Unconventional aircraft designs have the potential to lower the impact of aviation on emissions and climate as compared to conventional aircraft designs. However, the flight dynamics behaviour of such unconventional configurations must be carefully evaluated by studying Stability and Control (S&C) characteristics to design safe aircraft and mitigate risks in flight. Various methods, that are a combination of numerical and experimental methods, have been used in the literature to predict the Stability and Control (S&C) characteristics. Sub-scale Flight Testing (SFT) is one such method that can predict aircraft flight behaviour, especially in the case of unconventional designs for which legacy information is unavailable and wind tunnel tests can partially predict aircraft dynamics. In order to successfully use SFT, the Sub-scale Model (SM) used in SFT must be carefully designed such that the results of SFT can be scaled-up to predict full-scale flight behavior. Furthermore, the SM should be able to complete the required SFT mission safely (the model is trimmable, statically stable and dynamically stable throughout the flight envelope). Finally, the SM must be designed with a short leadtime, as the time available for SFT in the overall design cycle is limited. Thus, the design of sub-scale models is a multidisciplinary task. In this thesis, an appropriate methodology is identified and developed to design the structural components of SM, position Commercial Off-The-Shelf (COTS) components and estimate the mass, inertia and the associated Center of Gravity (CG) of the SM. These are important inputs to determine the flight dynamics behaviour of the SM. Secondly, the structural analysis capabilities are automated to ensure that the structure does not fail in flight under critical load conditions. To shorten the design lead-time, methodologies developed in this thesis are formalized using a Knowledge Based Engineering (KBE) system. This KBE application automates the estimation of the weight & balance of a SM, which includes software modules for structure generation, flight equipment selection and positioning and the automated pre/post-processing task for Finite-Element (FE) analysis. Such a KBE application enables structural studies for different SM scale sizes, design variables such as rib pitch or frame pitch, and load cases. This KBE application to estimate the weight & balance properties of the SM can be coupled with other disciplines such as aerodynamic analysis, flight dynamics toolbox, etc. as part of a Multidisciplinary Design Analysis and Optimization (MDAO) workflow to quickly design sub-scale models that can be used to predict full-scale flight behaviour. 
Three different case studies are performed to demonstrate the effectiveness ofthe methodology and the KBE application. Each case study predicts the different aircraft configuration namely, a conventional Citation II and two unconventional models being the Prandtl-Plane and the Flying V. The methodologies can therefore be used for future SFT activities and can help in successfully comparing the subscale aircraft model behavior to the full-scale aircraft behavior, thereby making Subscale Flight Testing a step closer to reality.

The Flying V is a concept proposed and patented by Benad and Airbus Operations GmbH, which is a flying wing aircraft in the shape of the letter V. Multiple researches already showed promising results, with a 25% increased lift over drag ratio compared to conventional aircraft like the Airbus A350-900. Before research of sub-scale flight testing can be conducted, research into the flight characteristics is necessary. Based on the identification of the problems, the research question for the thesis research is defined as: "What are the flight characteristics of the Flying V sub-scale model at approach speed and high angles of attack?" In total, three different wind tunnel campaigns were conducted at the Open Jet Facility of the Delft University of Technology. During the wind tunnel campaigns, experiments were conducted to acquire data through balance measurements and flow visualisation techniques by means of oil, tufts ans smoke. The untrimmed maximum lift coefficient was estimated to be 1.09 at 41 degrees angle of attack and positive stability was found up to 20 degrees angle of attack. Flow visualisation results showed that large leading edge vortices acted on the surface of the wing, originating from the root towards the crank of the wing. A leading edge separated vortex spread over the wingtip, which originated at the kink of the leading edge. Investigations into the trimmed flight of the aircraft concluded that due to trim limitations the centre of gravity is bound between 1.345 and 1.425 meter behind the nose. The centre of gravity has an optimal location at 1.365 meter behind the nose, providing a static margin of about 9%. With the trimmed lift curve and the optimal centre of gravity location, the flight speed of 35 meter per second can be flown at 3.6 degrees angle of attack. Due to limitation in maximum deflection of the control surfaces, the maximum lift coefficient is estimated at 0.95, at 28.5 degrees angle of attack at a stall speed of 14.8 meter per second. At 20 degrees angle of attack, where the static stability switches from positive to negative, the lift coefficient is estimated at 0.73 at 17.2 meter per second and is defined as the 'safe' stall speed. Taking the FAA and ICAO regulations into account, the approach speed is estimated at 19.2 meter per second, at an angle of attack of 15.9 degrees at a lift coefficient of 0.58.

Despite widespread research into the possibilities of improving aerodynamic efficiency, a plateau seems to have been reached for the conventional configuration. Hence, the potential of unconventional configurations are being investigated in recent years. Flying V is one such configuration that promises a lift to drag ratio about 24 in cruise conditions, an improvement of 25% with respect to the NASA Common Research Model that was used as conventional configuration benchmark. In addition, the flight dynamic characteristics of such an aircraft must be investigated to ensure flight safety. Sub-scale flight testing (SSFT) allows the characterization of flight dynamics using sub-scaled models. In order to mitigate the risk of loss of control situations, the static stability and control characteristics of the model must be investigated. This research work aims to support SSFT by designing a sub-scale model and assessing its aerodynamic characteristics by wind tunnel testing. The sub-scaled design is representative of a 4.6% geometrically scaled model of full-scale Flying V design based on Froude scaling laws.

To test the aerodynamics of the future flying model, wind tunnel testing have been conducted. Balance measurements of 4.6% scaled half-model have been collected in an open jet wind-tunnel. A total of three control surfaces are installed on the model and the two inboard have been prescribed, during the design phase, to provide pitching moment control authority. The shift in aerodynamic center at higher angles of attack has been registered and large deflections of the control surfaces have been noticed to influence the shift of the aerodynamic center up to 6% of the mean aerodynamic chord.

Aside of the experimental investigations, RANS simulations have been performed using the Spalart-Allmaras one equation turbulence model. Although discrepancies have been identified between the wind tunnel and numerical results, especially in terms of drag and pitching moment coefficient, the CFD results are used to get a better understanding of the influence of vortical flows on the genesis of lift and drag over the scaled model. Based on the performed CFD simulations, it can be concluded that the performed CFD simulations are insufficient to reproduce the pitching moment behavior recorded during wind tunnel testing.

The designed sub-scaled model proved to be able to substain flight loads up to more than 2.5g at MTOM conditions. Based on the performed analyses, the center of gravity is suggested to be located between 1.33 and 1.39 meters behind the nose of the configuration. The deployed control surfaces can trim the aircraft up maximum lift coefficients between 0.6 and 0.7, depending on the location of the center of gravity, with an ultimate static stability margin equal to -4.4%. The results highlight that a reduction in pitching moment control authority would cause a reductions up to 20 % on the maximum lift coefficient achievable in trimmed conditions due to lack of control authority for forward location of the center of gravity. The designed model can therefore be used for future SSFT activities and landing speeds are estimated to be lower than 20 m/s for the proposed range of center of gravity locations at MTOM conditions.

In early design stages of aircraft, component sizing is to a large extent based on semi empirical methods, which are generally lacking for unconventional aircraft configurations. This increases the uncertainty of the performances of the generated design, such as its stability and controllability. This work aims at using a more physics-based approach for predicting stability and control derivatives estimation during early design stages. A 3D panel method is chosen, which predicts the stability and control derivatives 32% more accurate than a vortex lattice solver. An acceptable run time is achieved by creating a new version of an in-house Knowledge Based Engineering tool, which reduced pre-processing time by 96% compared to manual use of the analysis tool. This methodology is then used to predict the stability and control derivatives of two box wing aircraft, and one conventional aircraft, which have been designed for similar requirements.