Propulsion-airframe integration for low-boom supersonic aircraft

Abstract

Integration of the propulsion system into the airframe is a critical process of aircraft design, especially for aircraft operating in a supersonic flow regime. Literature shows that the inclusion of a propulsion system in the aircraft design process impacts aerodynamic performance, structural mechanics, and noise characteristics while targeted design requirements become more difficult to be satisfied. This is due to several reasons such as; the external nature of the propulsion system which is generally needed to be designed separately to satisfy engine requirements, the complex flow regime around the engine nacelle, and the sharp velocity and pressure changes required to keep operating an aircraft engine during any possible mission scenario. In order to advance the state of the art in aero-propulsive design optimization research topic; high fidelity, high accuracy computing infrastructures, and accurately scoped design spaces need to be incorporated. One approach is to achieve this objective with a separate design process while locally tailoring aerodynamic shapes of the supersonic inlet and nozzle, limiting design space for nacelle location with trade-offs, and finally, finding the optimal location of the propulsion system on the airframe with an automated design optimization considering sonic boom. In this thesis, a methodology for designing engine components aerodynamically, assembling them as a full-scale nacelle body and finally integrating this geometry into the airframe while considering the sonic boom characteristics of the aircraft, is investigated. First, a validation study is performed to observe the performance of the computational framework. The SU2 open-source multi-physics solver is used for computing the pressure field for the C25D benchmark model. Sonic boom calculations are performed with NASA Langley Research Center's sBOOM software and pressure change through longitudinal distance is compared with literature data. A sufficiently agreeable pressure signature is obtained by considering a disadvantageous calculation environment. A coarser mesh and Euler equations are used which are due to the limited computing power at the very beginning of the research. A small-scale trade-off study is also conducted during this initial phase to examine the effects of nacelle location on pressure signature propagation and noise generated by the aircraft. A representative engine nacelle without internal hollows is positioned in four different locations on the JAXA Wing Body benchmark model with boundary conditions of the C25D's engine. This comparative study concluded that the "shielding" effect of the fuselage is useful for low-boom designs where configurations near the aircraft bodies produced more favorable near-field and propagated pressure signatures. Two novel supersonic, two-dimensional, external-compression inlets are tailored to operate under critical conditions for the given mission requirements. The feasible aerodynamic shape of these air intakes is achieved by using the usual shock relations. These inlet configurations are distinctive by their number of "ramps" to finely tune oblique shock angles in order to concentrate supersonic shocks on the "cowl lip". This operative condition is called "the critical condition" for a supersonic engine inlet. While it is almost impossible to operate an engine inlet under critical conditions during every stage of the mission without a moving surface, the cruise stage of a mission is where this condition must be held for the lowest spillage and highest total pressure recovery. For this purpose, engine inlets are designed to operate under critical conditions for flow parameters that would correspond to the cruise condition of the aircraft. Benchmark geometries shared by NASA for sonic boom prediction workshops are used to determine these flow parameters. The method of characteristics is used to design a minimum-length convergent-divergent duct to be used as a nozzle for the propulsion system to achieve a stable plume shape. Throat and engine plenum areas are based on the provided benchmark model data. Preliminary performance prediction and validation of critical conditions for these inlets and nozzle are done with two-dimensional CFD computations before moving on and integrating full nacelle to the JWB airframe and computing a pressure signature. Two-ramp configuration operates slightly better than a three-ramp by starting under supercritical conditions while maintaining a better total pressure recovery. A 10 dB of difference is computed as an impact of the propulsion system on the perceived loudness level in total as compared to the lean airframe. However, the effect of inlet configuration on the noise level could not be observed as significant since the aircraft body shielded inlet shocks. Preparation to extend design space from rigid aerodynamics to aeroelasticity is started with the inspection of the SU2 code as a fluid-structure interaction driver as it includes a novel elasticity solver which can be coupled to flow solver internally. Examination of the ALE method and methodology of the coupling scheme used in SU2 provides an understanding of FSI methodology in general. The third chapter of the thesis is where previously obtained scientific background, computational experience, and propulsion system configuration are applied in different combinations to C25D and AeroMDO concept aircraft. The first section is reserved for the investigation of the effect of the engine on an aeroelastic aircraft where an in-house developed inner-wing structure at AeroMDO laboratory is used to compute aeroelastic response of C25D concept geometry with its own propulsion system. The maximum displacement value (on the trailing edge of the wing tip) increased 2.04 times due to the pressure changes on the wing for powered configuration. The currently designed wing structural model -which is not in the scope of this thesis- together with the integrated propulsion system, produced relatively low displacement values considering the flight conditions. However, considering that the deformation is doubled, it is important to propose a propulsion system integration in the design of the wing structural model for similar design processes. The second section includes a comparative study to experiment on the position of the nacelle on an aeroelastic aircraft geometry. Representative nacelles are located mid-wing and wing-root to evaluate: displacement of wing structure, nearfield pressure signature and ground-level pressure signature of the aircraft. Considering the engine weight and position, it is observed that the geometry integrated into the middle of the wing increases displacement in the region where it is located but reduces the maximum displacement value as the net force acting on the wing tip is reduced. Lower pressure variations are observed in wing root configuration as a result of the shocks produced by the nacelle located on the wing root being "shielded" by the fuselage. Therefore, wing root geometry is found to be advantageous in terms of noise as it produces lower pressure changes. Finally, an optimization study is conducted using an already optimized in-house low-boom concept airframe. The AeroMDO Lab concept developed a low boom airliner configuration under a TUBITAK-funded scientific project with grant number 218M471. Even though the main scope of this final section is to explain how the propulsion system is located on this airframe, the aerodynamic and structural design procedures of AeroMDO airliner are explained briefly to provide a foundation for engine location optimization study. Two parameters are used to optimize the position of the propulsion system. Namely, longitudinal and later distances of the nacelle. Since the computational burden of engine-activated CFD simulations is drastically high, 19 simulations result is detected as suitable to use while establishing a surrogate model. Then, optimization is performed with the sole purpose of minimizing the noise level produced by the propulsion-integrated airframe. As a result, the noise level of the aircraft is successfully reduced comparing the initial location of the nacelle. Also, the optimum geometry obtained as a result of this study is integrated with the optimum structural model of AeroMDO airliner, and an aeroelastic analysis including all components is performed.