Unconventional Propulsive Empennage - Future or Fiction?

Abstract

Flight testing is an activity that exists since the beginning of flight, and is still seen as an essential part of the aircraft development process. While numerical simulation has been improved significantly over the last decade, flight testing remains attractive in the exploration and evaluation of the flight envelope, especially in the region where numerical simulation lacks prediction accuracy. Free-flying model testing is a potential technique, which is recognized by some of the largest research institutes. Using such relatively cheap scale models aids in the determination of the flight behavior of full-scale aircraft in an early stage in the design process. Furthermore it takes less time to develop a scalemodel and is also safer to fly. A 5.5% scale model of the Boeing 737-700 is created by the TU Delft as a first step towards investigating the potential of dynamically scaled flight testing. This Delft University Unconventional Concept (DUUC) has a conventional tube-wing configuration and features ducted fan engines mounted aft on the fuselage, which replace the conventional empennage. A horizontal and vertical jet vane is placed in the duct, which guide the propeller slipstream and thereby control the attitude of the aircraft. Successful flights have already been performed with this model, which proves the propulsive empennage concept. However, the stability and control behavior has only been assessed at a low level. Flow characteristics entailed by the ducted fan are accounted for by means of approximate functions and empirical relations instead of looking at the physical interaction between the components within the duct. This study focusses on the processing of said interaction by performing a CFD study of which the results are used in the longitudinal equations of motion of the DUUC. This way, certain motion can be simulated from which the effect of the unconventional empennage on the stability and control can be assessed. In order to study the aerodynamics of the propulsive empennage and subsequently its stability, a CADmodel is developed using PARAPY. This is a knowledge based engineering platform written in the Python language and allows for parametrization of the model such that geometric alterations can easily be implemented. Previous studies within the TU Delft have lead to a multi-model generator written in PARAPY. The fuselage and wing generation modules that were developed can readily be applied in the generation of the DUUC model. The propulsive empennage module is created by the author of this thesis, consisting of several sub-parts: (1) outer duct that forms a shielding around the propeller, (2) the pylon that connects the duct to the fuselage, (3) a center body to which the (4) propeller and (5) control vanes are attached. The full aircraft model has been used in a series of CFD simulations with ANSYS FLUENT. Inviscid steady calculations are performed using a pressure-based solver with a SIMPLE pressure-velocity coupling scheme. The propeller is modeled as an actuator disk whereby the thrust is defined by specifying a pressure jump across the thin surface. The thrust produced by the ducted fan is based on the turboprop installed on the ATR72-600 aircraft, which has similar dimensions compared to the Boeing 737-700. Both the full-scaleDUUC as well as the 5.5% scale model are subjected to this aerodynamic analysis. Minor discrepancies were found between the two models, which is a result of the artificial viscosity that is implemented in the Euler scheme to smoothen strong gradients in the solution (numerical dissipation). Due to the location of the engine, the thrust line is located above the center of gravity which contributes to a stabilizing effect. The effect of several parameters on the stability and control has been analysed, such as the center of gravity location, mass and inertia, and geometric scaling factor. The slight difference in aerodynamic performance between the full-scale and subscalemodels, means that the trimmed solution for steady horizontal flight is also slightly different with a maximum error of 2.5 degrees in elevator deflection. More drag is obtained as the aircraft scale goes down, resulting in a higher required thrust to achieve force balance in a trimmed flight condition. Furthermore it is found that by scaling the aircraft down according to Froude scaling theory, the response becomes much faster. This response can in turn be scaled back, which proved to be a very useful estimate of the full-scale simulation.