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Aviation History
1912
1912 - 0078.PDF
ITycHf JANUARY 27, 1912. ROUGH DESIGNS FOR By S. IT is not here intended to treat of the inside nature of the bomb, but of its outside form alone. The size, the proportion of iron to explosive (also the shape of point), e.f.c. depend on the use to which the bomb is to be put; here only the best form for accurate dropping is considered. For accuracy, the bomb must offer a small resistance to the air in its direction of flight, for the smaller the resistance the shorter the time of flight, and, therefore, the less the errors arising from faulty estimation, or variation of wind, and speed of enemy. And the shorter the time of flight the greater the striking velocity. An ideal bomb would offer no resistance to the air. It would fall under the law s= I6-I/2, and offering no horizontal resistance to the air would carry the speed of the aeroplane over the ground through out its flight, and hit the earth vertically below the aeroplane at that moment (provided the aeroplane held on its course). This, in the matter of horizontal resistance, would simplify sight setting, as will be seen when the sight is described. A sphere, 7 ins. in diameter and weighing 33 lbs., after falling 3,000 ft. in 15 sees., has a velocity of 346 ft./sec. ; after falling 6,000 ft. in 23 sees., the velocity is 400 ft./sec, and soon after this it attains its maximum velocity of 430 ft./sec., the speed then becoming constant. The ideal bomb, offering no air resistance would fall 3,000 ft. in AND SIGHT. 00 f+. •I— TRIJIICTOP.Y OF fl ?OOlb. GUNCOTTON SFHEKt WITH f\K INITIAL VELOCITY OF 50 MILES FER MOl'Fs <D 1000 a. 2D0O ft. 50O0 ft. A BOMB H. S. M. The trajectory of a 7 in. sphere, weighing 33 lbs., is shown in Fig. 2. As said before, the axis of the bomb is always a tangent to the trajectory. At the moment of letting go the trajectory must be horizontal (the bomb has the speed of the aeroplane horizontally and no speed vertically), and, therefore, the bomb should be let go in a horizontal and not a vertical position. (This suggests the possibility of a gliding bomb). The factors to be considered in deciding the time and position to let go the bomb are : (1) The speed and course of the aeroplane ; (2) the wind ; (3) the speed and course of the enemy, and (4) the height of the aeroplane. The wind will always be the hardest of these to estimate, and will have to be got by resolving the speed of the aeroplane over the ground into the speed of the aeroplane through the air, and the wind. This is one of the ordinary calculations of aerial navigation, but for bomb dropping purposes must be done with exceptional accuracy, even then there is no guarantee the wind will be the same all the way down to earth. Let us assume a case :—Height, 3,000 ft.; aeroplane's speed and course, south 50 miles/hour (73^ ft./sec.); wind from the west 30 ft./sec. The bomb is the one whose trajectory is given in Fig. 2. The bomb will take 15 sees, reaching earth. During that 15 sees, the speed given the bomb by the aeroplane will take it 900 ft. south, the wind will take it 450 ft. east. So the bomb will have to be let go 900 ft. north, and 450 ft. west of the target. If the target is BOMB SIGHT NOT TO SCHLE) 14 sees, and 6,000 ft. in 19 sees., having velocities of 440 ft./sec. and 620 ft./sec. Comparing these figures, it will be noticed that up to 3,000 ft. the spherical bomb is almost ideal, but dropped from higher altitudes it loses efficiency quickly; high altitudes may have to be used if the enemy has a good defence. The obvious improvement on a sphere is a stream-line form. The ratio of the resistance co-efficient of a sphere to that of a stream line form is not known, but Sir Hiram Maxim gives "0022 for a round bar and "000195 for a stream-line sectional bar of the same area, one over ten times the other; probably for a sphere and a stream-line form of the same volume the proportion is many times ten. Steadiness of the bomb in flight must also be obtained. A pro jectile gets its steadiness from the rotation given by the rifling of the gun. When it loses this rotation it turns over and over in the air varying its resistance and losing velocity in an irreguiar manner. To obtain steadiness the stream-line form must be fitted with a tail. Fig 1 is a suggested bomb. The tail of this bomb will cause it to always present the same face, its bluff bows, to the air, and so its axis will always be a tangent to the trajectory. Consequently the broadside aspect of the bomb does not enter into the calculation of the trajectory, which is the same as that of a sphere having the same co-efficient of resistance. The broadside aspect need not on this account, there fore, be considered in the design of the bomb. moving also, say N.W. 20 ft./sec., the bomb must be let go another 300 ft. N.W. of the target. To place the ship in this position it will be necessary to use some sort of sight, so that when the sight line is on the target (the sight being accurately set), the aeroplane is in a proper position to let go. A picture of a rough form of such a sight is shown in Fig. 3. It has four bars, an aero-bar, enemy bar, wind bar, and height pillar. The aero bar carries at one end the back sight, this bar should be rigidly fixed to the aeroplane in the fore and aft line, with the sight aft. Sliding on the aero bar is a carriage, carrying on its underside the height pillar, and on the height pillar, mounted so as to swing round, is the wind bar, carrying in its turn the enemy bar pivoted on a carriage. On the enemy bar slides the foresight. The aero, wind, and enemy bars are all graduated for speeds to a convenient scale, the zero of the aero bar being at the rear sight, and the zeroes of the other two at their pivots. In setting the sight the wind and enemy bars should be set to their respective speeds, and in the direction of their motions, as shown by the arrows in the picture ; the aero bars should be set to the mean horizontal speed of the bomb, due to the speed of the aeroplane. This mean speed 000 will vary with every height, being in our case —- ft./secs. = 60 ft./secs. If the bomb were ideal, an impossibility, offering no air resistance, this speed would for all heights be the speed of the aeroplane, saving calculation or the use of a table. The height pillar is graduated in heights with O at the heel. The length of each graduation from zero is the height divided by the
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