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Aviation History
1914
1914 - 0663.PDF
JUNK 19, i9I4. he machine will execute an undignified descent, to which the verb 'to pancake has been applied. The critical speed at which thU w,l take place is not necessarily related to the critical "least velocity angle of the aerofoil. Briefly, for a given machine the extent of the flight speed variation 15 f "T0"K°f ,'h£ reServe of thrust ov« the minimum resistance ^ ^•?^°1T^llue °f the HmitS WnB fixed by the »o2a that the aerofoil n called upon to sustain. In the case of a high-powered rgto^th^IeToToil"6 l0WCr llm'1 ^ * PreSCtibBd Critical The choice between monoplane and biplane is, in the main, a question of constructional engineering ; there is not a great deal to choose between the two from an aerodynamic standpoint, but with equally good design the monoplane gives a slightly better lift/drift ratio. The interference effect of the two members of a biplane aerofoil has been studied by many investigators. Langley showed, about 1890, that with superposed planes (aspect-ratio = 4) the interference was not serious when separated by a distance equal to more recent investigation t/jjGHT] their lesser dimension. The results of a _ by the staff of the N.P.L. are published in the report of the Advisory Committee for the year 1911-12 (p. 73), from which Table VIII has been taken. In addition to obtaining quantitative data for the particu lar aerofoil chosen (Bleriot, aspect-ratio = 4), an investigation was also TABLE VIII.— Table of Multiplying Factors to Obtain Coefficients from the Coefficients Jor a Single Aerofoil. Lift/Drift. 8\ io°. o'8i 0-84 0-82 o-86 0-84 0-87 085 088 0-89 o-9i made on the effect of staggering the planes. It is shown to be advantageous to arrange the upper foil in advance of the lower • IpSng. Lift Coefficient. Gap/Chord. 6°. 04 o'6i o-8 0-76 fo 081 1-2 o-86 16 0-89 8°. 0'62 0-77 0-82 o-86 0-89 10°. 0-63 0-78 0-82 0-87 C90 6°. 0-75 0-79 o-8i 0-84 o-88 the «am longitudinals. The first and simpler method has been used by several firms for many years past, and gives results which under ordinary conditions, are verv much on the safe side j the second method has t>een developed during the last few years by the N. 1'. L. see Report of the Advisory Committee, 1912-13, No. 83,) and has been adopted by the R.A.F., and more recently by other manufac turers. On the pin-joint hypothesis the stresses are solved by the well- known graphic stress diagram ; the alternative method is consider ably more complex"; reference should l>e made to the report cited. It is well to remark that though the pin-joint hypothesis gives results usually on the safe side, the extent of the factor of safety so introduced is not one that can be relied upon, and may in special cases be even negative. It is hardly necessary to point out that the more important and vital the problem, the less appropriate become methods of an approximate and inexact character. 9. Resistance of Struts, Wins, Wheels, &c—The question 01 the resistance of components such as are commonly embodied in the design of existing machines has been studied experimentally at the N.P.L., at the Aerodynamic Laboratory at Gottingen, and by Mr. F. Eiffel, in Paris. A few results relating to strut sections are given in Fig. 30a. The graph a a is a plotting from N.P.L. data (see Report of Advisory Committee, 1912-13, p. Ill), relating to the sections o, representing one of the best forms tested, graphs b and c relating to sections ^> and c as determined by Mr. F.iffel (see Assistance of the Air and Aviation, p. 1.S4). In Fig. 30a ordinatcs represent resistance coefficient in absolute units, also in terms of normal plane (the normal plane unit b;ing that of maximum section). In l_ig. 30/) are given two strut sections designed at the R.A.F. These were reported upon by the N.P.L. as giving less resistance forgive* strength than a number of others submitted. Approximately, strength for strength, these sections gave one-fourth the resistance of struts of circular form. (See Report of the Advisory Committee, 1911-12, p. 96.) The resistance oi wires and ropes has been investigated both z < O O z 0 0 id -•4-™ < Z -•a z. -1 a. J -28 a: s Z b. O - -| -j> £ E bj y-z \CL \ C ^^O!' (EIFFEL) ^err TR^rT-t c *^-^ STRUT SECTIONS O IO JO 30 4-0 50 VEUCny Fig. 30a. thus the combination a b, Fig. 29, is of the same efficiency as the combination a c. Considering the aerofoil, whether monoplane or biplane, from a structural standpoint, and in investigating the strength of the aero foil as a whole, it may be treated definitely as an inverted cantilever system. Thus, comparing the stresses in an aeroplane to the stresses in a cantilever bridge, we have the weight of ihe fuselage with its alighting chassis, motor, passengers, &c, the inverted equivalent of the supporting reaction on the central pier of a cantilever girder. We have the air-pressure force, by which the said load is sustained, distributed along the aerofoil length corresponding to the weights of the outstanding members of the cantilever. We have a variation of pressure from point to point due to gusts, eddies, &c., correspond ing to some degree to the movable loads representing traffic over tht bridge. In the case of the aerofoil, we have in addition some thing not represented in the analogy of the cantilever girder, *.*., the weight of the aerofoil itself directly supported by the pressure reaction ; we may, however, regard this equal and opposite distri bution of weight and pressure as superposed on the main system, and as not contributing to the stresses in the aerofoil members. So far as the analogy to the bridge holds good, it is evident we have a well-known engineering problem which is capable of being treated by well-known methods. In the calculation of stresses of the aerofoil members two alternative methods ate in current use ; in the one the aerofoil struts are treated as pin-jointed members, by the usual truss- girder construction ; according to the other method, in place of the hypothesis of the pin-joint, we have the hypothesis of continuity in SECTIONS Fig. 30b. by the N.P.L. and by Prof. Prandtl of Gottingen; the position may here be summarised by saying that the resistance of a rope or stranded cable, at right angles to the direction of motion, is virtually equal to that of its projected area in normal plane. The resistance of smooth wires is about 20 per cent. less. Both these results only hold good above a certain minimum value of /V, which may be taken at about 1*5 j thus at 100 ft. per second, the rule may lie taken as applying to cables or wires down to about V^ths inch (=0*015 ft.) diameter. (Compare Memoranda 40 and 75, Reports of the Advisory Committee.) Another interesting set of determinations, for which we are indebted to the N.P.L., is that relating to the resistance of alighting wheels ; these have been tested both in respect of resistance and lateral reaction. (Sec Memorandum 74, Report of the Advisory Committee, 1912-13.) The direct resistance of a 26-inch wheel fitted with 2 |-inch pneumatic tyres appears to be equal to about a third of its projected area in terms of equivalent normal plane, the projected area being taken to be that of the tyre itself. For fuller information reference should be made to the Memorandum cited. (To he continued.) ® ® ® ® The Vienna Meeting. FOR the flying week at Vienna, which opens on Sunday, 21st inst., 33 entries have l>een received. Among them is one from Great Britain—a Bristol—12 from France, 1 from Russia, 8 from Germany, 1 from Hungary, and 10from Austria, including one lady, Lilly Stein Schneider. 663
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