FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1914
1914 - 0634.PDF
[ftJGHT] JUNE 12, 1914. THE FLYING MACHINE FROM AN ENGINEERING STANDPOINT. By FREDERICK WILLIAM LANCHESTER, (Continued from page 608.) M.Inst.C.E. 6. Propulsion.—We are now in a position to consider the question of propulsion. Whether we appeal to experience or theory it would appear that there is only one method of propulsion available, namely, the screw-propeller.* The problem of propul sion, whether aeronautical or submarine, is essentially the same ; the laws of dynamic similarity with certain reservations are strictly applicable. Roughly speaking the conditions of usage of pro pellers in water and air may be compared by merely taking cognizance of the relative densities of the two media—approxi mately 800 to 1. The laws of dynamic similarity indicate that this relation is not exact, but any refinement of theory on this score is of academic rather than of practical importance. Apart from fine points of this kind there is a limitation that renders the air-propeller and the marine-propeller not strictly comparable ; this limitation is due to the appearance of the phenomenon known to the naval engineer as cavitation. The law of the relation of pressure to velocity for least resistance applies to the blade of the screw- propeller precisely as it does to the aerofoil itself, so that if a propeller is being designed for least resistance the pressure per square foot at any point of the blade must bear its constant relation to the square of the velocity of the blade through the fluid at that point. In the case of the marine-propeller this results in a speed being reached (at about 20 or 25 knots speed of vessel) at which the velocity of the blade tips is such that the negative pressure (on the back of the blade), based on the law of least resistance, is greater than the hydrostatic (absolute) pressure. Under these conditions a vacuum is formed in the vicinity of the blade extremity, and the system of flow is impaired ; this is the condition of incipient cavita tion, and as the speed is progressively increased the vacuum invades more and more of the blade area until the greater part of the propeller becomes ineffective. From the critical speed upwards the design of the marine-propeller becomes a compromise. The extremity of the blade is first designed broader to avoid developing pressures sufficient to initiate cavitation, and then, owing to the additional skin friction thereby involved, it is found desirable to adopt higher pitch/diameter ratio to prevent the extremities cutting the water with excessive velocity. Eventually the propeller for high-speed craft becomes one of extremely coarse pitch with blades of short or saucer-like form. No such thing as cavitation is expe rienced in the aeronautical propeller ; if we should require to deal with propeller-blade speeds approaching the velocity of sound we might find something analogous, due to the high rarefaction of air, but at present the aeronautical designer can afford to ignore all question of cavitation. It U frequently stated that the theory of the screw-propeller is entirely empirical and quite unsatisfactory ; this is not my opinion. The theory of the screw-propeller based on the theory of the aero- •«r.r Fig. 24. oil as laid down in my Aerodynamics (see Aerial Flight, vol. i, ch. ix), appears to fully meet the requirements of the aeronautical designer. According to this theory the propeller blade is treated as an aerofoil, its P/V2 ratio at every point of the blade is fixed by the same law as that of the aerofoil as given ; following this the gliding angle of the propeller blade is constant from root to tip. "Nature's method of propulsion—wing-8apping—besides being very objec tionable from a mechanical point of view, shows certainly no higher degree of mechanical efficiency than the screw-propeller (Engineering, February 26th, 1909). The section of the blade is at every point designed as an aerofoil in which the true helical surface corresponds to the horizontal plane in flight, t Under these circumstances it is shown in my woTk that each point of the propeller blade has efficiency proper to itself and is represented by a curve as plotted in Fig. 24, which corresponds to a gliding angle of 6°, or, approxi mately, 10 per cent. Under these conditions it will be seen that in the region of maximum efficiency is just over 81 per cent. Unfor tunately we cannot use only the region of maximum efficiency ; we have to employ the blade of considerable length, and consequently parts of the blade have an efficiency below the maximum. If we take a propeller of the usual proportions in which the pitch is about \\ times the diameter, that is such a blade as represented in Fig. 24, we see that the marine engineer declines to employ any portion of the blade with an efficiency of less than about 92 per cent, of the maximum, that is to say, the efficiency at different points of the blade varies from 77*5 to 81 per cent., or theoretically the limit of efficiency of such a propeller should be round about 77 per cent. Unfortunately, a propeller "in being" cannot consist of blades alone, it requires a boss and a connection between the boss and the blades, and in driving these functionally useless parts through the water a considerable further loss is inevitable. Probably it is for this reason that the actual efficiency of a marine propeller rarely exceeds 70 per cent. In my work a design is given of an aerial- propeller based on theory alone, in which a very conservative estimate is taken of the gliding angle. If in the light of present knowledge we assume the propeller blades being of the aspect ratio corresponding to that of my 1894 gliding model, the gliding angle or resistance coefficient will be about 5 or 6 per cent., and we might anticipate a theoretical limit to the propeller efficiency of 88 or 90 per cent. We have here, as in the marine propeller, to provide a boss and arms, and we require to take into account the fact that it never pays in practice to take the full diameter of the propeller that theory would indicate (it is better to sacrifice a few per cent, efficiency to save weight and clearance diameter). Everything con sidered, I am disposed to put a limit of efficiency of an aeronautical propeller at about 85 per cent. ; this is higher than has been found possible in marine engineering. My method of propeller design has been adopted and employed for some years by the Superintendent and staff of the R.A.F. with very satisfactory results; at present there is but little available information on the question of efficiency owing to the fact that the arrangements at the disposal of the R.A.F. do not permit of the testing of full-sized propellers. Working drawings of a propeller, designed at the R.A.F. by this method, are given in Figs. 25 and 26. For the full exposition of the system of " lay out," reference should be made to the work already referred to. As an alternative and purely empirical basis of treatment, we may fall back on our experience in marine propulsion. There is a practical rule which appears to be commonly adhered to in the design of successful marine propellers for moderate speed sea-going craft. The area of the propeller disk is approximately 1 per cent, of the total wetted surface. This rule has been found by me to repre sent a rough average of the practice in various cases, but whether it is an accepted rule or not I do not know. Let us take the case of a flying machine involving, say, a thrust of 200 lbs. at 80 ft. per sec. ; at this speed the frictional air-resistance will be approximately OTJ35 lb. per sq. ft. of surface-(o-07 lb. per sq. ft. of lamina, i.e., double surface); thus the resistance of the machine is approximately represented by 6,000 sq. ft. "wetted" surface, and, following the rule given in the case of water, the propeller disk should be 60 sq. ft. ; this corresponds to a propeller diameter of about 9 ft. In an actual machine of about this size the propeller is commonly of about 7 ft. to 8 ft. diameter, which, taking everything into account, is in substantial agreement. The propeller employed in flight is of necessity (from considerations of the engine revolution speed) of finer pitch than that of best efficiency. Under these condi tions theory shows that the correct diameter is less than that of the propeller of best diameter pitch ratio, such as employed by the naval architect. t There is one factor which affects the analogy between the aerofoil and the propeller blade ; the latter is not able to the same extent to hold or accumulate a dead-water wake, the propeller blade sheds its dead water.continuously by centrifugal force. The extent to which this affects the problem has yet to b& determined. 634
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
