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Aviation History
1949
1949 - 1404.PDF
i68 PYTHON FLIGHT August nth, 1949 !was by vaporization. Furthermore, as the speed of com- bustion is dependent upon the initial temperature of the mixture, it pays to have this as high as possible—again pointing the way to vaporization. The construction of the combustion chamber and the air- and gas-flow paths can be seen in the illustrations. Housed coaxially within the "combustion chamber is a dome-headed flame tube incorporating a vortex vaporizer. •jAir delivered from the compressor enters the upstream i mouth of the combustion chamber, the greater percentage of it passing through the annular passage between the flame tube and outer casing. The air required for combustion of the fuel is admitted to the flame tube in a variety of ways; the vaporizer volute has a ram intake aperture pro- jecting into the annulus between flame tube and casing, and in passing into the vaporizer, the air picks up fuel from the four-hole jet which projects into the ram throat. This primary charge is considerably too '' rich'' for complete combustion, but it spills over the sharply edged orifice in the forward face of the vaporizing volute and, on entering an envelope of burning gas, combustion of the charge is started. The jet of mixture travels up the flame- tube toward the baffle-plate behind an inlet aperture in the crown of the flame-tube dome. Air admitted through this hole is diffused by the plate into a continuous sheet which flows initially in a direction parallel and close to the sur- face of the flame-tube, so forming an insulating layer be- tween the metal and the flame, as well as gradually adding fresh air to the burning charge. As the burning gases are turned back by the curvature of the dome, further air is added through a series of shoulder holes ai the tangent point between dome and tube. The turbulence produced by this method of air intro- duction accelerates the combustion and, in fact, combus- tion is substantially completed when the gases pass around the outer surface of the vaporizer. Then, in order to ensure that air is available in excess of that required for combustion at the richest operating mixtures, further air is added through rings of holes around the flame-tube lower trunk. (Incidentally, an insulating film of cold air is introduced immediately upstream of the first ring of holes to alleviate the effects of high temperature on the material of the flame-tube. Curiously enough, this bound- ary layer flow does not affect the airflow through the holes.) As the Python has abnormally long combustion trunks, it has been possible to keep the mixing losses low. The process is initiated by four rectangular deflectors on the interior surface of the combustion chamber, imme- Ihe method of retaining turbine blades by pegs and spot-welded segment strips is a novel feature of the Python. diately downstream of the flame tube ; these direct wedges of cold air into the centre of the flame and cause an eddy- ing motion which persists down the duct to the turbine. The resulting peak temperature at the entry to the tur- bine is approximately 4 per cent higher than the mean temperature. Two of the Python's eleven combustion chambers are fitted with torch igniters (which are virtually sparking plugs through which fuel is sprayed), the remaining nine chambers being fitted with starting jets. When the engine is started, the total fuel pump delivery is given to the torch igniters and starting jets until such time as the engine has speeded up sufficiently for a 50 lb/sq in difference to be given across a valve in the fuel delivery line, at which point the valve opens and passes fuel to the main burners. Then, when running is stabilized, a solenoid valve cuts off supply of fuel to the igniters and starting jets. The starting jets produce a fuel/air mist in each of the nine combustion chambers, whilst the torch igniters in the remaining two chambers produce a flame in the chamber head, this flame spreading rapidly through the bridge tubes interconnecting the chambers, so igniting the mixture sprayed in by the starting jets. The fuel pump fitted to the Python is a Lucas C-size swashplate unit which, delivers through a flow control unit of Armstrong Siddeley design. Fuel is thence given to an isolator which is, in fact, virtually a high-pressure cock, and from which a line is taken off to the torch igniters and starting jets. The main fuel flow from the isolator goes to a distributor via a pressure-increasing valve (both of Armstrong Siddeley design), the latest type of which employs gradient slots, so that the curve of fuel pressure plotted against flow for the main burners can be made substantially linear. The pilot has a single-lever control which jointly operates the fuel flow control units and the airscrew constant-speed- unit via a hydraulic controller. Movement of the pilot's lever alters, through a cam, the setting of the flow control unit to govern the amount and rate of fuel supply to the burners. Into this linkage is tied a hydraulic vane-type follow-up servo which, through another (matched) cam, alters the setting of the c.s.u. . governor. The purpose of this form of control is that, when open- ing the throttle, the airscrew blade pitch will coarsen rather than fine-off during the acceleration and so prevent the hunting which would occur when the selection of full throttle made full fuel flow available. The maximum fuel flow is of the order of 400 gall/hr, and, as the Python re- quires no more than 150 gall/hr at 8,000 r.p.m. when the airscrew is in fine pitch, without the incorporation of this system to deceive the airscrew, so to speak, the constant- speed unit would try to overspeed, which would in turn bring in the overspeed governor of the fuel pump to shut down fuel flow, so causing the r.p.m. to drop, whilst the airscrew blade pitch continued to coarsen, so opening full fuel flow again to continue the unhappy cycle. By build- ing deceit into the control, however, the increment of en- gine r.p.m. is matched with the acceleration characteristics of the airscrew. C. B. B-W. PYTHON DATA Max. dia. over cowling 54.5in Length, from C/L of front A/S blades to end of exhaust cone 18X67inMinimum length of exhaust cone 26in Dia. of jet pipe, lagged 25inNet dry weight 3,1501b Summary of Se* Lavel Performance Rating Max. take-off ... Max. climb Max. cont. cruise Engine r.p.m. 8,000 7300 7,600 Speed, nvp.h. 0 100 200 300 -400 200 300 400 Airscrew ch.p. 3,670 3,2(0 3,450 33004,230 2,960 3.260 3,680 Net jet thrust 1,150 850 660 490 320 570390 220 Fuel cons. 355 322 329 342 357 2% 304 321 4
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