Whirling Arms and the First Wind Tunnels

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

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 of flight.

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

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. The unique 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 aerodynamic experimentation.

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 expensive.

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

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 combined.

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 immense aspirations.

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 other."

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 flight.

The Wright Brothers Put It All Together

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 over.

The first tests were exploratory and utilized an unconventional testing machine: a bicycle 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, powered aircraft.

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 taking data.

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 lift and a second for drag. 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 many configurations.

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 pilot.

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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