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Aviation History
1949
1949 - 0188.PDF
124 FLIGHT FEBRUARY 3RD, 1949 Tt.e large twc-seat cockpit with power-operated canopy is a dominant feature of the Boulton Paul Balliol I trainer. Mamba Balliol in the Air automatic cycle energized by a single control. Before describ- ing the flight and analysing the reason for certain of the actions it must be mentioned that a Rotol airscrew is fitted, and that the basic fine-pitch stop, which is the subject of one of the current series of trials, was set at 4 degrees for the particular flight. The stop setting is likely to be fixed at nearer 10 deg. It may be remembered that on the occasion of Mr. Lindsey's lecture on the development of the Mamba, before the R.Ae.S., the Armstrong Siddeley system of power and pitch control was subjected to some criticism and referred to as "devices." Mr. Lindsey accepted the remarks, but held that it was suit- able at this stage and capable of satisfactory operation although open to improvement. Our short experience in the air would lead us to confirm this from the pilot's point of view. To understand something of the airscrew pitch control problems of a turboprop such as the Mamba, reference should again be made to Mr. Lindsey's paper, summarized in Flight of Decem- ber 9th, 1948. Both the Mamba and its large relation, the Python, are at present .started with the aid of gas starters, developed by Arm- strong Siddeley and driven from a battery of ground com- pressed-air bottles. The air line it attached in much the same manner as the plug of a ground starter accumulator. Electric starters will almost certainly be fitted as soon as suitable types have been delivered. Climbing into the Balliol presents no difficulties, and one is soon strapped comfortably into one's seat in a cockpit which has been criticized on the score of elbow room, though this was in our opinion quite acceptable. Seat adjustment is good, and on the levers provided for this purpose there are smaller controls (bicycle brake type) which permit rudder reach adjustment. Two sticks with spade-type grip are provided. Field of view on the groifhd is average, but one is forced to face the fact that five sturdy screen pillars, which are pre- sumably essential if such a large screen and sliding canopy are to stand up to very high speeds, cannot fail to restrict vision, particularly obliquely forward where one locks for part of a landing. Moreover, the addition of two-stage amber with its permanently coloured screen top further interferes with view, particular! ' on turns. One feels that the Balliol is not alone in this weakness. It must be added that the canopy— closed or opened by pressure 011 a button—operates excellently, closes tight and excludes both noise and draught. Starting Up Several of the actions for starting (reference was made to this earlier) are under examination at present, so each must be executed separately. (Chocks in position—All clear.) First the power master switch goes on, and this is followed by fining-off the airscrew pitch to its minimum stop. Paradoxic- ally this is done with the aid of the feathering button after the high-pressure cock has been eased off its stop (explained later). Pitch drops back to the 4 deg stop very quickly and the low-pressure cock (equivalent to the normal fuel cock) is then turned on. Next, to feed fuel exclusively to the two chambers having igniter plugs, ,the Lucas valve for the purpose is put to "on." At this Stage, if not before, the power lever (''throttle") should be checked for closure against its "off" stop—if the throttle is at all open the Mamba will not start. When the ground crew are ready the starting (ignition) cycle is energized and starter air called for. With the noise of a small motor cycle the gas starter accelerates the engine rapidty, and at 1,000 r.p.m. the high-pressure cock is pushed to "on." As the turbine picks up, the Lucas switch is put off and air to the starter motors is stopped. Immediate Readiness By the time all instruments are checked and ground crew and chocks are clear, the Mamba is ready for taxying. Quick starts and take-offs with gas turbines are limited only by oil temperature, and by power and instrument checks. The engine oil not only lubricates, but operates the airscrew and engine power control device (cam box). Temperature must, therefore, not be too low. In operation, an electric immersion heater would be used to keep oil temperature up prior to starting. Fuel consumption is so high on large units that a lengthy run up might seriously affect range. This is specially true of fighters with their restricted tankage. Pre-heated oil and, in the case of large transports, a tow to the take-off runway, may be the answer to this problem. On the Balliol, which, like its prototype, has very powerful brakes, the 4 deg minimum pitch setting happens to be just right for taxying at idling r.p.m. The balance to be struck by alteration of the pitch setting is between minimum power absorption by the airscrew in very fine pitch for starting, and the ability, when airborne, to absorb sufficient power in the event of C.S.U. failure and the associated risk of overspeeding. After the simple standard take-ofF check for the Balliol has been completed the withdrawable fine-pitch stop is re-selected on the engagement switch. It then comes into play as soon as the pitch exceeds its limit of 18 deg, and remains in engage- ment until manually withdrawn again. This control of the withdrawable fine-pitch stop will later become automatic, operated according to whether or not there is a load orl the landing wheels or, perhaps, interconnected with the flap lever. The Mamba gives insufficient idling power to drive the air- screw at r8 deg pitch, so if the stop were engaged on run-up prior to take-ofl, and the engine subsequently closed down, before moving off, it would simply stall and have to be re- started in very fine pitch. It may have been wondered how a pilot knows whether he n 6
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