Giulio Romeo

HELIPLAT: Design, Aerodynamic, Structural Analysis of Long- Endurance Solar-Powered Stratospheric Platform

This paper presents the design and manufacture of the first European Very Long-Endurance Stratospheric Unmanned Air Vehicle, HeliPlat® (HELIos PLATform). This vehicle is a monoplane with eight brushless motors, a twin-boom tail type and two rudders. A computer program has been developed to carry out the platform design. To minimize airframe weight, high modulus carbon fibre composite material has been used extensively. Airfoil coordinates and wing planform have been optimized in order to achieve the best possible aerodynamic efficiency by using integral panel/boundary-layer methods and also to obtain the minimum possible induced drag with respect to local Reynolds airfoil. To this effect, several wind-tunnel tests were carried out so as to compare the analytically predicted airfoil performances. After an initial configuration had been worked out, a scale technological demonstrator (wing span 24 m) was designed, manufactured, and tested under shear, bending, and torsion loads. Finite element analysis was also carried out in order to predict the static and dynamic behaviour of both the full-size and scale model versions of the HeliPlat structure. The preliminary static test resulted in a high correspondence.

Post-buckling behaviour of graphite/epoxy stiffened panels with initial imperfections subjected to eccentric biaxial compression loading

The post-buckling behaviour of anisotropic stiffened panels with initial imperfections is investigated. Since buckling of the skin between the stiffeners often occurs first, a non-linear analysis is developed for symmetric panels under biaxial compression in order to obtain the out-of-plane panel deflection in the post-buckling range. The non-linear differential equations are expressed in terms of the out-of-plane displacement and the Airy function. They are solved with the Galerkin method for various boundary conditions by imposing an edge displacement control. The theoretical and experimental results obtained by the present analysis show that the transverse load can greatly influence the buckling loads and halfwave number. Since no experimental results have been found in the literature, several tests have been carried out on graphite/epoxy blade stiffened panels 900 mm long and 620 mm wide applying simultaneously biaxial compression loads with several combined ratios. An eccentricity results between longitudinal and transverse load, because the longitudinal compression is applied along the centroidal axes of the stiffened section while the transverse compression is applied to the skin panel. The correlation between the experimental and analytical results has been quite good; the experimental results demonstrate the influence of eccentricity of the transverse load on panel deflection in the pre- and post-buckling range.

Design of a High-Altitude Long-Endurance Solar-Powered Unmanned Air Vehicle for Multi-Payload and Operations

Abstract Several researches are being carried out at the Politecnico di Torino with the aim of designing a high altitude very-long endurance/unmanned air vehicle (HAVE/UAV). Being able to fly in the stratosphere (15-20 km) and with an endurance of about 4 months offers an advantage and possibility that is presently not available with conventional aircraft or satellites. A computer program has been developed to design the platform. The change in solar radiation over a period of a year, the altitude, masses, and efficiencies of the solar and fuel cells, as well as the aerodynamic, structural, flight mechanics, and aeroelastic performances have all been taken into account. Extensive use has been made of high modulus graphite/epoxy when designing the structure in order to minimize the airframe weight, but also to guarantee the required stiffness and aeroelastic performance. A blended wing body (BWB) configuration has been selected for solar HAVE aircraft multi payload and operation (SHAMPO) with eight brushless electric motors, as the result of a preliminary design. The BWB solution has been designed according to the conventional procedures and airworthiness regulations. It seems to be the best compromise between performance, available surfaces for solar cells and volume for multi-payload purposes, compared to conventional design. Several profiles and wing plans have been analysed and optimized to achieve the best efficiency using the Xfoil and Vsaero computational fluid dynamics (CFD) software. A finiteelement method and a classical theoretical analysis was carried out using the Msc/Patran/Nastran code to predict the static and aeroelastic behaviour of the SHAMPO. Aeroelastic analysis has been performed starting with a classical linear flutter analysis and considering an undeformed equilibrium condition. Classical linear flutter speed show as the airworthiness requirements has been achieved in the case of SHAMPO configuration. A preliminary non-linear aeroelastic model is introduced in the design process in order to deal with specific phenomena correlated with high static structural deflections occurring during standard flight conditions. Important flutter speed reduction (i.e. up to 42 per cent in special cases) are possible including such kind of phenomena.

Stability and Control of a High-Altitude-Long-Endurance UAV

This paper addresses stability analysis, control design, and simulation of high-altitude, long-endurance unmanned aerial vehicles. These aircraft are highly flexible and have very low structural frequencies. Hence, the separation between the elastic and rigid body motions no longer exists, and an approach that unifies these two motions must be used. The available data of the aircraft considered include the geometry of the aircraft, aerodynamic data, and mass, flexural rigidity, and torsional rigidity distributions. The wing of the aircraft is modeled as a flexible beam undergoing bending and torsion, whereas the remaining members are assumed to be rigid. The equations of motion are obtained by means of the Lagrangian equations in quasi coordinates. A perturbation approach separates the problem into nominal dynamics and perturbation dynamics. The equations for nominal dynamics are used to design desired maneuvers and to determine the corresponding structural deformations. The equations for perturbation dynamics are used to address stability of the aircraft on the desired flight paths, to design feedback controls to maintain stable flights, and to simulate the motion of the aircraft.

Nonlinear analysis of anisotropic plates with initial imperfections and various boundary conditions subjected to combined biaxial compression and shear loads

Abstract The postbuckling behaviour of anisotropic panels with initial imperfections is investigated. Nonlinear analysis is developed for symmetric panels under combined biaxial compression and shear loads in order to obtain the out-of-plane panel deflection in the postbuckling range. The nonlinear differential equations are obtained by using the principle of the stationary value of the total potential energy and are expressed in terms of the out-of-plane displacement and the Airy function. They are solved with the Galerkin method for various boundary conditions. The theoretical results are in good agreement with the few results concerning isotropic plates and the simply supported anisotropic results found in literature. A new and original test facility was built in order to apply simultaneously both biaxial compression and shear loads. An anisotropic panel, clamped along the four edges, has been tested under different combined loads; the correlation between the experimental and analytical results has been quite good. The results demonstrate the influence of initial imperfections on panel deflection in the postbuckling range: the curves obtained experimentally in the presence of imperfections cross those obtained theoretically, where imperfections are ignored (producing lower out-of-plane deflection).

Heliplat®: high altitude very-long endurance solar powered UAV for telecommunication and Earth observation applications

Research is at present being carried out at the Turin Polytechnic University with the aim of designing an HAVE/UAV (high altitude very-long endurance/unmanned air vehicle). The vehicle should climb to 17-20km by mainly taking advantage of direct Sun radiation and thereafter maintain a level flight; during the night, a fuel cells energy storage system would be used. A computer program has been developed to carry out a parametric study for the platform design. The solar radiation change over one year, the altitude, masses and efficiencies of the solar and fuel cells, and the aerodynamic performances have all been taken into account. The parametric studies have shown how fuel cells and solar cells efficiency and mass have the most influence on the platform dimensions. A wide use of high modulus CFRP has been made in designing the structure in order to minimise the airframe weight. A first configuration of HELIPLAT® (HELIos PLATform) was worked out, following a preliminary parametric study. The platform is a monoplane with eight brushless electric motors, a twin-boom tail type with an oversized horizontal stabiliser and two rudders. The co-ordinates at the root and along the wing span as well as the wing planform were optimised to achieve the best efficiency. Several profiles and wing plans have been analysed using the CFD software Xfoil and Vsaero. Several wind-tunnel tests were carried out to compare the analytically predicted performances. A preliminary design of a scale-sized technological demonstrator was completed with the aim of manufacturing a proof-of-concept structure. A FEM analysis was carried by using the Msc/Patran/Nastran code to predict the static and dynamic behaviour of the UAV structure.

HELIPLAT: Aerodynamic and Structural Analysis of HAVE Solar Powered platform

Research is at present being carried out at the Turin Polytechnic University with the aim of designing an HAVE/UAV (High Altitude Very-long Endurance/ Unmanned Air Vehicle). The vehicle should climb to 17-20 km by mainly taking advantage of direct sun radiation and thereafter maintaining a level flight; during the night, a fuel cells energy storage system would be used. A computer program has been developed to design the platform by taking into account the solar radiation change over one year, the altitude, masses and efficiencies of the solar and fuel cells, and the aerodynamic performances. The parametric studies have shown how fuel cells and solar cells efficiency and mass have the most influence on the platform dimensions. A wide use of high modulus CFRP has been made in designing the structure in order to minimise the airframe weight. A first configuration of HELIPLAT (HELIos PLATform) was worked out, following a preliminary parametric study. The platform is a monoplane with 8 brushless motors, a twin-boom tail type with a long horizontal stabilizer and two rudders. The airfoil coordinates at the root and along the wing span as well as the wing planform were optimised to achieve the best efficiency. Several profiles and wing plans have been analysed using the CFD softwares Xfoil and Vsaero. Several wind tunnel tests were carried out to compare the analytically predicted performances. A preliminary design of a scale-sized technological demonstrator was completed with the aim of manufacturing a proof-of-concept structure. A FEM analysis was carried by using the Msc/Patran/Nastran code to predict the static and dynamic behaviour of the UAV structure.

Nonlinear Aeroelastic Modeling and Experiments of Flexible Wings

High Altitude Long Endurance (HALE) aircraft exhibit aeroelastic behavior that is quite different from conventional flight vehicles. For such aircraft, large static structural deformations that occur during standard flight conditions could severely reduce the flutter speed. Limit cycle oscillations may occur, reducing their operational life. In this paper, a non-linear aeroelastic model is proposed as a design/diagnostic analysis tool. A geometrically exact non-linear structural model for large deformations by Hodges and Dowell, modified according to Da Silva second order geometrical non-linear terms, has been adopted. The structural model has been coupled with an unsteady aerodynamic model for an incompressible flow field, based on the Wagner aerodynamic indicial function, in order to obtain a non-linear aeroelastic model. Within the analysis, linear and non-linear static and dynamic aeroelastic responses have been considered. The approach enables one to expedite the calculation process. In addition to analytical and computational results, a high aspectratio wing model has been selected as a candidate for a feasibility study case and to validate the theoretical model. Preliminary tests have been conducted and more experiments are underway.

ENFICA-FC: Preliminary Survey & Design of 2-Seat Aircraft Powered by Fuel Cells Electric Propulsion

[Abstract] The main objective of the ENFICA-FC project (ENvironmentally Friendly Inter City Aircraft powered by Fuel Cells), funded by European Commission, is to develop and validate the use of a fuel cell based power system for propulsion of more/all electric aircraft. The fuel cell system will be installed in a light sport aircraft which will be flight and performance tested as a proof of functionality and future applicability for inter city aircraft. A feasibility study will be carried out to provide a preliminary definition of new forms of commuter aircraft propulsion systems that can be obtained by fuel cell technologies. In parallel, a two-seat electric-motordriven airplane powered by fuel cells will be developed and validated by flight-test. The high efficiency two-seat existing aircraft Rapid 200, manufactured by Jihlavan Aircraft, was selected over more than 100 light sport aircrafts and will be used for the conversion from internal combustion engine. The fuel cell system and the electric motor will be integrated on board. The following items shall be pursued: A fuel cell system shall be designed, built and tested in laboratory ready to be installed on board for flying. (provided by Intelligent Energy). A high efficiency brushless electric motors and power electronics apparatus for their control shall be designed and manufactured ready to be installed on board for flying Efficiency greater than 90% would be obtained by an optimised aerodynamic propeller design. A study of the flight mechanics of the new aircraft will be carried to verify the new flight performance. Flight test bed of the aircraft capable of remaining aloft for one hour will be the main goal of the project to validate the overall high performance of an all electric aircraft system. The ambitious results will be to present, in a public event within the scheduled time (Summer 2009), the flight test bed of the aircraft capable of remaining aloft for one hour to validate the overall high environmental performance of an all electric aircraft system.

Design and realisation of a two-seater aircraft powered by fuel cell electric propulsion

The main objective of the project is to develop and validate the use of a fuel cell based power system for the propulsion of more/all electric aircraft. The Rapid 200-FC two-seater electric-motor-driven aeroplane which is powered by fuel cells is at present being completed and will be validated during a flight test in Autumn 2009. Several configurations have been evaluated in order to install the new energy and propulsion system on board while maintaining the centre of gravity within allowable limits. The fuel cell system and the electric motor are being integrated on board. The FC stack will be able to deliver a maximum continuous power of 20kW. A battery pack has to guarantee another 20kW of maximum continuous power for a limited time period (15 minutes), during take-off, climbing and, in the case of emergency, during landing. The main goal of the project is to validate the overall high performance of an all electric aircraft system which is capable of remaining aloft for one hour. A parametric analysis has also been carried out to evaluate which key technologies influence the performance of future aircraft to the greatest extent.