Chapter 1 - Whirling Arms and the First Wind Tunnels

Flight Before Flying


[3] 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...

illustration of forces acting on a wing in flight

 [4] A wing generated lift by deflecting a mass of air downward producing an upward reaction force on the wing in accord with Newton's third law of motion. The downward turning of air is shown graphically by the streamlines around the wing. The lifting force can be calculated by measuring the mass of airflow affected by the wing and the downward component of velocity added to it. The lifting force can also be obtained by integrating air pressure along the top of the wing and subtracting it from that along the bottom of the wing. The net lifting force for the wing of a modern jet transport is about 150 pounds per square foot.


....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 impossible. 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 500 000 pounds.) Two...

3 graphs illustrating the relationship between CD and CL

 [5] The curves show typical wind tunnel-derived values of the drag coefficient (CD), the lift coefficient (CL ), and the lift-to-drag ratio (equal to CL / CD) as functions of the wing s angle of attack.

Early aerodynamicists had to develop a methodology for applying scale-model data to full-scale aircraft. Obviously, the drag forces depended on model size, air density, and airspeed. Newton showed that the aerodynamic force on a given shape (e.g., an aircraft at a given angle of attack) is directly proportional to the area (S), air density (e [Greek letter rho?, Chris Gamble, html editor]) and the square of the air velocity (V2). In equation form: D = CDSeV2 where D is the drag force and CD is a nondimensional drag coefficient determined experimentally from scale-model tests.

Drag coefficients obtained from wind tunnel tests can be used to predict the drag force on a full-scale aircraft by inserting in the equation the full-scale values of S, e and V. Although there are recognized pitfalls in applying the drag equation, they can be circumvented, and this approach is used today to scale up wind tunnel test data to full-scale aircraft.

Newton showed that the term ( 1/2 e V2) represented the energy of the air due to its motion. It is referred to as the "dynamic pressure." Throughout the modern world, the numerical value of the aerodynamic drag, lift, and other coefficients (CD, CL, et al.) is referenced to the dynamic pressure as follows:

D = CDS (1/2 e V2)



...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 100-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.