FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1915
1915 - 0267.PDF
In the present connection the most important fact to be accepted (whether taken on trust or otherwise), is that the vortex motion may be represented by a dynamic substitute in the form of a Fig. 1. cylindrical body of the fluid of circular section, whose diameter is equal to the span of the aerofoil, or in the case of the propeller equal to the effective length of the blades. We may obtain a graphic idea of what this means by reference to Fig. 2 ; here we see a blade arranged diametrically across the mouth of a slightly bent pipe ; the blade represents the aerofoil, the pipe represents the measure of the column of air that it con trols. Now we know that the question of skin-friction or surface Fig. 2. resistance is one of vital importance in flight ; the only real advan tage in fact of a foil of high aspeet ratio over one of low is due to the reduction of surface ; hence, we are in a position to appreciate the enormous importance of the cyclic component in the periptery, and the positive and ludicrous futility of the many schemes proposed from time to time to utilise pipes or scoops in place of the simple aerofoil or propeller blade. Such suggestions are not unlike those of the patentee of years gone by who wanted to pipe electric current through a tube so that less of it should be lost. -The circular form of the peripteral " cylinder " is, it is true, due to the particular character of the impulse distribution assumed as basis ; at present there is not much choice in this respect owing to the backwardness of the mathematician, but when attention is focussed on the point doubtless other solutions will be found ; the author is at the present time attacking the problem by graphic methods. In any case, the particular solution in question does undoubtedly fit the actual conditions very closely. lieyond this, it is becoming evident that even when widely different types of impulse distribution are in question the difference in the peripteral area equivalent is not so great as might be thought, thus it is not to be supposed that the result of assuming the circular cylinder as the vortex equivalent will be seriously in error. 3. When we apply the same concept to the case of the propeller we have each blade in its spiral path forming the diameter of a circle, its peripteral area, whose trace in space defines the equivalent of the fluid coming in effect within its grasp ; thus the whole region of the propeller stream must be considered as resembling a rope, whose number of strands is equal to the number of blades. The author finds this conception of the propeller race or stream one of considerable utility ; it should be kept constantly in mind. In the author's theory, vortex motion plays an important part in the determination of blade section. As in the case of the aerofoil the camber is dissected into two elements : the primary camber, which is related to the two-dimensional motion, that is the ulti mate motion left in the fluid : and the secondary camber, which is considered as superposed on the primary and which is concerned with the cyclic motion about the foil itself. In effect this is pre cisely the same as the treatment given in the author's earlier work, "Aerial Flight," Vol. I., but there the division was not made, neither was it realised what assistance to the theory of the subject would result from the separation of the two functions. In connection with the propeller theory as set forth in the present paper, the primary camber is that with which we are mainly concerned ; the role of the secondary camber only becomes of interest when we come to the discussion of blade form. This is to some degree dis cussed in connection with the helicopter and again in Part IV of the paper, but it is a subject which, to do it justice, would need a paper entirely devoted to its consideration. The blade section de.ilt with in the present theory is thus, generally speaking, that of the primary camber. It is, however, well to bear it constantly in mind that the actual blade is something quite different, although a dynamical equivalent. The necessity of the cyclic or vortex theory becomes the moie evident the closer we are acquainted with the propeller, and on any other basis the idea of the earlier workers in. the problem of propulsion would have been perfectly sound ; thus Fig. 3A would have been a perfectly correct and logical design. On the other hand, how would it be possible to justify the highly efficient pro pellers of the modern aeroplane (Fig. 3B), unless by an alternative assumption involving action at a-disiuncc ? Again, it is well known that the cores of the vortex pair left by each blade may become Fig. 3. visible if a little air is introduced, as, for example, when the pro peller of a steamship is not fully immersed. 4. In the present paper the symbols employed are those adopted in the author's previous work ; notably his " Aerial Flight and the recent papers read before the Institution of Naval Architects and this Institution. In the latter paper, however, the syml>ol fl was inadvertently used to denote the angle of trail of the primary camber of the aerofoil ; as this symbol was used for the effective pitch angle of the propeller blade in the previous work, it is being retained in the present paper in its earlier usage, and the symbol v is adopted for the trail of the primary camber. Again, objection has been urged (perhaps with justification) to the use of the skin- friction coefficient as being expressed in terms of normal plane pressure in lieu of being expressed as a constant dealing direct with the quantities density and velocity squared, as is usual in other cases. This objection is most easily met by combining the co efficient with the normal plane constant, thus—SC, it being under stood that when these symbols occur thus in juxtaposition, the value of C shall be taken as 0-62. As a matter of convenience the present paper is presented in four parts. (To be continued.) ® ® ® ® Fatalities in America. WHILE doing " stunts " at Los Angeles on March 19th in connection with the making of a cinematograph film representing a battle in the air, Frank Stites lost control of his biplane, which fell to the ground from a height of 150 ft, the pilot being instantly killed. A cable from Washington announces that Cecil Peoli was killed there on April 12th while testing a new armoured biplane, which he had built and submitted to the U.S. Army authorities. Military Flyer Killed in Mexico. LIEUT. GEORGE PUFI.A, formerly an instructor at the Moisant school, was killed at Yucatan, Mexico, on March 18th, through one wing of his machine buckling up. He was engaged dropping bombs during the fighting in Yucatan, and fell from a height of 1,500 feet. Fatal Accidents in Germany. ACCORDING to information to hand from Geneva and Lausanne, two new German monoplanes, while being tested at Freiburg, Baden, were caught in a sudden squall and dashed to the ground, both the officer pilots being killed. '7
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
