to monowing types such as the Cessna 152.
No free lunch here, long slender wings (high aspect ratio) are needed for better efficiency.
Mounting the top wing in a parasol fashion may help the "quickie", as would streamlining the bi-plane with retractable landing gear.
answered Jul 2 at 0:34