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About a Small Wind Tunnel, Cloud Chambers and "Primitive" Techniques of Measurements
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by Albert G. Ingalls
April, 1953
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IF THE POPULARITY of a scientific avocation can be judged by the number of its followers, there can be no doubt about which one stands at the bottom of the list. Amateur aerodynamics wins without challenge. This is rather surprising, considering how intimately aerodynamics is related to everyday experience and how wide open one phase of the subject is to amateurs. That phase is the slow-speed flow of air. While the professionals give plenty of attention to high-speed air flow, almost nothing is known precisely about the forces generated by slow air currents. Yet no one amateur, so far as this department can learn, is investigating this fascinating subject. Nor is a single low-speed wind tunnel, professional or amateur, in operation anywhere in the U. S. If anyone knows where such work is going on, we would like to hear about it.
One does not need to look far for examples of low-speed aerodynamics. It enters into the physics of space-heating and of air-conditioning and ventilating systems generally. The design of several meteorological instruments involves the micro-ounce forces set up by movable surfaces that comprise their sensing elements. All of these mechanisms have been fashioned largely by cut-and-try methods rather than on scientific principles. Perhaps the professional neglect of this basic science can be explained in terms of dollars and cents: it may be felt that the small results would not be worth the time spent.
But this explanation can scarcely apply to amateurs. Time is the amateur's greatest stock in trade. Many boys (aged 8 to 80 spend endless hours building and flying kites. Still, with few exceptions the kites they fly are aerodynamically no improvement over those flown 3,000 years ago. Even the Navy continues to use the classical and grossly inefficient box kite to haul aloft the radio antennas of its emergency life rafts. It is true that some of these are fancy affairs with aluminum tubing and fabric substituted for sticks and paper. But aerodynamically the Navy's 1953 box kites are a thousand years old. Even more surprising is the lack of active interest in low-speed aerodynamics by the multimillion-dollar model-airplane industry. A significant percentage of its estimated 100,000 enthusiasts are gifted laymen, professional pilots and others who hold degrees in science and engineering. Each year these energetic hobbyists build and fly tens of thousands of model aircraft. Yet the miniature wings they construct are inappropriately patterned on large-scale airfoils designed for speeds above 50 miles per hour or on models put together by cut-and-try methods.
Partial cutaway of a low-speed wind tunnel
Some of the curious effects caused by the motion of air can be demonstrated with simple household objects. Suspend two apples by strings, like a pair of pendulums, and hold them close together. When you blow between them they will move toward each other, instead of flying apart as might be expected. Take a piece of paper an inch or so square, stick a pin through it and drop it on the end of a spool with the pin in the spool opening. You will find it difficult to dislodge the paper by blowing through the other end of the spool. Set an electric fan on the floor and let its air stream blow toward the ceiling. If you drop an inflated rubber balloon into the air stream, it will not be blown away but will stay in the air stream and hover over the fan, even when the fan is tilted at a considerable angle.
All these effects are accounted for by a common property of moving air-one which explains why airplanes fly and how it is possible for a good baseball pitcher to throw a slight curve. Less pressure is exerted on a surface by air in motion than by air at rest. Airplane wings are shaped so that air flows faster over the upper surface than the lower one; this reduces the pressure above the wing and produces a lifting force. The effect was first described in precise terms by Daniel Bernoulli, of the celebrated family of Swiss mathematicians, in 1737.
Another interesting property of air is its stickiness. It clings to objects, "wets" them and thus tends to retard their motion through it. In general these drag forces, as well as those of lift, increase with increasing velocity. At speeds from about 50 to 400 miles per hour, the thin film of air that clings to the surface influences the forces in significant ways. At higher speeds the forces change: at the speed of sound, for example, moving objects literally rip the air apart, compressing that in front and creating a vacuum in the wake. The professionals today are largely occupied by the effects that lie beyond the so-called sonic barrier.
But no one appears to be in the least concerned with the equally interesting effects in what may be called the region of the gentle breeze. Only once, at least during recent years, has anyone ventured into that region. Just before the beginning of World War II a group of amateurs in Boston, headed by Captain W. C. Brown of the Army Air Force, decided to explore the behavior of aerodynamic forces set up by velocities under 10 feet per second. Several members of the group were majoring in aerodynamics at the Massachusetts Institute of Technology. The group spent many months building a precision wind tunnel for low-speed investigations. Unfortunately the tunnel was in operation for only a brief period before the war started, and the group completed only two studies. They plotted the characteristics of a family of airfoils worked out mathematically for indoor airplane models, and investigated the effect of streamlining the structural elements associated with these profiles. After Pearl Harbor most of the group went into military aviation, and that ended the project. But the few prized scraps of information that emerged from it continue after more than a decade to be published all over the world.
Captain Brown, who is now with the U. S. Office of Education, writes of the historic Boston tunnel as follows:
"One of the failures of the past 35 years of aviation has been the inability of man to conquer the low-speed field. The slow autogiro and helicopter represent two of the few successful innovations in conventional design since aviation became a fact. Who can predict what other discoveries in this field may revolutionize present design?
"Before the war several attempts were made with various types of equipment to gather data in the low-speed aeronautical field. One notable project was a -tunnel of about three feet diameter with the air stream driven by an ordinary fan. The famous B-7 airfoil came out of this work. Another project, more ambitious, was a tunnel in the Midwest which produced some interesting tests, although numerous corrections had to be made. But the Boston instrument continues to hold the record as the largest and most accurate low-speed wind tunnel ever constructed, and it could serve as a model for further work in this field today.
"John P. Glass, in those years a student at M.I.T., started it all, and to him goes much credit for the tunnel's design. Glass's design was executed by members of the Jordan Marsh Aviation League. William H. Phillips, also a former M.I.T. student, now with the National Advisory Committee for Aeronautics at Langley Field, Va., started designing the balances about a year after work was begun on the tunnel proper.
"The Boston tunnel was 18 feet long with a standard diameter of five feet at all points. The air was forced through the tunnel, instead of being sucked as in most high-speed tunnels. (Roger Hayward's drawing at the left shows the general arrangement.) This method was dictated largely by economic considerations. A tunnel of the conventional sucking type would have required an entrance cone about 18 feet in diameter and a length of 60 feet to get a smooth air flow. Even so, air flow at the low speeds contemplated by the designers would doubtless have been disturbed by eddies originating outside the tunnel By compressing the air at the propeller end of the instrument and permitting it to seep through blanketing layers of fabric into the test chamber, the tunnel achieved a smooth air flow with a structure of reasonable size. The pressure drop through the blanket, about three pounds per square foot, overcame any irregular pressures arising from turbulence created by the propeller and kept out of the test chamber eddies caused by persons moving about in the room.
"The tunnel was driven by a propeller five feet in diameter with six overlapping blades connected through a belt to a direct-current motor of 440 revolutions per minute and three horsepower. The velocity of the air stream could be varied between 2 and 12 feet per second by means of a shutter placed between the propeller and the blanket. This system of control offered a distinct advantage over regulating the speed of the motor, because it tended to offset slight velocity changes caused by variations in power line voltage, belt slippage and related factors.
"Air speed through the tunnel was measured by two gauges: a calibrated pendulum vane and an anemometer of the Richard type. Pressure in the tunnel during the calibration period was measured by a manometer arrangement, built by Phillips, which utilized a pair of mill; bottles. It was extremely accurate but was abandoned after it was found too sensitive to temperature changes for prolonged use.
"The test models were suspended from an airfoil balance. The first balances, intended for use with outdoor models, could weigh a force up to four ounces and were sensitive to three hundredths of an ounce. They were of the automatic spring type. It was found that a different type would be required for work with indoor models, because the forces to be measured were so infinitesimal. This problem was by far the most difficult encountered during the tunnel's design and construction. A successful design was developed after much work by Phillips [see drawing above]. The new balance, of the automatic torsion type, was sensitive to one thousandth of an ounce and had a capacity of one tenth of an ounce. It achieved its extreme sensitivity by using an electromechanical amplifier, incorporating the feedback principle, whose main features were derived from an instrument used at M.I.T. for measuring the surface tension of liquids. Any force tending to disturb the equilibrium of the balance's master beam was, in effect, counteracted by an equal force derived from a reversible electric motor actuated by a set of contacts carried by a secondary beam.
"The Boston tunnel employed five of these balances. One measured the vertical force, or lift, acting on the airfoil under test, and two others measured the drag forces. The two remaining balances measured pitching, rolling and yawing."
The test objects investigated by the Boston group consisted of a series of rectangular airfoils 30 inches long by five inches wide. They were not true wing sections, like those of an aircraft but merely thin sheets, bowed like a wind-filled sail. The curve was stiffened by a set of lateral ribs. Starting with the arc of a circle as the curve of the basic airfoil, the experimenters derived mathematically a family of related curves in which the peak of the curve was progressively shifted aft from the leading edge. The curves are described by the N.A.C.A. system, in which the diameter of the airfoil, or "chord," is taken as unity and the remaining dimensions are expressed as a percentage of this length. Five numerals define the curve: the first digit gives the highest point reached above the chord, the second and third give the distance of this maximum height from the leading edge, and the last two specify thickness.
Results of experiment were plotted
The experimenters found that the most successful airfoil aerodynamically was the one in which the peak of the curve (8 per cent) was located 40 per cent aft of the leading edge [see top of chart at the right] . Because this airfoil has no thickness (being formed of a single sheet of material), it is designated 84000. (For convenience the last two zeroes are frequently omitted.) A two-surface airfoil of the same shape with a thickness of 15 per cent is designated 84015.
The basic objective of these investigations is to measure two characteristics of a given airfoil: how variations in the speed of the air stream and the angle at which the airfoil meets the stream affect its lift and drag. The airfoil, or if desired a complete model of the airplane, is suspended in the test section of the tunnel from a T-shaped structure which in turn is coupled with the balances. After a series of readings at a predetermined range of air-stream velocities, the tunnel is shut down and the angle of attack is increased. A second set of forces is then recorded. The procedure is repeated through any range of attack angles desired.
One of the five balances for the wind tunnel
The forces so observed are recorded in thousandths of an ounce. The observations are transformed by simple equations into coefficients of lift and of drag usually designated Cl and Cd) and plotted as a set of curves, one showing the lift coefficients, another the drag coefficients and the third the "L/D" ratio of the two through a range of angles of attack. The main chart at the upper left shows a set of these curves derived for the 84000 airfoil.
The Boston tunnel of course can investigate the aerodynamic behavior of test objects of any shape. The instrument also opens boundless opportunities for the exploration of jet effects at low speed and of the drag effects of various surface textures.
