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Aviation History
1949
1949 - 1523.PDF
FLIGHT, 8 September 1949 Exhaust Reheat for Turbojets . : ? ' - ." maximum permissible nozzle size was severely restricted owingto the fact that sufficient thrust had to be maintained for take-off without reheat. This limited the amount of thrustboost obtainable and both units were passed out for flight with a thrust increase of 9 per cent above standard. This valuewas obtained for an increase in overall specific fuel consump- tion of 33 per cent. The inherent loss due to the pilot chamberwas 2.5-3 Per cerjt °f the standard take-off thrust. The method adopted in these and subsequent flight testswas to operate the reheat chamber at conditions giving maxi- mum turbine outlet temperature, this temperature beingrecorded by four thermocouples located in the turbine exhaust cone assembly. Level flights were made at varying altitudesand measurements taken of maximum and minimum reheat fuel flows for stable combustion. All readings were taken atmaximum turbine speed. The results of these tests showed that the range over which combustion remained stable wasconsiderably reduced at altitude, this effect being more marked on the port unit. It was evident from these results that themaximum ceiling for operation on both units was approxi- mately 13,000ft, although for the starboard unit alone itsceiling was in the region of 20,000ft. The aircraft performance tests consisted of level flights atmaximum reheat conditions at various altitudes and the results were compared with those of a standard aircraft. A gain intrue airspeed of 46 m.p.h. was obtained at all altitudes up to ] 2,000ft for an increase in overall fuel consumption of about65 per cent. An estimation of the rate of climb with reheat indicated an increase of approximately 46 per cent when com-pared with the standard aircraft. The transfer of the work to Power Jets (R. and D.), Ltd., marked the end of this seriesof tests. Major Modifications At this stage in the development of the halo system, theopportunity was taken to make several major alterations to the reheat installation. The diameter of the reheat pipe wasalso increased, as it was known that a higher combustion efficiency could be obtained in this way. To avoid unnecessarydevelopment, the reheat burner system was initially kept the same as for the previous tests. To obtain flight approval, a 25-hour acceptance test wascarried out on the test bed. During these tests a thrust in- crease of 13.5 per cent was recorded, for an increase in specificfuel consumption of 46 per cent. The reheat specific fuel con- sumption corresponding to these values was 5.25 lb/hr/]bthrust increase, thus indicating a high combustion efficiency. The only mechanical trouble experienced was that due to crack-ing of the material, both on the exhaust cone struts and the jet pipe difiuser. This was overcome by increasing the gaugeof the former and by inserting an 18 s.w.g. liner in the diffus- ing section between the exhaust cone outlet and the enlargedreheat pipe. The main object of this series of flight tests was to obtain TABLE I. Performance of Meteor I Aircraft with reheated W2/700 turbojets. Aircraft mean weight= 10,600 Ib. more information about the behaviour of the halo system alaltitude. Initially, the fuel system was arranged so that the reheat fuel was supplied from one of the emergency pumps onthe turbine unit but, owing to a sudden drop in speed at high altitudes when operating at high reheat pressures, the reheatfuel system was separated from the turbine circuit. Only one unit was reheated during the preliminary tests, and it wasfound possible to maintain stable burning up to 30,000ft. Both were then connected to the reheat control panel andclimbs were made to 30,500ft without difficulty. Tests were then made to determine the optimum number ofjet holes in the halo fuel ring. Originally there were twelve 0.032m dia. jets equally spaced around the halo and thisnumber was reduced to ten and later to nine. The best stability range was obtained with ten jets as used for the finaltests. It was found that, at ground level, the stability range covered fuel pressures from 130-200 lb/in2, the upper limitbeing set by the final nozzle size. At 30,000ft, however, a variation of +5 lb/in2 at 130 lb/in2 nominal was sufficient tocause blow-out. During a climb it was necessary to reduce turbine speed to keep within the jet temperature limitations.This would normally be done by reducing reheat fuel flow, but here the poor weak limit of the burner system prevented sucha remedy. Conditions R.P.M. 16,000 16,000 16,500 Alt. (ft) 5,000 25.000 5,000 Standard Aircraft Speed (m.p.h.) 412 433 440 Net Thrust (Ib) 2280 1450 2620 Fuel Cons. (Ib/hr) 3590 1900 4100 Reheated Aircraft Speed (m.p.h.) 462 500 485 Inc. (per cent) 12.0 15.5 10.0 Net Thrust (Ib) 3000 1920 3500 Inc. per cent 31.6 32.4 34.6 Fuel Cons. (Ib/hr) 6180 4400 6760 Inc. per cent 72.0 132.0 65.5 Test Procedure The procedure adopted for the level-flight performance testswas as follows: The altitude and reheat fuel pressure were Wset to give a conrtanf — for the nominal altitude under con- pa sideration, where W was tne mean all-up weight of the air-craft throughout a flight of average duration and pa the atmospheric pressure. In this particular case W was 10,600 lb.The throttles were then set to give maximum jet temperature and after completing a level at these conditions the units werethrottled back in 500 r.p.m., steps without altering the reheat flow, and the tests repeated. This procedure gave the maxi-mum range of conditions at which the reheat could be operated at a given altitude. During the performance tests with standard units, an ex-pression was obtained for the aircraft drag. Values were then obtained for the net thrust, both for the reheated and standardunits. Table I shows the results obtained, together with corre- sponding values for aircraft speed and fuel consumption. To conclude these tests, several take-off runs were madewith full reheat. A reduction of 25 per cent of the normal length of take-off run was recorded. Measurements of rateof climb showed an increase of 46 per cent compared with the standard aircraft. Summarizing, it was found that the halo reheat systemwas far too sensitive to changes in gas flow in the exhaust duct and that its stability range was too narrow to cover thedesired range of altitudes and turbine speeds. It should be noted that the maximum altitude of 30,500ft could only beobtained by reducing the turbine speed. The effect of exhaust reheat on aircraft performance was quite marked. At 25,000ftan increase in forward speed of 67 m.p.h. was obtained, but again at a reduced turbine speed and ahigh reheat fuel consumption. The rate of climb and take-off distance were alsoimproved by a considerable amount. On the completion of this work it wasgenerally agreed that the faults in the halo system were of a fundamental natureand that little could be done to eliminate them without making extensive changesin the design. (To be concluded next week) THE MINISTRIES AT FARNBOROUGH "DEPRESENTING fulfilment of some of the less-secret acti--^*- vities of its research establishments, the Ministry of Supply's exhibits at the S.B.A.C. Show (Stand H) cover awide field of investigation. Thrust augmentation by jet-pipe re-heat is shown by a one-third-scale system. A second recent development is an exhi- bition of ground-launched rocket technique for the investiga-tion of flutter at high speeds; a liquid bi-fuel rocket is also on view. Radio items on show include a Type 1522 transmitter-receiverand communications equipment Type A.R.I. 533"2- Instru- mentation is represented by continuous-trace recording, atwelve-channel galvo-camera recorder and a six-channel A.C. bridge amplifier and power-unit. Also shown are a new methodof examining metallographic specimens subject to fatigue stress; lightweight plastic roller-bearings; and a high-energy ignition system. Safety developments depicted include mag-netic fire-detectors and the fuel-tank explosion suppressor now being further developed by the Graviner Company. A number of the Ministry's exhibits were displayed for thefirst time at the Paris Salon last May. In, of course, a different sphere of activity, the display bythe Ministry of Civil Aviation (Stands 72 and 73) is equally interesting/ The central feature of the exhibit is the DeccaNavigator system, the operation of the master and three slave stations being graphically shown; there are also two liveDecca aircraft panels which, in conjunction with an appro- priate Decca map of the Farnborough area, enable the visitorto determine his exact position in that area in the same way that an aircraft pilot or navigator would do. Other items on the two stands include various types of com-munication and D.F. equipment use^ in civil aircraft control.
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