The integration of the engine with the airframe is investigated in the German-Dutch Wind tunnels (DNW) using special scale engines called Turbofan Powered Simulators (TPS). The bookkeeping of thrust and drag must be clear. The TPS thrust is determined under static conditions in a calibration facility, and then subtracted in the wind tunnel test from the balance force of the aircraft model (airframe + TPS) to obtain the airframe drag including jet interference drag. A critical assumption valid for traditional turbofan engines is that the external flow does not affect the statically calibrated thrust. However, with the rise of more efficient engines with higher bypass ratio this may not be the case due to the close coupling between the engine flow and the wing. The objective of this Thesis is to identify the current limitations of the testing procedure as well as to produce scientific basis to deal with these limitations. This is achieved by means of a consistent thrust/drag bookkeeping combining numerical methods and a modified experimental setup. External flow effects are identified by means of the analysis of theoretical models, the comparison with testing procedures undertaken in similar facilities, the review of standard bookkeeping techniques of full-scale turbofan engines and the analysis of previous test data. The change in thrust is quantified using a mathematical model integrated in an error propagation study by means of Monte Carlo simulations. The influence of the external flow and wing pressure field is further studied through a numerical analysis in RANS-SST for a very high bypass ratio TPS unit, and a Through-Flow Nacelle respectively. The latter configuration is also tested in the Low Speed Tunnel in DNW to investigate which instrumentation can be used to detect external flow effects in the future. In this case the velocity in the fan exhaust plane is measured with a hot wire and static pressure sensors are placed in the intake, exhaust and boattail. The theoretical, numerical and experimental approach show that the external flow and wing pressure field change the conditions in the TPS exhaust with respect to static calibration. In the wind tunnel, the nozzle exhaust shear layer decreases in size as the difference in velocities between plume and free air is decreased, reducing the flow spreading rate and increasing the local pressure at the nozzle exit plane. The local Nozzle Pressure Ratio is reduced. This leads to flow suppression, the reduction of the fan mass flow and exhaust velocity. In addition, the scrubbing and boattail drag, currently bookkept as loss of thrust in the modified standard net thrust, are changed from static to wind tunnel conditions. These effects change the TPS thrust leading towards an improperly bookkeeping of the aircraft installation drag. The bias error produced by external flow effects is one order of magnitude higher than the random instrumentation error and should be corrected for, especially at low power settings. Differences decrease proportionally to the Fan Nozzle Pressure Ratio until chocked conditions are reached, where the freestream velocity has no influence in the TPS performance. A possible solution lies in the advanced derivation of thrust and drag. The current approach neglects the thrust contribution from the nozzle exhaust to infinite downstream. According to the definition of the Jones thrust, a better solution lies in the assumption that the flow is expanded from the exhaust to infinity downstream without any transfer of energy of momentum. The decrease in mass flow and velocity can be effectively captured by pressure taps located at the intake or fan plane. A linear correlation exists between both stations, that can be used for the bookkeeping of the TPS thrust in the wind tunnel according to additional calibration in the wind tunnel. The new bookkeeping method can also be used to correct for the decrease in local jet exhaust Mach number from design conditions, the parameter of interest in engine/airframe integration tests. The research presents and solves the limitations of the new generation of turbofan engines by accounting for the local conditions at the TPS exhaust due to external flow effects. The new thrust/drag bookkeeping method leads to optimized configurations by improving the accuracy of engine/airframe integration tests.