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.
By Gayle Ehrenman, Associate Editor
The period between the end of World War I and
the United States' entry into World War II has been described as The Golden
Age of Flight. Barnstorming tours, major trophy races, and record-breaking
flights all captured the public's attention.
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.
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| Wiley Post wore a pressure suit while flying a Lockheed Vega (top). Curtiss AT-5A (bottom) was one of the first planes to use a NACA engine cowling. |
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.
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| NACA's Cowling No.10, which completely covered the cylinder heads and engine, cut engine drag by as much as 60 percent. |
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.
Engine Cowling
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.
High-Lift Devices
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 takeoff 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.
Pressurization
As airplanes became more powerful, the need grew for 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.
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| The P-51 Mustang was the first aircraft designed to use laminar flow airfoils. The finish was not smooth enough to maintain much laminar flow. |
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 in 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.