IT is the precise methods of engineering that will eventually place flying on a successfully commercial basis. As in all other branches of engineering, the theorists and the physicists point the way and give the methods that lead the practical man to the definite solution of perplexing problems, and in this aviation differs in no respect from any other art or science. The determination of the fundamental characteristics of air flow and air pressure on different kinds of surfaces and forms has led without doubt to a quicker and surer success in actual aeroplane flight, but it is qualitative rather than quantitative results that have been obtained so far. Up to the present few if any experiments in measuring the actual values of pressures on surfaces have been conducted on full-sized aeroplanes. The results that have been obtained come chiefly from extensive indoor laboratory experiments conducted on planes and shapes of small size, often only one-thousandth of the size of planes used on successful machines. But these small-scale experiments have in most cases been performed with great care and refinement, and from their results there are established the following empirical and fundamentally important laws and equations: A. Any air pressure on any plane or shape varies within the range of speed used in flight, substantially as the square of the velocity, and directly as the size of the surface. For the simplest case, a flat plane placed normal to the air stream, the air pressure P may be expressed as P = KS V2 where S = the surface area, V = the velocity of the moving air against the fixed surface or conversely of the moving surface against the still air, and K = a numerical constant, the most probable value of which is 0.003, when S is expressed in square feet and v in miles per hour. This is an empirical relation, derived from the results of many experiments, and upon it is based practically all of the theory of aerodynamics that finds application in actual practice. B. Air passing a surface, or conversely a surface moving through air, causes a frictional drag on the surface, which varies almost as the square of the velocity and directly as the length of the surface. Many formula? have been proposed, some based on experimental data and some on theoretical conclusions, but they differ widely from one another, and the value of this skin friction is still a subject of controversy, many experts claiming that friction is negligible, many others that it is of considerable value Here is a branch of aerodynamics still open to investigation, although the excellent results of Prof. Zahm seem almost conclusive evidence of the large value of frictional resistance. C. The pressure on an inclined flat plane varies with the angle of inclination to the air stream, but bears a fixed relation to the pressure on the same plane, when placed normal to the air stream. If we let P = normal pressure, and P1 = pressure acting on a plane when it is inclined below normal, at an angle a above the horizontal, then we may express this fixed relation between P and P1 as 2 sin a P1 P 1 sin2a This is known as Duchemins formula, and has been verified again and again by actual laboratory experiments on small flat planes. A very simple approximate relation suggested by Eiffel is The normal pressure P may be determined for any plane at any velocity by the relation of P = KSV, this pressure being gradually reduced as the plane is inclined to a value P', corresponding to that angle of inclination. D. The pressure on arched planes is much greater than on flat planes, and maybe equated to the value of the pressure on a flat plane of larger area. Whereas in inclined flat planes the pressure P1 is always perpendicular to the plane, on inclined curved planes at low angles P' is inclined in front of the perpendicular to the chord of the plane. Unlike flat planes, the pressures on curved planes cannot be reduced to intelligible formula?, and therefore in order to determine the pressures on curved inclined planes, resort must be had to tables of air pressures obtained from actual measurements on test surfaces and not to any formula? based on such measurements. The pressure P' on curved planes is usually tabulated as some percentage of the normal pressure P. It is in the determination of the pressures on curved surfaces that the results of aerodynamical experiments on small surfaces have been of most use in aeroplane designing. Lilienthal, Wright, Prandtl, Eiffel, Maxim and Stanton have all made determinations for curved planes that have found much practical application. E. On curved planes, as well as on flat planes, the total pressure P' acting on the plane when the latter is inclined at an angle a, may be resolved into a vertical component L and a horizontal component D. The component L is called the “lift,” and is equal to the weight of the aeroplane, while D is called the “drift,” and is the dynamic resistance to motion over-. come by the thrust of the propeller. F. On curved planes, as the depth or amount of curvature or arching is increased, the drift resistance increases. In other words, flatter planes have less resistance than more highly arched ones. The experimental results of Prof. Prandtl of Gottin- gen are particularly definite on this point. G. On curved planes, as the aspect ratio or ratio of span of plane to chord is increased, the lift increases greatly for the same area. Both Eiffel and Prandtl have amply verified this. H. Experiments show that on a plans there exists a point at which all the pressures on the plane may be considered as concentrated without disturbing the equilibrium. This center of action of the forces is called the “center of pressure.” On flat planes the center of pressure moves steadily forward from the center of the plane to a point near the front edge as the plane is inclined from the normal or 90-degree position to 0 degree. On curved planes a totally different action is observed. The center of pressure moves steadily forward from the center of figure to a point about one-third the width of plane from the front edge, as the inclination is reduced from 90 degrees to 15 degrees, . but at this point it turns abruptly and moves rapidly to the rear, passing the center of figure at about 5 degrees. J. Experiments in aerodynamic laboratories have further enabled forms of least resistance to motion to be determined and show what kind of torpedo or fusiform shapes give the least disturbance of the air streams. K. In addition, experiments on propellers have added immensely to the knowledge on this branch of aerodynamics, and have enabled air propellers to be designed that give a higher efficiency than is obtained in marine practice. The great French propeller manufacturing companies have had elaborate experiments conducted on full-size propellers, and have used the results to great advantage. L. The experimental photographing of the action of air streams on different planes and shapes has been a valuable contribution to aerodynamics and holds promise of becoming a field of much larger scope within a few years. These are fundamental “qualitative” results. In many cases 'the values of'different experimenters for the same thing show wide variations. In determinations of the constant K, for example, various' values from 0.0025 to 0.0046 are obtained, and many other differences are found. The “quantitative” results of experiments in aerodynamics are as yet not fixed, and it must be conceded that until reliable numerical values are obtained the precise engineering design that is looked forward to is hardly possible, even though excellent approximations may be made. The chief sources of error or difference in the experiments conducted thus far appear to be in the method and size of planes used. Whether size of surfaces has any effect on the nature of the pressures or their unit values is a problem in aerodynamics that is still to be solved. Many claim that the majority of experiments have been conducted on such small test surfaces that their results are of little value. Like all other aerodynamical problems, the answer is to be found in experiment only. There are five different methods of conducting experiments on test surfaces or models. These are: 1. Dropping the surface from a height in open air or in a closed room. M. Eiffel conducted a series of experiments on the Eiffel Tower in 1905, in which he allowed the test surfaces to fall about 300 feet. Lilienthal made some use of this method In the open air, and at the Russian Aerodynamic Institute of Koutschino, built by M. Riabouchinsky of Moscow, it has been employed. 2. Attaching the surface to a carriage moved on rails, as done hy Prof. Prandtl at Frankfort, or sliding down a long inclined railway, as performed by M. Canovetti in Italy. 3. Mounting the surface and testing apparatus on an automobile and driving at high speeds, taking careful record of the pressures. This method was employed to great advantage by M. Esna ult-Pelterie some years ago in France. 4. By means of a “whirling table” or large rotating arm, at the outer end of which are carried the forms and planes to be tested. The first aerodynamic testing apparatus, the old 8-foot whirling arm of Rouse used in 1758, was of this type. Later Langley, Maxim, Renard and Lilienthal used it to determine pressures on planes, and to test propellers. The huge new Vickers-Maxim testing table is the latest device of this kind. 5. By a wind tunnel. There are three main kinds of wind tunnels. In the first, used by Eiffel, Prandtl and Maxim, a huge fan blows air into a restricted passageway, and the air is then conducted through various screens and chambers until it issues out past the test surfaces in a more or less uniform steady' current. At Gottingen the elaborate character of the air passageway and screens renders the air stream practically perfect in its evenness of flow as it passes the test planes. In the second type of wind tunnel the air is drawn in past the test surfaces by a powerful fan placed in back of them. This is designed to avoid the “churned” air that is exhausted from a fan or propeller. Dr. Stanton in England and Prof. Zahm in this country use wind tunnels of this type. The third type consists of a fan blowing air through a chamber and screens as before, but at the end of the chamber is a nozzle which contracts the air stream and greatly increases its velocity. M. Rateau has used a wind tunnel of this type in his laboratory in Paris. The actual devices for measuring the pressure vary greatly with the different experimenters, and this plays, no doubt, an important part in the variations observed in their results. Pressure gages, often inaccurate, hydraulic apparatus, aerodynamic balances of great sensitiveness, pendulum devices capable of very exact calibration, graphical records on cylinders by movable pointers, electrical contact devices and comparison systems with standard flat planes, are some of the many means employed. To measure the actual velocity of the air stream, anemometers are used, and are either of the rotary cup type, recording on dials, or of the pressure type, in which the pressure of a surface acts through a spring and operates a large pointer. Only recently the great differences between conditions of air pressure and air flow inside a room and out in the open air have been recognized. The air in a closed room is perfectly quiet and lacks the characteristics of turbulent motion of the open air, characteristics that very likely have much to do with pressures on a surface of an aeroplane. Although a simple means of determining air pressures, the wind tunnel resembles only slightly actual free flight conditions, and many of the results obtained by this method of experiment are seriously open to question. Whirling arms, if small, or if rotated at too high a velocity, cause the air about them to assume a rotation, and thus render the results of the experiments inexact. Movement in a straight line in the open air is now recognized as the best means of experiment in aerodynamics, and the one that holds the greatest promise of establishing fully and exactly the great laws of flight. The next step forward after experiments with small test surfaces has already been taken. M. Eiffel, for the past few months, in his splendidly equipped laboratory at the Champ de Mars, Paris, has been making determinations on reduced reproductions of actual aeroplane types, models equipped with propellers, motors, wheels and all. These experiments, despite their small scale, are the most valuable conducted so far, and have enabled M. Eiffel to lay down many more fundamental laws that have a direct and important bearing on aeroplane work. One of the most interesting facts be has brought out is that when two identical planes are superimposed as on a biplane, the lift per unit of surface is less than if the same surface is used as a mono-plane. These significant results show that as the distance between the planes is increased from 2/3 to 3/3 and to 4/3 of the depth, the corresponding reduction of unit pressure due to placing the two planes one over the other is 65 per cent, 70 per cent and 75 per cent. M. Eiffel made extensive measurements on model wings of eighteen different aeroplane types, and made especially complete measurements on models of the new R. E. P. and the Nieuport monoplanes. His investigations on the distribution of pressure over a plane show definitely that the pressure at the front edge is very high, much higher than most aeroplane constructors suppose, and that at the rear it is very low, having often a negative value; i. e., the air is pressing on the upper face of the plane instead of the lower face. Flight is being rapidly advanced and made safe by the instructive experiments of searchers in this new and fascinating science of aerodynamics. The Moon and the Clouds IN Symons's Meteorological Magazine, July, 1911, Mr. William Ellis, F.R.S., discusses the widespread belief that the full moon possesses the faculty of clearing away clouds. This belief, which is expressed in the French proverb, “La lune mange les nuages” ("The moon eats the clouds") was held by Sir John Herschel, Humboldt, and Arago, but it has no foundation in fact. Many years ago Mr. Ellis tested the question by examining the observations of the amount of cloud made every two hours at the Royal Observatory, Greenwich. Grouping together, during the period discussed, the observations on the five days about full moon, and similarly the five days about new moon, there was found in both cases to be, on the average, a maximum amount of cloud in the forenoon, and a minimum amount in the evening. Now, if from any cause there was found to be a greater dispersion of evening cloud under a full moon than at the time of new moon, the evening amount of cloud at full moon should be considerably less than that at new moon, but there was no significant difference, the amount at the time of full moon being slightly greater than at new moon; this giving no support to the supposition of lunar influence to disperse evening cloud. The same result was arrived at a few years ago, but by* a different method, by Rev. S. J. Johnson. He noted the state of the sky at moon-rise and at midnight on the day of full moon for fifteen years. Of 186 occasions of full moon, in 126 there was a similar sky when the moon rose and at midnight; in 33 the sky was clearer about midnight than at moonrise; and in 27 the sky was more overcast about midnight; hence he found nothing to support the belief referred to. The belief is easily explained. There is, on an average, a progressive diminution of cloudiness toward evening—quite regardless of the moon's phase. The physical explanation of this phenomenon need not be given here. It was set forth a few years ago by Dr. W. N. Shaw, in the Quarterly Journal of the Royal Meteorological Society. Now, the change from a cloudy to a clear state of the sky is much more likely to be noticed when occurring near full moon; for with no moon the disappearance of the clouds, though making the stars visible, does little to change the general aspect of the evening; while under a full moon the disappearance of the evening cloud changes the whole appearance of nature, and attracts the attention of the most casual observer. Brightness of the Rising and Setting Sun CAMILLE FLAMMARION undertakes to answer, in LAstronomie, the following question, proposed to him by a correspondent: Why are our eyes less dazzled by the sun toward sunset than just after sunrise? Is the early morning sun really brighter than the late afternoon sun? There are two answers; one physiological, the other physical. The retina becomes progressively more sensitive in the dark. A sudden illumination at night dazzles our eyes, whereas the same absolute intensity of light would have much less effect in the daytime. During the day- the eye becomes gradually more and more accustomed to the light; in other words, less sensitive to it. However, the setting sun is probably actually less bright than the rising sun, because of the diminished purity of the atmosphere through which it shines. Solar radiation pumps up an enormous amount of moisture from the earth during the day.
This article was originally published with the title "Progress in Aerodynamics" in Scientific American 105, 16, 340-341 (October 1911)