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Biplanes became the first airplanes to enter service due to their structurally efficient design being more suitable for the weak materials then. This efficiency comes from the two wings acting as the chords of a very deep truss.

Another structurally favorable factor was the reduced wingspan and thus moment allowed by distributing lift across stacked wings.

Yet another weight saver was the reduction of wing loading, which enabled thus use of less powerful engines for slower flight.

However, the extra wing only produces 20% more lift with 100% parasitic drag increase. Ironically, this interference drag is even more severe at the lower speeds which biplanes are optimised for.

A braced monoplane has most of the structural advantages of a biplane, with less of the aerodynamic disadvantages. So it would seem to be a more logical design than a biplane, given the same materials and engine.

Yet biplanes were built in large numbers. Why?

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  • $\begingroup$ Related: Can biplane or triplane designs be revived with modern materials? $\endgroup$
    – user14897
    Mar 4, 2021 at 10:48
  • $\begingroup$ BTW I honestly question the lower wing gives only 20% more lift. It may be more, if set up properly, the lower wing puts the upper wing in perpetual ground effect. It would be a great study, but, as Peter says, the upper wing does most of the lifting. $\endgroup$
    – Robert DiGiovanni
    Mar 4, 2021 at 17:33

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Back in the pioneer days both types competed fiercely. The braced monoplane has similar structural depth to the braced biplane but less than twice the total wing area, so many argued in its favour. On the other hand, the shallower angle of its bracing wires introduced greater compressive forces in the wing spars, so they had to be strengthened and that added weight.

The monoplane suffered its first drawback around 1911 when, first in France and soon after in Britain, examples started falling out of the sky and for a time they were banned from military service. The French problem turned out to be a mistaken design assumption leading to over-thin bracing wires, the British to be the Ministry-specified cowling fasteners coming adrift. They were soon flying again, but the damage to their reputation was done.

Nevertheless, a few types were still in service when war broke out, especially with Germany. The Etrich Taube and Fokker Eindecker helped to restore their reputation, especially when Fokker developed interrupter gear for the machine guns, unleashing the Fokker scourge. But this would prove the high point of the braced monoplane's career.

The advantages of the biplane you mention - high strength and short span - proved to give greater manoeuvrability, stiffness and damage tolerance when it mattered, and the monoplane quickly became obsolete.

The biplane remained dominant until engine powers rose sufficiently to take planes beyond 200 mph or so, where the greater weight of the unbraced or cantilever monoplane was a price worth paying for cleaner aerodynamics.

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  • $\begingroup$ The biplane lost its dominance already during WW I, at least in Germany. Most new types were monoplanes: Fokker DVIII, all new Junkers, Dornier or Rohrbach (Zeppelin Staaken) designs, you name it. New biplanes were mostly designed for aerobatics and pilot training. $\endgroup$
    – Peter Kämpf
    Mar 7, 2021 at 8:49
  • $\begingroup$ @PeterKämpf That is a false picture full of mistakes. The parasol monoplane mounted only a modest challenge, and that mainly in France. The most significant Dornier was the D.1, a cantilever biplane, Junkers had built cantilever monoplanes throughout but only in penny numbers, Rohrbach were a postwar company; biplane houses such as Fokker and Albatros dominated German skies to the end. French fighters would serve your case better, but companies such as Lioré et Olivier long continued the biplane tradition in other sectors. $\endgroup$
    – Guy Inchbald
    Mar 7, 2021 at 10:44
  • $\begingroup$ Rohrbach worked at Zeppelin in Staaken. That's why it included the subtle hint that you close to overlook. The Dornier D.1 was partially responsible for the demise of biplanes. None of the many Albatros biplane types after the D.V were ordered in numbers. Fokker changed to monoplanes in 1918. Dornier and Rohrbach designed the planes you call postwar in 1918. Should I continue or do you see where the misconceptions are? $\endgroup$
    – Peter Kämpf
    Mar 7, 2021 at 11:58
  • $\begingroup$ Zeppelin Staaken did not build Rohrbach's monoplane until the war was over, and one obscure prototype doth not a revolution make. The Fokker D.VIII monoplane was smaller and lighter than the D.VII biplane but no faster, and the company went back to biplanes for what little was left of the war. Yes, I think we can all see where the misconceptions lie. The simple fact of history is that monoplanes remained a curiosity until engine powers rose to justify the structural inefficiency of the cantilever. $\endgroup$
    – Guy Inchbald
    Mar 7, 2021 at 14:39
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enter image description here "A braced monoplane has most of the structural advantages of a monoplane with less of the aerodynamic disadvantages"

Very surprising statement since, in the previous paragraph it is said: "the extra wing produces 20% more lift with 100% parasitic drag increase".

Actually, with spars, cantilever wing structure, and the aerodynamic benefits of thicker wings, biplanes were surpassed by high wing monoplanes in the 1920s.

Increase in thrust available from more powerful engines and variable pitch props rendered the weight savings but increased drag design of the biplane obsolete.

It is highly doubtful a biplane with similar power and fuel supply could have followed the $Spirit$ $of$ $St$ $Louis$ across the Atlantic.

A few pictures for our commenters:

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And finally:

enter image description here

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  • $\begingroup$ Structural advantages of a monoplane?? $\endgroup$
    – Abdullah
    Mar 4, 2021 at 11:01
  • $\begingroup$ yea, spelled obsolete wrong. not my favorite word. $\endgroup$
    – Robert DiGiovanni
    Mar 4, 2021 at 11:04
  • $\begingroup$ Biplanes are very strong and light, but slower. When you go past a certain point in speed, much more thrust must be devoted to form drag. This is why, at great speeds, wings are no longer needed (not even one) and lifting bodies suffice. $\endgroup$
    – Robert DiGiovanni
    Mar 4, 2021 at 11:06
  • $\begingroup$ don't get me wrong, I actually built and flew a biplane with stacked very high aspect wings and it flew well. $\endgroup$
    – Robert DiGiovanni
    Mar 4, 2021 at 11:11
  • $\begingroup$ surprisingly, the biplane has advantages in cross wind roll control because of shorter torque arm and partial shielding of the upwind wings. anhedralling must be added to the monowing to help this. $\endgroup$
    – Robert DiGiovanni
    Mar 4, 2021 at 11:14

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