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Aviation History
1920
1920 - 0844.PDF
JULY 29, 6 ANGLf BETWEEN STABILIZER AND «1M6 CHORD - OE6REES EFFECT OF ANGLE OF STABILIZER SETTING i P'B * OF FORE AND AFT LOCATION OF 4* VECTOR 'CHANCE Of BALANCE.) Fig. 3 is a graphical representation of a wind tunnel test of a model of a flying boat. It shows how Gm diminishes for larger angles of incidence until it vanishes at about 12°. Table I, of Appendix II, gives the metacentric heights calculated from wind tunnel tests on complete models (ex propellers) of 27 types. Aeroplanes both small and large are included, as well as float seaplanes and flying boats. We have a truly miscellaneous collection, with British, French, German and American examples, which should give a good test of the naval architect's method of classification by means of metacentric height. Designers have, in general, been giving a greater meta- centric height for great chord length, which appears reason- able. Plotting the metacentric height against chord length on Fig. 4 brings this out clearly. The points group them- selves about a straight line. It therefore seems possible from a knowledge of the relation of chord to metacentric height fox any design to say with some confidence whether the machine will be more or less stable (stiff) longitudinally than the average, and whether such stability will be reasonable or abnormal. Effect of Change in Angle of Stabiliser Setting.-—In Table II of Appendix II are assembled calculations of Gm from wind tunnel tests in which the setting of the stabiliser has been altered. To bring out the effect of such changes, Fig. 5 has been made where Gm is plotted against the angle of stabiliser setting, i.e., the angle between the stabiliser and the wing chord. It will be noted that the points fall near straight lines whose slopes are the change in Gm per degree change in angle of stabiliser setting. This slope is denoted by AGm/Afi and is plotted against the ratio of horizontal tail area/to wing area in Fig. 6. It is found that AGmJAB depends on the area of the hori- zontal tail surfaces in a straight line function. The slope of each line of Fig. 5 when plotted on Fig. 6 falls near either one or the other of two straight lines which have substantially the same slope. It is to be noted from these lines that the tractor type tail surface is less efficient than the N.C., a pusher type tail surface, a conclusion consistent with previous experimental work. (A. C. A. R. and M. 438 and 505.) Effect of Change in Stabiliser Area.—The values of Gm for changes in stabiliser area are given for various machines in Table III of Appendix II. These values when plotted against percentage of the original area see Fig. 7) lie near straight lines. The variation in the slopes of these lines is small and seems to be independent of the original size of the stabiliser. Fore and Aft Movement of Centre of Pressure.— In order to study the movement of the centre of pressure, the 40 vector location, or line of action of total air force for an angle of inci- dence of 40, has been determined for a number of machines and for various stabiliser settings. These locations, which may be found in Table IV of Appendix II, are plotted on Fig. 8. For each machine the points are found to lie near' a straight line, the line for the larger machines having, however, a greater slope than those for the smaller machines. In Fig. 9 the slopes from the lines of Fig. 8 are plotted against the wing chord of the machines which they represent. The fact that all of the points so plotted lie very nearly on a straight line indicates that the movement of the 40 vector with change in angle of stabiliser setting is almost entirely dependent upon the wing chord. The average value of the movement of the 4° vector per degree change in stabiliser setting is found to be 0-033 x chord. This movement is forward if the normally negative angle between stabiliser and wing chord is in- creased, and vice versa. In the foregoing discussion I have tried to bring out the fact that an aeroplane actually has a metacentric height which is entirely under the control of the designer. It "would very much clarify our knowledge of aeroplanes if we knew the initial metacentric height the designer gave it. Knowing •: r 9 SUo UJ z 0 BRIS JN-Z(» TOLH6 V ISA-PS H CP-10 ;C§J( P»GE<j>/'' ERE -2 (lAH&I TAIL 1 FIG. 9. 4 fc 8 10 12 14 It 16 20 Zi. 21 WIN& CHORD-FEET RELATION BETWEEN WING CHORD AND FORE AND AFT MOVEMENT OF 4° VECTOR FOR T CHANGE IN 5TABILIZ.EF? 5ETTIN6 (CHANGE OF BALANCE) this, we could judge with some degree of confidence whether the machine is reasonably stable for its type. Further, if we knew the metacentric height as designed, the effect of stabiliser adjustment could be considered more intelligently than it is at present. The adjustable stabiliser is a very useful and convenient device for altering the balance of a machine, but it also alters the metacentric height. It is quite possible, by excessive use of this means to balance up an. otherwise tail heavy machine, entirely to destroy the metacentric height and hence the stability. The tables and charts I have given here may prove useful, I trust, in the consideration of, such a possibility for a particular design. (To be Continued) An Aircraft Disposal Souvenir A UNIQUE publication is to hand in the shape of a booklet —if it is permissible so to call a thing measuring 20 ins. by 12 ins.—entitled " The Wings of Commerce." It has been got out as a souvenir of the great deal by which the Aircraft Disposal Company recently acquired the huge accumulation of Government aircraft and material and for which the Handley Page Co. are the sole selling agents. Photographs are given of the various types of machines available, and the specifications are in English, French and Spanish. Some fine designs by Garth Jones decorate the opening pages, which also contain extracts from the speeches of Lord Londonderry and Mr. Handley Page at the luncheon held to celebrate the deal. The edition of the souvenir is limited to 2,000 copies and each one is signed by Mr. Handley Page. An Aviation Company in Burma THE interest taken by the Burmese in the possibilities of commercial aviation has resulted in the first aviation com- pany being registered at Rangoon with a nominal capital of one million rupees. The objects of the Company are to acquire aircraft, carry on passenger services and commercial transport of every description. -..,- _., .--:•"••- -; " , .- 846
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