FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1915
1915 - 0906.PDF
Taking first the question of wing resistance, this is commonly in the order of I lb. for every 12 or 14 lbs. supported in flight; but, according to Lanchester,* "if it is found practicable to employ a really high aspect ratio.t there is every reason to suppose that a resistance coefficient as low as 6 per cent., or even 5 per cent., may prove to be attainable." In his "James Forrest" Lecture to the Institution of Civil Engineers last year, Mr. Lanchester expresses the aggregate body resistance as an equivalent flat surface measuring, in the case of modern machines, about 5 sq. ft. in area. His remarks on the future are as follows :— "If we take an aerofoil coefficient of 7 per cent., and a curve representing 3 square feet equivalent normal plane, we find that at 80 miles per hour the gliding angle, or the resistance coefficient, should be approximately 12 per cent., and at 60 miles per hour TO per cent. ; I believe this figure to be in sight, though it may not yet have been actually reached " " If we try, in the light of present data, to look into the future, it seems probable that the limiting gliding angle, or, rather, the minimum total coefficient of resistance, may even be materially less than 1 in 10; thus, if it is found possible, in spite of structural difficulties, to obtain in an actual machine results equal to those obtained in wind channel model tests, namely, a coefficient of resistance for the aerofoil approximating to 5 per cent., and if the body area equivalent, for a machine of 1,200 lbs. gross weight, can eventually be reduced to 2 square feet, a total coefficient of resistance as low as 8 per cent, may prove well within reach. Whether the sacrifice necessary in order to achieve such results in practice would be justified, the future alone can decide. The solution of an engineering problem is always to some degree a matter of com promise, and it would be rash to suggest that in the case of the flying-machine there are not considerations of sufficient importance to render it inadvisable to run after the last 1 per cent, reduction in tractive effort." The Power Required for Flight. Taking the case of any particular aeroplane already constructed it is apparent that the nature of the resistances it encounters in flight renders it necessary to plot a graph in order fully to record the conditions. Thus, on the one hand, there will be the resistance of the wings, which will at first decrease with increasing speed as the attitude approaches the speed of least resistance, and afterwards will increase again. On the other hand, there will be the body resistance, which increases rapidly with the speed from first to last. These two resistances form separate and intersecting graphs on the resistance chart, and must be combined to form the total resistance. If the thrust available for the propeller is superimposed, a characteristic chart is thereby constructed, which forms a key to the anticipated speed and climbing qualities of the machine. Graphs of this character were introduced by the Royal Aircraft Factory to express the anticipated and actual results of their machines. (See Fig. 70 The two points of intersection between the propeller thrust and aeroplane resistance curves indicate the speed range, and the lower of these two limits is that in which the machine assumes the cabri attitude that has been described as so potentially dangerous. At any given speed on the chart the reserve thrust is indicated by that part of the ordinate, measured to scale, which is intercepted by the two curves. The product of this thrust by the flight speed gives the reserve power available for acceleration. If the attitude of the machine is adjusted to a path of ascent, so that the flight speed through the air remains constant, this reserve power may be utilised for climbing, and the rate of vertical ascent can be estimated from the reserve power available and the total weight to be lifted. From a mere consideration of two facts, viz., that a machine must be able to fly slowly (40 m.p.h. or thereabouts) in order to alight with safety on indifferent ground and when piloted with very moderate skill, and yet must be able to fly fast in order to make headway through a strong wind or to escape a superior force of its enemy in war, it is apparent that a wide speed range is of funda mental importance in any generally useful aeroplane. This, in turn, as can be seen from the characteristic chart of aeroplane resistance, calls for a reserve of power and accounts for the fact that the engines have steadily tended towards larger sizes, especially of late years. If we take an imaginary case of an aeroplane weighing 1750 lbs. J experiencing a resistance of 1 in 7 at 60 m.p. h., the power necessarily expended on level flight is 40 h.p. If the power available at the propeller is 60 h.p., the reserve at 60 m.p h. is 20 h.p. and the estimated rate of climbing is nearly 380 ft./min. In the military * See Lanchester on "Th« Flying Machine from an Engineering Point of View," Proceedings of the Institution of Civil Engineers, yol. CXC^III, 1913-14. t Ratio of span to chord in a wing. . % The weights of aeroplanes in the Military Aeroplane Trials, 1912, varied from 1,481 lbs. to 2,680 lbs. with pilot and 4! hours' fuel and oil. aeroplane trials of IQI2,§ the climbing rate ranged from 105 to 365 ft. /min. The present climbing value of one of the British Govern ment's aeroplanes;! is from 400 to 450 ft./min. IT Prior to the war, one of the most used- engines was the 80 h.p. Gnome, which develops 64 h.p. on the brake. It is now less widely used, owing to the growing demand for engines capable of giving at least 90 h.p. effectively and continuously. In the near future, engines of over 200 h.p. will undoubtedly be in use on the larger types of aircraft. As most people are aware, the aeroplane motor, although similar to the motor-car engine in its fundamental principles, is, as a rule, of a very different description. In the early days it was not unknown for sportsmen to take engines out of their cars in order to equip their aeroplanes, but the engine in such cases continued to evince a distinct preference for the support of terra firma. Exigencies of weight forced designers of aeroplanes to seek lighter motors, and the rKc-r **** se-c. Fig. 7.—Chart serving as an analysis, or svnthesis, of aeroplane resistances. The resistance of the body, •which comprises also the struts, wires, landing carriage, &c, is shown by its graph to increase rapidly -with the speed. The resistance of the wings is shown by its graph to decrease rapidly until an angle of about 5° is reached, beyond which point the resistance increases again as the angle becomes finer. The sum of the two resistances produces the total resistance curve, which is seen to have a minimum value at about 75 m.p.h. The product of the total resistance by the speed produces the graph of horse-power required. The graph of horsepower available at the propeller is superimposed on the chart from known data. The ordi' nates intercepted between the two horse-power curves, measured to scale, gives the reserve power available for acceleration or climbing. The above chart is for a par ticular aeroplane and is taken from the technical Report of the Advisory Committee. The method, however, can be applied to all machines, and is regularly employed at the Royal Aircraft Factory. Particular attention is drawn to the obvious danger of forcing the machine to fly very slowly at very steep angles of incidence. The proximity of the two horse-power curves on the left of the diagram Illustrates the absence of reserve power in case of emer gency, and Figs. 2 and 6 show that the wing may suddenly pass the critical angle and so lose its lift in this region. Wrights, it will be remembered, had to build their own engine for their very first machine. Many attempts to produce a satisfactory aeroplane engine was made in different quarters before the Gnome rotary motor was produced. The advent of this remarkable machine gave a great stimulus to practical flying, but its influence on competitive engine design seems rather to have damped than inspired progress. Being a type apart, and having a quality of lightness only readily to be attained by following the basic principles of its design, its success rapidly developed into a virtual monopoly that was somewhat deterrent to new enterprise. Neither its lack of economy in fuel and oil consumption nor its need for frequent attention have much militated against its utility under flying conditions as these have existed hitherto, but it is quite certain that aeroplane engines of the near future must attain in these respects more nearly to the standard of those employed on automobiles. Indeed, fuel and oil economy rapidly attain a greater importance than the actual weight of the engine when long duration flights are considered. (To be continued.) § For an analysis of these trials see " Aviation," p. 272. | See ,; Technical Report of the Advisory Committee for Aeronautics," 1912-13, p. 248. % One of the R.A. F. experimental machines climbed 900 feet in one minute (see " Technical Report," 1912-13, p. 265). 906
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
