Lesson 4 · 40 min

The Streamlined Monoplane

Between the late 1920s and the mid-1930s, a handful of ideas came together: the cantilever wing, the smooth metal skin, the engine cowling, retractable gear, flaps and the variable-pitch propeller. Each was a modest gain. Together they halved the zero-lift drag coefficient, quadrupled the wing loading and created the shape of every propeller airplane since.

Learning objectives

The ingredients

No single invention made the modern airplane. Loftin and most historians point to a group of technologies that matured between about 1925 and 1935, many of them tested in the wind tunnels of the US National Advisory Committee for Aeronautics (NACA), founded in 1915:

  1. 1927

    The clean cantilever monoplane. The Lockheed Vega had a smooth molded-plywood monocoque fuselage and a thick cantilever wing with no struts. Loftin gives it \(\CDz = 0.0278\), against about 0.04 for the biplanes of the decade before.

  2. 1928

    The NACA cowling. Fred Weick's tests at NACA Langley showed that a carefully shaped ring around a radial engine both cut its drag sharply and improved its cooling. It won the Collier Trophy for 1929 and appeared on almost every radial-engine aircraft afterward.

  3. c. 1930

    Stressed-skin metal structures. A smooth aluminum-alloy skin, riveted to frames and stringers, carries the loads itself (semi-monocoque construction). Herbert Wagner's tension-field theory showed that thin skins can carry shear even after they buckle, making such structures light. Smooth skins also cut drag.

  4. 1933

    The Boeing 247 and the Douglas DC-1. Twin-engine, all-metal, cantilever-wing transports with cowled engines and retractable gear. The 247 is often called the first modern airliner; the DC-1 led to the DC-2 (1934) and the DC-3 (1935).

  5. 1930s

    Flaps, variable-pitch propellers and supercharging. Split and later Fowler flaps raised the maximum lift coefficient for landing; controllable-pitch and then constant-speed propellers stayed efficient at takeoff and in cruise; superchargers and higher-octane fuel kept engines powerful at altitude.

  6. 1940

    Refinement: the P-51 Mustang. A NACA laminar-flow airfoil, a carefully ducted radiator and flush riveting gave the lowest \(\CDz\) in Loftin's propeller tables, 0.0163.

Flaps, stall speed and wing loading

At the stall the wing is at its maximum lift coefficient, \(\CLmax\), and lift still equals weight:

Stall speed

\[ W = \tfrac12\rho V_s^2 S\,\CLmax \qquad\Rightarrow\qquad V_s = \sqrt{\frac{2}{\rho\,\CLmax}\,\frac{W}{S}} \]

For a given stall (and so landing) speed, the wing loading can only rise if \(\CLmax\) rises: \(\WS = \tfrac12\rho V_s^2\CLmax\).

Landing speed was limited by the length of grass airfields and by safety. A designer who wanted a small, low-drag wing for a fast cruise therefore needed a high \(\CLmax\) for landing. Flaps provided it: deflected for landing, they raise \(\CLmax\) from about 1.3 to about 2 on the aircraft of the 1930s, and retract for cruise. That is the main reason wing loadings could keep rising after the drag stopped falling.

Figure 4.2 Published stall speed against wing loading at gross weight, for the propeller aircraft whose stall speed Loftin lists (sea level). The curves are \(V_s = \sqrt{2(\WS)/(\rho\CLmax)}\) for \(\CLmax = 1\), \(1.5\) and \(2\); the slider adds a curve of your own. Points of the 1930s and 1940s lie toward higher \(\CLmax\): flaps. A few points imply an implausibly high \(\CLmax\): their published stall speed was probably measured at a lower, landing weight.

Example 4.1 — What flaps are worth

A transport must stall at no more than \(110\ \text{km/h}\) at sea level. What is the highest wing loading it can have with \(\CLmax = 1.3\) (no flaps) and with \(\CLmax = 2.0\) (flaps down)? By how much can the wing shrink, for the same weight?

Show solution

\(V_s = 110/3.6 = 30.56\ \text{m/s}\), so \(\tfrac12\rho V_s^2 = \tfrac12(1.225)(30.56)^2 = 571.9\ \text{Pa}\).

\[ \CLmax = 1.3:\ \ \frac{W}{S} = 571.9(1.3) = 743\ \text{N/m}^2\ \ (75.8\ \text{kg/m}^2) \] \[ \CLmax = 2.0:\ \ \frac{W}{S} = 571.9(2.0) = 1144\ \text{N/m}^2\ \ (116.6\ \text{kg/m}^2) \]

For the same weight, \(S \propto 1/(\WS)\): the flapped wing needs \(1.3/2.0 = 0.65\) of the area, a wing 35% smaller, with 35% less skin-friction drag in cruise.

The DC-3: why it won

The Douglas DC-3 combined every ingredient: a cantilever wing with flaps, stressed-skin construction, two cowled engines in nacelles faired into the wing, retractable gear and variable-pitch propellers. It was often described as the first airliner that could make money by carrying passengers alone, without a mail subsidy. More than 10 000 were built in the United States alone, most of them as the military C-47.

Example 4.2 — The Ford Trimotor and the DC-3

The Ford 5-AT Trimotor of 1928 had a drag area of \(f = 39.33\ \text{ft}^2\) (\(3.654\ \text{m}^2\)) and a gross weight of \(6123\ \text{kg}\); the DC-3 had \(f = 25.58\ \text{ft}^2\) (\(2.376\ \text{m}^2\)) and \(11\,340\ \text{kg}\). Find the power each absorbs in zero-lift drag at \(240\ \text{km/h}\) at sea level, the Ford's top speed. Compare per tonne of gross weight.

Show solution

\(V = 240/3.6 = 66.67\ \text{m/s}\), so \(\tfrac12\rho V^3 = \tfrac12(1.225)(66.67)^3 = 181\,500\ \text{W/m}^2\).

\[ \text{Ford: } P_0 = 181\,500(3.654) = 663\ \text{kW}, \qquad \text{DC-3: } P_0 = 181\,500(2.376) = 431\ \text{kW} \] \[ \text{per tonne:}\quad \frac{663}{6.123} = 108\ \text{kW/t}, \qquad \frac{431}{11.34} = 38\ \text{kW/t} \]

The DC-3, almost twice as heavy, needs about two-thirds of the Ford's drag power, and about one-third per tonne. It also cruised well above the Ford's top speed. Lower drag per unit of payload is lower fuel cost per seat: that is the economics of streamlining.

Figure 4.3 From the Ford Trimotor to the B-29, to scale (grid squares are 5 m). In fifteen years, the largest aircraft went from a 6-tonne high-wing trimotor with a corrugated skin to a 54-tonne pressurized bomber with a wing of aspect ratio 11.5. Choose one to compare its numbers.

The Second World War: refinement and scale

Wartime development pushed power, speed and size rather than new configurations. Engines grew from about 750 kW to more than 1600 kW; fighters such as the P-51 reached about 700 km/h; bombers such as the B-29 combined a high aspect ratio wing (\(A = 11.5\)), Fowler flaps and a pressurized cabin, and reached \(\LDmax = 16.8\), the highest value in Loftin's propeller tables. The Lockheed Super Constellation airliner of 1950 reached 16.0.

By 1945 the piston-engine propeller airplane was close to its limits. Near 700 to 750 km/h, propeller tips approach the speed of sound and lose efficiency, and the drag of the airframe begins to rise steeply as the air around it approaches the speed of sound. Going faster needed both a new engine and a new wing: the subject of Lesson 5.

Check your understanding

Key takeaways