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Aviation History
1913
1913 - 0023.PDF
JANUARY 4, 1913. Another form which the motor might take is that of a Saxon, or as it is more generally termed, Chinese flyer, in which the actual screw or propeller-blade and boss was hollow, with suitably placed orifices at the side of the tips, the hollow being filled with the proper composition. Our personal Guy Fawke»' Day experiences with Chinese flyers is that they are almost invariably successful, more especially if of the double Saxon type, i.e., where the fiery jet issues from both ends simultaneously. Naturally it lasts only half as long, but is twice as powerful while it does last. It may be perhaps just worth while mentioning that if anyone can secure a flight of over half a minute's duration with any of the above types, or indeed any type of gunpowder motor, he undoubtedly becomes eligible as a competitor for the power-driven model contest at the Olympia Show next February. The greatest difficulty appears to be the discovering (by actual experiment, in all probability) the most suitable composition, to give as lasting, steady and powerful a jet as possible. Gunpowder itself is a mixture of charcoal, sulphur, and nitre, the latter constituting three-fourths of the weight. The nitre supplies the oxygen for the combustion of the charcoal, which is converted into carbonic acid, and the sulphur, which is added to increase the rapidity of the combustion, is also oxidized. The products of the action are both numerous and complicated, but the important result so far as we are concerned is the sudden generation of a quantity of carbonic acid, nitrogen, carbonic oxide, hydrogen and other gases, which at the oxidizing temperature and pressure of the air would occupy a space 300 times greater than the powder from which they are set free, but the intense heat attending the chemical action greatly dilates the gases, so that at the moment of explosion or combustion they occupy a space at least 1,500 times greater than the gunpowder. The fineness to which the ingredients are ground has an important bearing on the rate of combustion. It is these dilated gases throttled more or less by the " choke " rushing out through the choke-hole which, on the principle of action and reaction being equal and opposite, propel the rocket in an opposite direction. A rocket containing a pound weight of charge will rise from 300 to 600 yards in the air, large rockets from 800 to 1,200 yards. The reader must not conclude from the above that we are advocating " rocket " motors for model aeroplanes—we merely wish to place a few quiet unbiassed facts before our readers, leaving them to draw their own conclusions; this type of motor possesses one point in its favour, which we have so far omitted to state, and that is that in a model propelled by such the weight can be distributed similarly to that on a full-sized machine. (AHEil Scientific Model Building. IV.—Propellers, their Design and Construction. The design and construction of propellers is, without doubt, one of the most difficult, as it is also one—if not the most important part of model aeroplaning. The function of a propeller is to produce dynamic thrust, and the great advantage of the use of a propeller as a thrusting or propulsive agent is that its surface is always active. It has no dead points, its motion (unlike that of flapping wings) is continuous and not reciprocating, and it, therefore, requires no special mechanism in its construction and operation. It should always be borne in mind that a propeller is nothing more nor less than a particular form of aeroplane ; it may, in a word, be regarded as a monoplane whose wing-tips have an extreme warp. Similarly, the converse is also true that we may regard a monoplane wing as a propeller of infinite radius and infinitely fine pitch. (Questions of aspect-ratio, stream-line form, plan form, positive and negative pressure, &c, affect propellers in very much the same way that they do wings, and the question of the " dipping front edge " also enters into the construction of propellers just as it does into that of wings or aerocurves. Since all the power of a motor is supplied through the propeller, it at once follows that the efficiency of any machine as a whole must depend as a whole on the efficiency of its propeller or propellers, ® ® NOTES ON PAPER By G. supposing more than one is used ; we propose, therefore, dealing with the subject at some length. The pitch of a propeller is the linear distance the propeller moves, backwards or forwards, in one complete revolution. In Fig. 3, the line, AB, represents a linear distance equal to the travel of the blade-tips 22 i.e., the diameter of the propeller x - . The dotted line, AC> the theoretical course of the propeller-blade during one revolution ; the perpendicular line, BC, represents the consequent pitch. The FIG,.5 reader will probably obtain a clearer idea of this by cutting out a piece of wedged-shaped paper similar to ABC ; of such a size as to just meet when wrapped round some convenient cylindrical object, i.e., for A to just reach to B ; AC then forms a spiral or helix, i.e., a screw. If a horizontal radius is kept in contact with this spiral, AC, it will sweep out a spiral surface as it rotates around the centre of the cylinder. A propeller is a part of such a surface. Theoretically, of course, a propeller need have but one blade, but practically more than one must be used, owing to lack of balance, &c. This distance is, however, a purely theoretical one. When such a propeller screws itself forward the air yields and slips away. Theoretically, if we state that such and such a propeller has a pitch of I ft. or 12 ins., we mean that the model would advance I ft. through the air for each revolution of the propeller; this is only true, however, if it were mounted in solid guides like a nut on a bolt with one thread to the foot. In a yielding fluid-like air it does not practically advance this distance, and hence occurs what is known as :— Slip, which may be defined as the distance which ought to be traversed, but which is lost through imperfections in the propelling mechanism, or it may be considered as power which should have been used in driving the model forward. It should be noted that in the case of a motorcar running on a good hard, dry road nothing is lost in slip, since there is none. In the case of a model aeroplane held stationary whilst the motor is allowed to run down and drive the propeller in so doing, all the power is used in slip, i.e., in putting the air in motion, and none in propulsion or driving or dragging it forward. Let us suppose the propeller on our model has a pitch of 1 ft., and we give the rubber motor 500 turns, then theoretically it should travel in calm air 500 ft. before the propeller is run down ; no pro peller yet designed will do this ; supposing the actual distance 385 ft. then 23 per cent, has been lost in slip. For this to,be actually correct, the propeller should stop at the precise instant that the model comes to the ground. Taking " slip" into account, The speed of the model in feet per minute — pitch in feet x revo lutions per viinute — slip feet per minute). Or briefly put— Theoretical speed = pitch > r.p.m. Actual speed = pitch x r.p.m. -slip. This slip wants to be made small, just how small is not known. If made too small the propeller will not be so efficient, or, at any rate, such is the conclusion come to in marine propulsion, where it is found most economical results are obtained when the slip is from 10 to 20 per cent. In aerial propulsion there are some reasons for assuming that about 15 per cent, may be the best. While it is true that slip represents energy lost, some slip is essential, because without it there would be no " thrust," this same thrust being derived from the reaction of the volume of air driven backwards. (To be continued.) GLIDER EXPERIMENTS. H. KILSHAW. [This contribution has been awarded the first FLIGHT Certificate of Merit. See pages 12 and 13.] PART I.—Dihedral and Incidence Angles. A REVIEW of some recent experiments of mine in natural stability may be of interest to others who conduct such researches. Paper gliders formed the subject of the experiments in question, which proved highly interesting. Four gliders were made from a stiff cartridge paper, and to ensure each being of the same shape and dimensions, they were cut in one from a sheet folded into quarters. The accompanying sketches and descriptions clearly show their appearance and characteristic;. Each glider was subjected to three tests as follows :—Test I, launched level; Test II, launched at an angle of 45" over to left wing ; Test III, released with wings vertical without forward motion as a test to side-slip, and the following results were obtained :— Glider I.—Test I, very good stability; Test II, turned to left, slight oscillations, gradual recovery to level ; Test III, side-slip and nose-dive before gliding. Glider II.—Test I, good stability; Test II, turned to left, oscil lating badly but gradually diminishing ; Test III, oscillated badly, followed by a nose-dive. Glider III.—Test I, good stability; Test II, quick recovery but overturning to right wing ; Test III, completely overturning, landing upside down without gliding. Glider IV.—Test I, excellent stability ; Test II, steady turn to left ; Test III, quick recovery and nose-dive before gliding.
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