MODERN aircraft engines must be designed to give the maximum output with the minimum possible weight. To help attain this goal, it has been found desirable to have a more definite knowledge of the temperatures and stresses under which the engine parts operate.
To establish the operating temperatures of aluminum cylinder heads and pistons, the authors have developed a method that takes advantage of the fact that the Brinell hardness of the cylinder head and piston decreases with the number of hours of service and the temperature at which the part operates.
The stresses under which parts operate have also been studied by the authors, who used the brittle lacquer method, extensometers, and wire strain gages. Of these three, they found the brittle lacquer method the most satisfactory. Its ability to operate in constricted areas around the ribs
and interiors of the piston is a great asset.
Since most of the stresses in pistons are of a fatigue nature, pistons should be designed on the material's fatigue strength at the temperature of operation.
To study fatigue stresses, the authors use a hydraulically operated fatigue testing machine, which can produce up to 100,000 lb of load in either or both directions between the master piston and the bolster plate at 1300 cycles per min. The part to be tested is placed in the machine with its head against a cushion with an appropriate connecting rod.
Electric heaters, properly controlled, simulate operating temperatures of the piston. It is quite necessary to maintain the same temperature gradients throughout the piston to produce failures comparable to those occurring in service.


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