Lesson 3 · 35 min

Wood, Wire and Fabric: The Biplane Era

In fifteen years the airplane went from a curiosity at Kitty Hawk to tens of thousands of fighters and bombers. Almost all of them had two wings braced by struts and wires. This lesson asks why, what that choice cost in drag, and how the first cantilever monoplanes pointed the way out.

Learning objectives

Describe how the airplane layout settled between 1905 and 1914: tractor propeller, tail behind, ailerons.
Explain the structural reasons for the braced biplane and its aerodynamic penalties.
Use the drag area \(f = \CDz S\) to find the zero-lift drag and the power it absorbs, \(P_0 = \tfrac12\rho V^3 f\).
Compare aircraft by their drag polars and \(\LDmax\), and relate the induced-drag term to Prandtl's lifting-line theory.

The layout settles: 1905–1914

The Wright Flyer was a canard pusher: elevator in front, propellers behind. Within a decade almost every successful aircraft had turned it around. Louis Blériot's monoplane crossed the English Channel in July 1909 with the propeller at the front (a tractor), the pilot behind it and the tail at the back of a long fuselage. That arrangement gives natural stability in pitch, keeps the propeller in clean air and protects the pilot in a crash, and it remains the standard layout today.

Other features also settled. Ailerons, hinged surfaces near the wing tips, replaced wing warping, which could not work on stiffer wings. A control stick and a rudder bar became standard. And by 1913 Igor Sikorsky had flown a four-engine aircraft, the Russky Vityaz, followed by the Ilya Muromets bomber.

Why two wings?

A wing must carry bending moments to the fuselage. With the thin airfoils of the time, a single wing deep enough to do that alone was too heavy. Two wings joined by struts and crossed wires form a braced box truss: a structure as deep as the gap between the wings, very stiff and very light, built from wood and steel wire that any furniture maker could work. Chanute had used it in 1896 and the Wrights copied him.

What the biplane gave

A light, stiff structure; a large wing area on a short span, so a low wing loading (slow, safe landings on grass with no flaps); small span, so a fast roll rate for a fighter.

What it cost

The drag of struts, wires and fittings (a high \(\CDz\)); interference between the two wings; and a low effective aspect ratio, so more induced drag for the same lift.

Loftin's First World War aircraft have \(\CDz\) between about 0.032 and 0.077, wing loadings of 28 to 59 kg/m² and \(\LDmax\) of 6.4 to 10. Every one of those numbers changed dramatically in the next twenty years.

Figure 3.1 First World War aircraft, to scale (grid squares are 5 m). The single-seat fighters are tiny next to the Handley Page O/400 and Caproni Ca.42 bombers, whose spans approach 30 m. Biplanes show the lower wing dashed; the Fokker Dr.I and Caproni Ca.42 are triplanes, the Fokker E.III, D.VIII and Junkers D.I monoplanes. Choose one to read its numbers.

Drag area and the power it absorbs

The zero-lift drag of a whole aircraft is easiest to think about as a drag area \(f = \CDz S\): the zero-lift drag is \(D_0 = q f\), with \(q = \tfrac12\rho V^2\). The power needed just to push that drag through the air grows with the cube of the speed:

Zero-lift drag and power

\[ D_0 = \tfrac12\rho V^2 f, \qquad P_0 = D_0 V = \tfrac12\rho V^3 f, \qquad f = \CDz S \]

To double the top speed with the same drag area takes eight times the power. Reducing \(f\) is the only cheap route to speed.

Example 3.1 — Where does a Camel's power go?

The Sopwith Camel has \(f = 8.73\ \text{ft}^2\) (\(0.811\ \text{m}^2\)) and reached \(105\ \text{mph}\) (\(46.9\ \text{m/s}\)) at \(10\,000\ \text{ft}\) (\(3048\ \text{m}\)), where \(\rho = 0.905\ \text{kg/m}^3\). Find the zero-lift drag and the power it absorbs, and compare with the engine's \(130\ \text{hp}\).

Show solution

\[ q = \tfrac12(0.905)(46.9)^2 = 997\ \text{Pa}, \qquad D_0 = qf = 997(0.811) = 808\ \text{N} \] \[ P_0 = D_0 V = 808(46.9) = 37.9\ \text{kW}, \qquad P_{\text{engine}} = 130(0.7457) = 96.9\ \text{kW} \]

The zero-lift drag alone takes about 40% of the engine's rated power. Induced drag adds only about 10 kW at this speed (\(\CL \approx 0.31\)). The rest of the gap is altitude and the propeller: an unsupercharged engine at 3000 m gives roughly 70% of its sea-level power, and a propeller of the time turned perhaps 70 to 75% of the shaft power into thrust. Then \(96.9(0.70)(0.72) \approx 49\ \text{kW}\), close to the 48 kW the drag needs.

Induced drag and the drag polar

Why should drag grow with \(\CL^2\)? Ludwig Prandtl's lifting-line theory, published in Göttingen in 1918–1919, gave the answer: a wing of finite span sheds trailing vortices that tilt its lift backward, producing an induced drag coefficient \(\CL^2/(\pi A e)\), with \(e = 1\) for an elliptic lift distribution. For the first time designers had a theory that said how much span was worth. Prandtl's student Max Munk extended it to biplanes, whose two wings interfere and act like a single wing of lower aspect ratio.

For the drag polars below, Loftin used \(e = 0.70\) for the First World War aircraft and \(0.75\) for the later monoplanes, and an effective span for the biplanes and triplanes. With those values, \(\LDmax = \tfrac12\sqrt{\pi A e/\CDz}\) reproduces his tables.

Figure 3.2 Lift-to-drag ratio against lift coefficient, \(L/D = \CL/(\CDz + \CL^2/\pi A e)\), from Loftin's \(\CDz\) and \(A\). Each curve peaks at \(\CL^* = \sqrt{\pi A e\,\CDz}\), marked by a dot. Change \(e\) to see how much the result depends on it, and compare the First World War fighters with the Lockheed Vega of 1927 and the Douglas DC-3 of 1935.

Example 3.2 — A monoplane is not automatically better

Compare the Fokker D.VII biplane (\(\CDz = 0.0404\), \(A = 4.70\)) with the Junkers D.I, an all-metal cantilever monoplane of the same year (\(\CDz = 0.0612\), \(A = 5.46\)). Use \(e = 0.70\).

Show solution

\[ \text{D.VII:}\ \ \LDmax = \tfrac12\sqrt{\frac{\pi(4.70)(0.70)}{0.0404}} = 8.0, \qquad \text{D.I:}\ \ \LDmax = \tfrac12\sqrt{\frac{\pi(5.46)(0.70)}{0.0612}} = 7.0 \]

The Junkers had the higher aspect ratio, but its corrugated metal skin, chosen for stiffness, had so much more drag that the biplane won. The cantilever monoplane only paid off once it was combined with smooth skins and streamlining, the subject of Lesson 4.

Thick wings and the first cantilever monoplanes

Two German developments of the war foreshadowed the future. In Göttingen, Prandtl's wind tunnel showed that thick airfoils, long assumed to have high drag, gave more lift and a gentler stall than the thin sections in use. Thick wings have room for deep internal spars, so they can be cantilevered: carried with no external bracing at all.

Hugo Junkers flew the J 1, the first all-metal aircraft, in December 1915, a cantilever monoplane of sheet steel. His later aircraft used corrugated aluminum alloy (duralumin), including the J.I armored ground-attack biplane and the D.I monoplane fighter.
Anthony Fokker's designers used thick wooden cantilever wings on the Dr.I triplane and the D.VII biplane, which needed almost no bracing wire. The D.VII's handling at altitude made it so respected that the armistice of November 1918 specifically required Germany to surrender its D.VIIs.

Check your understanding

Key takeaways

By 1914 the standard layout was set: tractor propeller, fuselage, tail behind, ailerons.
The braced biplane was a structural choice: a deep, light truss with thin airfoils, a large area on a short span. Its price was drag: \(\CDz \approx 0.03\) to \(0.05\) and \(\LDmax \approx 7\) to \(9\).
Zero-lift drag is \(D_0 = qf\) with \(f = \CDz S\); the power it absorbs grows as \(V^3\).
Prandtl's lifting-line theory (1918–19) explained induced drag, \(\CL^2/(\pi A e)\), and gave span a measurable value.
Thick airfoils made cantilever wings possible; Junkers and Fokker showed the way. Next, Lesson 4 combines them with streamlining to create the modern airplane.