FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1915
1915 - 0305.PDF
APWL 3°. WS- the opener gave them credit for, on account of the comparatively small bombs they were able to carry, whilst the Z-ppehn could, he pointed out, carry about a ton and a half of explosives, and had the further advantage that if brought down when over a town they would in their very fall do an enormous amount of damage. Altogether, the op.ioser could not agree that aeroplanes and sea planes had proved themselves to be the most useful factor, and air- ships the most useless. After the opposer had finished his remarks, several of the visitors spoke, mainly against the opener's contention that aeroplanes and n>»nlanes had Droved themselves the most useful fartnr in rhe war \fwm sea] one JKC, ui»*"ij »&«.««.. ...w — f.~u~. ^ w^Li.uiiyU mat ttciuiuaues ana planes had proved themselves the most useful factor in the war, • of the visitors maintaining that battleships must be considered the most important factor, whilst another expressed the opinion that the internal combustion engine had done more than any other single factor to be given this place of honour. Another visitor pointed out that it was hardly fair on the part of the opener to claim that the British pilots were the best in the world, since, during the months preceding the war, a number of world's records had been established by German pilots. In replying to the opposer and visitors who had expressed their disagreement with his views, the opener pointed out that he was willing to admit that the aeroplane could be beaten by other factors for special purposes, but that what he mostly wished to emphasise was the great variety of purposes for which the aero plane could be used. ® ® ® ® THE SCREW PROPELLER. By F. W. LANCHESTER, M.InstC.E. (Continued from page 288), Now, when we are concerned with the problem of superposed aerotoils or planes, the conditions (as pointed out in the author's " Aerodynamics," § 210) are not the same as where the blades of a propeller are in question. In the former case it is the quantity termed by the autbor the " sweep" of the foil which is important, and the practical separation distance of two aerofoils is in fa:t the nearest approach to a positive measure of this quan ity ; in the latter case it is the peripteral area which is decisive. The point of view of the author, as harmonising the otherwise rather ambiguous position is, in brief, that if experiments were conducted with numbeis of superposed foils the individual spacing would require to be increased as the numbers become greater until in the limit the spacing would require to be that corresponding to the peripteral area, that is to say, in the ordinary way the spacing, when the number becomes sufficient, would approximate to from f to | of the span. It is difficult to say that interference would cease even at this distance, but it can be definitely stated that it must become sensible and even serious if the distance is appreciably less; the propeller blade conditions are analogous to those of an aerofoil system of superposed members when the number becomes indefinitely great. The basis, therefore, on which we shall compute the number of blades permissible will be as follows". An expression will be found for the volume --wept per second by the peripteral area of a single blade ; complementary to this an expression will be given for the volume passing per second through the propeller or rotor disc area; the latter in terms of the farmer will give the appro priate number of blades. Owing to the fact that the above is not of necessity a whole number, and that fractional blades are inad missible, the designer will usually have the option of adopting a number of blades wnich will result in interference in some degree, or of adopting one less blade, in which case the whole of the sup porting reaction as computed from the propeller disc area will not be realised. In the former case the mechanical efficiency will suffer to some (usually small) degiee ; in the latter case a rather larger propeller or rotor diameter will have to be adopted than otherwise necessary. Let "span," i.e., effective blade length, = § diameter of rotor, take periptery as defined by a circle whose diameter = span, thus, peripteral area, = Q a 04 = 0-14 a. Now we will call the path of the centre of the blade the peripteral axis—it is a spiral—and the velocity along the peripteral axis is clearly the z>, of Fig. 4, hence the volume included by the periptery per second = 0*14 a Vx. Now the effective area of the rotor or propeller disc is its total area less that central portion not swept by the blades ; that is to say, a circle \ of the diameter of the disc requires to be deducted to correspond with the assumption that the effective blade length is § of the diameter ; in any case this central portion cannot in theory be utilised so long as the instrument is a screw of any kind. Thus, effective area of disc = -f a 01 = 0-937 a. And the velocity « of the air through the disc depends upon the value of Q ; in the limiting values, Q = unity u = ^2/2 Q = 2 u — T/J. In the first case, denoting the number of blades by the symbol A7, v-, Vy sin y 2 we have o-i4 al\N = 0-937 a —= 0-937 a (12) or> AT = 3 -33 sin 7) In the second case the number is twice as great, or, N = 6-66 sin i). We will assume—as it is certainly fair to do—that on the perip teral axis the angle t) is given a value in or about that of highest efficiency. If, in any case, this assumption does not represent the fact, it is open for the designer to give the angle in question whatever value he please, and to revise the present calcu'ation to suit his own particular case. Thus, in the first case, referring to Figs. 5 and 6, n = 2*5, »j m 35 degrees, sin tj -e 0422 N - 1-4. n = 5-0, TJ = 21$ degrees, sin m 0*366 Ar = I'*, In the second case, n = 2'S, it 21 degrees, sin ij • 0*358 Ar= 2-36 n = 5-o, «j = 18 degrees, sin i| = 0-309 N m 2*o6, From the foregoing we are justified in stating that two blades, in practice the minimum which can be adopted, are more than sufficient ; any greater number will have no advantage, and can only result in a lowered efficiency. The particular condition in which the number comes out materially above 2, i.e., with the maxi mum po-sible value of Q in combination with a low aspect ratio, is an extreme which can be reasonably ignored: even in this case the provision of three blades would result in considerable interference. 9. Our next consideration will be that of the camber and sectional form of the rotor blade : we have also to consider the question of the variation to be assigned to IJ at different points along the radius. In connection with the latter point it is to be remarked at the outset that the rotor of a helicopter is not bound down by the ordinary considerations which control the angle variations in the ordinary screw propeller. The whole angle v, in the case of the helicopter, is analogous to a part only of the gliding angle 7, as used in the author's " Aerodynamics," the angle 8 representing the effective pitch angle in the screw propeller theory (comp. Part II), is zero in the helicopter, since the latter is presumed to sustain •without axial advance through the air. In practice, it is true, the machine requires to lift, it must be capable of a certain vertical velocity, but this is not the essential feature of the problem ; it is rather incidental. Given that the requirements of alternation are properly met by the design, it is only necessary to supply the needed additional horse-power and drive the rotor above its normal speed to obtain a definite upward velocity. The detail of the regime under these conditions need not trouble us. In the extreme, if it be con sidered possible and desirable to obtain a high speed rate of ascent, then the problem ceases to be in fact a special case, and may lie treated as an ordinary matter of screw propulsion, and the more general theory of Part III will then apply ; at present, however, the h.p.(weight problem becomes far too severe if a high rate of ascent be contemplated ; we must remember that no machine of the type under consideration has yet been even made to lift successfully, let alone to soar like a rocket. Thus, the angle f) being zero, the angle of the blade in the heli copter may be graded from point to point along the blade length according to the will of the designer, in the same way as the addendum or slip angle in the screw propeller, and in deciding this point the designer requires to take into account, on the one hand, the distribution of the load over the disc area required by theory, and, on the other hand, the falling off in efficiency when the optimum angle—as shown by Figs. 5 and 6—is departed from. If the angle be made constant from root to tip, the resulting pressure per unit area on the blade will manifestly include a multiplier varying as the square of the distance from the axis, thus imparting an unduly high velocity to the outer concentric elements of the stream. If, alternatively, the angle TJ be made variable along the blade length, in such way as to be represented by a true helical surface, then this multiplier becomes proportional to the radius itself, which is that required by theory to give a correct distribution to the momentum in the wake stream, but at the same time results in the employment of angles of somewhat low efficiency. To some extent the conditions may be reconciled by a tapering off of the blades towards their outer extremities, by which means the pic-Mure 305
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
