Evaluation of the overall installation penalty of aero-engines for single aisle aircraft

Abstract

The past 70 years has shown an incredible growth in air transport. The increase in flights has resulted in an increased strain on the environment. Climate change is a fact. Designers have been working on improving the aircraft, and in particular, the engine efficiency. By improving the internal components through better materials, new design methods and new fabrication techniques, the engine specific fuel consumption has decreased. A major factor however is the engine bypass ratio. By increasing the bypass ratio the engine specific fuel consumption (SFC) can be drastically reduced. As a consequence the engine weight and size increased with subsequent ramifications.

Evaluating the installation penalty is not a new topic. In existing literature, studies on the various aerodynamic effects have been published. The largest contributor to the drag is the friction drag from the nacelle. Interference drag due to wing-pylon-nacelle flow interaction can be reduced with proper engine placement. The increase in drag due to a larger engine can also be minimised by the engine location and the shape of the nacelle. All studies show a change in drag when engine size or location are varied. Consequently all studies have been done using finite volume Reynolds Averaged Navier Stokes (RANS) solvers. Friction drag is relatively easy to predict, but if complex flows and separation areas are present, high fidelity tools are required.

An answer to the main research question: For narrow wing/body aircraft, can the installation penalties from an increase in bypass ratio outweigh the gain in specific fuel consumption? is found when a full aircraft redesign is allowed. Using the Initiator toolbox it was found that the lower fuel burn, due to a SFC improvement, results is a lighter aircraft for a given mission. The reduction in fuel burn negates the increase in engine weight, drag, and possible interference.

The stability analysis is done for an engine retro-fit scenario, like the 737 MAX. The method used requires several assumptions or estimations based on empirical relations. In order to improve the accuracy the vorticity solver FlightStream was used. It was not possible to validate the A320 design by using both FlightStream and the empirical methods, however the trends observed can be used to evaluate the effect on the c.g. margin and the required tail size. The c.g. margin decreased by max 7.1%, which would nullify a typical stability margin of 5%. The calculated maximum increase in tail size is 18%. Compared to the A320 tail size, it is only a 2% increase, and in most cases the tail becomes smaller. This can be explained because the A320 uses an oversized tail, originally designed for the smaller A318.