Airframe induced flow suppression effects on turbofan engines

Abstract

A wide variety of aerodynamic effects must be considered to achieve suitable integration of engine and airframe of transport aircraft. When turbofan engines are installed under the wing, the static pressures at the engine nozzles are often locally higher than that of the undisturbed environment. An increased static pressure downstream of an unchoked propelling nozzle leads to a reduction in nozzle pressure ratio. The nozzle mass flow rate and the nozzle gross thrust reduce as a consequence which is referred to as "flow suppression". The conventional thrust accounting method for flight test and also aerodynamic wind tunnel testing with Turbofan Propulsion Simulators (TPS) is based on a mass flow and momentum consideration and does not take the local static pressure at the nozzle of the installed engine explicitly into account. Consequently, the determined thrust is overestimated in most practical cases. This thrust accounting simplification was permissible for earlier engine generations with higher fan pressure ratios because only small errors incurred. However, the mass flow rate and thrust bias increases exponentially for newer designs with increasing bypass and decreasing fan pressure ratios. Aerodynamic data is corrected for nacelle external flow induced suppression effects dominating in low-speed conditions according to standard industry practice unlike wing induced flow suppression which dominates in low-speed conditions. Based on parametric studies, the impact of the back-pressure effect on the determination of mass flow rate and thrust as a function of aircraft design, high-lift configuration, operational and flight condition parameters is explained. A straight forward data reduction approach avoiding systematic errors due to nozzle backpressure effects based on the continuity equation is suggested. This approach can not only be employed for wind tunnel tests utilizing TPS and Through Flow Nacelles (TFN) but also for full scale in-flight thrust determination.