A Vectoring Thrust Coaxial Rotor for Micro Air Vehicle: Modeling, Design and Analysis

The growing interest of rotary wing UAVs, for military and civilian applications, has encouraged designers to consider miniaturized configurations, more efficient in terms of endurance, payload capability and maneuverability. The purpose of this paper is to study a new configuration of coaxial rotor as applied to a micro aerial vehicle (MAV) with the intention to guarantee the vehicle maneuverability while removing unnecessary control surfaces which would increase wind gust sensitivity. Coaxial rotor configurations maximize the available rotor disk surface and allow for torque cancelation. Tilting rotors may allow for the vehicle control

On the positivity of fvs schemes

Over the last ten years, robustness of schemes has raised an increasing interest among the CFD community. One mathematical aspect of scheme robustness is the positivity preserving property. At high Mach numbers, solving the conservative Euler equations can lead to negative densities or internal energy. Some schemes such as the flux vector splitting (FVS) schemes are known to avoid this drawback. In this study, a general method is detailed to analyze the positivity of FVS schemes. As an application, three classical FVS schemes (Van Leer's, Hänel's variant and Steger and Warming's) are proved to be positively conservative under a CFL-like condition. Finally, it is proved that for any FVS scheme, there is an intrinsic incompatibility between the desirable property of positivity and the exact resolution of contact discontinuities

Hybrid micro air vehicle for complex environment missions

Modern urban reconnaissance missions dictate the need for a Micro Air Vehicle (MAV) platform capable of performing a complex mission : rapid and efficient ingress to a target location followed by slow loiter for quality image capture. This may be achieved using a tilt-body fixed-wing vehicle which combines the speed, range, and gust-hardiness of a fixed wing with the loiter and precision capability of a rotorcraft vehicle

A cure for the sonic point glitch

Among the various numerical schemes developed since the '80s for the computation of the compressible Euler equations, the vast majority produce in certain cases spurious pressure glitches at sonic points. This flaw is particularly visible in the computation of transonic expansions and leads to unphysical "expansion shocks" when the flow undergoes rapid change of direction. The analysis of this flaw is presented, based on a series of numerical experiments. For Flux-Vector Splitting methods, it is suggested that it is not the order of differentiability of the numerical flux which is crucial but the way the pressure at an interface is calculated. A new way of evaluating the pressure at the interface is proposed, based upon kinetic theory, and is applied to most current available algorithms including Flux Vector Splitting and Flux-Difference Splitting methods as well as recent hybrid schemes (AUSM, HUS)

The aerodynamics of micro air vehicles: technical challenges and scientific issues

Micro air vehicles raise numerous design problems associated to the size reduction: lower aerodynamic and propulsion efficiencies, higher sensitivity to atmospheric turbulence, low-density energy of electric propulsion, etc. The paper discusses some of the most important design issues and analyses the aerodynamic challenges encountered in the field of MAVs. A number of novel aerodynamic configurations combining rotors and fixed-wing are proposed and discussed in order to recover efficiency and maneuverability at low speeds

Positivity of flux vector splitting schemes

Over the last ten years, robustness of schemes has raised an increasing interest among the CFD community. One mathematical aspect of scheme robustness is the positivity preserving property. At high Mach numbers, solving the conservative Euler equations can lead to negative densities or internal energy. Some schemes such as the flux vector splitting (FVS) schemes are known to avoid this drawback. In this study, a general method is detailed to analyze the positivity of FVS schemes. As an application, three classical FVS schemes (Van Leer's, Hänel's variant, and Steger and Warming's) are proved to be positively conservative under a CFL-like condition. Finally, it is proved that for any FVS scheme, there is an intrinsic incompatibility between the desirable property of positivity and the exact resolution of contact discontinuities

A matrix stability analysis of the carbuncle phenomenon

The carbuncle phenomenon is a shock instability mechanism which ruins all efforts to compute grid-aligned shock waves using low-dissipative upwind schemes. The present study develops a stability analysis for two-dimensional steady shocks on structured meshes based on the matrix method. The numerical resolution of the corresponding eigenvalue problem confirms the typical odd–even form of the unstable mode and displays a Mach number threshold effect currently observed in computations. Furthermore, the present method indicates that the instability of steady shocks is not only governed by the upstream Mach number but also by the numerical shock structure. Finally, the source of the instability is localized in the upstream region, providing some clues to better understand and control the onset of the carbuncle

Shock wave instability and the carbuncle phenomenon: same intrinsic origin ?

The theoretical linear stability of a shock wave moving in an unlimited homogeneous environment has been widely studied during the last fifty years. Important results have been obtained by Dyakov (1954), Landau & Lifchitz (1959) and then by Swan & Fowles (1975) where the fluctuating quantities are written as normal modes. More recently, numerical studies on upwind finite difference schemes have shown some instabilities in the case of the motion of an inviscid perfect gas in a rectangular channel. The purpose of this paper is first to specify a mathematical formulation for the eigenmodes and to exhibit a new mode which was not found by the previous stability analysis of shock waves. Then, this mode is confirmed by numerical simulations which may lead to a new understanding of the so-called carbuncle phenomenon

Equilibrium transition study for a hybrid MAV

Wind tunnel testing was performed on a VTOL aircraft in order to characterize longitudinal flight behavior during an equilibrium transition between vertical and horizontal flight modes. Trim values for airspeed, pitch, motor speed and elevator position were determined. Data was collected by independently varying the trim parameters, and stability and control derivatives were identified as functions of the trim pitch angle. A linear fractional representation model was then proposed, along with several methods to improve longitudinal control of the aircraft

Design of Optimum Torsionally Flexible PropRotors for Tilt-Body MAVs

This paper presents a methodology to design the optimum proprotor for tilt-body microair-vehicles (TB-MAV) with efficient global propulsion system and long flight endurance in both cruise and hover modes. The TB-MAV developed at ISAE, which is called MAVion, was used as a baseline in the design process. To acquire maximum performance of TB-MAV’s global propulsion system, an efficient optimization process of the proprotor propulsion system was carried out. The optimization process consists of two-step inverse design methods. The first step determines the optimal operating conditions in terms of power and rotational speed of proprotor and the second step designs the optimal blade geometry in terms of twist angle distribution. The optimal blade twist distribution along the blade was computed using the Glauert’s strip theory for minimum energy loss condition. Meanwhile, the optimal operating conditions were determined by the motor outputs corresponding to high motor efficiency. A comparison of performance in terms of total efficiency and flight endurance between the optimized flexible proprotor, the optimized rigid proprotor, optimized propeller and optimized rotor is presented