The Golden Age of Flight: the quarter-century following World war I yielded more engineering improvements than just about any other period in aviation history.
Former World War I aviators, hoping to make a living flying, hit upon an exciting new form of entertainment: aerial acrobatics. These daredevils bought up surplus Curtiss JN-4 Jenny biplanes made during the war and launched barnstorming tours that thrilled, terrified, and captivated the public.
Some of the greatest pilots of the era, such as Charles Lindbergh, Pancho Barnes, and Wiley Post, became well-known through their aerial acrobatics and general risk-taking.
It was during this period, too, that the major air trophy races took center stage. The Schneider Trophy Race was the first of the Golden Age's contests. This international competition, begun by wealthy French industrialist Jacques Schneider in the early 1910s, reached its pinnacle with the 1925 win by U.S. Army Lt. Jimmy Doolittle. Just a day after he won the Schneider, Doolittle set a new world speed record for seaplanes while piloting a Curtiss R3C-2.
It may have been the barnstormers and the great trophy races that captured the public's attention initially, but it was the record-breaking flights of individuals that truly distinguished this era. When Charles Lindbergh's Spirit of St. Louis took off from Long Island's Roosevelt Field on May 20, 1927, and touched down at Le Bourget field near Paris 33.5 hours later, the public's fascination with flight was off and soaring.
Besides Lindbergh, the era also saw the solo Atlantic crossing by Amelia Earhart, the around the world flights of one-eyed Wiley Post, and the first completely blind takeoff and landing, as executed by Jimmy Doolittle.
But behind the achievements of pilots, there was ingenious engineering. From the late 1920s through the 1930s, propeller-driven airplanes reached maturity, and paved the way for the high-flying aircraft that would follow.
The Cantilevered-Wing Monoplane
Lucky Lindy's Spirit of St. Louis, a Ryan Aeronautics M-2 strut-based monoplane, popularized the monoplane configuration in America and marked the beginning of the end for the biplane. But it was far from the first monoplane to take to the air.
The first monoplane to achieve successful flight was built by Trajan Vuia, a Romanian inventor who lived in Paris. Vuia flew his monoplane 40 feet in 1906.
In 1908, Louis Bleriot, a self-taught French pilot with an engineering degree, crossed the English Channel in a monoplane, the Bleriot XI, which had a 25-horsepower Anzani engine that drove a two-bladed propeller. Exterior wires supported the wings.
The Bleriot XI proved to be quite popular as a speedy alternative to the biplane. However, by about 1911, it started earning a reputation as unsafe and unreliable, because of a number of crashes related to its wings folding up.
The reputation of the monoplane was redeemed in 1915, by German engineer Hugo Junkers, who developed an all-steel low-wing monoplane, the Junkers J-1. This plane was covered with sheets of steel welded to the tubular fuselage. The center section of its fuselage and the center section of its wings were constructed as one unit. This made the wing structure stronger, and less susceptible to structural failure, than the semicantilevered wings of the Bleriot XI.
The first widely accepted monoplane in the United States was the Ford Trimotor. It was made entirely of metal, covered by a corrugated aluminum alloy skin. The prototype of the Trimotor made its debut in 1926, equipped with three 420-hp Pratt & Whitney Wasp radial engines. The plane was designed to maintain flight after the loss of one engine. In practice, though, its 13,500-pound gross weight made it unable to climb after takeoff following the loss of one engine.
The last production aircraft rolled off the line in 1933. According to NASA, one of these aircraft was flying in scheduled airline service as late as the 1970s.
Retractable Landing Gear
Not long after the Ford Trimotor appeared, another pioneering aircraft, the Boeing Monomail, made its debut. This plane, introduced in 1930, is considered one of the pioneers in the development of retractable landing gear.
The Monomail, Boeing's first all-metal monoplane, had a cantilevered wing design, a streamlined fuselage, and retractable landing gear.
The notion of retractable landing gear grew out of research conducted in the late 1920s by the National Advisory Committee for Aeronautics. NACA's Propeller Research Tunnel at the Langley Memorial Aeronautical Laboratory in Virginia allowed engineers to test a complete airplane, as opposed to testing scale models. Tests in the tunnel showed that the landing gear contributed up to 40 percent of fuselage drag. Reducing that drag would significantly improve the performance of an airplane.
The two most obvious solutions to this problem were either to retract the landing gear inside the airplane after takeoff, or to find a way to cover fixed landing gear so it produced less drag during flight.
Retractable gear posed some problems of cost, weight, and reliability. Landing gear that collapsed when a plane touched down clearly posed a hazard to both the plane and pilot. And, there was the issue of where to put the gear when it was retracted. Some aircraft pulled the wheels straight up, often into a cowling behind the engine. Others pulled the wheels into the fuselage horizontally, with a door covering the opening.
Getting the landing gear to retract was another issue. That required a hydraulic or electric drive motor, more machinery, and a bigger engine--all of which added weight to the plane, virtually negating the benefits of reduced drag.
Retractable landing gear was appealing, but the extra weight made it unattractive until the 1930s, when airplane speeds began to reach 200 miles per hour. At this point, the increased weight of the gear was less of an issue than reducing drag, and thereby increasing speed.
The huge radial piston engines powering the planes of the 1920s and 1930s created their own drag problems, which NACA was working hard to understand and eliminate. In these engines, the cylinders were arranged in a circular fashion around the crankcase; air flowing back through the propeller and over the cylinder heads cooled the engines. But, to get this cooling effect, the cylinder heads had to stick out of the fuselage, creating drag.
Besides its retractable landing gear, the Boeing Monomail was notable for being among the first planes to use a cowling over its engine. The cowling grew out of research conducted at NACA's wind tunnel.
NACA engineers, led by Fred Weick, started testing various cowlings, which covered the cylinder heads, in the Propeller Research Tunnel. Weick finally came up with one design, No. 10, that completely covered the cylinder heads and the engine, but allowed air to flow in through the front. The air was directed over the hottest parts of the engine, and then released along the sides of the fuselage. The engineers also learned that the cowling had to connect to the fuselage in such a way as to not disturb this flow of air.
The No. 10 cowling cut engine drag by as much as 60 percent. In tests, a Curtiss Hawk AT 5A biplane with a Wright Whirlwind J-5 engine saw an increase in maximum speed from 118 to 137 mph.
VARIABLE PITCH PROPELLERS
As engine horsepower increased post World War I, the wooden fixed-pitch propeller that operated efficiently only at its design speed was no longer enough.
Around 1917, mechanically controlled variable pitch propellers were under development in Great Britain and Germany. These propellers solved the problem of changing the engine thrust without having to change the engine power and speed of the propeller. However, the bigger and more powerful the engine, the faster the variable pitch propeller wore out.
By the 1920s, designers had abandoned the mechanically controlled variable-pitch propeller.
An American engineer, Frank W. Caldwell, developed a propeller that had detachable blades joined to a central hub. This allowed pitch adjustments to be made while the plane was on the ground. This ground-adjustable propeller proved to be crucial to the success of Charles Lindbergh's solo transatlantic flight in 1927. Essentially, though, this was still a fixed-pitch propeller.
In 1929, Caldwell joined the Hamilton Standard Propeller Corp. and began work on a hydraulic, two-position propeller that provided efficiency at takeoff and landing--the two most critical flight times. American aircraft designers adopted the design, and started adding it to planes in 1932. The B-10 bomber, Boeing Model 247, and Douglas DC-2 all used the variable pitch propeller. The propeller reduced the Boeing 247 commercial transport plane's takeoff run by 20 percent, and increased its climbing rate by 22 percent and its cruising speed by 5.5 percent.
Caldwell and Hamilton Standard went on to develop a propeller that changed blade angle automatically, according to engine speed. On multi-engine aircraft, this Hydromatic constant-speed propeller had the ability to "feather," stopping the propeller's rotation, to keep the propeller from windmilling after an engine failure. Almost all the planes used during World War II were equipped with Hydromatic propellers.
Planes of the early 1920s suffered from two serious problems: a tendency to stall, and to go into a spin and crash. Stalls result when planes travel too slowly or when the angle of the wing compared to the airflow is too steep. Spins and crashes are caused just before a wing stalls, when the airflow becomes turbulent over the upper surface of the wing, increasing drag and decreasing lift.
Two different sets of engineers sought to correct these problems through the use of slots, open spaces running along the wing, outward from the fuselage. In 1919, German pilot Gustav Lachmann applied for a patent on a single-slotted wing design that would help prevent stalls. In his design, air flowed between the slots when a plane was flying with a high angle of attack at low speeds; while in normal level flight, the air would pass over the slots, allowing the wing to act like a normal wing. Lachmann's patent was rejected because authorities believed the slots would destroy the wing's lift.
At roughly the same time, engineers at the British firm Handley Page were addressing the turbulent air issue by running a slot down the length of the wing, near its leading edge, from the fuselage to the wing tip. In their tests, this slot increased lift from the wing by 60 percent.
Yet another engineer, O. Mader of the German airplane manufacturer Junkers, was testing a wing design that reduced burbling and increased lift. Rather than using slots, Mader's design used an auxiliary airfoil mounted behind the main wing. The design had a larger slot between the airfoil and the main wing, running parallel to the wing and airfoil, yet worked similarly to the designs of Lachmann and Handley Page.
Slotted wings didn't have much impact until they were combined with flaps, extensions on the trailing edge of the wing that a pilot can extend during landing and take off to increase lift. By the 1920s, flaps and slots began making an appearance on commercial aircraft. But it wasn't until 1922, when Orville Wright received his last patent for the split flap, that slots and flaps really took off. By maneuvering the split flap, a pilot could increase lift or drag. That allowed pilots to perform steep dives at lower speeds, and to descend toward a runway at a steeper rate, making for easier landings. The split flap consisted of a hinged section on the trailing edge of the underside of the wing. The split flap saw use on the Northrop Gamma, Lockheed Orion, Boeing DC-1, and DC-3.
The other major development during this time was Harlan D. Fowler's Fowler flap. Fowler, an engineer who worked for the U.S. Army Air Corps, used his own time and money to develop a flap that slid back from the wing and rotated down to create a slot between it and the wing. This flap increased wing area and lift. It ultimately saw use on the Lockheed 14 twin engine airliner in 1937. A variation, the triple slotted Fowler flap, is in use today on the Boeing 727 airliner.
As airplanes became more powerful, the need grew lot them to make more efficient use of fuel. Flying at higher altitudes, where the air is thinner, allowed planes to burn fuel more efficiently, fly faster and longer, and fly above many storms. But, there was a catch: They could fly only at 18,000 feet or less; any higher than that, and the thin air and frigid temperatures would kill the pilot and passengers.
In 1932, B.F. Goodrich made a full pressure suit for pioneering aviator Wiley Post, who went on to set unofficial altitude records, and to discover the jetstream in the process, while wearing the suit in his Lockheed Vega aircraft.
By 1938, the pressurized airplane was in production. Boeing's little-known 307 Stratoliner, affectionately dubbed the flying whale, for its portly lines, was the first in-service pressurized airplane when it entered airline service in April 1940.
Laminar Flow Airfoils
When NACA was established in 1915, it set to work testing airfoil performance in a wind tunnel, with the hopes of developing an airfoil that reduced drag, while still affording a large angle of attack. It tested a number of brass airfoil models with a span of 18 inches and a chord (or maximum width) of 3 inches, and found that slight variations in airfoil design created large differences in aerodynamic performance.
In 1933, the agency issued Technical Report No. 460, "The Characteristics of 78 Related Airfoil Sections From Tests in the Variable-Density Wind Tunnel." The testing data gave manufacturers a selection of standard airfoils, many of which found wide use during World War II.
By the late 1930s, NACA began work on creating airfoils with maximum lift. It quickly focused on laminar-flow airfoils. Laminar flow is the smooth, uninterrupted flow of air over the contour of the wings. It is most often found at the front of a streamlined body. When the smooth flow of air is interrupted over a section of wing, turbulence is created, which causes a loss of lift and an increase in drag.
The NACA laminar-flow airfoils were shaped with a relatively thin leading edge, and maximum thickness as far back as possible. The curvature is the same on both the upper and lower surface of the airfoil. In tests, NACA's laminar-flow airfoils reduced airfoil drag by almost 50 percent. In practice, though, these airfoils performed much like traditional airfoils. They proved to have excellent high-speed characteristics and to deliver a high Mach number. Ironically, because of limitations in manufacturing, the airfoils never delivered much ill terms of drag reduction.
North American's P-51 Mustang was the first aircraft designed to use laminar flow airfoils.
Much of what we consider to be "modern" airplane design originated during the Golden Age of Flight. This brief period in aeronautics yielded more engineering improvements then virtually any other period in the history of flight.