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Aviation History
1958
1958 - 0078.PDF
80 11 12 «• TJJ and TS8 • • . 10 configuration that accomplished the 50-hr test in August of 1956. Thefirst shipment engine was returned to the factory for overhaul after having accumulated 135 hours of testing. We were pleased with this perform-ance of our first delivered engine, particularly in view of jet experience where initial engines are returned with substantially fewer hours. Since this 50-hr engine had already demonstrated production specifi-cation requirements for horsepower, fuel consumption and engine weight, the outstanding requirement for 150-hr acceptance was to obtain dura-bility of engine parts and improved power control stability margins. I would like to discuss a few of these improvements in some detail. After accumulating 2,500 hours of engine testing using 12 compressorassemblies, some with over 300 hours operating time, an interesting design problem was encountered with some of the rear-stage com-pressor rotor blades. After this extended amount of testing, the rear stages in several compressors began to exhibit a fatigue condition in thedovetail. On the three occasions in which blades cracked, only one of the failures resulted in complete separation of the blade and dovetail. Theengine data sheets disclosed that the engine operated for eight hours on military specification test schedule after the failure occurred, and beforethe damage was suspected. A change in compressor efficiency was noted when the data was processed by the analytical group—the personneloperating the engine were not aware of any power change or mechanical malfunction at the time. The stage of blades was removed and a newset installed without removing the engine from the test stand. The test was back on schedule within 24 hours, and an additional 130 hours ofcycle testing was accumulated without further difficulty. The cause of the fatigue problems was determined by strain-gaugingthe rotor blades. Investigation showed the fatigue was due to excitation in the first torsional mode at compressor speeds corresponding to thenormal rated power condition. In every development manufacturing programme, the shop personnelmust learn how to make the new parts which, in many cases, are signifi- cantly different from similar parts previously designed for older engines.Initially, the parts may vary substantially from the dimensions and toler- ances established by the design engineers. Our attention was initiallyfocused on the improvements being made in rotor-blade dimensions when it became apparent that the test engines were delivering 50-100 h.p.more than was obtained when using compressor blades manufactured earlier in the programme. The shop improvements in obtaining designlimits for thickness and contour changed the mass and vibration charac- teristics of the blades. When these blades were excited by a pulsatingload such that it twisted torsionally, we discovered the dovetail would break when the natural frequency was obtained. Anyone familiar with small high-speed compressors can appreciatethe problem in developing suitable blade strain-gauge instrumentation. At the start of the T58(programme, an attempt was made to strain-gaugethe rotor blades using an adaptation of a slip-ring assembly developed and built in Germany. However, because the assembly was not designedto operate at the 27,000 r.p.m. speed of the T58, and with corresponding lubrication and vibration characteristics not compatible with this slip-ring design, it was impossible to get the micro-volt signals from the gauges to the oscillograph recorders. Since no blades were breaking,testing was continued and the instrumentation designers went to work on a slip-ring designed specifically for small-engine applications. For-tunately, they came up with a design that worked and was available at the time the fatigue phenomena occurred. A potentially important feature of the compressor has been demon- TS3 (above) TS8 (opposite) 1 Drive shaft and ball bearing 2 Roller bearing 3 Planetary reduction gear 4 Heated intake casting 5 Main ball bearing on independent power shaft 6 Accessory drive bevel 7 Compressor front bearing > Fixed inlet guide vanes 9 Five-stage axial compressor 10 Centrifugal compressor It Diffuser air path 12 Powerplant mountings 11 Secondary cascade 14 Fuel injector 15 Walking-stick vaporizing burner 14 Reverse-flow chamber 17 Compressor turbine (overhung) 18 Power turbine 19 Compressor-shaft rear bearing 20 Power-shaft rear bearing 1 Starter drive clutch 2 Heated intake casting 3 Scub-thaft bearing and accessory drive bevel A Accessory drive 5 Compressor front shaft and roller bearing * Variable stators 7 Ball thrust bearing 8 Annular chamber * Dual fuel gallery 10 Two-stage compressor turbine 11 Rear roller bearing 12 Power turbine 13 Output roller bearing 14 Main ball bearing 15 Bifurcated exhaust duct It Exhaust duct 17 Inner wall of duct 18 Duct bracing strut 19 Idler pinion 20 Final drive shaft
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