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Aviation History
1935
1935 - 1298.PDF
SUPPLEMENT TO FLIGHT 5«8£> 30 THE AIRCRAFT ENGINEER MAY 30, 1935 it impossible for the pilot to use mixture strengths at or in the neighbourhood of W.M.M.P., and the engine is thereby safeguarded against seizure due to abnormally high temperatures, and the utmost range is secured for a given fuel tankage. The device operates the altitude valve for changes in altitude whether put in the "rich automatic" (power) or in the "weak automatic" (economy whilst cruising) position. SERVO PISTON IN GROUND LEVEL" POSITION ROLLER FOR CAM TRACK IN ALTITUDE VALVE LEVER Ky FiG.9 Fig. 9 shows the internal construction of the Hobson- Penn Automatic Mixture Control, which is the subject of British and foreign patents. It will be seen that it contains a barometric bellows, valve gear and servo piston, very similar to Hobson Boost Control but with the following differences: — The bellows chamber is open to atmospheric pressure instead of being affected by boost pressure. One end of the bellows is linked up to the servo piston is such a way that a change in atmospheric pressure causes a small and not complete stroke of the piston, and surrounding the valve is a rotatable sleeve containing two sets of holes at different heights. Rotating this sleeve opens up one pair of holes, closes the other pair, and causes the servo piston to choose one or other of two "zero" positions, from which it is sensitive to changes in altitude. In the " rich automatic " position at sea level the piston is at the extreme top of its stroke, and has a total move ment of approximately \\ inches. Movement of the sleeve to uncover the second pair of holes causes the piston to take up a second " zero" position farther down the cylinder, and its movement with increase in altitude commences from this new point, the piston travelling to the extreme lower position at very high altitudes. The distance between these two pairs of holes in the sleeve varies with different makes of engines, and depends on the drop in mixture strength that is found to be most suitable between the " rich auto matic " setting and the " weak automatic " setting. Once this difference is found and the necessary adjust ments and certain mechanical details settled, they remain constant for any particular combination of engine and | 1 NORMAL 10-15/. RICH MIXTURE N. "RICH AUTOMATIC" WEAKEST MIXTURE FOR MAXIMUM POWER -HIOHESr TEMPERATURES- 1 V 1 • JMMSIH6 RAHOC __ A "WEAK AU WITH SAFE TEA F1G.10 * TOMATIC" 1PERATURES THROTTLE OPENING carburetter, and can be reproduced for production engines with every assurance of similarity in performance. Whilst altitude conditions can be simulated at ground level on the intake side of the engine, reduced air pressure conditions on the exhaust side can only be reproduced by the use of very large experimental plant, which is very costly and somewhat dangerous to use. It is necessary to make certain flight tests at different altitudes and speeds to determine the required engine charac teristics, and it is also necessary for the carburetter curve to be as flat as possible throughout the cruising range, with a power jet coming in when approaching full power condi tion, as shown in Fig. 10, in which three curves, viz., W.M.M.P., " rich automatic," and " weak automatic " are shown. In addition the altitude valve of the carburetter must give a straight-line curve, i.e., it must give progressive and equal changes in mixture strength for progressive and equal movement of the altitude valve, and must not be affected by varying throttle openings. Engines cruising at part throttle on extremely weak mixtures become increasingly difficult to accelerate, the ordinary plain accelerating pump being unable to cope with the demands of the engine, as most of its discharge is deposited on the walls of the supercharger. The Hobson Delayed-action Pump has been designed to overcome this difficulty and functions as follows : — The first movement of the pump operating lever throws a mass discharge of fuel into the carburetter intake, which thoroughly wets the walls of the supercharger, and is imme diately followed by a sustained discharge, which gives the necessary mixture enrichment while the engine accelerates from slow running to normal full-throttle conditions. Safety Devices Instances have been known where automatic mixture con trols of the single-stage type have failed through puncture of the pressure-sensitive bellows, with the result that the mixture strength remained in the altitude condition and forced landing had to be made owing to mixture weak ness causing the engine to cut out near the ground. The Hobson-Penn automatic instrument does not use a single bellows, but a stack of capsules housed in a retaining case which limits their maximum expansion. The failure of any one capsule causes only a slight weakening in mix ture strength—insufficient to make the engine unmanage able. Sticking of the valve is obviated by the spring sur rounding the capsule retaining case becoming coil-bouml and allowing the servo piston to exert the whole of its force on the valve. A summary of th,e advantages so far obtained by the combined use of a Variable Datum Boost Control and the Hobson-Penn two-stage Automatic Mixture Control is interesting: — (1) The pilot cannot give his engine excessive boost pressures and need not watch his boost gauge. Yet he can get additional power for take-off, as well as pro-
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