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.

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.

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.

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 C_l and C_d) 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.

THE ARTICLE on diffusion cloud chambers published in this department last September has brought forth hundreds of letters from amateurs, who made chambers of everything from whiskey glasses to fish tanks. The tricks they devised for getting around Murphy's law would fill a book.

Major Reuben B. Moody, an officer on duty at Wright-Patterson Air Force Base in Ohio, wrote:

"My brother Jerry, a graduate chemistry student at Ohio State University, and I have spent many enjoyable hours in the construction of cloud chambers of assorted shapes and sizes-some of which worked.

"Our first chamber was constructed according to the instructions in your article. For the chamber itself we used a wide-mouthed pickle jar with a metallic screw-top. A synthetic sponge was secured to the bottom of the jar by means of expansion clamps (manufactured from coat hangers), and the sponge was then thoroughly soaked with rubbing alcohol. Across the mouth of the jar we stretched a black cloth over which we screwed the metal lid. The jar was upended on a cake of dry ice. The cloth remaining outside the jar was spread out to cover the ice, thus providing a contrasting background and preventing the dry ice 'smoke' from interfering with our vision.

"We detected a miniature rainfall almost immediately, and within five minutes we could perceive the threadlike vapor trails. (My wife, who is not overly enthusiastic regarding scientific matters was disappointed because no lightning flashes were detected during our miniature rain storm.)

"Our first cloud chamber remained active for as long as the dry ice lasted- about seven hours. Although we were fascinated and elated with the results of our first endeavor, we began to think of ways to improve our results and to reduce the eyestrain attendant on observing them. In our pickle-jar chamber the sensitive region never exceeded a depth of about one inch. Also, the eyestrain was terrible, due to the ghostlike and short-lived appearance of the tracks and because of light reflections from the glass sides of the jar.

"Since metal is a better heat conductor than glass, we conceived the idea that the all-important temperature difference could be improved by constructing a metal cloud chamber. Accordingly, from the kitchen we obtained a coffee can about six inches deep and six inches in diameter. After painting the interior, of the tin can with blackboard slating, we cut three horizontal window slits in the can, one above the other. These windows, designed for observation and lighting purposes, were about one inch high and three inches wide. Over the windows we glued strips of cellophane (we found that scotch tape could not withstand the extreme cold without shrinking and without losing its adhesive properties). To ensure air-tightness we secured the edges of the cellophane windows with strips of plastic tape. On the bottom of the can we placed a cut-to-size disk of velveteen fabric, and to the lid of the coffee tin we glued the synthetic sponge [see drawing above].

"With the coffee-can chamber we obtained much better results. The eyestrain was eliminated; the sensitive region increased in depth to about 2.5 inches, and we were able to observe from 30 to 50 tracks per minute. Again the chamber remained active for as long as the dry ice lasted-this time about 18 hours with a cake of ice about two inches thick.

"As fascinating as our primitive apparatus proved to be, even more fascinating is the mystery surrounding the cosmic rays whose paths we were able to see. How do these rays obtain their tremendous energies and where do the rays themselves originate? Perhaps these questions will some day be answered by such a device as the cloud chamber."

A number of British amateurs also built successful chambers. R. P. Randall, a telescope-maker of South Harrow, Middlesex, England, reports:

"My chamber was a tall glass jar 18 by 5 inches. I carried out the whole experiment as a lecture to our scientific society. 'Rain' fell from a height of about five inches, and it was not until I had been watching for a few minutes that I realized that most of the rain was due to faint cosmic ray tracks about five inches from the bottom. The tracks could only be due to cosmic rays, as I had no active material available to produce them. The sensitive region was four to five inches deep, and I wonder if this was not due to using a tall cylinder and ethyl alcohol."

RECENTLY the Johns Hopkins University experimental physicist John Strong, in the course of designing his highly refined machine for ruling diffraction gratings [SCIENTIFIC AMERICAN, June, 1952, et seq.], was led to delve into the origins and history of modern high precision in the mechanical arts. He wrote in an article: "I find that the construction methods of greatest precision are all primitive methods." Just what he meant by primitive has been the subject of lively arguments among those who heard the term for the first time.

The literal meaning of primitive is first. In the sense in which Strong used the word, the dictionary defines primitive as "having something else of the same kind derived from it but not in itself derived from anything of the same kind." In other words, a primitive method of construction is distinguished from a derived method.

Let us consider a concrete example. Strong discusses the dividing of a circle into equal parts such as degrees. If it is done with a dividing engine, the accuracy obtained is derived from the master circle on the engine. Therefore this is not a primitive method of construction. If we put the circle in a lathe and divide it by rotating the lathe spindle by equal increments, this again is a secondary method. The increments are determined by the gears in the lathe, and their own precision, such as it is, was originally derived from a master circle.

In contrast consider this purely primitive method. Draw a circle on paper and cut it out with scissors, as shown in the drawings on the next page. Drive a spike through its center and into a mahogany table. Make a first mark anywhere on the circle's periphery and continue the mark on the table, as in the left-hand drawing. Estimate the 180-degree point opposite, mark the circle there and extend the mark to the table. Now rotate the circle as in the right-hand drawing, placing the trial 180-degree mark on the paper at the first mark on the table. The true 180degree point lies between extension of the trial 180-degree mark on the table and the first mark on the paper. A second approximation by the same principle brings us much closer to precision, which, however, we can never quite reach.

In the experimental physicist's encyclopedia edited by Sir Richard Glazebrook, entitled A Dictionary of Applied Physics, E. O. Henrici and G. W. Watts describe the procedure, at once primitive and modern, used in placing the degree and finer marks on the large circle for a precise dividing engine. They used the cut-and-try principle just described, with micrometer microscopes for locating the marks. After placing the 180 degree marks, they filled in the 90-degree, 45-degree and 22-1/2 degree marks and then the marks for all the 4,820 five minute divisions, each within one half-second of arc, the whole task requiring six months of tedious application.

This was a truly primitive method because precision was not derived but created, as it were, "from the blue." The unfortunate fact that the word primitive is often used in a secondary or derived sense (for many words have their own primitive and derived meanings) to denote "antiquated," or "out-of-date," has misled many. Some supposed that Strong had said in effect that the construction methods of greatest precision were the outmoded, crude or clumsy ones. Not at all; he was simply separating primary methods of construction from those "derived from other things of the same kind." As a matter of fact, there is still no better method for dividing a circle than by a series of approximations made by cut-and-try.

To the list of primitive methods could be added the generation of an accurate sphere by smoothing a rough ball on the end of a tube-a method by which the Chinese have made crystal balls for centuries and amateur telescope-makers have made Coddington lenses ["The Amateur Astronomer"; SCIENTIFIC AMERICAN, August, 1948]. The same method can be used in reverse to generate concave spherical surfaces [Amateur Telescope Making-Advanced, page 241].

Those who enjoy philosophical contemplation of the primitive methods often speak of them as "elegant," in the sense of refined. It is fun to imagine oneself a Robinson Crusoe and to speculate on how many of the amenities of science and technology one could re-create on an island without modern technical resources. Perhaps "Robinson Crusoe methods" most clearly defines the primitive principles.

Optical flats are ground by a primitive method. If three equal disks of glass or metal are ground together in pairs with abrasive grains between them-the first against the second, the second against the third, and the third against the first- they will approach ever closer to true plane surfaces. In his list of primitive methods Strong describes this as "the generation of flat surfaces, three at a time, by Whitworth's method of lapping." Mechanical engineers use the term "Whitworth's method" for the method of making the flats they call "surface plates." The term is less familiar to optical workers.

Sir Joseph Whitworth was described by Joseph Wickham Roe of Yale, in his classic English and American Tool Builders, as "the most influential machine-tool builder of the l9th century." He standardized the screw thread, and his tools became the standard of the world. Though Whitworth's name is attached to the three-disk method, James Weir French says, in his article on the working of optical parts in A Dictionary of Applied Physics, that the method appears to have been known to earlier opticians. They did not, however, describe it clearly; they were secretive. James Nasmyth, another l9th-century English machine-tool builder (whose hobby was astronomy), says the three disk method was in use early in that century in the shop of Henry Maudslay, the greatest of the old English tool builders. Whitworth had worked in Maudslay's shop. The three disks were then ground with emery grains. Noting that this usually resulted in "bell-mouth form," or what is now called "turned-down edge," Whitworth substituted local scraping of the metal with a hand tool. Though Whitworth contributed only the scraping (and improved on Maudslay's surface plates) his name has been attached to the whole three-disk principle. This is no more than just, after all, because he made the method public for the use of all instead of keeping it secret. He described it in 1840 before the British Association for the Advancement of Science in lucid language:

"Let one of the plates now be selected as the model, and the others be surfaced to it with the aid of coloring matter. For distinctness they may be called Nos. 1, 2 and 8 [see drawing above]. When Nos. 2 and S have been brought up to [made as nearly flat as] No. 1, compare them together. It is evident that if No. 1 be in any degree out of truth, Nos. 2 and 8 will be alike, and the nature of their error will become sensible on comparing them together by the intervention of color. To bring them to a true plane, equal quantities must be taken in both from corresponding places. When this has been done with all the skill the mechanic may possess, and Nos. 2 and 8 are found to agree, the next step is to get up No. 1 to both, applying it to them in immediate succession, so as to compare the impressions. The art here lies in getting No. 1 between the two, which is the probable direction of the true plane. It is to be presumed that No. 1 is now nearer truth than either of the others, and it is therefore to be again taken as the model, and the operation repeated."

Whitworth accomplished far more than to tell the world how to make a better flat. In an age that was still advancing from the mechanical crudity of the 18th century, when millwrights' wooden rules were graduated only to eighths of an inch and the best of the workmen could go only to "32nds bare" and "32nds full," and when the principal tools of the machinist were a hammer and cold chisel, he set forth the underlying significance of the surface plate or flat in controlling practically everything manufactured. Thanks to this control the parts of lathes and planers could now be made much more nearly plane; other machines built on these primary tools could for the first time be precise, and everything made on these in turn could be precise.

Today the optical flat makes possible the interchangeability of parts and thus controls mass production in all the mechanical industries, including the optical. This has come about in four decades, since industry adopted the gauge block for use with optical flats to test the tools that measure mechanical parts with high precision using light waves.

Anyone who has tried to make even a six-inch optical flat knows that it is much more difficult to make a flat to standard optical tolerance (one 500,000th of an inch) than to make a paraboloidal mirror of equal diameter, also that the difficulty increases with increasing diameter more than in mirror making. A dozen years ago Fred B. Ferson of Biloxi, Miss., was an advanced amateur telescope-maker, teaching himself how to make roof prisms. Each facet of a prism is a small optical flat, but the facets are not made singly: groups of them are attached to a single rigid metal backing and the whole is figured as a single flat of about 10-inch diameter. While making thousands of these 10-inch flats in wartime, Ferson also taught the advanced amateurs in this magazine's wartime roof-prism program how to make them.

Ferson remained in optical work. Recently he and his chief optician, Peter Lenart, Jr., made a flat of 10-5/8 inches diameter that was flat within one five-millionth of an inch. This has been acquired by the National Bureau of Standards as the standard for testing flats that are submitted to it for calibration. Most of them are submitted by industries that use flats to control the accuracy of gauge blocks, which in turn control the accuracy of shop tools.

There is a common impression that expert flat figurers use occult methods, generally behind doubly barred doors. Actually their methods are essentially no different from those available to beginners. Their "secret" is working with the best possible conditions. First, they use disks of fused quartz, to minimize differential expansion from changing temperature. Second, they work with thick disks (2-1/2 inches for a 10-inch), to minimize flexure. Third, they use correct methods of reading the interference fringes that measure the flatness, and of dealing with the residue of flexure. This factor, which the worker must understand before he can achieve accuracy of better than one millionth of an inch, has been reduced from an art to a science in an eminent new paper by Walter B. Emerson of the National Bureau of Standards titled "Determination of Planeness and Bending of Optical Flats" (N.B.S. Research Paper 2359, Superintendent of Documents, Washington 25, D.C., 10 cents). The key problem is to ascertain the true shape of the flat-the shape if it did not bend. Emerson accomplishes this by supporting the flat first at the edge, then at the center, pairing it with a flat of different thickness and working out equations. After studying his paper one wonders how many determinations of flatness previously made with master flats were correct, and what are the true contours of flats so measured.

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