We associate biplanes with the infancy of aviation, but monoplanes existed from the very earliest days as well. The Bleriot XI that was the first airplane to cross the English Channel was a monoplane, as was the first really effective fighter of World War I, the Fokker Eindecker. German industrialist and all-around genius Hugo Junkers even built an all-metal cantilever low-wing monoplane fighter in 1915—probably the most remarkable example of prescience in the entire history of aviation.
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A monoplane has aerodynamic advantages, but for one to achieve its full potential its wings need to be cantilevers, and they have to be built in such a way as to resist twisting. That combination of requirements was incompatible with very thin wings, and in the early days of aviation it was widely—though not universally—believed that thin birdlike airfoils were best. Junkers seems to have been the first to realize that thick wings would work as well as, and in some respects better than, thin ones. Fokker followed suit, and all his later designs used thick cantilever wings with load-bearing skins, like those we use today.
The prevalence of the biplane over the monoplane was due almost entirely to practical considerations. When engines were weak and it was hard to get into the air at all, multiple wings were an obvious way to add lift without making the airplane much bigger. Stacking wings above one another and connecting them with struts and wires also made for a structure that was strong, rigid, and light. The suggestion of the biplane truss structure is said to have been one of the contributions of Octave Chanute, a civil engineer, to the Wrights—although the brothers, who were not dopes, can hardly have been unaware of it.
Wood-and-fabric biplanes were also easily built by unskilled labor and could be easily rigged and repaired, important considerations in wartime. The principal disadvantage of the biplane truss was its drag, a penalty that most early monoplanes, including the Fokker Eindecker, shared because their thin, flexible wings were externally braced with wires. Early builders probably did not realize how great the drag of wires was. A notable later finding was that the drag of a round wire is equal to that of an airfoil 30 times as thick.
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The airplanes of WWI came in a great variety of shapes and sizes, with wingspans ranging from 23-foot fighters to a 113-foot bomber, the Ilya Muromets, designed by Igor Sikorsky. Most had upper and lower wings of approximately the same span and area. Usually the upper wing was placed a little farther forward than the lower—an arrangement called “positive stagger”—in order to provide visibility in turns. A big scoop was also taken out of the trailing edge of the upper wing for the same purpose.
Those modifications were necessary because for small airplanes to accommodate pilots of different weights the cockpit had to be placed close to the center of lift and therefore between the wings, with disastrous consequences for visibility in turns. Bombers, which were larger and less sensitive to CG shifts, did not bother to stagger their wings. There exists a memorable photograph of an Ilya Muromets flying at a height of 30 feet or while a couple of windblown men in overcoats stand rather casually on top of its aft fuselage.
Another reason for cutting a chunk out of the trailing edge of the upper wing was that prior to the widespread adoption of machine guns synchronized to fire forward through the propeller disk the scouts (as they called fighters then) had a machine gun mounted atop the upper wing. In order to use it, the pilot stood up in the cockpit, holding the stick between his knees. Fortunately, this is a skill that is no longer required of us—not even of ATPs.
In a few cases sweep was added to the upper wing in order to move the centersection forward. Sweep was found to enhance snap-rolling performance, and today many acrobatic sport biplanes have mildly swept upper wings.
Thomas Sopwith’s designer, Herbert Smith, may have been motivated at least in part by considerations of pilot visibility when he created the Sopwith Triplane in 1916. The plane’s three wings were unusually narrow and strongly staggered, so that the trailing edge of the upper wing was clear of the cockpit, even without a cutout. Notwithstanding a brief 1917 craze for triplanes on both sides, and the indelible association, thanks to Snoopy, of Manfred von Richthofen with the Fokker version, triplanes were a flash in the pan that had no advantage over biplanes.
Stagger, vertical spacing or “gap,” and “decalage”—the difference in angle of incidence between the two wings—could be varied to taste. For practical reasons, however, gaps were seldom much greater than one chord length, and stagger tended to be positive or nil. The famous Beech Staggerwing, which should have been called the Negativestaggerwing, reversed the customary arrangement and placed the lower wing ahead of the upper. In a four-seat cruiser with a closed cabin, negative stagger both put the pilot near the leading edge of the upper wing for visibility and allowed retracting the landing gear into the lower wing,
Designers relied on intuition until 1923, when German aerodynamicist Max Munk, then in the employ of the National Advisory Committee for Aeronautics (the predecessor to NASA), published his “General Biplane Theory,” which for the first time provided data on which to base rational choices. The questions that Munk answered had to do with the influence of stagger, gap, and decalage on the lift and drag, and specifically the induced or lift-related drag, of biplanes. One of Munk’s findings was that stagger had little influence. In principle, the gap was important to minimizing induced drag, which could theoretically be half that of a monoplane having the same wingspan. Unfortunately, the most effective gap was an infinite one, and did not lend itself to practical construction.
Munk for the first time quantified the fundamental deficiency of the biplane—the mutual interference of its wings. The high pressure and low velocity on the underside of the upper wing conflicts with the low pressure and high velocity on the upper surface of the lower wing. One of the results of interference is that the total lift of a biplane’s two wings is less, at a given angle of attack, than that of a monoplane wing of the same area. For a desired landing speed, therefore, a biplane’s wing area must be larger.
In the technical literature on wing arrangements, one encounters a curious fact. From an aerodynamic point of view, all of our monoplanes, except flying wings, are strictly speaking extremely staggered biplanes. By the same token, what we call biplanes are triplanes, triplanes are quadruplanes, and so on.
Better not to go there.
This column first appeared in the May Issue 958 of the FLYING print edition.
