Testing after Overhaul
TESTING AFTER OVERHAUL
An overhauled aero-engine must be submitted to and pass certain tests before it can be approved as satisfactory for a further period of service. The tests are divided into two groups, the first being an Endurance Test intended as a loading check of the engine, and as an indication of the power performance and the fuel and oil consumptions, and the second group, following the dismantling inspection and re-assembly of the engine, being a Final Test made to determine the performance of the engine prior to acceptance for service.
The tests are made, in accordance with the procedure specified in the appropriate Engine Test Schedule, with the engine either filled with a propeller or test fan, or coupled to a dynamometer or other approved means of absorbing the power output of the engine. As the engine type specification states the engine output in definite terms of brake horsepower, a newly manufactured engine receives its Final Test mounted on a dynamometer test bench or using an approved type of torque-meter. All Gipsy engines manufactured or overhauled by The de Havilland Engine Company are tested by the dynamometer test bench method, and operators who have similar dynamometer test equipment will be able to adopt the same procedure.
However, when testing an overhauled engine, it is not essential to measure the power output in terms of brake horse-power and, where dynamometer test equipment is not available or not practicable, all the requirements of the tests can be met by using a calibrated test fan, or a propeller adjusted to act as a test fan, and running the engine in an approved fan test cell.
When a propeller is used as a test fan the adequacy of engine cooling must be assured ; this may be done by fitting a larger airscoop with a variable air-inlet. As the propeller blade stresses may be more severe when operated under static engine running conditions in a test cell, a propeller, after being approved for use as a test fan, must not be used subsequently for flight purposes. Furthermore, the period of time that a flight propeller is retained in service as a test fan may also be restricted.
To ensure consistent test results by the fan test method the test cell and test equipment must be adequate for the purpose, and effects due to environ ment and climatic conditions must be accounted for in the observed results. The test fan must have been approved as being suitable for use for the particular type of engine, and be satisfactorily calibrated to establish its Standard r.p.m. on an especially calibrated engine in the. test cell in which the fan is to be used.
The calibrated test fan provides a convenient means of loading the engine during the test, and of assessing engine performance in terms of r.p.m. at specified running conditions, by comparison with the “Standard” r.p.m. of the fan. The acceptance of the fan tested overhauled engine is dependent upon the observed r.p.m. values obtained, after due correction for variation from standard atmospheric conditions, being within certain defined limits stated in the acceptance limitations of the Engine Test Schedule.
Separate test schedules are prepared for each mark of engine and where the test cell is situated at more than 1,000 ft. above sea level, individual test schedules are required for each locality. Therefore, operators contemplating overhauling Gipsy engines should write to the Service Department of The de Havilland Engine Company Limited at the address given on page ii at the beginning of this handbook, giving details of the engines they propose to test, the height of the proposed site above sea level, and requesting full details of the test cell equipment, test fan calibration and the procedure for testing overhauled engines.
