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Aviation History
1913
1913 - 0578.PDF
(AW"!] overcome* the chief error d :e to the tilting of the aeroplane, leaving only a small secondary error of no importance. If the manometer is placed 10' out of the vertical, this secondary error will make the reading be 101 '4 miles per hour instead of 100. Fig. 5 is another Fig- 5 orm of mam.meter designed by Mr. Short, using one liquid, and still eliminating the chief error due to the lilting of the aeroplane. If the aeroplane has an upward or downward acceleration or is changing its direction there is an error. An aeroplane flying at 100 milts per hour in a circle of 1,400 ft. radius will make one complete turn in one minute, and the banking of 26 will prevent side slip. In this case the manometer connected to the Pilot tube will read 94 5 miles per hour instead of 100, the correct speed. If the vertical acceleration is one-tenth gravity on 3'2 ft. per second, the manometer will read 5 per cen\ too low or too high according as the acceleration is downward, or upwards. Tnese enon due to vertical acceleration or flying in a circle are not large and they will be the same in any manometer in which the air pressure is balanced by the attraction of gravity on a liquid, or a weight. If a spring is used these errors do not exist. If a Pilot tube is lixed to the tips of the wings of an aeroplane and it is flying in a circle, the speed of the outer wing tip is greater than the speed tit the inner wing tip. If these Pitot tubes are joined together by a tube there will Ise a greater pressure at one end of the tul>e than at the other, and at first sight we should expect that there would be a How of air through the tube from the outer to the inner wing tip. But this is not the case, because the aeroplane is moving in a circle and there will be centrifugal force acting on the air in the tube, and this will tend to make it flow outwards and •will exactly balance the tendency of the air to flow inwards due to LECTURE AT ® ® BIRMI Wtc have received the following notice of a lecture delivered at the Birmingham University (Edgbastoni by Lord Dynast of Ropp. The lecturer gave a full mathematical theory underlying the con struction of aeroplanes. The subject was treated from three different standpoints. The dynamics of resistance were studied first, the head resistance being attributed to the inertia of particles in the horizontal component of the normal reaction on the supporting member or any streamline torm traversing a fluid. The vifcosity of air results in the skin friction, and head resistance can be convened into Skin friction as means of diminishing the total resistance. The normal resistance thus resolved is found to be expressed in most oonveaieal form by the formula R- V° (S1'1 K+LY), When <v> is a (actor depending upon V, and passing through the value of z. Then V s 74 m.p.h. K is a constant for given shape. L is a linear dimension depending on the perimeter of the cross section of the body in motion. V is a variable for dimensions and shape. S is area of cross section. V is velocity of motion. These two resistances of horizontal component and viscosity are negative in opposition to the subtensive force which is the vertical component of the normal reaction, and which is opposing the action of gravity. Thus efficiency of design may be expressed, p _ Resistance due to gravitation. Resistance due to velocity x head resistance. Expressing all the factors of this equation in terms of known MAY 31, 1013. the excess pressure in the Pitot tube on the outer wing tip, and there will be no flow through the tube. If there is a side-slip this statement is only approximately true, but if there is sufficient banking to prevent side-slip it is true. If both a Pitot and static tube are fixed at the tip of one wing and are connected to a manometer at the centre, its reading will give the velocity of the wing tip. The centrifugal force in this case will act equally on the air in both tubes, and as the manometer measures their difference of the pressures, the centrifugal force will produce no effect on the reading. When flying in a circle the velocity of this wing tip is not the velocity of the centre of the machine. This difference is usually not large, but if it is thought advi-able that the manometer should give the velocity at the centre, two Pitot tubes can be used, one at each wing tip, and both be connected to the manometer, the static tube* also both being connected. The manometer will then give the mean of the speeds of the wing tips, that is the velocity at the centre. This is done for accurate speed ii.ea*urements at the Royal Aircraft Factory. Ascending Spe"d. I have made an instrument for indicating the speed with which an aeroplane or airship rises or falls. It is roughly made, and at present is only in the experimental s-tage of development, and it will require remodelling in order to reduce its size and make it more convenient. I will, however, give a short description of it, as it may be of some interest. A clock is arranged to move a valve at equal intervals of time. The valve first connects a vessel to the open air ; it then closes and the air in the vessel is at the same pre?sure as the air out-ide. After a short time during which we will suppose that the outside air pres sure has fallen owing to the aircraft n-ing, the valve is again moved so that the vessel is connected to the indicating apparatus. This apparatus measures the excess of pressure of the air in the vessel over the air outside, and gives ihe change of barometric pressure during the short time between the last two movements of the valve ; that i.-, it measures the amount of rise in a fixed time or the verlical speed. It does not give the speed at each instant, but the average speed during a short interval which terminated a few seconds previously. The indication is a little late, and in order to reduce this lateness as much as possible two vessels are used which are opened and closed and connected up alternately to the indicating apparatus. Direction and Speed. Two of the instruments we have considered, the Pitot tube speed- meter and the compass, give us the direction and the velocity of the aeroplane through the air, but unless we know the speed and direction of the wind, we do not know the real direction of flight or the speed over the ground. These instruments give valuable results, but we want to know the direction the head of the aeroplane should point in order to get to a definite place, and the speed over the ground as well as through the air. I see no satisfactory solution of of this problem, and will not discuss the question, except to suggest that someone here present solves it during the coming year. (To be concluded). ® ® NGHAM UNIVERSITY. quantities they represent, we get the equation of efficiency and differentiating for maximum value limiting case, equation of least resistance. Next comes the investigation of the flight path as depending upon the factors dealt with in previous equation of least resistance. The angle of attack of a particle of air impinging on the surface of an aerofoil forms the next layer, which is at a greater (dihedral) angle to the horizon than the surface of the aerofoil itself, and thus the next particle of the flow impinges upon a plane of greater inclination than the one above it. Thus a curved section is generated which is a junction of the original angle and cross-section of the supporting member. In this way the necessity of camber surface was explained in an efficient and lucid way. Lastly, the equation of stability has been found as depending upon factors culminating in the expression, N a - * — Where N is the co-efficient of stability. L distance between C.P. of aeropol and C.P. of tail area. V velocity, o angle of supporting member. A tail area. M mass of aeroplane. These formu'a* prove that the angle of trail has no influence on the stability of a well-designed aeroplane, which is a remarkable result in itself. Another point of intense interest is that the co efficient of stability, N, is directly proportional to the cube of velocity, which has an important bearing on the construction of high-powered aeroplanes. The lecture was illustrated by many slides of bird's wings, aeroplanes, curves for mathematical work, and examples of application of the proved principles underlying this new science. 600
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