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International audience; In recent years, unmanned aerial vehicles applications have grown in various fields. New design configurations have emerged to address these needs. This paper focuses on the multidisciplinary design optimization of three popular concepts of drones under a common framework: multicopters, fixed-wings, and the more recent fixed-wing VTOLs. Low-fidelity but computationally efficient models for drone sizing are proposed, some of them being shared by the different concepts. The models are integrated with a design optimization approach, and an efficient sizing methodology allows for reducing the problem complexity. This approach is instrumental because of the large number of design variables and constraints involved in the multidisciplinary design process. Examples of sizing results are provided to validate the methodology for fixed wing UAVs. In addition, a study case demonstrates the benefit of a common framework for comparing the performances of different drone concepts. A key finding is that the VTOL propulsion of the fixed-wing VTOLs must be carefully optimized to avoid an undesirable mass penalty.

Two separate experiments are conducted on a three bladed NACA0012 rotor operating at low Reynolds numbers using phase-locked infrared thermography coupled with simultaneous force and torque measurements. The first, focuses on the effects of freestream turbulence on boundary layer transition over the suction side of the aerofoil of the rotor in an advancing configuration. Freestream turbulence (FST) was generated in an open section wind tunnel using grids and was characterized using Hot-Wire anemometry. In general, when the rotor was subjected to FST, an increase in thrust and efficiency was observed, which could be due to the FST suppressing flow separation or by inducing early transition. The second experiment, consisted of a parametric study on the impact of forcing boundary layer transition using roughness placed on the suction side of the aerofoil, in a hover configuration. The height of the roughness was varied from 52-220μm and was placed at all at at 10% chord, over the entire span of the blade. Force and torque measurements revealed that there could be a optimal roughness height that could lead to a performance increase.

International audience; The number of applications that use multirotor drones has significantly increased in the past year due to their high manoeuvrability and capability to perform hovering. The main limit of such configuration is its endurance especially when batteries are used as the energy source. This paper focuses on the evaluation of the endurance of multirotor drones in forward flight using a design optimization approach. Lightweight models to represent accuratly the physics of forward flight are proposed. One significant finding is that the design optimization code enables to rapidly obtain a design for a given set of requirements and has been validated on the Parrot ANAFI USA. The end of the article illustrates the sensitivity of maximum takeoff weight to the drag of aerodynamic frame.

The present work outlines efforts at ISAE-SUPAERO to reduce the acoustic footprint of rotors operating at low Reynolds and low Mach numbers typical of small and medium scale drones. An experimental campaign that aims at characterizing rotor noise is presented. The latter serves as a basis for validation of numerical approaches and to investigate the noise mechanisms. Data are obtained in an anechoic room with dimensions (wedge tip to wedge tip) 5.02x5.24x5.34 m3 and cut-off frequency 80 Hz. Aerodynamic loads as well as acoustic fields are recorded for two off-the-shelf (APC 9x6 SF and 11x4.7 SF) and four canonical rotors with different numbers of blades. Both near and far acoustic fields are measured using one microphone located one diameter away from the rotor plane and 13 microphones mounted on a directivity antenna. A range of rotation speed from 1,000 to 10,000 RPM is addressed, corresponding to Mach and Reynolds numbers up to 0.23 and 130,000 respectively. Results presented in this article are related to two-bladed rotors and are validated using existing experimental data from the literature. They are then used to assess in-house numerical simulations (NL-VLM and CFD) and unravel the physics behind rotor noise at low Reynolds and low Mach numbers. This fundamental understanding will help draw general guidelines for the design of low noise rotors.

International audience; This paper presents a numerical and experimental investigation of the aerodynamic characteristics of a vertical take-off and landing micro air vehicle designed to fly in the Martian atmosphere. Numerical tools are validated through experimental data from the literature where Martian conditions are replicated. An innovative airfoil is designed for the specific flying conditions of the Martian MAV, i.e. compressible and ultra-low Reynolds number flows. This airfoil is evaluated in different Reynolds and Mach number conditions using unsteady compressible laminar Navier-Stokes simulations. Single rotors and coaxial configurations are designed for different thrust constraints by optimizing the chord and twist distributions using a free wake lifting line method. Rotors are firstly evaluated numerically using Navier-Stokes simulations. To validate the simulations, the optimized configurations are tested experimentally in an evacuation chamber, recreating Martian atmosphere in terms of density and gas. Thrust and torque on each rotor are measured by a two-component set-up. Tests are performed up to transonic flow conditions at blade tip. Results are extrapolated to a suggested design for a Martian MAV.

International audience; An experimental investigation on the flow topology and performance of a rotor operating at low Reynolds numbers is presented. The feasibility of laminar to turbulent transition experiments over small rotors is demonstrated. Phase-locked infrared thermography coupled with simultaneous force and torque measurements were used to study a three bladed NACA0012 rotor with a radius of 0.125 m and an angle of attack of 10 degrees. Boundary layer transition was fostered using two-dimensional (2D) and threedimensional (3D) isolated roughness elements, placed at approximately 5% and 28% of the rotor blades chord. In the smooth rotor configuration, a 3D flow topology is observed, consisting of a clear laminar region closer to the blade root and a turbulent region at the blade tip. It was found that the state of the boundary layer can significantly affect the rotor’s performance, with the forcing of laminar to turbulent transition generally resulting in a loss of performance when compared to the smooth reference rotor case.

International audience; This paper presents a numerical and experimental study of the aerodynamic characteristics of a vertical takeoff and landing (VTOL) micro air vehicle (MAV) designed to fly in Mars' atmosphere. Numerical tools are validated through experimental data from the literature where Martian conditions are replicated. Innovative airfoils are designed for the specific flying conditions of the Martian MAV, i.e. compressible and ultra-low Reynolds number flows. An optimized airfoil is evaluated in different Reynolds and Mach number conditions using unsteady laminar Navier-Stokes simulations. Rotors are designed by optimizing the chord and twist distributions using a free wake lifting line method with different geometrical and lift constraints. Rotors are numerically evaluated using two different Navier-Stokes solvers. A coaxial configuration is also defined to achieve zero total torque and double lift. To validate the simulations, experiments are performed in the ONERA's low pressure tank, recreating Martian atmosphere in terms of density and gas. Thrust and torque are measured by a two-component setup developed by ISAE-SUPAERO, with independent measures on each rotor in coaxial configuration. Tests are performed up to transonic flow conditions at blade tip.

International audience

International audience; In the advent of electrical vertical take-off and landing aircraft development, a fast approach to predict variationof propeller axial and off-axis aerodynamic loads at large incidence angle has been desired. This paper presentedan analytical approach obtained by a simplified blade element method considering local blade section conditions.The theory has been further validated against available experiments for propeller at high incidence conditions, and was found applicable to a wide range of geometries and operating conditions.

International audience; A detailed experimental study of propeller-wing interaction was presented. Five-hole probe was used to obtain velocity distribution at a survey plane behind trailing edge of a high-aspect-ratio wing with tractor propeller. Significant deformation of propeller slipstream was observed compared to a free propeller. The deformation was more prominent at low advance ratio, where transition flight of vertical take-off / landing aircraft were concerned. Comparison with two recent reduced-order slipstream models revealed large discrepancies between theoretical predictions and wake survey results. An analytical model of slipstream transverse deformation was proposed at the end that might be incorporated into improve such models.

International audience

International audience; One of the major challenges of Mini-UAV flight is unsteady interaction with turbulent environment while flying in lower levels of atmospheric boundary layer. Following inspiration from nature we expose a new system for angle of attack estimation based on pressure measurements on the wing. Such an equipment can be used for real-time estimation of the angle of attack during flight or even further building of wind velocity vector with additional equipment. Those information can find purpose in control and stabilization of the aircraft due to inequalities seen by the wing or even for various soaring strategies that rely on active control for energy extraction. In that purpose flying wing UAV has been used with totally four span-wise locations for local angle of attack estimation. In-flight angle of attack estimation of differential pressure measurements have been compared with magnetic sensor with wind vane. Difference in local angle of attack at four span-wise locations has confirmed spatial variation of turbulence. Moreover, theoretical energy dissipation of wind fluctuations described by Kaimal spectrum has shown acceptable match with measured ones.

Micro-Air Vehicles (MAV) are becoming common devices in a wide range of operations while the optimization of their propulsion system is rarely addressed. On the one hand, an aerodynamic optimization would have a straightforward effect on the endurance. On the other hand, an aeroacoustic optimization might increase discretion in military operating conditions, reduce noise pollution in civilian, urban environment and allow sound recordings in dual applications. This contribution aims at presenting a complete methodology for the design of silent and still efficient rotors for MAV, from aerodynamic prediction to aeroacoustic optimization and experimental validation. This approach is suitable for engineering purposes. The aerodynamic and acoustic modeling are described and the optimization procedure is presented. A step-by-step optimization is achieved and measured on an experimental bench suitable for non-anechoic environment. A discussion on the results is proposed. Key parameters on the blade geometry for the reduction of rotor noise are provided at the end of the paper.

The IMAV 2017 conference has been held at ISAE-SUPAERO, Toulouse, France from Sept. 18 to Sept. 21, 2017. More than 250 participants coming from 30 different countries worldwide have presented their latest research activities in the field of drones. 38 papers have been presented during the conference including various topics such as Aerodynamics, Aeroacoustics, Propulsion, Autopilots, Sensors, Communication systems, Mission planning techniques, Artificial Intelligence, Human-machine cooperation as applied to drones.

International audience; The demand in Micro-Air Vehicles (MAV) is increasing as well as their potential missions. Either for discretion in military operations or noise pollution in civilian use, noise reduction of MAV is a goal to achieve. Aeroacoustic research has long been focusing on full scale rotorcrafts. At MAV scales however, the hierarchization of the numerous sources of noise is not straightforward, as a consequence of the relatively low Reynolds number that ranges typically from 5,000 to 100,000. This knowledge however, is crucial for aeroacoustic optimization. This contribution briefly describes a low-cost, numerical methodology to achieve noise reduction by optimization of MAV rotor blade geometry. That methodology is applied to reduce noise from a MAV developped at ISAE-Supaero and a 8 dB(A) reduction on the acoustic power is found experimentally. The innovative rotor blade geometry allowing this noise reduction is then analyzed in detail using high-fidelity numerical approaches such as Unsteady Reynolds Averaged Navier-Stokes (URANS) simulation and Very Large Eddy Simulation using Lattice Boltzmann Method (VLES-LBM). That strategy gives insight into the flow features around the optimized rotor and guidelines for the acoustic models used in a low-cost numerical optimization loop.

The paper proposes an overview of the physical models developed/selected and implemented in the ONERA aerothermodynamic (ATD) engineering code FAST. This tool is used to quickly determine the pressure and heat flux surface distribution at the wall, as well as aerodynamic forces and moments coefficients in hypersonic regime for free-molecular, transitional and continuum flows, for realistic designs of space vehicles ranging from capsules to spacecrafts and for generic shapes of orbital debris as well. An original feature of the approach is that geometrical components of the object are not separately processed but are investigated by a global method taking into account geometrical effects and flow history (shadow regions, surface heat flux propagation). Several application cases are displayed to rely on the engineering approach: ARD and AOTV capsules, Pre-X (an IXV-like vehicle) and CubeSat (a debris-like single object). The given results have been analysed by comparison with experimental and CFD data. The limits of the approach are discussed, paving the way for future developments.

International audience

This article presents the results of a wind tunnel campaign for a tilt-body UAV, the MAVion. The objective of this campaign is to develop a simplified flight model for use in control systems design and implementation. In order to achieve precise flight control during transition, stationary and cruise modes, the aerodynamic coefficients are identified for a wide flight envelope of angle of attack and sideslip. Additionally, the equilibrium transition is studied and the results validate the MAVion design. Moreover, an analysis of performance on aerodynamics due to addition of winglets in this platform is carried out.

International audience; A passive twist control is considered as an adaptive way to maximize the overall efficiency of a proprotor developed for convertible Micro Air Vehicles (MAV). Incorporated into a database of airfoil characteristics, Blade Element Momentum Theory (BEMT) is implemented to predict the performance of proprotors at low Reynolds numbers. Using this model, it is found that low twist allows for efficient hovering while high twist helps to forward flight. The Centrifugal Force Induced Twist (CFIT) concept is proposed to realize the required torsion of proprotor between hover and forward flight. Tip mass is used to provide the nose-down twisting moment by centrifugal force and stabilize the flexible blade. Classical Lamination Theory (CLT) is employed to estimate the torsion behavior of glass/epoxy laminate blade and to study the feasibility of CFIT concept. The results indicate that the predicted torsion of CFIT blade is of the same level with required deformation. The laminate blades were tested in hover and forward flight modes, with deformations measured by Laser Displacement Sensor (LDS). In rotor mode, the laminate blade can generate approximately -9◦ torsion at blade tip rotating at 1,300 RPM. By contrast, at 800 RPM and inflow velocity 8m/s, it is capable of providing around -5◦ torsion at blade tip in propeller mode.

International audience; In the present study, a passive twist control is considered as a potential way to improve the overall flight efficiency for proprotor of Micro Air Vehicle (MAV). This paper will focus on the aerodynamic performance and deformation behaviour of a flexible laminate blade. Incorporated with adatabase of airfoil characteristics, Blade Element Momentum Theory (BEMT) is implemented for performance prediction of proprotor at low Reynolds numbers. The preliminary procedure is based on finding optimum twist distributions for hover and forward flight, but keeping a given chord distribution.A numerical model is developed using a combination of aerodynamic model based on BEMT, and structural model based on anisotropic beam finite element, in order to evaluate the coupled structural and the aerodynamic characteristics of the deformable proprotor blade. The numerical model - Fluid Structure Interaction (FSI) was then validated by means of shape reconstruction from LDS (Laser Displacement Sensor) outputs. It can be concluded that the proposedexperiment technique is capable of providing a predictive and reliable data in blade geometry and performance for rotor mode. The FSI approach is also valid as a reliable tool for designing and analyzing the MAV proprotor made of composite material.

National audience; This paper is concerned with the evaluation of design techniques, both for the propulsive performance and for the structural behavior of a composite flexible proprotor. A numerical model was developed using a combination of aerodynamic model based on Blade Element Momentum Theory (BEMT), and structural model based on anisotropic beam finite element, in order to evaluate the coupled structural and the aerodynamic characteristics of the deformable proprotor blade. The numerical model was then validated by means of static performance measurements and shape reconstruction from Laser Distance Sensor (LDS) outputs. From the validation results of both aerodynamic and structural model, it can be concluded that the numerical approach developed by the authors is valid as a reliable tool for designing and analyzing the drone proprotor made of composite material. The proposed experiment technique is also capable of providing a predictive and reliable data in blade geometry and performance for rotor modes.Mots clefs

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.

Consumer-grade single-frequency GPS receivers with their known limitations are the predominant means of localization for small Unmanned Aircraft Systems (UAS). More intricate maneuvers such as automatic landings require a level of accuracy this class of receivers does not provide. As a contribution to improve the positioning accuracy without sacrificing the low-cost approach of this class of vehicles, a Local Area Augmentation System (LAAS) has been developed, based on consumer-grade single-frequency miniature GPS receivers both for the base station and airborne positioning. On the part of the airborne receiver, the conventional approach of carrier phase smoothing has been extended by incorporating Doppler measurements to propagate the position during carrier phase signal outages or in the event of cycle slips. Pseudoranges and the augmented carrier phase observations are merged by means of an indirect linear Kalman filter in the position domain. The characteristics of the error state allow for some simplifications that reduce the computing effort of the filter. To evaluate the system’s performance under dynamic conditions, raw GPS data have been collected on a ground based moving platform and processed with Simulink. The results show a significantly improved 3D position accuracy compared to the standalone receiver solution.

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.

International audience; This study focus on optimisation of electric propulsion system for a given mission with multiple working conditions. A program called Qpoptimizer is developed and presented which can analyse and couple numerous motors and propellers from databases for a specific mission. It can also design a custom propeller by using the motor and airfoil databases. Qpoptimizer uses Qprop and Qmil opensource propeller analyses and design programs from Mark Drela. Motor and propeller couples are simulated at each predefined working condition and given a score according to their total performance. This methodology ensures the optimisation of the selected motor and propeller couples to be valid and optimum not only for one working condition (for example: cruise condition) but for all of them (take-off,high speed,etc...). Theoretical models and experimental measurements are explained in order to generate the required databases for the existing motors, propellers and airfoils. Finally, an application of the Qpoptimizer program on a real mission is also presented where a custom propeller is optimised according to the weighted mission working conditions.

The numerical calculation of shock wave by the Euler equations can generate in some cases and with some numerical schemes a pathology: it is the so-called carbuncle phenomenon. The purpose of this paper is to show that this pathology can be interpreted by the existence of an unstable mode in the continuous Euler equations.