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Aviation History
1934
1934 - 1432.PDF
SUPPLEMENT TO FLIGHT 44 THE AIRCRAFT ENGINEER JUNE 21, 1934 A " ,; . . • :'••.'• "'••;. ;••••'? '': * * •'.'••''. *v TO C TDC c ...'•'' * FIG.13 o roc •V • State 4. #/» = 0. State 1, #/» = 0-85. State 3, A7/» = 3-35. State 5, J\T/» = 5-6 SWIRL INVESTIGATION: Full-throttle Indicator Diagrams with different Swirl Rates. Speed 1300 rpm. Large dots give T.D.C. and atmospheric lines. The vertical spacing is one dot per 100 lb/in2, the horizontal spacing one dot per 10 deg. crank. of gyration of the combustion chamber cross-section, since the moment of momentum is constant, but in practice the variation is somewhat less than this owing to friction and other factors. It is desirable, therefore, for the combustion chamber to be the full diameter of the cylinder and spherical. And conical sections and others of small gyration radius should be avoided. Inlet Port Timing A late inlet opening increases the swirl, since the inflow velocity is increased during the early part of the suction stroke, when the obliquity of inflow is greatest. The inlet ports, therefore, should open as early as possible. Inlet Port Depth As explained above, the swirl is produced by the one-sided fairing of the inlet opening by the cylinder port wall. The deeper the port, therefore, the greater the swirl, as shown in Fig. 14, and it follows that the inlet ports should be made as shallow as possible. This FIG 14. bort depth ratio SHALLOW PORT .APPROACH Air approaches or idee from both sides inflow obliquity sitohi OCEP PORT APPROACH Air approaches orifice from one side only, inflow Effect of Inlet Port Depth on Swirl Production. is particularly difficult to ensure in multi-cylinder blocks, owing to the possibility of core displacements, and in such cases, therefore, anti-swirl precautions should always be taken. Shape of Induction Passage As shown in Fig. 14, a considerable degree of swirl control can be effected by varying the direction of flow in the induction belt. Where possible the flow in the belt should be given an anti-swirl bias by suitably locat- ing the inlet to the belt. In multi-cylinder blocks, however, this is often difficult for mechanical reasons, particularly where the induction belt is common to several cylinders, and in such cases one can with advan- tage use short baffles near one or more inlet ports in each cylinder, giving the air an anti-swirl bias. In such cases " blowing " tests on a model form useful, if rough, guides to design. Swirl in Poppet Valve Engines There is no inherent swirl tendency in poppet-valve engines, but a certain amount is produced in overhead- valve engines with inlet bends directed tangentially to the cylinder axis. This is usually too slight to do any harm, but in a few cases excessive swirl has been traced to this cause. The remedy is obviously to modify the inlet porting so as to give a more nearly radial flow. Summary (1) Excessive swirl rarely occurs in poppet-valve engines, but is liable to do so in single-sleeve valve engines, especially low-speed engines. (2) The swirl ratio should not exceed a value of about 3, but, if below this, its exact value matters little. (3) The symptoms of " over-swirl " are loss of power and thermal efficiency, low-pitched detonation noise, small ignition advance requirement and smooth running. The combination of the last two features distinguishes over-swirl from over-turbulence, in which quick ignition is combined with rough running. (4) Factors tending to increase swirl and therefore to be avoided, are: —Pro-swirl induction-belt flow, small diameter of combustion chamber, late inlet opening, deep inlet ports. (5) If avoidance of the above pro-swirl factors still leaves the swirl too high, the remedy is anti-swirl induction-belt flow or anti-swirl baffles. Conclusion The writer would like to thank the Air Ministry, on whose behalf these experiments were made, and Ricardo & Co. Engineers (1927), Ltd., at whose experi- mental works they were carried out, for permission to publish the information given herein. THE POLAR DIAGRAM THE ELEMENTS OF A METHOD OF STRESSING BEAMS UNDER COMPRESSIVE END LOAD. By EDGAR H. ATKIN.* AMONG the methods devised for calculating the strength of beams under compressive end load, one of the most elegant, and certainly one of the simplest, is the method of the polar diagram. This method was first described by its discoverer, Mr. Howard, in the Aeronautical Research Committee's Report and Memorandum No. 1233. As there expounded, however, the presentation is more suitable for the mathe- matician than for the practical engineer, hence there is a • Mr. Atkin is on the Technical Staff of Boulton & Paul, Ltd. 608 d
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