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Aviation History
1932
1932 - 0405.PDF
APRIL 29, 1932 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT curve of the engine which we have considered in detail is normal. If the power curve were abnormally steep the variable pitch airscrew would show to still greater advantage as the holding down effect of a fixed pitch air screw at climbing speed would become more pronounced. Similarly, if the supercharged height is increased the holding down effect of the fixed pitch airscrew at ground level still further enhances the value of the variable pitch airscrew. Where the range in r.p.m. is considerable, as in the case of engine C, in which 20 per cent, over-revving is permissible, the variable pitch airscrew makes it possible to use the full available power, whereas, as we saw, a fixed airscrew will necessitate throttling on climb or loss of r.p.m. and power at top speed. In the case of tandem airscrews the rear airscrew is working in the slipstream of the leading one, so that the rear airscrew has to be designed for a greater airspeed than the machine attains. The range of airspeeds for the rear airscrew is much more limited than for the front one, and is bounded approximately by the limits of 0.9 and 1.1 to 1.2 of the maximum forward speed of the machine. With such a limited speed range the rear screw, if designed to give maximum r.p.m. at top speed, will not hold down to normal r.p.m. on climb, and throttjing will be necessary. This causes a very rapid drop in thrust horsepower, since, under constant conditions of loading, the horsepower varies as the cube of the r.p.m. Here, again, the variable pitch airscrew is of great value, as it eliminates this source of loss of power. It appears from our investigation that, with a fixed pitch airscrew, a low gear ratio will lead to a loss of r.p.m. and, therefore, of b.h.p., but that the increase in propulsive efficiency of the airscrew will more than counterbalance this loss and lead to improved perform ance. This would seem to supply the answer to the question so often asked as to what is the best gear ratio, namely, that which will give the biggest permissible airscrew diameter. Since the variable pitch airscrew gets over the trouble of holding down, the same applies even more strongly to its case. ENGINE MOUNTING STRESSES BY R. RODGER. By way of an introduction to the following article, we do not think we could do better than quote from the covering letter from Mr. Rodger: " I read with intercut the article in THE AIRCRAFT ENGINEER dated Auguxt 28, 1931, on ' Forces on the Engine Mounting of a Spin ning Aircraft,' by Mr. D. Williams, B.Sc, A .M.1 .Mech .E. I was particularly pleated to see the engine mounting receiving some attention in the technical press, as the unit is generally passed over in a rather casual manner in most treatises on aircraft structures. While admit ting that the actual principles involved in stressing are not very ' highbrow,' I do think that the subject em bodies sufficient tricky points, particularly as regards the allocation of the external forces, to warrant more detailed treatment. I have set myself the task of pro ducing a precis on the subject, including all the vital points in a manner which, I hope, will lend itself to ready reference." 1. Introduction Tnn severely practical engineer, whose academic know ledge may be limited, is almost invariably confronted by two major difficulties. The first is the absence of a comprehensive statement, for, though the literature on a particular subject may be voluminous, it is usually scattered amongst numerous sources. The second is that even when the information sought has been found its presentation is often such as to be of little immediate practical value without additional simplification. This state of affairs is notorious in aircraft literature and is probably due to the comparative youth of the science. So far as the writer is aware from a careful examination of British and American text-books treat ing of aircraft structures, the question of engine mount ing stresses appears to have been dealt with most casually, and a concise and comprehensive statement on the subject appears to be desirable. The object of this paper is to supply the apparent need by collating and presenting in an immediately useful form the infor mation on the subject contained in the various text books referred to above. 2. Types of Mounting After passing through various phases of development aero engines seem to have settled down into two well- defined classes of cylinder layout, viz. : — (i) Those in which the cylinders are arranged in banks along the crankshaft, (ii) Those in which the cylinders are arranged radially around the crankshaft. In modern practice the majority of the mountings to accommodate the above types of engines are space frames in steel tubing, although other variations do arise, e.g., bulkhead and plate mountings. However, the object of this paper is to examine the general routine methods adopted to estimate the strength of the engine mounting structure, and nothing will, therefore, be gained by going into detail variations in mounting layout. Consequently in what follows a specific example of a space frame mounting for each of the two classes of engines defined above will be considered, and the external and internal forces affecting same will be estab lished. 3. Properties of a Space Frame Throughout the paper it is assumed (i) That the point of intersection of two or more members of the frame is a pin-joint and per fectly frictionless. (ii) That all the members are capable of withstanding either tension or compression, i.e., they are all tie-struts, (iii) That each member is incapable of being per ceptibly deformed under the action of the load it may have to carry. 4. Cases to be Considered An aeroplane is capable of three-dimensional motion and the various attitudes which it may assume are, therefore, almost indefinitely numerous. It would be an impossible task to carry out stress analysis for all the possible attitudes and it consequently becomes neces sary to confine one's attention to a reasonable number of well-defined cases which past experience leads one to believe are representative of the most severe conditions normally likely to be encountered. These cases are specified by the Airworthiness Department of the Ail- Ministry in A.P. 970. They are as follow: — Case I.—Turning in flight, with the engine on, at stalling attitude. Case II.—Normal flight and landing, with the engine off, at stalling attitude. Case III.—Static thrust and torque with the aircraft at rest on the ground, tail down. Case IV.—Inverted flight, with the engine off, at inverted stalling attitude. Case V.—Side load, with the engine off, at vertical bank attitude. Generally it is sufficient to consider Cases I to III, inclusive, although it may sometimes be necessary to consider also Case V. Case IV need only be considered on aerobatic civil aircraft, and when called for in the specification for Service aircraft. 376 e
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