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Aviation History
1946
1946 - 0625.PDF
MARCH 28TH, 1946 FLIGHT running of jet engines wear ear defenders for the protec-tion of the ear drums from the high-frequency vibrations which are emitted during power running. When carrying out ground runs it is most importantthat the compressor intake or intake duct is facing into wind. This is essential for three reasons: first to ensurethat the exhaust gases do not recirculate to the air intake, which would have the detrimentaleffect of raising the jet pipe temperature (par- ticularly emphasised at idling r.p.m.), thelatter being dependent on the square of the in- take temperature change; secondly, cooling ofthe engine is assisted ; and thirdly, on shutting- down, the ram effect from the ambient air hasthe helpful result of purging the engine. Observation should also be made to ensurethat the exhaust gas stream has a clear path after emission. No area or objects subject toheat effect or deterioration should be in the path of the jet stream. In preparing a jet engine for storage, thenormal protective action is taken by inhibition spraying of the engine. It is unnecessary tocarry out any special precautions other than the normal blanking, etc. Corrosive reactionis not present, since leaded fuels are not used. If practicable, it is advisable periodically torotate the rotor to preserve the oil film on the moving components, though this is not considered abso-lutely essential. Operational Faults and Rectifications Due to the inherent simplicity of the simple gas turbine, it has been stated that little trouble is normally experienced concerning the servicing and operation com- pared with a modern reciprocating power plant. What troubles are experienced can be traced and analysed with- out much difficulty. The troubles encountered may be separated into four main categories: — (a) Failure to start. (b) Rough running. (c) Erratic operation. (d) Incorrect jet pipe temperature indication. Failure to Start.—When a gas turbine fails to start there is little object in continuing the attempt. Actually it may prove disastrous by possibly producing an excessively hot start, or alternatively aggravating existing mechanical failure. The cause will invariably be traced to one ot the following troubles: — (1) No Ignition.—Elimination of the ignitor plugs, booster coils, electrical starter circuit and accumulator state will define this cause. (2) Insufficient Fuel Pressure to the Burners.—Low fuel pressures may be due to one or more of the following causes: (a) A faulty fuel pump; (b) excessive fuel leaks; (c) an excess A.C.U. relief (if fitted); (d) failure for the pump valve or fuel accu- mulator to operate.; (e) insufficient r.p.m. to produce requisite fuel pressure and mass flow; (f) slow-running adjustment of the throttle, by-pass set at too low a value; (g) air in the fuel system. Items (b) (d) and (e) will be indicated visibly, while (a), (c), (f) and (g) can be amined independently. (3) Insufficient r.p.m.-—This usually in- dicates a low voltage of the accumulator. It may also be caused by the starter motor not receiving full electrical power due to the energising solenoid resistance not break- ing contact. Alternatively, the partial seizure of the rotor or an auxiliary drive may be the cause. (The r.p.m. indicator should first of all be checked for correct reading before sus- pecting any of the above faults unless they are obviously apparent.) (4) Mechanical Failure.—It is seldom that the rotor (combining the compressor rotor or impellor and turbine) gives rise to any mechanical failure. It will be appreciated that this is more apt to happen when running under " power" conditions. This failure, however, is a very rare occurrence. Secondary mechanical failures include the fracture of auxiliary drives and gear trains affecting components such as the H.P. fuel pump, BAROSTAT FUEL PUMP LUBRCATING OIL — FILTER PENDULUM CONTROLLING FEED DURING AEROBATICS DRIVE TO ROTOL AUXILIARIES GEAR BOX FUEL FILTER' FUEL ACCUMULATOR. H.P COCK C DUMP VW.VE UNIT Accumulator, trip valve andpressure cock. 1, Connection to pump inlet ; 2, Connections to manifold ; 3, Relief valve ; 4, High-pressure cock ; 5, Trip valve ; 6, Spill to atmosphere Fuel injection and control components grouped on the front of the Rolls-Royce Derwent compressor. starter motor, oil pressure and scavenge pumps and any auxiliarycomponents. Rough Running.—As a gas turbine produces its powerby rotation of the major components there is very little vibration compared with the reciprocating I.C. enginewhere high opposed inertia forces are present. There are in certain engines, however, the characteristic criticalperiods operating through certain ranges of r.p.m. which it is advisable to avoid in view of the resonance set up,which eventually may lead to an impellor or turbine blade failure in the form of fatigue. What little vibration doesexist in a gas turbine is of very high frequency, around the order of r.p.m. cycles per second. If there are anysigns of unusual roughness, this may possibly be due to the impellor and turbine rotor combination being out ofdynamic balance, probably due to a partial failure of an impellor or turbine blade. Boththese instances are of a serious character and necessitate a com-plete strip of the engine. Defec- tive rotor bearings or auxiliarydrive failures may also promote rough running. Erratic Operation.—Erratic run-ning will be attributed in practic- ally every case to the fuel system,and in such cases it is customary to change either the throttle valve or theA.C.U. (if fitted). When the fuel sys- tem is incorporated with an "all-speed " governor, this unit will be sus- pect. In certain cases, erratic run-ning may be due to one or more burners in the combustion system beingchoked, thus causing bad distribution : this fault is usually indicated by view-ing up the propelling nozzle from a dis- tance while the engine is running andobserving any local hot-spot areas which may be apparent around theannulus of the turbine blades. high-
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