FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1911
1911 - 0872.PDF
(/£GHT OCTOBER 7, 1911. SCIENCE AND THE AERONAUTICAL ENGINEER.* By L. BERN. THERE is a general tendency, which has found expression in certain aeronautical organs, to believe that since the aeroplane has been able to attain a certain development without the application of laboratory results and without an adequate knowledge of the theory of mechanical flight that the science of aviation does not require for its full expansion a complete theoretical knowledge of the different factors which influence it. Not only is it thought by some that a true insight of the different problems conneqted with aerial travelling can only be acquired by actual practice, but also that any theoretical study of the subject cannot in any way assist the new science. In view of the magnificent performances of some aviators who do not possess any scientific training we hear it proclaimed every day that aviation has been created by practical men and that it can be only developed by them ; let it be said, however, that all those who have a really scientific training and who have studied the development of any branch of science have not been dazzled by the magnificent flights that have been performed in the last few months, but, on the contrary, have seen in them the necessity for methodical and scientific research. The need of experimental data, obtained with a true scientific spirit, has already been strongly felt by those who are anxious to evolve, out of the existing machines, stronger and safer air-craft. If sensational records have been broken over and over again within the last few months, the aeroplanes themselves have not been very materially improved as regards their stability in wind eddies or as regards the safety of landing. A theoretical knowledge based on accurate experimental data is indispensable to any aeronautical engineer as dis tinguished from an aviator. M. R. Esnault-Pelterie, the designer of the R.E.P. aeroplanes has himself stated that, in his experiments with his aeroplanes, he lost quite a year for having ignored a principle which is now clearly established, namely that of the backward motian of the centre of pressure of a curved surface as its inclination diminishes. Aeronautical laboratories now exist in every country taking real interest in the possibilities of the flying-machine as a powerful weapon of war and as an industry. The best known are those of Cha'ais-Mendon and of the Eiffel Tower in France, of the Nadonal Physical Laboratory in this country, of Kutchinu in Russia, of Gottingen in Germany, and the military laboratory of the Brigata specialist in Italy. Important results have been obtained in those well-equipped laboratories quite recently and, in our lectures, we shall classify and examine the most important of them and see how to apply them to solve some of the numerous problems that face the aeronautical engineer. We shall particularly study the magnificent results obtained by Eiffel in the laboratory he has erected at the foot of the tower bearing his name, as well as those recently obtained by Prandtl at the laboratory of Gottingen, and we propose to devote a few evenings to the examination of those results which are sure to come to light before the end of the present session. The remarkable volume which has been published by Eiffel quite recently, and which contains the results of his researches, and some most important conclusions to be drawn from them,is no idle excursion in the realms of mathematical probabilities, but a treasure of practical information gathered from a large number of results co-ordinated with all mastery of modern scientific methods. It is now possible to design aeroplanes with fair accuracy; the time for guesswork is passed, and the sooner this is realised the better. The able manner in which the minutest details were studied and carried out leaves little doubt as to the trustworthiness of these experiments, and the results receive confirmation from those obtained by Mr. Staunton at the National Labora tory. Of all the facts estalished by these experiments, few can be more important than the simple relation which holds between aeroplanes of different sizes. The behaviour of a large or full-sized aeroplane can be deduced from the results of experiments on a small size model, by the use of a simple coefficient. Thus, what has long been done in the ship building industry can also be done in the case of air-craft. The behaviour of surfaces at various inclinations to the direction of the wind or of travel has received extensive * From the opening remarks of the lecturer at the beginning of the winter session of the Aeronautical Course at the Regent Street Polytechnic. DESBLEDS. treatment. The pressure in the direction of travel or the resistance to travel, the pressure at right angles to that direction or the sustentation, the total pressure, and the dis tribution of pressure on both sides of the surface have all been accurately measured. Results obtained by a balance method have been strikingly confirmed by the summation of local pressures on both sides obtained by means of manometers. The results have been embodied in a set of curves which it is essential to study carefully in order to arrive at an intelligent understanding of this intricate question. The interest attaching to a peak of high pressure in all these curves, is only surpassed by that attaching to the remarkable sustenta tion of curved surfaces, and their peculiarly unstable behaviour at small inclinations, especially at small negative inclinations. The suggestion that this may have been the cause of many accidents otherwise unaccounted for deserves serious considera tion, and shows how valuable such investigations are, and how essential it is that all concerned should make themselves thoroughly acquainted with the results. A polar curve has been ingeniously devised from which the following quantities can be read out at a glance, namely : The resistance to travel or horizontal component of the pressure as abscissa, the sustentation or vertical component of the pressure as ordinate, the resultant or total pressure as a radius vector, the inclination of this resultant, the inclina tion of the surface, and the position of least resistance. The distribution of pressures on both sides of the surface has also been represented by curves. It is worth noting that the negative pressure at the top of the plate, especially near the leading edge, is considerably larger than the positive pressure on the lower side of the surface. This fact is most important from a constructional point of view. It means that the canvas which covers the upper part of an aeroplane should be strongly fixed to the framework of the aeroplane and that its leading edge should be protected from the strong suction of this negative pressure. The position of the resultant centre of pressure also forms the subject of an interesting set of curves. Besides flat surfaces, and surfaces having one curvature only as commonly used, surfaces having two curvatures have also been experimented upon, and have been found to possess the striking property of offering a decreasing instead of an increasing resistance at higher speeds. The experiments have not been confined to surfaces of a few simple geometric forms, but have included small models of most of the aeroplanes in actual flying machines. All these were made 90 cm. x 15 cm. The results obtained with these, after being converted by the use of a coefficient ratio to make them apply to the full-sized aeroplanes, were compared and found to agree with the actual data obtained from the flights of the full-sized machines. It had been usually accepted or assumed that for small angles of inclination ranging from the angle which gives zero sustentation to angle of about 8°, the total pressure was proportional to the inclination. This is now proved to be incorrect in the case of Wright aeroplanes. For small angles up to 2 ° the rate of increase of the pressure for these aeroplanes diminishes gradually ; it is only between 2° and 8° that the rate of increase is practically constant. Beyond 8° the rate of increase diminishes again. An important factor from a practical point of view is the ratio of the horizontal component to the vertical component of the pressure, i.e., of the resistance to the sustentation. This factor also has been represented by curves. For the Wright aeroplanes the minimum value of this ratio occurs at 20. For smaller angles the ratio increases rapidly, while for angles up to 8° the ratio increases but slightly ; for larger angles the ratio again increases rapidly. It must be remarked that the angle for which this ratio is a minimum is not necessarily the best angle, for the sustentation may then be inconveniently small for the area of the surface, and it may be advantageous to sacrifice a small increase in the proportion of resistance to sustentation for the sake of a notable increase in sustentation for a given area ; thus in the above case, an angle of 6° or even 8° may be found more suitable. These are only a few examples of the value of labora tory researches and of their practical usefulness. It is only oa sound data that we can build up the new science of aeronautics and such a technology as is indispensable for the full and rapid development of the new branch of engineering4 874
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
