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Aviation History
1949
1949 - 0189.PDF
FEBRUARY 3RD, 194c FLIGHT r I25 The neat Mamba installation enhances the appearance of the Balliol I. is getting full power from a Mamba or, for that matter, a Python, on opening the single power lever, r.p.m. alone with- out knowledge pf airscrew pitch being no criterion. The answer is that a torquemeter pressure gauge is provided, which looks like a slightly enlarged oil pressure gauge, and reads up to about 600 Ib/sq in. The pilot can either memorize the mini- mum acceptable torquemeter pressure for take-off and the correct pressure for normal cruising power or, if a keen type, provide himself with a small card of curves for reference: r.p.m. x torquemeter pressure x k = s.h.p. The constant is fixed for the engine on the test bed. The result is given in s.h.p., ignoring the small amount of jet thrust. Take-oft On opening up the Balliol moved off under excellent direc- tional control and with an exhilarating push in the back for the occupants. It came unstuck after a very short run and, with wheels and flaps up, climbed away at a creditable rate with a relatively modest recommended climbing speed of 150 m.p.b. Three impressions registered at once on the first experience of flight behind one little Mamba: (1) There was a most pleasing smoothness and freedom from all vibration coupled with a rather unexpected sense of power. (2) The fact that, in spite of its airscrew, the Balliol definitely resembles a pure-jet- powered aircraft more nearly than one with piston engine. (3) That while noise level is pleasantly low—sufficently so to hear the wind scream around the canopy—there is nevertheless an individual power noise as of a rather distant power station. In this noise there is a deeper note for which the secondary reduction gear train may be accountable, in view of the low frequency. This impression was confirmed when taxying after landing. There was no doubt, however, about the quietness and smoothness of the Mamba by comparison with a piston engine. Having climbed above the mist and haze we closed in on an Oxford from which the accompanying aerial photographs were taken. The Mamba power lever can be harshly handled without ill effect, but, like other gas turbines, this unit takes a second or two to accelerate or decelerate. On opening up from around 12,000 r.p.m. to the full cruising speed of 14,000, a very definite acceleration is felt, there is also a noticeable torque swing to starboard. Formation flying in a Mamba trainer will call for experience for the first reaction to closing or opening the throttle is a momentary reversal of what would be expected, presumably due to pitch change leading that of the turbine speed. The ovcr-and-undershoot are very small indeed, and r.p.m. settle quickly. There seemed to be no need to keep a close watch on jet pipe temperature, which was seldom seen to exceed 520 deg C Cruising speed at 3,000 to 5,006ft is as high as 220 m.p.h. I.A.S. on about 700 s.h.p. This is less than Merlin cruising power, but the speed is considerably higher. —- We were unable to try any aerobatics on this particularaircraft, although the Mamba-Balliol has been put through its paces in this respect and, incidentally, tested for its 30 secondsof inverted flying. It was apparent within a few minutes of take-off that the Balliol, with its responsive controls, will bewell liked on the score of handling in all ordinary manoeuvres Naturally no attempt was made to try out the feathering ofthe airscrew. It has been proved, however, in other installa- tions that both the Mamba and the much larger Python readilyrelight in flight. This is due to their vaporizing-type combus- tion chambers. The action of the feathering button wasmentioned in the starting-up procedure, and to feather an engine in the air the high-pressure cock is first turned off. Asa result the airscrew moves automatically to the full coarse position. There is no longer any appreciable drag from theblades, but to prevent slow windmilling the feathering button is next pressed to turn the blades to the fully feathered atti-tude. As was explained earlier, if the high-pressure cock is part open and the fine-pitch stop withdrawn, the featheringbutton can be used on the ground to fine off the bJade angle prior to starting-up the engine. Mist started to close in over the airfield as we called up forlanding instructions—receiving the answer loud and clear, thanks to the quiet cockpit, and in spite of some outside inter-ference. A comfortable initial approach speed was about 120 m.p.h., and this was reduced safely to 90 m.p.h. during thefinal stages. Like that of the P. 108, the approach angle is steep and the view of the runway over the nose excellent.When holding off, purposely on the ^fast side, the fine-pitch stop was withdrawn and at once the blades moved round to4 deg, the effect being similar to that caused by opening power- ful air brakes. This was accompanied by a slight nose-downmoment, and immediate touch-down at about 75 m.p.h. The withdrawing action was .taken late due to the very fine settingof the basic stop for this flight. As explained earlier, with a rather coarser setting, a more convenient time might be earlyon the approach. Due partly to an attempt to analyse and explain reactionsand reasons for the various control movements, the foregoing account tends to make the Mamba-Balliol appear most com-plicated to operate. This is not true in practice, and, in fact, in most respects, once airborne, the machine and its power unitare unusually simple to handle. Merlin-engined Balliol 2 and Athena 2 trainers are now under-going comparative trials at Boscombe, and, no doubt, fairly prolonged pre-service trials will follow for one or both. Theturboprop Balliol 1, which may follow the . piston-engined variant into service, will by then have a great deal more ex-perience behind it. Apart from its primary purpose, the Balliol will also give the Services and engine manufacturersinvaluable experience in the operation and maintenance of small turboprops.
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