Howard F. Calvert, Gordon T. Smith
An investigation was conducted in a turbojet engine, mounted in a sea-level test stand, to determine the vibrational characteristics of four representative types of air-cooled turbine blades. Two of the types were standard-span (4 in.) blades for the engine used in this investigation, and the other two types were long-span (6.25 in.) blades for high mass-flow turbines. Static fatigue tests were also conducted with these blades. The blades were all brazed assemblies of cast and sheet-metal components. The vibratory stresses were measured with NASA high-temperature strain gages. In all four blade types, the first-bending-mode frequencies were increased from 2 to 3 percent by a coolant-flow rate of approximately 8 percent. With the standard-span blades, cooling had no noticeable effect on the maximum vibratory stress measured; however, with the long-span blades, the vibratory stresses were reduced at high speeds by introducing cooling air. Also, the long-span blades vibrated at all speeds and the vibratory stresses increased rapidly with speed. The results of the long-span-blade tests may have been influenced by the testing conditions at the tip regions of the blades.
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