What justifies a biplane over a braced monoplane?

Question

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?

Answer 1

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.

Answer 2

Four elements of the aviation matrix compete, as always: structures, aerodynamics, engine power, and stability/control. The very first powered flight of the Wright brothers took place in a biplane with a 12 hp engine, and was hard to control. It took almost a year after the (brilliant) first flight before they could execute a full 360 degree turn, and six of their aircraft delivered to the US Army crashed. From the wiki:

Army accidents

In 1912–1913 a series of fatal crashes of Wright airplanes bought by the U.S. Army called into question their safety and design. The death toll reached 11 by 1913, half of them in the Wright model C. All six model C Army airplanes crashed.

How to make aeroplanes stable and easy to control? One solution was implemented by the young Anthony Fokker: simply eliminate controllability in the roll axis. `In the pic below from the Wiki article about the Fokker Spin, Anthony sits behind the engine structure, and the two wing halves can be distinguished supported by a spider web of bracing wires (Spin translates as Spider). Large wings => low wing loading!

The two wing halves can be seen to tip upwards, providing passive roll stability. No ailerons, the flight direction was commanded by the rudder only. Flying slowly in a straight line, turning gently with rudder only. And then WW1 started. At first aeroplanes were just observers, then came to be pursued by enemy aeroplanes to eliminate the flying artillery scouts. Airborne manoeuvring was now applying large loads on the structure, and the monoplane consisting of two wing halves held in place by wires could not compete anymore. For one, more wires were required, creating way more drag.

So the solution was the inherent strength and stiffness of the biplane structure. First the rectangle was crossed out by wires, later by more aerodynamic wooden bracing. At the end of WW1 the most feared plane was the Fokker DVII, which was particularly mentioned in the peace treaty.

Does the lower wing of a biplane account for only 20% of the lift? For sure if its area is 20% of total wing area. Aerodynamically an unbraced monoplane is superior, structurally a biplane is simpler. If the function of the plane is transport for low costs, a single wing with a large aspect ratio is the design goal. A small aerobatic plane might still be a biplane though (Pitts Special, from the wiki).