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Aviation History
1915
1915 - 0344.PDF
\fm HT i;. In all propeller design the question sooner or later arises as t<i h'.w much of the efficiency curve to include in the blade length selected. If we take, say, the 7 value 0"07, the graph for which is given in Fig. to, it (j quite clear that we have the option of utilising as much or a* little of the curve as we please or may deem expedient. For example, t/ we select just the very best portion of the curve with a view to obtaining the highest possible efficiency, we shall have a propeller with a pitch diameter ratio round about 3 to 1, and the arms to carry the blades will require to be considerably longer than the blades themselves ; clearly the losses in the resistance of the arms will lie too serious, and the proportion of the disc area usefully employed will be so small that any such design is out of the question. This question has been discussed very fully in the author's previous work $ it is a matter in which there must always be some latitude and discretion left to the designer ; it is equally a matter in which some convention is necessary as representing normal practice. For the purpose of the investigations forming Part III. of the present paper, the convention taken is that the diameter of the pro peller is in every case twite1 that at whi, h its efiieieney is greatest; the meaning of this, as to the amount of the efficiency curve included, will be clear on reference to Figs. 8 to 13. A further MAY 14, igI5, Now the value of 7 may be its minimum value ; that is to say, it may be the least gliding angle, but this is not essential; the equation applies equally whatever the angle happens to be ; it applies equally in fact to the case of worm gear or the common screw press—the angle 7 representing the effective angle of friction. In the case of the propeller, as in the case of the aerofoil, the condition of least gliding angle corresponds definitely to a certain PJ V* value, ex pressed, as we know, in absolute units, thus P = C o V-, the value of the constant C being (for least resistance) approximately equal to «/$ n and commonly lying between 0-25 and o'3$ under ordinary conditions of design, Evidently when conditions permit we shall make use of the most advantageous P; V'z value and design on the lines of the preceding section for optimum efficiency, choosing our effective blade length and corresponding piteh\diameter ratio appro priately ; it is when there are requirements to be considered other than efficiency (and in some degree there are always other require ments), that we have to go more deeply into the problem and effect something in the direction of a compromise. 17. There are two particular limitations more commonly met with as imposed by engineering conditions (as apart from purely scientific considerations), both of which as it chances tend in the same direc tion ; these are a diameter limit and a pitch limit. The first of these AKOLX 9; (Diaama; 80 70 00 60 4ft 40 3ft 80 28 2ft S4 TJ SO 1» 18"17 16 1ft 14 ICO 0) w 70 60 to 40 so 90 10 0 1 p. fc f -"T-" IBXJ: _L__L - • n "="•7—;—r : i ! ii ! i -T"'l — 4 "" '-' ' • -J 1 —1 r • ! 1 ] 1 0 O'l os n-.-i EADIWI T!J TMtfS OF PITCH 1 «• 0-09 (ii! <>-7 0-8 0-9 l'O Fig. 12. 1-3 AHOLS 9; (Dsouse) 80 70 40 no »ft «o an 30 28 J« U W 20 19 18 17 Iti 15 14 100 so 80 70 •0 eo 40 so 20 JO 0 -; 1*= | 1 ' 1 = I ! 1 -J—I—I r — (• L J J T 1 1 1 I r 1 l 0 01 OS OS llADIrS IN TKKJIM OF PITCH 1 >- 0-10 : 1 Fig. 13. convention has been adopted to the effect that the active portion of the blade is three-quarters a'its total radial length ; thus, the central portion of the propeller, of one-quarter the diameter of the disc, is regarded as " blind," the blades within this circle are treated as spar sections, or alternatively the central region may be supposed occu pied by a l»ss as actually the case in the Gifford propeller. The convention as to the type of vortex motion in the blade periptery remains as before, the periptery Icing taken as represented dynamically by the content of a cylinder whose diameter is equal to the effective blade length. The justification for this will be found in the author's preceding contribution, " The Aerofoil in the Light of Theory and Experiment."' PARI' III.— The General Theory and Solution «/ the S, Frontier. 16. We have seen that for any value of the gliding angle 7, the efficiency of propulsion is given by the expression where 9 is the effective pitch angle for any given element of the blade, and that the maximum efficiency is that corresponding to a value of * — , the mean over the whole blade being less than this value by an amount depending upon the portion of the curve of efficiency utilised by the blade length selected. is usually due to questions of ground clearance, or in the case of the marine propeller, limitations imposed by draught or proximity ot hull, &c. ; the second is due to the incompatibility between the engine or prime mover and the propeller as to revolution speed— more frequently than not the engine or motor constructor asks for a higher revolution speed (in order to save weight) than that required for the propeller of optimum efficiency. Under the conditions of restricted diameter, the thrust reaction has to be sustained by a lesser total area representing a lesser mass of air than that corresponding to highest efficiency, and if any attempt be made to design to the same constant C as before, the blade area will be found to represent a greater proportion of the total disc area than under the unrestricted conditions. If, as we may assume to be the case, the whole column of air or fluid be fully occupied by the periptery of the blades in the initial design, it is clear that the maintenance of the pressure con stant at its theoretically best value will result in interference, and ultimately it becomes necessary to adopt a higher pressure con-tant and put up with a higher value of y with a corresponding drop in the value of ^^ + that is to say, a loss of efficiency. It may not be considered self-evident that the restriction as to pitch, i.e., the adoption of a pitch less than that of highest efficiency, tends a/"necessity in the same direction, but investigation 344
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