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Aviation History
1949
1949 - 2041.PDF
FLIGHT, 22 December 1940. 807 TAILPIPE REHEAT . . . (c) portion of nitrogen in the gases, together with thepresence of CO, and H 2O, tends to slow reaction, in-crease the flame" length and narrow the stability band. The effect of velocity tends to concentrate the fuelaround the stabilizer. Consequently the rich-extinction point will occur earlier at high velocities than lowIt is impossible to lay down precise rules regarding stabilizer design and methods of fuel injection. The practical approachis that of trial and error. The temptation is to effect ignition of the mixture spontaneously. Air and fuel are well mixed,the fuel will be vaporized when it reaches the stabilizer, and under sea level conditions a pressure of 26-30 lb/sq in will existin the tailpipe. Cool flames form at temperatures up to 700 deg C, but no true combustion will take place until about800 deg C is reached. However, despite the possibility of spontaneous ignition at take-off, it is very unlikely that itwould occur at altitudes where the maximum abso-lute tailpipe pressure may not exceed 6 lb/sq in. It is fairly well estab-lished that fuel injection can be effected satisfac-torily by the use of plain jets. The major difficultyappears to be localization of the fuel in the body ofthe air, giving rise to uneven distribution in thejet pipe. This can be mitigated by positioningthe jets well upstream of the stabilizer, but radiallocalization of the mixture .is then more difficult andignition may occur o-n4y when quite a large pro-portion of the gas is above the weak limit.This implies a sudden, uncontrolled rise in thrustat low boosts and is better avoided. Combustion - chambervolume may be determined from . bustion intensity, the criterion for this being the heat input per atmosphere per hour per cubic foot as given by: — W'f (lb/hr) X CV (C.H. U/lb) O 1O 2O 3O 4O ALTITUDE (ft x 1OS) Fig. 7. Effect of reheat on specific fuel consumption. a consideration of com- I = P(atn) X Volume (cu ft) An approximate method of determining minimum combustionr \- ..,_.. • i „„ nommintinn r>f eftertive nameof effectiv flam velocity. This is'dlfficuTt to assess, but a figure 0^30 ft/sec. jszone diameter involves an assumptionreasonable. In a high-speed gas stream an explosion flame front would move radially at the relatively slow flame speedand downstream at the gas speed plus the flame speed. In a reheat chamber this would be influenced by the measure ofrecirculation essential for a self-supporting flame. Accord- ingly the flame is envisaged as a hot central core surroundedby a'conical envelope, the solid angle of which will be deter- mined by the resultant of the gas velocity and the flame velocity. The design aims in the Goblin reheat system were:(a) Good ignitability under all flight conditions, the reheat lighting up without shock and subsequently beingopened smoothly to full thrust. Wide stability limits with special emphasis on the weakend to ensure smoothness and pilot's control. Low "cold" loss due to stabilizers, in order to giveapproximately normal operation without reheat. Reasonable combustion pressure-loss and efficiency,consistent with a realistic tailpipe for installation in single-engined fighters. . Good mechanical reliability and cool operation of pipe (b) (c) (d) (e) (f) and nozzle. Variable degree of reheat under pilot's control to be interconnected with a variable nozzle to ensure optimum operating conditions.Not all these aims have yet been achieved. In the early work, efforts were concentrated on obtaining good stabilization andlight-up. The system of individual stabilizers was soon dropped and a vee-section ring was selected for reasons of stability,good light-up and reliability. Two ignition plugs were fitted, although one proved sufficient to give, smooth ignition with athrust increase of only 100-150 lb. A variable nozzle was not fitted since the design presented many difficulties not yet over- come. The amount of thrust increase was therefore dictatedby the thrust available without reheat which had to be suffi- cient for take-off. A series of bench tests was conductedusing nozzles of three different diameters to obtain a range of operation up to 20 per cent thrust increase. Investigation into the general behaviour of reheat at alti-tudes can only be carried out in this country by installation in an aircraft, and accordingly a Goblin-engined Vampire wasadapted as shown in the heading illustration. The fuel system cofnprised two Lucas pumps supplying fuel through twobranches: one to the normal control box and the other to a special reheat shut-off cock and control box operated by a singlelever in the cockpit. Flight clearance was obtained on com- pletion of a 25-hr special-category test, and installation andpreliminary clearance flights were completed without incident. General performance proved to be good. A thrust increaseof 13 per cent at sea level was obtained, which would increase to nearly 20 per cent at 400 m.p h. and 25 per cent at 600m.p.h. These thrust gains resulted in an increase of over 40 m.p.h. in maximum forward speed and a 30 per cent reductionin the time required to climb to operational height. There was no sign of rich extinction when the air flow was cut downby reducing the engine speed, and weak extinction occurred only when the fuel was shut off completely. Aircraft handle-ability was slightly affected as in order to accommodate the extended tailpipe it was necessary to tilt it upwards at 6 degfrom the longitudinal axis of the Goblin unit. This gave a nose-up effect which altered the trim. Apart from a slightvibration when reheat was in operation the aircraft was other- wise normal and there were no signs of overheating of the tail-pipe. This research programme has made clear the inherent possi-bilities of the reheat system. It provides proportional increases of thrust up to 45 per cent for take-off and up to 85 per centfor combat at 600 m.p.h. Specific fuel consumption will rise to a value of some 3 lb/hr/lb for the above conditions, butwhen considered in relation to the improved rate of climb, the amount of fuel required to reach operational altitudes is notexcessive. The installation of stabilizers and the variable nozzle will decrease the tailpipe efficiency under normal con-ditions. The use of very high temperatures—some 2.200 deg K— renders difficult the mechanical construction of a variablenozzle. There appears no reason why a reheat system should increase installation weight by more than 30 per cent of thenormal engine weight. Since in this manner a small engine is given a maximum power which could only be equalled by amuch larger power unit, this does not seem a high price to pay. NAVAIDS ENTITLED Navigational Systems and Instrument Aids, apaper of considerable merit was presented by Doctors D. E. Adams and A. M. Uttley before the R.Ae.S. onDecember 15th. After dealing with the general requirements and dilatingupon the essential differences between civil and military navigational problems, the paper went on to include a com-prehensive survey of the current navigational systems. Some useful information was given on the influence ofnavigational equipment on aircraft design, particularly in relation to weight-saving and the reduction of drag. In thelatter connection, the employment of dielectrically isolated sections of the aircraft itself as an aerial (for dekametre wave-lengths) and the use of slot aerials for metre wavelengths was particularly noteworthy. In relation to the future, it was suggested in the paperthat, as equipment becomes more reliable, it would seem likely that the number of crew members would be reduced; wealready had the automatic pilot and this would be followed •by the automatic navigator and the automatic engineer; butjudgment and decision would always remain. For passenger travel, aircrews would be essential for anindefinite period, but it was possible to foresee the automatic delivery of mail and freight with take-off and landing con-ditions outside those which could be tolerated by human beings, so easing the serious problems of airfield space andthe utilization of existing airfields. The military counterpart would be the expendable pilotless bomber, which would presenta much more difficult problem, for the very reverse of co- operation could be expected at the receiving end. With ground assistance, there was no fundamental problem;the civil problem was more one of organization. When active opposition had to be encountered, the case was quite different;here, navigational effectiveness, taking into consideration other aspects of the operational role—reconnaissance, inter-ception or bombing—must inevitably take precedence over simplicity or cheapness.
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