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Aviation History
1917
1917 - 0161.PDF
FEBRUARY 15, 1917. THE CASE FOR THE LARGE AEROPLANES By F. HANDLEY SECTION I. General. THE question of aeroplane size is a most important one.It raises the whole question as to whether there is, or is not, a limitation to aeroplane size, and therefore whether progress inconstruction will be limited to improvement on present-day small types of machines, or whether there is an infinitepossibility in the extension of designs to much larger types. It has been argued by many that, just as ships, trains, andother machines for transport purposes increase in size as years go on, so will the aeroplane progress, and that the largeaeroplane will have a definite place in the field of aviation. Others have adopted the opposite view. The general consideration in favour of the large machine isthat although there is a heavier initial capital outlay, large machines are much cheapr to build, cheaper to maintain, andcheap r to run than small ones, and thus progress is seen in every type of mechanical transport towards the employmentof larger and larger machines with a view to taking full advantage of the economies effected. In an aeroplane there could, however, be no advantage in .the use of large machines if that increase in size gives a disproportionate increase in weight which would more thannullify constructional advantages, or if the large aeroplane had aerodynamical disadvantages. The whole case needsmost careful examination from all points of view. In the arguments set forth below I have endeavoured tocompare machines of different size and review their relative advantages, determining first of all bases of comparison toenable a true picture to be obtained. As these necessitate the explanation of a new method of aerodynamical comparison,I have set this forth at rather greater length than is necessary for the development of the argument proper. After adiscussion of the aerodynamical problem, I have dealt with the effect on structural weight of an increase in the size ofaeroplane, and then turned back to find the effect on the aeroplane's performance of the weight variation with sizeincrease. Lastly, there are a few notes on the large machine from a flying standpoint. It is a matter of some difficulty toobtain a true basis of comparison from pilots' opinions. Pilots are, as General Brancker remarked in his paper, a very con-servative body, opposed to innovation, and the machine of the moment's design is not necessarily the one of the future,or the one from which future machines will be developed. SECTION II. Aerodynamical Bases of Comparison.To determine the calculated performance of any machine, it is necessary to have available the wind-channel experimentson the lift and lift/drag of a large number of planes as well as the resistance for various types of bodies similar to thatproposed to be used. The curves of lift and lift/drag are usually plotted in absolute units and in the form shown inFig. i. From these wind-channel curves the performance curve ofthe whole machine is obtained. After the general details of a machine's design are settled,such as the weight to be carried, the area of the planes, &c, the plane resistance at various speeds is found from the liftand lift/drag curves. To these values are added the correct ones for body resistance, the values of the two curves addedtogether and the total horse-power required calculated for different speeds. When the engine power and propellerefficiency are known the curve of available horse-power can be plotted and the points of intersection of the two horse-powercurves mark the limits of aeroplane speed variation. It is quite easy to see that this method, although exceedinglyuseful for any particular aeroplane, does not afford a quick means of comparison between a machine with planes ofdifferent section or different shape or loading. I have there- fore adopted a different method of plotting so that theperformance of any machine can be directly predicted from the wind-channel tests, on the lift and lift/drag of theplanes used, and on the body resistance, the new method taking into^account the effect of altered loading or varyingair densities at various heights. I will deal first of all with the|plane calculation.The following is the notation adopted :— V—velocity of the aeroplane in feet/seconds. W—total weight in lbs. of the aeroplane. A—area of main planes in square feet. L —density of the air in lbs. cubic feet. g—32-2. Ky—absolute value of the lift coefficient. Kx—absolute value of the drag coefficient. * A pap«r read Wfore the Aeronautical Society on February 7th, 1917. PAGE, A.F.A6.S. Rb—total body resistance in absolute units per foot per second of the aeroplane considered, i.e., resistance of the chassis, body, struts, in fact all the resistance of the aeroplane except that of the planes. The following equations may be written :— whence or where w V ir -Ky. 1 "VKy p ~g 1 .A , V . F5 W A • g >P FfV A Resistance > • Kx . -? . Ag V* h.p. - Kx . -?g 55O Inserting the value of V from equation"(2) above Whence where h.p. Kx g 55° h.p. - b KxKyV Ky b- 35O A • (0 (2 (3) 1 (4) (S) (6) .:• •" (7) (8) (9) (10) Summarising F~V' Ky- , Kx / 1 h-V--bKy\/ Ky :urves: (3) (9) Instead of the usual Kx and Ky curves for a plane there will now be plotted #&&,'•..$*... a-, Ky which is equivalent to plotting horse-power required againstvelocities. A curve for the section known as R.A.F. 6 and one for the section known as R.A.F. 3 have been plotted outin this way. It is well to examine these curves to see their general application before proceeding to deal with the questionof plane comparison. In Fig. 4 are plotted the ordinary Kx and Ky curves for R.A.F. 6 and R.A.F. 3. R.A.F. 6 hasthe lower value of Ky maximum and a higher value for Fig. 1.
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