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Aviation History
1910
1910 - 0240.PDF
MARCH 26, 1910. horizontal component of thrust in the direction of flight. It must be borne in mind, however, that such a favourable force has to be paid for by engine power, and there is no particular point, so far as we can see, in utilising the direct thrust of the propeller for the purpose of indirectly /AN^LE OF /^DEFLECTION REL. WIND COMPONENT recreating another forward thrust in the aeroplane. The purpose of the propeller thrust is to overcome resistance, which is essentially a retarding force and is made up of two factors, the aerodynamic resistance that provides the venical lift, overcoming the force of gravity, and the direct resistance represented by the skin friction of the struts, planes and spars. Laws of Flight Resistance. It may be of interest while dwelling on this subject to recall the laws that govern these resistances. For a given weight supported, the aerodynamic resistance varies inversely as the square of the speed, while the head resistance varies directly as the square of the speed. The reason why the aerodynamic resistance varies inversely as the square of the speed is because the angle of incidence varies inversely as the square of the speed, and because the aerodynamic resistance is proportional to the product of the angle and the weight. In other words, if the speed of flight be doubled, the aerodynamic resistance will be reduced to a quarter of its former value. Now it is most important to bear in mind that this law applies to aerodynamic resistance alone, and that it has no meaning whatever unless taken in conjunction with the law of direct resistance. This law, as mentioned above, states that the head resistance varies directly as the square of the speed, consequently the faster the machine flies the greater will be the power expended on this resistance. Langley, the renowned American investigator of aero dynamics, seems to have regarded skin friction as negligible, and to have been under the impression that the inverse law relating to aerodynamic resistance could be accepted by itself; at any rate, his published state ments certainly imply that the higher the speed the less the power required. It is quite easy to understand, however, that it is only economical to reduce the aerodynamic resistance to an amount that is determined by the relative head resistance, because, when two factors obeying absolutely opposite laws have to be taken simultaneou-ly into consideration, there is one value for each, and one value only, that produces a minimum sum in combination. The great problem that confronts the designer of an efficient aero plane is the selection of angles, artas and dimensions of struts, so that the total resistance of the machine in flight shall be the least possible for the load supported. The Dihedral Angle Problem, Reverting again to the table, which forms the basis of our present remarks, the next column to come under consideration is that representing the dihedral angle made by the wings of many of the monoplanes. A dihedral angle is an angle made by two surfaces that do not lie in the same plane. In respect to aeroplanes, it might be described as the V-setting of the wings. Its purpose is to endow the machine with a certain amount of natural lateral stability, and the theory, or, rather, theories, associated with the dihedral angle, formed •the subject of a most in teresting correspondence in FLIGHT last year. In our description of the A n t o i n ette monoplane (FLIGHT, Vol. I, p 662), which was the first of the monoplanes to have the dihedral angle in a very marked degree, we sought to explain the theory of natural stability associattd therewith by means of simple diagrams. These diagrams are again reproduced, and as we have seen no cause to change our original theory, we will again put forward the same explanation. The air pressure on an aeroplane is assumed to be normal, ie., perpendicular to its surface, hence in the accompanying diagrams the lines, P, represent the air pressures on each wing sepa rately. These pressures are inclined from the vertical, but they produce, by the ordinary laws of component forces, two vertical components, P1, that are sufficient to overcome the force of gravity and support the machine in flight. When the machine is canted over, as shown in the lower diagram, where the right-hand wing is repre sented in a horizontal position, the full value of P acts vertically upwards on the right wing, whereas the vertical components, P1, of the pressure on the left wing is reduced. The values of P, however, are still equal to one another on both wings. Now, if the machine were pivoted upon an ax\s fixed in space, the mere fact that the two values of P remain unchanged would be sufficient to keep the machine in equilibrium in any position and would therefore prevent a restoration to the attitude represented in the first diagram. A flying machine, however, does not proceed along a fixed axis in space ; it can rise or fall as a whole, and this fact causes the lower diagram to represent an unstable condition embodying a couple tending to restore the machine to its original attitude. The canting of the machine has not shifted its centre of gravity, C.G., which, for convenience, is assumed to be situated at the junction of the wings, and consequently it has not altered the proportion of the load carried by each wing. It will be observed, therefore, that the upward force on the right wing is superior to the load it has to carry, whereas the upward force on the left wing is inferior to its load. The result is obvious ; the right wing will lift a little and the left wing fall until the loads and the lifts are again balanced. It is not necessary, nor is it probable, that this restoration of equilibrium takes place through an axis passing through the centre of gravity; it is quite likely that the centre of gravity falls a little through space in the operation. In the table we have, for convenience, given the amount of the dihedral angle as a slope of 1 in "X." It will be noticed that there is a considerable difference in the values. The Diameter and Pitch of Propellers. The next series of figures in the table relate to the propeller, or tractor screw, as the case may be. A tractor 238
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