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Aviation History
1914
1914 - 0662.PDF
I/OGHT JUNE 19, 1914. THE FLYING MACHINE FROM AN ENGINEERING STANDPOINT, By FREDERICK WILLIAM LANCHESTER, M.Inst.CE. (Continued from page 635.) We may now proceed to consider the interrelation and compati bility of engine and propeller. It has already been pointed out that in order to get the full output from a given engine (as is also well known to be the case in marine propulsion), a propeller pitch has often to be selected far from that proper to highest efficiency. The difficulty has (as in the early Wright machine) been met by adopting a reduction gear ; alternatively (as also in the Wright machine) a multiplicity of propellers may be employed. It is evident, for example, that, if four propellers be used in place of one propeller, the individual diameter may be halved, and conse quently for a given pitch (and therefore revolution speed) the pitch diameter ratio doubled. The original Wright machine furnished a good example of a case in which the propeller pitch diameter ratio QiGairi umniaiii o o 2 * r 9 Fig. 28. was made approximately that of best efficiency, and this result was obtained, in spite of the low velocity of the Wright machine, by a combination of both methods : that is to say, two propellers were used instead of one, and these propellers were geared down from the engine in the relation 10 to 33. -"• "'• ••• The incompatibility at present existing between the engine speed and the propeller pitch becomes less as the flight velocity is increased, so that, in the case of an ordinary machine of about 1,400 lbs. total weight, the propeller speed (for best efficiency) for a single screw machine becomes appropriate to the normal engine speed at about too miles per hour. Since the loss of efficiency for a fine pitch propeller, even down to half the pitch ratio of best efficiency, is not great, it may be taken that for flight speeds of 50 miles an hour upwards the balance of advantage lies definitely with the direct-coupled propeller ; this agrees with experience. A point of interest in connection with propellers of comparatively fine pitch and somewhat reduced diameter, such as commonly in use to-day, is the fact that, with the engine opened fully out, there is very little difference between the thrust and the revolution speed whether the machine is standing or is in full flight—it is commonly reported that the revolution speed does not increase more than 10 per cent, from " standing " to full normal flight speed—the thrust variation also is slight. This fact constitutes the only justification for the static test of aeronautical propellers, frequently resorted 10 when approximate data are required. There is no doubt that in a propeller of theoretically perfect proportion, or in an existing propeller, if fitted to a machine of less resistance, there would be a far greater response to flight speed variations. Actually this is the case in marine propulsion where the propeller revolution recorder is commonly found to give more reliable readings than the ship's log. 8. Relating to the Design of the Aerofoil.—We shall now proceed to the discussion of the more detailed arrangements and structural features of the machine. First, the aerofoil. The pressure appro priate to least resistance we have already seen to be given by the pV2 expression o-32 />V* in abs. units, or <—— in lbs. per square foot. (Compare Figs. 17 and 18 and text. The constant 0*32 is empirical.) Consequently if w is the weight (in flying order) the area required IOO TV is —. as appropriate to least resistance, p Vi The above is the whole basis of any initial " lay out" ; there are many refinements, however, to be considered which enter into the complete problem ; the principal of these are :— The fact that part of if is a function of the aerofoil area—the quantity we are determining—means that the best area will be less than given by the foregoing expression. This point has been dealt with by me in Aerial Flight, 1907, vol. i, §§ 171, 194, 195, 196; also more recently by the staff of the N.P.L. (See Report of the Advisory Committee, 1911-12, p. 78). Beyond the above the specification of flight velocity for any machine consists more often than not in the prescription of higher and lower limits rather than of a set fixed speed. Under these circumstances the final values and proportions are based on a lay out of graphs of resistances, thrust, &c, on the lines of the diagrams already given, Figs. 11 and 12. It is evident from the general character of the resistance velocity curve as shown diagrammatically in Figs, it and 12, that whereas considerable departure may be permitted from the normal velocity of flight on either side of the minimum without incurring appreciable increase in resistance, at the limits of the flight speed range, the slope of the resistance curve is considerable, and there will Fig. 29. be sharply defined points at which the resistance is equal to the maximum propeller thrust and no liberties can be taken. It is important to note that at the maximum limit of flight speed the equilibrium of thrust and resistance is stable, whereas at the mini mum limit the conditions are tho«e of instability, so that should the machine at any time fall below the minimum, the aeronaut can only recover his power of flight by calling upon gravity to assist him, that is to say, by taking a downward course. If, as when near to the ground (or an obstacle), the downward course is not permissible. 662
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