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Aviation History
1945
1945 - 1177.PDF
JUNE 14TH, 1945 FLIGHT 643 Marshall Cabin Blower Qualities and Performance of the Blower : Some Facts on Pressurising and the Conditioning of Pressure Cabins ALTHOUGH for many years the pressurising of air-craft cabins has been a familiar topic in the aero-nautical world relatively little has actually appeared in print about it, and of course, during the war years, although great strides have been made in the practical employment of pressure cabins for various military types virtually no publicity on the subject has been allowed. However, the time has now come when we can divulge a certain amount of the development work that has been going on, and in spite of the fact that the actual aircraft concerned are not yet fully cleared from security restric- tions, some general facts on the installations and disclo- sure of future trends is permissible. Since 1939 research and development work on cabin blowers, in this country, fell almost entirely to Sir George Godfrey and Partners Ltd. This company made proposals to the Air Ministry before the war that they should investi- gate the possibility of utilising an adaptation of the stan- ALTITUDE X I00O FEET IO 15 2O 25 30 I4O1 ' IO 15 2O ALTITUDE X IOOO FEET 25 This set of typical performance curves applies to the Type 15 blower, which has an output of 15 lb./min. of air at 21,500ft. Two such units are fitted on the Avro Tudor I. Externally there is little to be seen. Intake and output ductsare bolted to opposite sides of the finned casing and rotation is imparted to the rotors by means of a quill drive from theengine to the timing gears. dard Marshall Blower of their manufacture for employ- ment to pressurising. Tests were begun and were the prelude to a long and arduous period of development in which the co-operation and resources of the Royal Air- craft Establishment played a large part. Prior to 1938, the Marshall Blower had 2-blade rotors and a rotational speed of 3,000 r.p.m., and with this, com- pression ratios did not normally exceed 1.5 : 1. To adapt the unit for operation at high altitudes and to reduce its weight presented many problems, not the least of which was uneven case expansion due to the inordinate tempera ture rise across the blower—this actually reaching the 200 deg. C. mark at great height. Anqther difficult problem was oil sealing. Nevertheless, by considerable research and refinement of the design the difficulties were overcome, and in 1941 blowers were supplied in quantity for a particular mark of Spitfire for which the require- ments were a 2 lb. /sq. 'in. pressure rise at 40,000ft. Internal Rotor Cooling It is not possible to dilate upon the history of the blower development in the space at our disposal. How- ever, some idea of the work put into the venture can be gained from the appreciation that blowers have been made for operation at 50,000ft. and also of output capacities ranging from 15 up to 80 lb. of air per minute. Rotor ' design has also progressed from the ordinary 2-blade type to the 3-blade straight and 3-blade helical types. The present-day blower such as will be used on the new pressurised civil airctaft is an extraordinarily compact and efficient unit. Uneven case expansion has been taken care of by improvement of the external cooling and by intro- ducing internal cooling of the rotors. For the latter, air is taken in through the rotor shafts and fed to the lobes of the rotor vanes whence it is evacuated via slinger discs to isolate it from the main airflow through the blower. The internal cooling air in the rotors is circulated automatic- ally on the thermo-cycle system. Just how important the control of case expansion is will be realised when it is appreciated that the relative face clearance between the rotor blades is about o.ooSin. and the biade-crown case clearance is o.oo6in. Gearing has been another problem for, with the small clearances demanding operational accuracy in the face of the working temperatures and the period between over- hauls, only gears made to a master gear standard could be accepted. Nevertheless, such quality brings its own reward. The Marshall Blower is of the positive displacement type and, as many motoring enthusiast readers will recog nise, is of the Rootes variety. Such a blower, although not specifically a compressor, has many advantages over
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