An experimental study of single-expansion-ramp-nozzle (SERN) models has been conducted to investigate how the nozzle flow was affected by the flap length variation at nozzle pressure ratios (NPR) from 4 to 60 under wind off condition.Wall static pressure distributions were obtained.A three-dimensional numerical simulation was also conducted to predict the performance which was unattainable in the test facility of four models with different flap lengths.The results show that the flow exhibits strong three-dimensionality and crossflow separation of the exhaust flow is apparent in the ramp side regions at high NPR.The peak value of the axial thrust coefficient (ATC) increases with the increase of the length of flap;moreover

the corresponding NPR becomes larger and approaches to design NPR.The ATC increases with increasing flap length at NPR between 30 and 80 and the gap between maximum and minimum reaches 1.8% at the design NPR

whereas the model with shorter flap has higher ATC at certain low NPR.The thrust vector angle increases along with the flap length increase

and the maximal and minimal values of the thrust vector reaches 6.03° and -1.83° at the design point respectively.