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Aviation History
1912
1912 - 0595.PDF
JUNK 29, 1912, direction in which the machine will most likely strike. From this it is apparent that the fast machine should have an undercarriage much further forward at the base than should a slow machine. A strut or post which is vertical from a side elevation is distinctly bad, and in many cases the wire holding it forward at the lower end has a much greater load to carry than has the strut. Nothing shows a greater lack of knowledge of elementary mechanics than the majority of undercarriages which are being used in this country. One sees enormous beams, struts, skids and axles on machines simply because the materials are not used to the best advantage. Probably the strongest undercarriage in existence on well-known machines is theNieuport, and no one will say it is not the simplest. Nothing shows greater inefficiency on the part of the designer than an elaborate undercarriage. Struts of streamline section are practically useless if strength per lb. of weight is of any importance, as the strength of such a strut is proportional to the square of the thickness across the thinnest place, and even assuming a strut to be bent across its deepest section its strength is considerably reduced by having a sharp trailing edge. It is also absolutely essential that a strut is thicker in the centre, and tapers towards each end. Designers often estimate what thickness of strut is required in the centre, and keep it parallel to the ends to save labour or trouble, or for some unaccountable reason, ignoring all theltime thelfact that it is weakening the strut to leave the material there. Take a strut of the kind just mentioned, and put a compressive load on each end until the strut bends in the centre (which it will do). Now we all know without going into the mathematics of the problem that there is not the same tendency for it to bend towards the end as there is in the centre. Therefore the strut is considerably stronger everywhere else than it is in the centre, and for that reason the deflection is concentrated on the one spot, and the fibres are stressed to a much sharper angle than they would be if the strut were tapered exactly in proportion to the load each part of it had to carry. In this latter case the strut would assume the arc of a circle, and each particle of the strut would be bent a much smaller amount for the same load than in the former case. The struts should be of circular section, preferably, and should be made streamline, if it is desired, by some light material. It is practically as important to avoid over strengthening in places as it is to avoid weak spots. This makes us again wonder why manufacturers : connect their machines to the point of contact with earth by the agency of beams, girders, wires and strainers, &c. As a matter of fact, the probable cause is that they have thought out some ingenious method of springing, and have overlooked the whole point in the design in their endeavours to give publicity to their ingenuity. (TQGHT If they neglected springing altogether, except as a method of removing the wheels at the instant when it is required to bring the parts designed to take the shock into contact with the earth, they would probably achieve a greater measure of success and incidentally relegate to the scrap heap about three-quarters of the weight they are lugging about in the air. It would be better for both the pilot and machine as well as for the scrap iron merchant. It is very doubtful if any springing device, except for the purpose mentioned above, is of much real use on an aeroplane, and ceitain it is that the few machines that have been built without any springing device at all have been quite successful. The early Wrights, with skids alone, were quite a success, but of course one cannot very well alight on rigid wheels, and wheels are necessary nowadays. If the undercarriage struts are designed so as to be able to support, say 13 times the weight of the machine, then springs are useless unless they are about as strong (and they never are), because even with the springs removed the machine would alight successfully up to their load without fracture, and if with the springs they close up tightly the whole weight of the shock is transmitted to the struts. It is certainly true that with efficient springs the machine is brought to rest in a greater distance and the shock thereby reduced, but it is much simpler to design a strut to withstand the whole of the shock than to design springing devices as capable. A long well-designed strut will have enough spring in it to make the shock gradual (as far as stresses in the material are concerned) without in any way distressing the fibres of the strut, provided a landing is made that does not exceed twelve times (or whatever the strut is designed to stand) the machine's weight. It should easily be possible to design four light struts that will stand 6 lo 8 tons as a momentary load. With reference to bolts used in aeroplanes, those subjected to tension stresses ought to be machined down on the unthreaded portion to the diameter at the bottom of the thread, or slightly less. This has been proved in numerous laboratory tests to increase the actual breaking load, and the writer has seen instances where the increase amounted to 150 per cent, or equal to 2\ times the breaking load of the ordinary bolt. One can only come to the one conclusion, and that is that aero planes as at present constructed are not (except in very few cases) sound mechanical structures. It may be that the fault does not altogether lie with the constructor but with the progress made in plane design. One manufacturer recently told the writer that he would be pleased to put in all the details I specified and work to my factor of safety on the condition that I did not specify that the machine had to fly. GRANVILLE E. BRADSHAW. ® ® ® ® THE, AERO ENGINE. By G. H. CHALLENGER. Lag in Combustion.—An example, making use of Table IX, will show more clearly how the effect of lag in combustion is more marked at high revolution speeds. In a motor running at 300 r.p.m. each stroke is completed in T\jth second, if combustion is complete in 2-^isth second with ignition at dead centre, the crank pin will have passed through 9° or the movement of piston will have been through -oo6i volumes. TABLE IX. ... Volume Swept Volume Swept ^"Velocity Angle a. Angle fl. b piston.' from 0» to p at 0 1 Average 1 r = 1,000. 216 614 816 926 1,114 1,274 1,407 1,498 1.556 - . 1.575 If the motor is speeded up to 1,200 r.p.m. each stroke will occupy ^th, second—if the richness of mixture is such that complete combustion still requires 7rbn second with ignition at dead centre, then the crank pin will have moved through 36' or the piston will have moved through '0955 volumes. If the weakening of mixtures either by dilution with air or residue exhaust, or attenuation by throttling, or a weaker igniting spark had resulted in T1,, second being required for complete combustion, then in the former case the piston would have moved through '0244 volume whilst in the latter case the piston displacement would be no less than '3455 volumes. If the mixture is sufficiently weak combustion will be so prolonged 0 9 " 18 - 27 36 - 45 " 54 - 63 - 72 - 81 - 9 - 18 - 27 - 36 - 45 - 54 - 63 - 72 - 81 - 90 •0061 •0183 •0300 •0410 •0508 •0596 •0669 •0725 •0763 •0782 •0061 •0244 •0545 •0955 •1464 •2061 •2730 •3455 •4218 •5000 (Concludedfrom page 572.) that it is not completed when the piston reaches the end of its stroke and so may result in ignition of the mixture in the induction pipe, when the inlet valve is opened. The Spark Gap.—Slight inequalities in the spark gaps of the ignition plugs or in the electrical inductance of the plugs or the leads from the magneto to the various cylinders of a multi-cylinder engine and slight inequalities in mixture between one cylinder and another which would pass unnoticed at lower revolution speeds will be considerably magnified at high revolution sjMieds and the eradication of these differences will become the most serious part of the "tuning up" process, i.e., supposing that the defect is not inherent in the design of the motor. Table VIII indicates that for a given mixture supply the time of combustion decreases almost in propor tion to the increase in piston speed, so that if the other factors do not vary, a fixed point of ignition, as is now Incoming common practice on automobiles, is justified where simplicity takes prece dence to maximum efficiency. The table only goes up to a piston speed of 14*1 ft. per second, whereas speeds of 20 ft. per second are becoming common on aero engines. As shown on Table V, attenuation of the charge becomes a serious matter at high piston speeds, and if fixed ignition is used the mixture supplied must be much richer than at lower speeds in order to ensure complete combustion in a reasonable time, because the compression pressure will be less and the dilution with residue exhaust greater. Spark Advance.—As shown by the experiment of Prof. Watson already mentioned, the mixture can be weakened if the spark is advanced and an increase of power obtained in spite of the back pressure due to combustion during the compression stroke. Forced feed as compared with ordinary suction feed would require less advance, because by limiting the adulteration with residue exhaust and ensuring a full cylinder charge, the compression pressure would be maintained, thus more rapid combustion would ensue. 595
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