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As I understand from answers I've read here 1, 2, 3, biplanes create more lift with less wing span when compared to single wing aircraft, because the combined wings have greater surface area overall.

However this is usually at the expense of added drag as both wings are accelerating the same air downwards causing interference and creating inefficiencies. Also the wings commonly have supporting wires/structures between them causing more drag.

With that said biplanes do have some desirable characteristics like shorter roll response. I also realize when looking at how nasa calculates for the drag coefficient, wing aspect ratio is a major factor in their equation:

enter image description here

With this information it would seem to me if you could separate the wings of a biplane far enough apart that they don't cause much interference, you could possibly get a better performing aircraft with the same wingspan.

I almost imagine it being as simple as making the tail a full sized wing and giving each wing a thinner aspect ratio. However I've never seen this done before or even attempted. Is there a reason for this? Would the wings still interfere too much with each-other.

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  • $\begingroup$ Frankly as far as I can tell, the disadvantages of biplanes greatly outweigh the advantages you'd get through increased lift i.e. surface drag, unnecessary weight gains, etc. Biplanes just aren't efficient enough in the end game for most all applications. There are a few where an optimized design perhaps like your idea where it might prove useful, but in the mainstream no I don't think so. $\endgroup$ – Jihyun Aug 7 '18 at 20:08
  • $\begingroup$ And yes also having a wing in the tail would not only offset the CoL too much, but it would almost definitely receive interference and turbulent flow from the front wing, unless it's mounted low/high enough too where a whole new series of issues would arise. Sorry but it's just too much trouble to implement radical designs. $\endgroup$ – Jihyun Aug 7 '18 at 20:12
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Moving the wings vertically apart would mean more and heavier supporting structure, but also a higher pitch moment of inertia. Since this grows with the square of the vertical expansion, a biplane with a lot of distance between both wings will be hard to control and very sluggish.

The real advantage of a biplane is reduced induced drag because less structure needs to be lifted. When cross-bracing enables the top wing to work as the compression member and the bottom wing to work as the tension member of a truss, a very lightweight structure becomes possible, especially when the size of the aircraft grows.

If all you have to power your airplane is a heavy, inefficient piston engine like those at the beginning of the last century, and you can tolerate that your aircraft won't fly fast or far, a biplane is indeed the lowest-drag solution.

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    $\begingroup$ Your answers always explain things from an angle I've never considered. Now I have to look up what effects pitch inertia have. Thank you! $\endgroup$ – YAHsaves Aug 7 '18 at 22:05
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    $\begingroup$ " a biplane with a lot of distance between both wings will be hard to control and very sluggish." - I disagree. Pitch resistance due to moving the wings apart would be nominal, both because they represent only a tiny amount of the pitch mass (they are both lightweight and located near the center of rotation) and because they are moving in the direction of the relative wind. The engine, on the other hand, is both heavy and a long way from the COR. Contrast this with roll resistance, where the engine is now at the COR and the wing surface is moving against the relative wing. $\endgroup$ – Maury Markowitz Aug 8 '18 at 21:10
  • $\begingroup$ @MauryMarkowitz: The question is about moving them apart. And then they are not close to the center of rotation. $\endgroup$ – Peter Kämpf Aug 8 '18 at 23:58
  • $\begingroup$ How does the lower wing act like the tension member of a truss,I understand how the upper wing is a compression member(because the flying wire attached to it is at an angle) but what I don't get is how the lower wing is a tension member?...pls can u explain just a little bit @Peter Kämpf $\endgroup$ – Jessica Ham Jul 9 at 12:38
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As for "making the tail a full sized wing", this is a reasonable idea and it has been tried on quite a few designs. Such configuration is called tandem wing. Perhaps the best known example is Quickie:

Quickie Wikimedia

Purely aerodynamically, tandem is better than biplane, due to both the wing-wing interference and the lack of horizontal tail. However, tandem doesn't have the structural benefits of the biplane (see Peter's answer) and so must be heavier. In which conditions it may be preferential to the classical design deserves a separate analysis, but given the scarcity of such airplanes, they must be quite narrow.

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    $\begingroup$ Wow I can't believe you actually found a plane like this! That's incredible thank you. I'm wondering with today's composite materials could a plane be made without the weight concerns? I'll try to look into how well this plane performed $\endgroup$ – YAHsaves Aug 8 '18 at 2:50
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    $\begingroup$ @YAHsaves, well, there are always weight concerns: if you can make this design lighter, you can potentially make biplane even lighter. That said, there are many other considerations (for example, wall thickness: you can't make it too thin), and the composites do shift the balance towards clean console and monocoque designs. It's not an accident that Quickie is a composite airplane (even though it's from the '70s). $\endgroup$ – Zeus Aug 8 '18 at 3:12
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Your question of "better performance" can be yes or a no, depending on what you're after. Engineering is a management of compromises. Understanding the pros and cons of a biplane design will prove that the disadvantages usually outweigh the advantages, but perhaps not- again it depends on what you're after.

Engineers have learned things over the last 100+ years- Stacked wings will not generate twice the lift of a single wing, their aerodynamics interfere with each other BUT you will absolutely get more lift than a single wing of the same span.

Remember wingtip vortices kill lift, and you'll have four wingtips. Stacked wings always add drag. If you have two cantilever wings you could reduce the drag from whatever struts and wires you eliminate over the older designs, but it will still have more drag of all sorts than a monoplane. You might like simpler wing design as biplanes have greater STOL performance without flaps, The compromise here- you'll have a tougher time handling that much wing in a crosswind.

Flaps give you flexibility of higher lift and a lower stall without so much wing area. Plus you'll really need to slam the stick forward if you have an engine failure, these planes really slow down fast.

Back in the day biplanes were hard to beat. The first monoplanes had just as many wires, the first cantilever wings were poorly made and killed a few people, which made pilots deathly afraid of them. Change is hard to push onto people when the tried and true works so well like good lift and high strength, but the solutions are so well understood today the biplane's only advantages are maybe some margin of higher strength, Easy inexpensive strength, greater lift in a given span, short wings roll faster and an awesome look and experience.

Novelty can be fun, and you do gain some things with trade offs. I'd say don't quit if you like the idea, do it. They certainly do fly and can fly well, and you could improve on it. If you just want to fly Cable trussed biplanes give you inexpensive, easier to design strength. You might have a lot of fun- but you won't improve fuel economy, safer ground handling, speed or a longer glide.

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  • $\begingroup$ @David Bloxham, Thank you for your response. That's an interesting point to think about high cross winds when on the ground if you have no way to offset the lift. Regarding wing tipped vertices, I actually found out recently from this site they don't cause drag directly, but are simply part of the lift process. You can read about this here: aviation.stackexchange.com/questions/31996/… $\endgroup$ – YAHsaves Aug 8 '18 at 2:55

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