The stability analysis of an aerial vehicle is an of great importance integral part of its design procedure. It is of even greater importance in the case of tailless aircraft, which are prone to stability issues. In the present study, the aerodynamic and stability characteristics of a Vertical Take-off and Landing (VTOL) fixed wing Unmanned Aerial Vehicle (UAV), designated as MPU RX-4, are investigated. The MPU RX-4 has a flying wing layout and is capable of performing, both conventional flight, like a regular fixed wing aerial vehicle, as well as vertical hovering, like a multicopter, adapting on different operational demands and achieving rapid field deployment. In this study, the preliminary design phase of the MPU RX-4 is presented and the aerial vehicle’s aerodynamic performance, as well as, its stability and control behavior are assessed using both semi-empirical correlations, specifically modified for lightweight flying wing UAVs, and Computational Fluid Dynamics (CFD) analyses. These correlations are employed to estimate the non-dimensional aerodynamic coefficients for various flight conditions (e.g. cruise, loiter, maximum speed, etc.) of the MPU RX-4 flight envelope. Furthermore, the correlations are validated with dedicated CFD analyses in order to assure their level of accuracy. Finally, the MPU RX-4 stability and control derivatives, and the required control surfaces deflection for steady level flight are computed, in order to assess its overall aerodynamic performance and flight characteristics.

Unmanned aerial vehicles (UAVs) are at the forefront of this century’s technological shift, becoming ubiquitous in research and market areas. However, as a UAV navigates autonomously, there are unanticipated occasions, e.g., collisions with dynamic obstacles or loss of data provided by the global navigation satellite system, where the aircraft has to change its mission plans. In particular, to be protected from possible accidents, the platform’s geofence protection system should adjust the trajectory appropriately when obstacles are detected and select the proper ground surface when emergency landing is prompted. As these processes require fast reaction times, utilizing low latency sensors and algorithms is necessary. This article proposes a complete and low-complexity geofence protection system for recognizing moving objects and assessing the ground surface’s suitability using onboard sensing and processing modules. The proposed system is implemented on a novel fixed-wing UAV, designated as MPU RX-4, which features an unconventional flying wing layout and vertical take-off and landing (VTOL) capabilities. Our system is based on a forward-facing laser imaging, detection, and ranging (LIDAR) sensor and three downward-facing laser range-finders. We take advantage of the high-precision distance measurements and operational speed to identify moving obstacles using the LIDAR module, while the ground’s slope and the existence of any obstacle therein are computed through the range-finders. First, the article describes the UAV design procedure and its aerodynamic performance characteristics, which allowed us to evaluate our approach on a testbed aircraft. Then, the evaluation protocol shows that our system can perform robustly and under real-time constraints reaching an overall latency of only 165.5 ms, sufficient for reliable detection and avoidance of moving obstacles.

Flow control methods for aerodynamic drag reduction have been a field of interest to aircraft designers, who seek to minimize fuel consumption and increase the aircraft’s aerodynamic performance. Various flow control techniques, applied to aeronautical applications ranging from large airliners to small hand-launched unmanned aerial vehicles (UAVs), have been conceptualized, designed and tested in the past. Among others, the concept of riblets, inspired by the shark’s skin morphology, has been proposed and evaluated for airliners. In this work, the implementation of riblets on a medium-altitude long-endurance UAV (MALE) is investigated. The riblets can offer drag reduction due to the decrease in total skin friction, by altering the boundary layer characteristics in the near-wall region. The riblets are implemented on specific locations on the UAV (main wing, fuselage and empennage) and appropriately selected, on which the boundary layer becomes transitional from the laminar to the turbulent flow regime. For this reason, computational fluid dynamics modelling is performed by solving the Reynolds-averaged Navier–Stokes equations, incorporating the k-ω SST eddy viscosity turbulence model. The effect of the riblets in the near-wall region is modelled with the use of an appropriate wall boundary condition for the specific turbulence dissipation rate transport equation. It is shown that a drag reduction benefit, for both the loiter and the cruise flight segments of the UAV mission, can be obtained, and this is clearly presented by the drag polar diagrams of the air vehicle. Finally, the potential benefit to flight performance in terms of endurance and payload weight increase is also evaluated.

The current work investigates the effect of canard geometric characteristics on the performance of a lightweight flying wing Unmanned Aerial Vehicle (UAV), capable of both conventional and Vertical Take-Off and Landing (VTOL) flight. The canards are sized as horizontal stabilizers to enhance the UAV’s longitudinal stability and minimize trimming requirements during cruise. Using a Design of Experiments (DOE) approach and, specifically, the Taguchi method, six canards’ design parameters are investigated on three different levels. These parameters are the sweep angle (Λ), aspect ratio (AR), taper ratio (λ), vertical position in relation to the main wing (v_pos), incidence angle (i_c), and dihedral angle (Γ). An L₂₇ orthogonal array (OA) is used to investigate the influence of these key design parameters using two performance criteria, namely the Lift-to-Drag ratio (L/D) and the pitching moment coefficient (C_m), at cruise conditions (designated as L/D_cruise and |C_m|_cruise). The investigation is conducted by using high-fidelity Computational Fluid Dynamics (CFD) methods for each of the 27 configurations defined by the L₂₇ OA, over a range of angles of attack. Based on the CFD results, two distinct combinations are derived for maximum L/D_cruise and minimum |C_m|_cruise using the Signal-to-Noise ratio (SNR) analysis. The optimal design parameter combinations for the two performance criteria are A₂B₁C₁D₁E₁F₁ and A₂B₂C₁D₂E₂F₃, respectively. Finally, the Pareto Analysis of Variance (ANOVA) technique is conducted to define the contribution of each of the six design parameters on the L/D_cruise and the |C_m|_cruise. More specifically, i_c seems to have the most significant effect on L/D_cruise, whereas Λ is the most important parameter for |C_m|_cruise.

A novel control technique is investigated for hypersonic aerial vehicles. The technique is based on the use of active shock bumps (SBs) as a form of control device. The SBs deflect to create shockwaves on–demand, at specific locations around the aerial vehicle. As a result, a force is applied on the aerial vehicle, which in turn is used to provide the necessary moment for pitch and roll manoeuvres. In this work, a preliminary aerodynamic analysis of the SB device technique is made by means of CFD. For this purpose, and taking the large corresponding Reynolds numbers of the flow into consideration, the two–dimensional Euler equations are solved. A parametric investigation is carried out, by examining the effect of key parameters, namely the Mach number (M) and device deflection angle (δSB) on the produced force acting on the vehicle, serving as a proof of concept. Using a specific interpolation method, the resultant force is presented as a function of the Mach number and the device deflection angle, on three–dimensional charts, where the effect of each parameter is shown (force–Mach–deflection maps). Furthermore, a preliminary feasibility study is performed, including a kinematic analysis and some key material considerations. Additionally, a kinetic analysis is also conducted to secure the dynamic rigidity of the actuating mechanism and provide an initial estimation concerning weight and basic geometrical parameters of the SB mechanism components.