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Aviation History
1920
1920 - 0208.PDF
the strain is distributed vertically over a transverse frame,and horizontally along the corridor and framework of the ship. Distribution of Fixed and Non-Dischargeable WeightsThe fixed weights of the ship are mostly determined by structural considerations, and their position cannot be altered.In the design stage, however, a certain amount of latitude is allowable in fixing the position of the cars. These shouldbe so arranged that the propellers do not mask each other, and also that when the ship is at rest at her maximum staticheight, with all gasbags full, and with the non-dischargeable weights in their appointed positions, there is a momentby the stern. This is always preferable to a moment by the bow. At least two wing cars, one on either side, should be fittedas far up the side as possible, so as to enable manoeuvring to be carried out near the ground if necessary without anydanger of the propellers hitting the ground. The landing ballast should be distributed so as to allowthe pilot to change trim quickly if necessary when landing. For this purpose, it should be divided as far as possible aftand forward consistent with strength. Average figures are : Forward landing ballast 1-5 to 1-75 per cent, of total lift.Aft landing ballast 1 • 5 to 1 • 75 per cent, of total lift. Midships landing ballast 1 per cent, of total lift.Total, 3-5 to 4-5 per cent, of total lift. The landing fuel is, of necessity, stored as near as possibleto the various power units in the fixed petrol tanks. Distribution of Dischargeable Weights The dischargeable weights can be divided for the purpose of distribution under three headings :— Ballast,: . Petrol and oil, Stores. The petrol and oil should be distributed in relation to the position of the engines, and should be such that at any trim that the ship may take up, there is always an adequate supply to the engines. In recent ships the petrol has been carried in aluminium tanks suspended from the corridor, each tank carrying 200 or 300 kg. petrol. The tanks are so arranged that they can be slipped and discharged overboard in case of emergency. Certain tanks are, however, made fixed for use as service tanks. The water ballast is carried in fabric bags, each carrying approximately 1,000 kg. (one ton) of water, suspended in the corridor. The ballast is discharged outside the ship through a valve at the bottom of the bag and a fabric hose or aluminium pipe, time of emptying bag being 65 sees. In general, an excess number of positions for ballast bags and slip petrol tanks are provided beyond the number actually FEBRUARY 19, 1920 required in the " standard " conditions, to allow for varia- tions in initial lift due to atmospheric and purity conditions and to allow the pilot as wide a range as possible for trimming the ship. Horse-Power and Speed The resistance to motion of an airship is made up of :— 1. Resistance of the ship's form. 2. Resistance of the fins, rudders and elevators. 3. Resistance of the cars and propellers, if any of the latter are fixed. 4. The resistance of the external wires, such as fin inter- bracing wires, gondola suspension wires, struts, etc. 5. Miscellaneous resistances, such as radiators, ladders, etc. 6. Augmented resistances due to wake of propellers. Model results are usually given for (1) only. They are expressed in terms of the resistance coefficient C = R/pw^V* and vV* where R = resistance of ship's form. v = speed. V = volume of air displaced by ship's outer form. From theoretical considerations, using the principle of dynamical similarity, C should be the same for similar ships for the same value of 1/V"5. The maximum practical value of vV5 in the wind channel for, say, a I/120th size model,which is obtained for an air speed in the channel of 80 ft./sec, is only i/88th of that for the full-sized ship at 40 m.p.h. The law of variation of C for values of v\T3 between these two limits is not known. Hence the prediction of the resistance results from model experiments cannot yet be done with any degree of accuracy. From trials on ships running at a constant height, it has been established that by running various combinations of engines at full power, then Total B.H.P. oc (speed)a. If speed is measured in miles per hour, then for a ship similar in form to R 33, and with the same efficiency of pro- pellers and gearing, approximately, B.H.P. =0-0058 x (speed in m.p.h.)3. To obtain the b.h.p. required for a similar ship of different displacement it is sufficiently accurate to a first approximation to take the b.h.p. to vary as (total lift)3. The weight of machinery and engine car may be assumed to vary directly with the horse-power, though care must be exercised in using this rule owing to improvements in design. An average figure given by Mr. Campbell before the Institute of Naval Architects was 10 lbs. per b.h.p. Proportion of Fixed Weight to Total Lift The efficiency of a design is measured by the relative values of the fixed weights to the total lift—the lower thi General arrangement of control car, R 33, showing position of water ballast and manoeuvring valve controls. steering and elevating hand-wheels, etc. .:-••••• \ -.•::•• • . 208 . -^:'i""Y>v:;-'''7-- v • -->' >
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