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With larger wings comes more drag. So why don't large planes have 4 smaller wings instead of 2 very long ones?

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    $\begingroup$ Possible duplicate of Why are there no longer any biplanes? $\endgroup$ – fooot Jul 24 '17 at 22:30
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    $\begingroup$ Voting to leave open, the question is not specifically about the classical biplane configuration. $\endgroup$ – Koyovis Jul 25 '17 at 5:26
  • $\begingroup$ @mins What if each wing has a bigger aspect ratio than the single wing? The bending moments are lower due to shorter span. $\endgroup$ – Koyovis Jul 25 '17 at 9:42
  • $\begingroup$ @mins That's not how I read it. $\endgroup$ – Koyovis Jul 25 '17 at 20:18
  • $\begingroup$ Not a large plane, but there have been 4-wing aircraft, such as the SuperMarine Nighthawk $\endgroup$ – TomMcW Jul 25 '17 at 22:49
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With larger wings comes more drag. True, because the drag is a function of the wing area A: $$ D = C_D \cdot \frac {1}{2} \rho \cdot V^2 \cdot A $$

You need a certain wing area to support the weight of the aircraft. Now the question is: over how many wings will we distribute this wing area? For over 80 years, the answer has been: one wing (the two wing halves make up one wing).

The classical biplane is a thing of the past, however there have been incredibly clever aircraft builders who have looked at the long coupled canard configuration. This one really makes a lot of sense:

enter image description here Image source

So to answer your question: there are aircraft with two wings. It's just that people don't buy them, and go for boring old Cessna's instead.

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    $\begingroup$ It's not so much that people buy boring 2-winged Cessnas (and Pipers &c), as that they buy old ones. Mainly because the old ones can be had for 10-20% of the price of new, and not that many people have a spare $250K or so to spend on a new 172. $\endgroup$ – jamesqf Jul 25 '17 at 6:35
  • $\begingroup$ Is the plane in the picture missing its front gear? $\endgroup$ – Burhan Khalid Jul 25 '17 at 10:28
  • $\begingroup$ @Burhan: Burt Rutan designed it with a fixed main gear but retractable nose-wheel. However you'd be a brave man (or very very knowledgable) to challenge his design decisions. $\endgroup$ – RedGrittyBrick Jul 25 '17 at 15:04
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    $\begingroup$ Your “drag” expression includes only parasite drag. Adding the term for induced drag sends anything else than single main wing straight into the land of bad ideas. Including most of the Rutan's ones—they were pretty efficient in spite of the unusual layouts, not because of them. $\endgroup$ – Jan Hudec Jul 25 '17 at 19:01
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    $\begingroup$ @Koyovis, actually, induced drag is a function of lift and span. The aspect ratio comes about when you divide by wing area to get the scale-independent lift coefficient. Also, it is not really per-wing: tandem wings with AR 1:20 behave approximately as one wing with AR 1:10, but as two separate wings it will require more structure. And then there is the inherent inefficiency of canards in stable designs—since stall of the canard is limiting factor, the main wing is not used to its full potential, so it needs to be a bit bigger. $\endgroup$ – Jan Hudec Jul 25 '17 at 21:02
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Biplanes are a thing and have been since the earliest days of flight.

However, they fell out of favour because they actually have more drag than a corresponding monoplane. Not only do you need essentially the same amount of wing but there's also the extra supporting structure.

Alternatively, if you're thinking of two wings one behind the other then the turbulence from the front wing will dramatically decrease the lift of the second.

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I think there are could be a couple of reasons why one wing is the norm. Firstly, from an induced drag perspective a longer wing, if structurally possible, will provide less drag than a shorter, fatter one. The longer the aspect ratio (the ratio of wingspan to the cord (width) of the wing) the less effect the wingtip vortices have.

Secondly, as mentioned in another answer, a wing behind another wing sees disrupted flow. How this flow is disrupted could be hard to predict when designing the aircraft

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  • $\begingroup$ Yes the aerodynamics of a single wing with high aspect ratio are very good. However, the bending moment at the wing root is proportional to the wingspan squared... $\endgroup$ – Koyovis Jul 27 '17 at 4:57
  • $\begingroup$ But people still overcome that to reap the aerodynamic reward $\endgroup$ – user74671 Jul 27 '17 at 22:58
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Biplanes and other multi-wing configurations suffer from the airflow coupling of the wings. Specifically, lift is due to deflection of the air downwards by the wing. Another nearby wing now is dealing with deflected air, reducing its possible lift. As mentioned earlier, the real lift function depends on span per unit lift, regardless of how many wing (or horizontal tail) surfaces are involved. A Cessna wouldn't fly with 36 one-foot wings...

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  • $\begingroup$ Another nearby wing now is dealing with deflected air, reducing its possible lift. The other nearby wing could be mounted at a different angle, to keep Angle of Attack constant. $\endgroup$ – Koyovis Jul 27 '17 at 21:17
  • $\begingroup$ @Koyovis: Now this other wing will produce much more induced drag. Local flow is pointing downwards, so lift will point backwards. It will be much better to stick that extra pair of wings on the tips of the original wing, even when the resulting wing is heavier than the sum of both. $\endgroup$ – Peter Kämpf Jul 27 '17 at 21:39
  • $\begingroup$ @PeterKämpf Good point about induced drag. $\endgroup$ – Koyovis Jul 27 '17 at 21:50

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