An experimental investigation of forced surge by water injection in a twin-spoolturbojet engine has been performed on the ground thrust bed. It is demonstratedthat water injection into the main combustion chamber coupled with the flow area scale-down of both turbine nozzle and jet nozzle is satisfactory for the test. An optimum scale-down percentage of both areas was found from a large number of tests.During such combined forced surge process

the engine operating condition can approach the surging boundary accurately along the line of the corrected constantrotation speed. Hence

exact stable operating margin as well as stable operatingboundary for the compressor can be obtained.