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A triplane, mainly used in WW1 on the german side, was a biplane with 3 wings.

What were the advantages to having more wings?

And, why don't countries use them today?

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    $\begingroup$ Really very similar to the characteristics of biplanes. $\endgroup$ – fooot Jan 29 '16 at 18:57
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    $\begingroup$ but, if the charictaristics are so simillar, why add another wing, what advantage does it gain $\endgroup$ – Nathan Cook Jan 29 '16 at 19:04
  • $\begingroup$ It gains stronger characteristics. $\endgroup$ – fooot Jan 29 '16 at 19:25
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Narrower wing chords versus a biplane of similar span and area. More efficient and increased lift Potentially faster rate of climb, tighter turning radius

Just to name a few. You're essentially increasing the surface area without increasing the wingspan

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    $\begingroup$ Increased drag, probably increased weight as well. $\endgroup$ – Dan Pichelman Jan 29 '16 at 19:36
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    $\begingroup$ Drag is dependent on velocity, triplanes were common when aircraft were slow and also needed slow stall speeds. When planes got faster they became obsolete due to the drag. (And improvements in wing support structures.) $\endgroup$ – Ville Niemi Jan 30 '16 at 1:45
  • $\begingroup$ @VilleNiemi, induced drag is inversely proportional to velocity and biplanes (and triplanes) have higher induced drag as well! $\endgroup$ – Jan Hudec Feb 1 '16 at 10:40
  • $\begingroup$ @JanHudec True, but IMHO higher induced drag is acceptable price to pay if you need low stall speed with limited wing span. Monoplanes operating from bad runways (or small bodies of water) still have to do the same trade-off, right? $\endgroup$ – Ville Niemi Feb 1 '16 at 11:22
  • $\begingroup$ @VilleNiemi, no, they don't. Bad runways don't mean your wing span would be particularly limited. In fact you want long span to get most of ground effect and you want to utilize ground effect for soft-field take-off (soft/rough-field technique is to unstick as soon as possible and accelerate in ground effect). And long span gives you lower induced drag and lower best glide speed (as the form drag then grows faster). $\endgroup$ – Jan Hudec Feb 1 '16 at 12:59
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The accepted answer is wrong.

Triplanes have all the disadvantages of biplanes, only more so. But they share their advantages as well, and the main reason to build them in WW I was roll maneuverability:

  • The reduced wingspan reduced roll inertia, which helps to accelerate quickly into a high roll rate. Note that roll authority changes linearly with span (if the ailerons are scaled with the span), but inertia will change with span squared.
  • The reduced wingspan reduced roll damping, so the achievable maximum roll rate is higher. The maximum roll rate is reached when the aileron-commanded rolling moment equals roll damping, and roll damping is proportional to span squared.

When the Sopwith Triplane became operational early in 1917, German pilots felt at a disadvantage and Reinhold Platz at Fokker created the famous Dr.I. The next generation were biplanes again, however, and the latest Fokker creation of WW I was actually a monoplane. The high climb rate of the Triplane was caused by its low wing loading and high power-to-weight ratio and more due to the lightweight construction and the advances in engines than the triplane layout.

What were the advantages to having more wings?

Adding a second wing creates a lightweight, stiff box girder. As for why people added even more wings, see this excellent answer.

And, why don't countries use them today?

Because countries don't use airplanes, pilots do.

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