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Aviation History
1912
1912 - 0262.PDF
i/jjGHT MARCH 23, 1912. EXPERIMENTAL RESEARCH AT THE N.P.L. ACCORDING to custom, the National Physical Laboratory celebrated the conclusion of its official year by holding a reunion at Bushey Home on March 15th, which was attended as usual by most of the leading engineers and scientists in the country. Aeronautics, which is the only department of the N.P.L. that properly comes within the scope of editorial recognition in these columns, forms but a small fraction of the scientific ground that is covered by the work of this invaluable institution, yet, as may be seen from the report of this section published elsewhere, the aero nautical work accomplished and in progress is large both in variety and importance. Experi mental research of the kind that is undertaken at Teddington requires the most painstaking preparation, and a year seems but a moment of time when it comes to laying down and calebrating the apparatus, and making all the preliminary investigations that are essential to ensure accuracy. As yet this department of the laboratory is only on the fringe of its great subject, but it is making good progress and the time is not far off when the English aviation world will realise that in the N.P.L. it has an asset unsurpassed for value by the laboratories of any other country in the world. The bare report that we give practically in extenso, is not, perhaps, so informative as it might be, but there is much in it that will arouse interest. The references to tests on wing sections in particular will attract the notice of all our readers, and we feel justified even at the risk of error, in supplementing what is there stated by the gist of the information that it was possible to obtain in the course of conversation during the visit. Thus far, the experiments on wing sections have been mainly confined to sections having a flat underside, and the tests have investigated the effects of varying the altitude (angle of incidence of chord to line of flight) and varying the camber of the upper surface alone, by altering the maximum thickness of the section. It seems that the upper and lower surfaces of a wing section should be separately investigated, as they have different characteristics. It is also evident that the upper surface is both the most interesting and the most important ; it contributes perhaps three times as much lift as the lower surface to the support of the machine, and it is on the upper surface that the critical changes of flow, causing sudden variations of lift, takes place. The nature of the flow over the under surface conforms to the contour of the surface, and the distribution of pressure is such that a maximum value not exceeding 'tyv- is obtained in the vicinity of the leading edge for small angles of incidence. Over the upper surface, the flow adheres to the fabric for a short distance and then becomes turbulent. At angles of incidence exceeding a critical value, some where about 10' to 16', this turbulence becomes "dead water," due to discontinuity of flow, and gives rise to a sudden loss of lift coupled with an increase in the drift resistance. The distribution of negative pressure is such that a maximum value, perhaps in the order of three limes ('5pz'2), occurs in the vicinity of the leading edge. This distribution accounts for the forward inclination of the resultant pressure, which is the secret of the low co-efficient (ratio of drift to lift) of the cambered wing as com pared with the flat plane. Any wing section that might be considered reasonable for practical use ought to begin to manifest a positive lift in a negative altitude of about ij0. The lift and the co-efficient vary with the altitude, but they also vary with the camber for a constant attitude, consequently the angle of incidence is not the only significant angle in calculations affecting the cambered plane. The lift increases and the co-efficient decreases with an increase in the maximum thickness of a section that has a flat under surface : in other words some function of the camber represents the effective angle over and above that obtained from the altitude of the wing. Variations in the position of the maximum camber along the chord are of relatively small importance, as also are changes in the surface •contour of a section having a given maximum camber. Eor an attitude of zero degrees angle of incidence, the thinnest plane that is structurally strong enough to do its work will have the lowest co-efficient: in other words the angle of least resistance for a given loading has a very small numerical value. The lift of a wing obeys the v- law, and the co-efficient of flight is independent of speed. At present the lowest co-efficient obtained from experiments is -fa (= "0625). It should be easy to construct a wing having a co-efficient of ^ ( = "077), but it may be assumed that any good practical wing has a co-efficient not greater than i\s (= 0"i). Skin friction has been found from experiments on model dirigible envelopes to approximate to the formula and co-efficient established by Zahm, when corrected so as to conform with the theory of aero dynamical similarity ; that is to say it may be regarded (for rough- and-ready calculation only) as somewhere in the order of the values obtained by the simplified expression R = "000018 Vs lbs. per sq. ft. of double surface for speeds up to 70 m.p.h. In an average biplane it has been estimated that the body resistance (struts, wires, undercarriage, engine and pilot) is some what more than half the total resistance, hence the practical co-efficient of flight (gliding angle) may be expected to be in the order of (2 x -1) = -2 = I in 5. _ Also, it is evident that the greatest opportunity for reducing the power required for a given speed lies in the reduction of body resistance rather than in the improvement of wing section. Any improvement in this direction has an immediate and important effect: the mere shaping of the struts might make all the difference with regard to being able to carry a passenger under given conditions. In struts, the best practical section, having regard to strength and weight, is probably one having a fore-and-aft length about three times the greatest breadth. For stream-line bodies, however, this ratio might be extended to, say, 6 to I, with advantage. The entry must be blunt, but slightly pointed rather than hemispherical. A hemispherical head should always be followed by a parallel body for a short distance before the sides close in for the run. The run should not be curtailed, even if the flow does not adhere to its surface, for by doing so the resistance is increased by the region of dead water thus established. In the cambered wing section, it is similarly important to distinguish between the turbulence, that is a normal characteristic of the flow over the upper surface, and the dead water that is enclosed by the surface of discontinuity, created when the attitude of the plane exceeds its critical angle. The dead water region is also characterised by turbulence, but it is not part of the stream-line system as in the case of the turbulence attending the normal attitude of the plane. In connection with propellers, it would seem that the general conclusions relating to wing sections again apply. Almost any fair description of two-bladed propeller should give an efficiency between 65 and 70 per cent., beyond which improvement is the result of a combination of refinements that it is difficult properly to correlate. Progressive research still continues to support the doctrine that all fluids are fundamentally alike in their behaviour, and that model experiments in water are a reliable index to the full-scale conditions in air. The following details are abstracted from the official report of the N.P.L. for the year 1911 :— Satisfactory progress has been made with the researches carried out in accordance with the programme laid down by the Advisory Committee for Aeronautics. Among these researches may be mentioned specially the investigation of the efficiency of a number of aeroplane surfaces, including a careful study of the distribution of air pressure, and the nature of the flow round such surfaces. The determinations of the air resistance of smooth wires, of standard ropes, and struts of different forms, are also of interest. The tests of motors entered in the second competition for a prize of ^1,000 offered by Mr. Patrick Alexander were completed towards the end of the year. A very complete series of weathering tests of balloon fabrics has been completed by Mr. Barr. Wind Channels.—The following investigations will be carried out in the existing 4 ft. channel and the new 4 ft. channel when completed. 1. A complete series of tests on the lift and drift of wing forms, together with observations on the effect of aspect ratio. 2. Study of the effect of warping the trailing edge of a wing form up and down. 3. Determination of the most suitable " gap " in biplanes. 4. Continuation of the work on the best form of strut. Water Channels.—It is proposed to continue the investigation into the stability of dirigibles. Air Channel for Visual and Photographic Work.—A new channel for this work will be completed early in the year. The channel will be mainly employed for visual and photographic observations intended to serve as a guide to the direction in which progress may be expected in the experiments in the larger air channels. Whirling Table.—It is proposed to continue the propeller experiments, especially as to the effect of blade area and pitch on the amount of power available from a propeller of given diameter. Wind Towers.—It is proposed, if the research on the lateral variation of wind velocity is completed this year, to make experi ments on the resistance of a full-sized aeroplane wing in the natural wind for the purpose of comparison with the resistance of a similar model in the wind channels. Strength Tests of Fabrics.—The following work is proposed :— Compound stress tests on diagonally double fabrics in continuation of the work on parallel doubled fabrics. Tearing testson large specimens 262
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