Whirling Arms and the First Wind Tunnels
Taken from the book "Wind Tunnels of NASA"
by Donald D. Baals and William R. Corliss
Hyperlinks added to Wright Brother's material
The would-be aeronauts of the nineteenth century closely
studied the flight of birds and began building flying machines patterned
after avian structures. Their birdlike craft failed miserably. They quickly
realized that in reality they knew nothing about the lift and drag forces
acting on surfaces cutting through the atmosphere. To fly, man first had to
understand the flow of air over aircraft surfaces. This meant that he had
to build instrumented laboratories in which wings, fuselages, and control
surfaces could be tested under controlled conditions. Thus it is not surprising
that the first wind tunnel was built a full 30 years before the Wrights'
success at Kitty Hawk.
The wind tunnel is indispensable to the development of modern aircraft.
Today no aeronautical engineer would contemplate committing an advanced aircraft
design to flight without first measuring its lift and drag properties and its
stability and controllability in a wind tunnel. Tunnel tests first, free-flight
tests later, is the proper order of things.
On the End of a Whirling Arm
The utility of the wind tunnel is obvious today, but it was not the first
aerodynamic test device. Early experimenters realized that they needed a
machine to replace nature's capricious winds with a steady, controllable
flow of air. They recognized, as Leonardo da Vinci and Isaac Newton had
before them, that they could either move their test model through the air
at the required velocity or they could blow the air past a stationary model.
Both approaches were employed in the early days of aeronautics.
First, relatively steady natural wind sources were searched out. Models
were mounted above windswept ridges and in the mouths of blowing caves.
Even here, the perversity of nature finally forced expert-menters to turn
to various mechanical schemes for moving their test models through still air.
The simplest and cheapest contrivance for moving models at high speeds was
the whirling arm-a sort of aeronautical centrifuge.
Benjamin Robins (1707-1751), a brilliant English mathematician, was the first to
employ a whirling arm. His first machine had an arm 4 feet long. Spun by
a falling weight acting on a pulley and spindle arrangement, the arm tip
reached velocities of only a few feet per second.
Robins mounted various blunt shapes-pyramids, oblong plates, etc. -on the
arm tip and spun them in different orientations. He concluded that "all
the theories of resistance hitherto established are extremely defective.
" Different shapes, even though they presented the same area to the
airstream, did not always have the same air resistance or drag. The
manifestly complex relationship between drag, model shape, model orientation,
and air velocity defied the simple theory propounded earlier by Newton and
made ground testing of aircraft all the more important to the infant science
Sir George Cayley (1773-1857) also used a whirling arm to measure the drag and lift of
various airfoils. His whirling arm was 5 feet long and attained tip speeds
between 10 and 20 feet per second. Armed with test data from the arm,
Cayley built a small glider that is believed to have been the first successful
heavier-than-air vehicle in history. In 1804 Cayley built and flew an unmanned
glider with a wing area of 200 square feet. By 1852 he had a triplane glider
design that incorporated many features of modern aircraft, but manned, powered
aircraft were still half a century away.
Although Cayley performed many aerodynamic experiments and designed precocious
airplane models, his major contribution to flight was one of design philosophy.
Before Cayley, would-be aeronauts believed that the propulsion system should
generate both lift and forward motion at the same time, as birds and
helicopters do. Cayley said, "Make a surface support a given weight by
the application of power to the resistance of air. " In other words, use
an engine to create forward motion and let the motion develop lift via
the wings. This separation of propulsion and lift functions, simple though
it sounds, was a revolutionary change in the way people thought about aircraft.
One need not build planes with flapping wings! A whole new horizon in
aircraft design opened up.
Looking for Something Better
The whirling arm provided most of the systematic aerodynamic data
gathered up to the end of the nineteenth century. Its flaws, however, did
not go unnoticed. Test results were adversely influenced as the arm's
eggbeater action set all the air in the vicinity in rotary motion. Aircraft
models on the end of an arm in effect flew into their own wakes. With so much
turbulence, experimenters could not determine the
true relative velocity
between the model and air. Furthermore, it was extremely difficult to mount
instruments and measure the small forces exerted on the model when it was
spinning at high speeds. Something better was needed.
That something better was a "wind tunnel." This utterly simple device consists of
an enclosed passage through which air is driven by a fan or any appropriate
drive system. The heart of the wind tunnel is the test section, in which a
scale model is supported in a carefully controlled airstream, which produces
a flow of air about the model, duplicating that of the full-scale aircraft.
The aerodynamic characteristics of the model and its flow field are directly
measured by appropriate balances and test instrumentation. The wind tunnel,
although it appears in myriad forms, always retains the five identifying
elements italicized above. The wind tunnel's great capacity for controlled,
systematic testing quickly rendered the whirling arm obsolete.
role and capabilities of a wind tunnel can best be appreciated by recognizing
the aerodynamic forces and moments acting on an aircraft in flight. The three
basic forces are lift, drag, and side force as measured in an axis system
referenced to the direction of flight of the aircraft. The drag force is
along (but reversed to) the flight path; the lift and side forces are at right
angles to it. In a wind tunnel, the axial centerline of the test section
defines the direction of the oncoming wind-the aerodynamic equivalent of
the flight path. The ease of measuring aerodynamic forces relative to the
tunnel axis on a model held stationary in the airstream opened a new era in
Frank H. Wenham (1824-1908), a Council Member of the Aeronautical Society of
Great Britain, is generally credited with designing and operating the first
wind tunnel in 18 7 1. Wenham had tried a whirling arm, but his unhappy
experiences impelled him to urge the Council to raise funds to build a wind
tunnel. In Wenham's words, it "had a trunk 12 feet long and 18 inches square,
to direct the current horizontally, and in parallel course." A fan-blower
upstream of the model, driven by a steam engine, propelled air down the
tube to the model.
Wenham mounted various shapes in the tunnel, measuring the lift and drag
forces created by the air rushing by. For such a simple experiment, the results
were of great significance to aeronautics. Wenham and his colleagues were
astounded to find that, at low angles of incidence, the lift-to-drag ratios of test
surfaces could be surprisingly high-roughly 5 at a 15 degree angle of attack.
Newton's aerodynamic theories were much less optimistic. With such high
lift-to-drag ratios, wings could support substantial loads, making powered
flight seem much more attainable than previously thought possible. These
researches also revealed the effect of what is now called aspect ratio:
long, narrow wings, like those on modern gliders, provided much more lift
than stubby wings with the same areas. The wind tunnel idea was already
paying off handsomely.
With the advent of the wind tunnel, aerodynamicists finally began to
understand the factors that controlled lift and drag, but they were still
nagged by the question of model scale. Can the experimental results obtained
with a one-tenth scale model be applied to the real, full-sized aircraft?
Almost all wind tunnel tests were and still are performed with scale models
because wind tunnels capable of handling full-sized aircraft are simply too
In a classic set of experiments, Osborne Reynolds (1842-1912) of
the University of Manchester demonstrated that the airflow pattern over a
scale model would be the same for the full-scale vehicle if a certain flow
parameter were the same in both cases. This factor, now known as the Reynolds
number, is a basic parameter in the description of all fluid-flow situations,
including the shapes of flow patterns, the ease of heat transfer, and the
onset of turbulence.
Flight Before Flying
"It is easy to invent a flying machine; more difficult to build one; to make
it fly is everything."
----- Otto Lilienthal
Otto Lilienthal (1848-1896) has been called the world's first true
aviator. Although he built no powered aircraft, his hang gliders made him
world famous and generated great enthusiasm for manned flight. Starting
in 1891, Lilienthal flew-actually glided-over 2500 times, covering 270 yards
in his longest attempt. He amassed more air time than all his predecessors
Lilienthal's hang glider experiments were preceded by his
whirling arm tests of various lifting surfaces. Between 1866 and 1889 he
built several whirling arms, ranging from 6-1/2 to 23 feet in diameter. On
the basis of these tests, he concluded incorrectly that flight using flat
airfoils was definitely' ' possible. He turned next to cambered surfaces.
Even here, his test data were discouraging with respect to powered flight.
Undaunted by his pessimistic lab results, Lilienthal could not resist trying
to fly. And he really did fly in the sense that he could control his glider's
course over long distances. He lacked only an engine and propeller.
In stark contrast to the delicate birdlike gliders of Lilienthal was the
steam-driven Goliath of Sir Hiram Maxim (1840-1916). An American living in
England, Maxim had made a fortune with his famous machine gun. His goal in
aeronautics was powered, manned flight. With considerable wealth behind
him, he built large elaborate testing facilities and aircraft to match his
Maxim first tested airfoils. His whirling arm was 64 feet in
diameter, as befitted his brute force approach. The arm boasted elaborate
instrumentation to measure lift, drag, and relative air velocity. A wind
tunnel, however, was the main focus of Maxim's experimental work, and he built
it in heroic dimensions. It was 12 feet long, with a test section 3 feet
square. Twin coaxial fans mounted upstream and driven by a steam engine blew
air into the test section at 50 miles per hour . The tunnel and whirling arm
proved to Maxim that cambered airfoils provided the most lift with the least
drag. He obtained a lift-to-drag ratio of 14 for a cambered airfoil at 4
degree angle of attack-a spectacular performance for the late 1800s. He was
also the first to detect the effects of aerodynamic interference, where the
total drag of a structure exceeded the sum of the drags of the individual
components. He cautioned, therefore, that "the various members constituting
the frame of a flying machine should not be placed in close proximity to each
Consistent with his no-nonsense philosophy, Maxim built an 8000-pound
flying machine with a wing area of 4000 square feet. (The wing area of today's
DC-10 is only 3550 square feet, but it Supports an aircraft weight of 550,000
pounds.) Two 180-horsepower steam engines turned propellers 17.8 feet in
diameter. For 1894 this was a fantastic machine. It was propelled along a
2000-foot track that was designed to hold the craft down and keep it from
actually flying. In a test, the aircraft developed so much lift that it tore
loose from the test track and wrecked itself Maxim considered the experiment a
success and turned his attention elsewhere.
The scene shifted to America. Samuel P. Langley (1834-1906) was the first
major aeronautical figure in the United States. Mathematician, astronomer,
and Secretary of the Smithsonian Institution, Langley turned to aeronautics
in 1886. Like his contemporaries, he began by assessing the performance of
various airfoils. Langley built a whirling arm 60 feet in diameter that was
spun by a 10-horsepower engine and was capable of attaining speeds of loo-mph.
Langley covered much the same ground as Wenham, Maxim, and others. He was
optimistic about powered flight, stating that "so far as mere power to sustain
heavy bodies in the air by mechanical flight goes, such mechanical flight is
possible with engines we now have."
Samuel Langley's whirling arm experiments were not without their
frustrations. Located outdoors, the apparatus was frequently disturbed by
winds and the self-created mass of air swirling around the arm. So annoying
were Langley's problems that the Wright brothers, watching from Dayton,
turned to the wind tunnel as their major test facility.
Langley is perhaps best known for the failures of his Aerodromes, but his
highly successful unmanned, powered gliders have been slighted by many
aeronautical historians. His late-model gliders were propelled by tiny
1-horsepower steam engines that carried them for distances up to 3 / 4 mile.
Langley believed that these flights proved the potential of manned, powered
The Wright Brothers Put It All Together
Wilbur (1867-1912) and Orville (1871-1948) Wright, operating from the unlikely
background of bicycle manufacturers, built their first flying machine in
August 1899. It was a simple, 5-foot span, unmanned biplane kite rigged so
that it could be maneuvered by twisting or warping the wings (somewhat like
birds do for control). Kite tests led to the construction of their first
unpowered manned glider in 1900. Twelve test flights with glider No. 1 proved
that their pitch and roll controls worked. The glider, however, was generating
far less lift and more drag than they expected.
To find out why their first glider did not perform as predicted, the Wrights
set up a remarkably simple experiment using natural winds to compare the relative
lifting forces of flat and cambered surfaces. In effect, they built an
aerodynamic balance that showed unequivocally which of two test airfoils
developed more lift. This "wind tunnel without walls" confirmed the Wrights'
growing belief that the accepted aerodynamic design tables they were using
were seriously in error.
Sobered by these revelations, the Wrights increased the wing area of glider
No. 2 to 290 square feet. The initial trial flights at Kitty Hawk
disappointed them still further. The highly cambered wings created pitching
movements that could not be controlled. After several near disasters, airfoil
curvature was reduced, and the craft behaved much better.
The Wrights returned to Dayton with mixed feelings. Glider No. 2 had flown,
but, from the standpoint of their expectations, the 1901 Kitty Hawk tests were
a disaster. Their morale sagged. "Having set out with absolute faith in the
scientific data, we were driven to doubt one thing after another, till finally
after two years of experimentation, we cast it all aside, and decided to rely
entirely upon our own investigations.
They began with a comprehensive series of experiments with a wide variety of airfoils.
In the short span of 3 months these tests produced the basic data needed for
building their 1902 glider and the powered aircraft to follow. During this
short span of time, the Wrights leapfrogged other aerodynamicists the world
The first tests were exploratory and utilized an unconventional
testing machine: a
with a third wheel mounted horizontally on the
front of the frame. Two test shapes were mounted on the wheel, and the bicycle
was pedaled rapidly (up to 15 mph) up and down the streets of Dayton.
The airfoil being tested would produce a torque in one direction, but this
was counterbalanced by an opposite torque from a reference shape. The rotating
balance was brought into equilibrium by changing the airfoil's angle of attack.
Data from the impromptu rig were crude, but they reinforced the Wrights'
decision to reject existing handbook data. They had to write their own
handbook, and for that they needed a wind tunnel.
The first tunnel
consisted of a square tube for channeling the air, a
driving fan, and a two-element balance mounted in the airstream. One balance
element was a calibrated plane surface; the other was a cambered test surface
inclined at the same angle but in the opposite direction. When the wind
tunnel was brought up to speed, the vane-type balance turned one way or the
other, thereby indicating the relative lifting forces. The preliminary results
from the makeshift tunnel were so encouraging that the Wrights immediately built a
larger and more sophisticated facility
with a 16-inch-square test section.
Here they obtained the critical data they needed for their first manned,
They did make one mistake-they installed the tunnel's two-bladed fan upstream.
Shields, screens, and a honeycomb grid did cut down the turbulence, but it
was a curious lapse for the detail-conscious Wrights. Recognizing that their
laboratory itself was the return path for the air rushing out of the tunnel
test section at 25-35 mph, they forbade the moving of objects and people while
The heart of any successful wind tunnel is its balance system-the apparatus
that measures the aerodynamic forces acting on the model. The Wrights built
two balances-one for
and a second for
The balances never measured
actual forces; they simply compared test airfoils with reference airfoils or
the forces on calibrated flat surfaces. This approach allowed the Wrights to
rapidly pit one airfoil against another and select the best from
The Wright brothers returned to Kitty Hawk in late summer 1902 to build glider
No. 3. It was only slightly larger than the 1901 version, with a wing area of
305 square feet, a 32-foot wing span, and a weight of 116.5 pounds minus the
For straight-ahead gliding the craft worked well. The lift-to-drag ratio was
approximately 8, a one-third increase over their earlier gliders. Pitch control
was excellent, but turns were a problem. To turn, the plane had to be rolled
in the direction of the turn. This was accomplished by warping the wings;
that is, one wing panel would be twisted to increase the tip's angle of attack,
while the other wing's panel would be twisted in the opposite direction. The
high wing, however, created excessive drag and tended to wheel the craft in a
direction opposite from that intended. The addition of a rudder linked to the
wing-warping control solved this problem.
The famous 1903 Wright Flyer followed the 1902 glider design closely, except
for the addition of twin counter-rotating propellers 8-1/2 feet in diameter
driven by a 12-horsepower gasoline engine. Back again at Kitty Hawk, on the
morning of December 17, 1903, with Orville at the controls, the Flyer headed
into a 20-mph wind. After a short run of 40 feet, it rose into the air under
its own power and flew for 120 feet. Three more flights were made that morning,
with the longest lasting 59 seconds and covering 862 feet on the ground, or about
1 / 2 mile in the air. The Flyer was slightly damaged on the last landing, and
before repairs could be made a gust of wind turned it over and destroyed it.
It never flew again.
The historic Wright Flyer has been rebuilt and is now on display at the National Air and
Space Museum in Washington, D.C.
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