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Aviation History
1940
1940 - 0918.PDF
MARCH 28, 1940 ways movement to the engine. This gives the second and third degrees of freedom, since movement in the plane of the ring in two directions at right angles is provided. The restraining force for this movement is also gener- ated by the rubber "springs." Consider a radial move- ment at the points of attachment at the end of a horizontal diameter—in other words, a horizontal vibration of the engine. This implies a tangential movement at the points of attachment at the end of a vertical diameter, and such movement puts the rubber in shear and the restraining force is brought into play. There is no fourth degree of freedom. Lateral movement in both directions and angular movement in the plane of the ring will isolate most of the vibration. In the thrust direction, of course, no relative movement of engine and mounting can be permitted, and vibration in this direction is relatively unimportant, as it could only be caused by variation of thrust. Reduction of Vibration Small tapered roller bearings are provided in the hinge of the link to ensure freedom without static friction. Length of the link has only a small effect on the elastic constants, unless it is so short as to cause torsional restraint in the mounting spring. Also a short link would produce a fore- and-aft motion as a result of angular motion, and this would be undesirable. Fig. 1 shows the first dynamic suspension installation which was made in 1937 for a double-row Wright Cyclone. The rubber shear mounting springs were i\ in. diameter and the links were of forged steel 3 in. long. A test was made to compare this mounting with the earlier type which had only angular freedom. Removable rods were made and fitted, so locking the seven links and preventing their movement. Vibration on the ground and in flight tests was reported to be quite acceptable. Sperry-M.I.T. vibrograph records were taken of engine motion, airscrew stress, mounting tube stress and rear cockpit motion. The links were then unlocked by removing the rods and all tests repeated. The records showed a general reduction in engine and cockpit motion other than torsional, a marked reduction in mount- ing tube stress variation (shown in Fig. 3), and no change for the worse in airscrew stress. The improvement as measured by the pilot's physiological reaction was definitely better than the instrument records indicated, so appar- ently reduction of vibration is reflected by a considerable increase in passenger comfort. Cylinder Head Attachment Mounting springs with rubber in shear have been chosen for this work because of certain desirable characteristics. They are light, they have an increasing spring rate for large deflections (so acting as "snubbers"), they damp Fig. 4. Mounting the engine by the cylinder-head. Fig. 5. Sketch of strut-type cylinder-head mounting. the vibrations, and even if the rubber fails completely the engine cannot fall out of its mounting. Certain aircraft requirements have now made it desirable to mount large engines from the cylinder heads, and this is a later development. The details of a pair of mount struts as they appear on the test stand are shown in Figs. 4 and 5. The crossbeam is a means of carrying the strut attachment to the rocker bolts, the strongest point on the cylinder head. The size of the spring may appear excessive, but the weight of the rubber required for vibration-isolating engine mountings varies in general as the square of the horse power. This requirement results from the reducing airscrew r.p.m. asso- ciated with increase in horse power and the necessity for obtaining natural trequencies lower than airscrew speed. A double-row Cyclone has been run on the test-stand with this new type of mounting. The latest Waco, the Model E for 1940, adopts dynamic suspension for its Pratt and Whitney engine. Another novel feature is the design of the cowling integral with the fuse- lage and independent of the engine. Though the engine may move, the cowling will not. It is to Prof. E. S. Taylor, of the Massachusetts Insti- tute of Technology, that credit is due for the conception of the mathematical principles of dynamic suspension. He has made valuable contributions to theory and analysis. A complete mathematical treatment of the subject from the design point of view is given by K. A. Browne in the S.A.E. Journal for May, 1939, to whom acknowledgment for this article is made. Baltic Aviation SINCE the occupation of Poland the Riga-Stockholm serviceoperated by ABA has been carrying large numbers ofPolish refugees from Lithuania and Latvia across the Baltic, and some weeks ago the German Government protested against this practice. This protest coincided with an Order issued by the Swedish Government refusing transit visas to Poles and Czechs, and the trans-Baltic air transport business has since been somewhat irregular. The Estonian company, AGO, is now reported to have com- pleted all preparations for the opening of two air services, viz., from Tallinn to Stockholm and from Tallinn to Konigsberg. A plan to introduce a new service from Stockholm to Kaunas, Lithuania, is being considered by the authorities concerned. The Swedish ABA opened a temporary service from Stockholm to the island of Gotland which the formation of ice in the Baltic had cut off from the mainland. There was one daily schedule. Air Lanes to Eire AIR traffic lanes for aircraft entering or leaving Eire (DublinAirport, which is not open to public use) have been pre- scribed by the Eire Minister for Industry and Commerce (Mr.Sean McEntee) in the Emergency Powers (Air Navigation and Restriction) (No. 3) Order, 1940, which has just been issued. Aircraft entering Eire on the east coast and proceeding tothe Dublin municipal airport at Collinstown must do so within an area bounded to the north and south by two straight linesdrawn parallel to, and at a distance of one mile on each side of, a straight line from the airport to Malahide and fromMalahide bearing 90 degrees true to the limit of territorial waters. , Aircraft entering from Northern Ireland must cross wboundary at Dundalk and then follow a route within an area bounded to the east and west by two straight lines P^11*to, and at a distance of one mile on each side of, a straig line from Dundalk to the airport.
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