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Aviation History
1917
1917 - 0144.PDF
axis passing through the centre of gravity of the section.The curves of stress given are for a three-blade propeller of about 8$ ft. diameter, 5 ft. pitch, absorbing 150 h.p. at1,300 r.p.m. The curves are not accurate, as they are intended merely to give a general idea of the order of mag-nitude of the stresses likely to occur in such a propeller. The 3tres3 caused by centrifugal force is uniform over anysection of the blade and varies in intensity at points along the blade, as shown approximately in Figs. 13, 14 and 15.Steady bending due to aerodynamic forces is caused by torque and thrust. These forces act along X — X and Y — Y,respectively for any section, such as shown in Fig. 12. When resolved along I — I and II — II they induce bendingmoments that) cause the fibre stress as shown in Figs. 13, 14 and 15. Gyroscopic moments are only induced when theaeroplane is changing its direction of flight. In order to estimate the stress set up in the blades an assumption mustbe made as to the angular velocity of the propeller axis ; 4000 FEBRUARY 81 1917. Many American crank-cases are not sufficiently rigid inconstruction. It is believed that crank-case castings are not designed and built in this country with sufficient care.Some of the jigs for boring crankshaft and camshaft bearing seats are not so accurate as desirable. In some cases it hasbeen found that pistons are not of uniform weight and are not carefully made. Lack of interchangeability of partsand careless workmanship have been great faults in this country. Oiling system.—This should be by pressure to all importantbearings, preferably from a gear pump. Screens should be provided to protect the suction pumps. For engines thathave push-rod and rocker-arm valve mechanism, means should be provided to reduce the friction on the exhaust-valve rocker-arm bearing, especially if the valves are more than ij ins. diameter. Ignition.—All military aeroplanes, except possibly the 2000 RADIUS IN FEET 0 HUB 2 IRADIUS IN FEET Fig. 14.—Stress in propeller blade at point B (see Fig. 15.—Stress in propeller blade at point C (see Fig. 12). Fig. 12). tliat is, as to the precession. There is some question as tothe assumption it is reasonable to make in computing the stresses. The type of aeroplane size and disposition of thelarger masses, such as engines, &c, will affect the rate at which a machine can be turned in flight. In general, theangular velocity in yaw will not greatly exceed 0-35 radian l>er second. It must be remembered, however, that asteeply-banked turn also involves rotation in pitch. The maximum angular velocity attained in coming out of asteep dive can be estimated, from the characteristics of the. aeroplane and the factor of safety, which determine themaximum high speed attainable and the radius of curvature of the path along which it is possible to pull the machineout of its dive safely. A safe value for the angular velocity in pitch for the usual type of present-day aeroplanes isabout 1 radian per second. Loops have been turned in about 6 sees., which gives about the value mentioned of the angularv'ocity. A precession of I radian per second at the normal speed of the engine should, therefore, be assumed in com-puting the stresses. The* stresses set up by gyroscopic forces are alternating, changing in sign (tension to compression)twice in each revolution of the propeller about its axis. The fibre stress caused by the gyroscopic moments is given inFigs. 13, 14 and 15. Algebraically adding the fibre stress at the three points chosen gives the approximate value ofthe resultant fibre stress at those points, as shown in the same figures. It will be noticed that the maximum stressesoccur at some distance out from the hub. To ensure a good wearing blade, which will stand up under the necessarilyhard usage given it in the field, a factor of safety of not less than 5 is suggested as being the minimum consistent withrequirements when the three principal stresses are taken into consideration. SUGGESTIONS FOR IMPROVEMENTS IN DESIGN. These suggestions on power plants are based on/theexperience of the First Aero Squadron, United States Army, in the field. It is considered extremely poor practice touse shims under the caps of crankpin and crankshaft bearings. Air Work in Macedonia. IN spite of rain, mist and latterly snow, the Britishairmen on the Macedonian front have been very active, says the Times correspondent. Numerous successful raids have pursuit type, should have two complete and independentignition systems. Engines larger than 140 h.p. should have a booster system for starting on battery spark, if a starteris not provided. It is believed that our magnetos would have much longer life if a more suitable shock-absorbingdevice between the driving gear and the magneto shaft were provided. A magneto mounting should be machined sothat the magneto shaft will be exactly in line with its driving shaft; dowel pins and dowel-pin holes to preserve thisalignment should be provided. No shims should be used here. We have had considerable trouble because of non-uniform and warped carbon brushes. Fuel supply.—^Carburettors should be located in such away that oil, water and impurities cannot enter them. They should be supported from the engine, and not from theframework of the aeroplane. They should be supported independently of the intake manifolds, if practicable. Gasketsfor connections in intake manifolds should be as thin as practicable. Manifolds built of copper, brazed, or of steel,welded, are considered preferable to cast manifolds. Steel is considered preferable, but should, of course, be heattreated after welding. It is urged that more study and care should be put into the design relating to shape and finishof the interior of intake manifolds and passages. It is believed, in this connection, that much greater efficienc 'can be obtained by attention to fluid flow. Cooling system.—Radiators should preferably be placedat the leading edge of the upper wing, the header being shaped so as to form part of this leading edge. If it isnecessary to place the radiator between the engine and the propeller, the radiator should be circular. The radiatorshould be provided with a sufficient number of points of support to prevent deformation of the shell owing to shockson landing. Care should be taken with the alignment of tubes at the connections in the water-circulating system.A ring reinforcement might be welded to a flanged end of the thin tubing and the face machined so as to make a goodfit to the cylinder jacket. It is considered bad practice to expand thin tubing. been carried out, and four enemy machines had beenbrought down since January ist. Three of these were brought down by one officer. The observation officer on thethird was Count Schwerin, who was taken prisoner. '44
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