Design, manufacturing and flight test of innovative UAV (unmanned Air Vehicles) powered by solar energy and fuel cells fueled by hydrogen

1.1 HeliPlat®:  High Altitude Long Endurance RPAS

A research is being carried out at POLITO aiming at the design of Very-Long Endurance (4-6 months) Solar Powered Autonomous Stratospheric RPAS (VESPAS-RPAS) and manufacturing of solar powered prototype. The main advantage of VESPAS-RPAS is that this system has less climb and descend events, important when considering interference with the aviation traffic. An area of 300km of diameter would be monitored (24h by 24h, seven days per week)by each one of this platforms. All the Mediterranean Sea could be electronically controlled by 8-9 platforms (Fig. 2), drastically reducing in such way the service cost (800-1000 €/hour flight) in comparison to other manned aircraft (7.000-8.000 €/hour flight) or satellite systems, and tedious work . Any other high-altitude UAV configuration for border surveillance has a very limited endurance (24-48 hours) that would drastically increase any potential collision risk with civil aviation traffic.

This RPAS could play the role of pseudo-satellite,with the advantage of allowing more detailed land vision, due to the relative closeness to the land, with continuous earth observation and at a much lower cost than real satellites. Several satellite systems used for earth observation are useless for continuous real-time border surveillance because of their limited spatial resolution.

Figure 1

Figure 2

Under coordination of the Scientific Responsible, the research is being carried out since 1995 as part of several funding obtained by ASI (Italian Space Agency), ESA (Stratos project) and European Commission (projects Helinet, Capecon and TANGO). A multi-disciplinary optimization computer program was developed for carrying out the HeliPlat® design. The solar radiation change over one year, the altitude, masses and efficiencies of solar cells and fuel cells, aerodynamic performances, structural mass, etc. were taken into account. A wide use of ultra-high modulus graphite/epoxy material was made in designing the structure in order to obtain a very light - high stiffened structure. A first configuration of HeliPlat® was worked out, as a result of the preliminary design study (wing span 73m). The platform is a monoplane with 8 brushless motors, twin-boom tail type with a large horizontal stabilizer and two rudders (Fig. 1).

The possibility of long endurance (4-6 months)for a stratospheric platform can be realised with the application of an integrated Hydrogen-based energy system. It is a closed-loop system: during daytime, the power generated by thin high efficiency solar cells that cover the aircraft’s wing and horizontal tail supply power to electric motors for flying and to an electrolyser which splits water into its two components, hydrogen and oxygen. The gases are stored into pressurized tanks and then, during night-time, used as inlet gases for fuel cells stack in order to produce electric DC power and water to be supplied to the electrolyser. Since fuel cells represent the promise of clean and efficient power generation, they are a suitable alternative to conventional energy sources.

A 1:3 scaled size prototype (wing span 24 m , length 7 m) (Fig. 3) was built in order to show the technological feasibility. CASA Space-EADS, Spain, realized the single graphite/epoxy elements of the whole structure (wing tubular spars and ribs, horizontal and vertical tail tubular spars and ribs, booms) and the metal fittings by machine manufacturing. The Politecnico di Torino, Dept. of Aerospace Eng., was in charge of assembling the different parts of the aircraft (wing, horizontal and vertical tails, booms) and of assembling the whole aircraft (made in cooperation with the company Archemide Advanced Composites) in order to perform on it static tests up to the design loads and find the correlation with the numerical analysis. The single main spars of wing, booms, horizontal & vertical tails, were made by Sandwich structure with high modulus graphite/epoxy material and Nomex core; ribs were made by Rohacell PMI material reinforced with carbon/epoxy Strips; fittings were made in aluminium alloy. The wing was made in five sections connected one to each other by special properly designed curved metal insert. All the different parts were correctly positioned in order to define the bonding process and assembling. A best arrangement was defined in order to maintain the surfaces performances as expected by design calculations. A rear spar was manufactured to complete the wing and tail structures. A sandwich glass/epoxy leading edge was designed and manufactured in the centre part of the wing (total length 12m) long for increasing the torsion stiffness. The lifting surfaces were completed with the positioning of a solid form skin. The level of assembling was assessed by several tests in different conditions. All the different parts were then transported to the Politecnico di Torino for the subsequent testing activity. A special device was designed and manufactured for carrying on a shear / bending / torsion test on the whole manufactured prototype. The scaled-size prototype exceeded the static tests up to the limit loads (n=3) without any residual detrimental effect. A static test was then carried up to the ultimate load (N=7.5) to show reliability of technological process & theoretical analysis.

Figure 3

Within the EC Project “CAPECON: Civil UAV Applications & Economic Effectivity of Potential  Configuration Solutions” seven potential configurations were evaluated: 3 HALE with Politecnico di Torino (Scientific Responsible: Prof. G. ROMEO), as Work Package Leader of Solar, Modular and Blended Configurations Design, and Task leader for Solar configuration Design; 2 MALE and 2 Rotary UAV concepts. A preliminary development and design of Solar HALE Aircraft Multi Payload & Operation (SHAMPO) was also performed. 

Actually the research group is involved in the optimization design of VESPAS-RPAS, mainly: a) Aerodynamic optimization; b) Innovative structural solutions; c) High wing flexibility and its influence on flight dynamic and aeroelastic behaviour; d) design of propulsive system and of solar cells system.

A scaled-sized (40m wing span) demonstrator is being designed and would be manufactured (after agreement and funding with a private company) for a 30 day continuous flight at an altitude between 17-24km aiming to the world endurance record.

1.2 RAPID-200-FC: Electric power Aircraft powered by fuel-cells fuelled by hydrogen

The research project  ENFICA-FC (ENvironmentally Friendly Inter City Aircraft powered by Fuel Cells) was funded by the European Commission within the 6th FP in the Aeronautics Action (2006-2010, Coordinator Politecnico di Torino, Prof. G. Romeo). The overall cost of the project is 4.5 million Euros of which 2.9 million Euros is financed, for the first time in Europe, with public funds allocated by the European Commission.

The team is made up of the Politecnico di Torino (Coordinator, design of the modified aircraft and experimental flight tests), Skyleader (CZ) (constructor of the aircraft), Intelligent Energy (UK) (designer and constructor of the hydrogen fuel cells), APL (UK) (in charge of the tanks and supply of the high pressure hydrogen), Mavel Elettronica (IT) (designer and constructor of the inverter and electronic control of the power) and the University of Pisa (IT) (design engine case and preliminary lab. tests on the electric system). Israel Aircraft Industry, Université Libre de Bruxelles and Evektor (CZ) carried out indeed a more theoretical type study for designing a 20-30 passengers aircraft in the regional and intercity sector aiming of using zero emission propulsion systems fuelled by hydrogen. a SME in the field of administrative management (Metec) carried out the administration of the project.

Several flight tests were successfully carried out by POLITO on the two-seat aircraft RAPID200-FC, first all-electric aeroplane worldwide powered by fuel cells (EC Funded project ENFICA-FC) (Fig. 4). Six test flights were successfully carried out showing the positive handling qualities and satisfactory engine performances of this airplane and the results shall be applied for VESPAS-RPAS.  The aircraft has an entirely electric 40 kW propeller; power is supplied to the propeller through 20 kW hydrogen gas fuel cells. In order to guarantee absolute safety of the operations, the aeroplane also has a second source of energy that consists of a set of 20 kW lithium polymer batteries (supplied by Air Energy GmbH) which are able to guarantee alternative or supplementary power during take-off and initial climbing. The electric engine is fed through the generation of high energy currents in an ionization and hydrogen re-combination system (PEM) which has a final product of 100-110 Amps of electrical current at 200-240 V, plus air and water vapours emitted at environmental temperature. Fuel cell stacks (completely developed by Intelligent Energy) give their power at 65°C; the cooling water is circulating within the transparent plastic tube, placed in the cabin, that supply the fuel cell system from the water tank placed in the right wing.

Figure 4

The completely electrical power system was successfully tested during the experimental flights. The rotation speed of 84 km/h has been obtained within 184 m of taxi at power of 35 kW. After take-off, speed was than increased up to indicated values of 110-120 km/h. Level flight at 700ft and 130 km/h was attained by mean of only fuel cell power setting. Positive handling qualities and satisfactory engine performances of these two flight tests let the team to consider this successful early flight as good starting point for the long endurance high speed flights. The flights, fuelled by Air Product by gaseous hydrogen at 350 bar pressure, reached an endurance of 40-45 minutes.

The Light Sport Aircraft RAPID 200-Fuel Cell, is the first European airplane, and technically also worldwide, that is fuelled by hydrogen and successfully concluding flight tests. The speed and endurance world records were established for Electrically-powered Aeroplane (FAI Class C) fuel cells operated (ZERO EMISSION). A new speed world record of 135 km/h (flown of 4 consecutive runs over a 3 km course - according to FAI Sporting Code, Class C Aeroplanes) was established. Also several higher speed at 145-150 Km/h for tens of seconds were measured during free flights. with a top speed of 180 km/h, which was measured during several diving and pull-up manoeuvre tests. The results obtained during flights can be considered as a further step in the European and World Aeronautics Science in introducing a completely clean energy (ZERO EMISSION).

1.3 SESA: Remotely Piloted Aircraft, Mini category, powered by Solar Energy

The flying model SESA (Small Electric Solar Unmanned Airplane) (Fig. 5) was manufactured within the EC funded projects TANGO (Telecommunications Advanced Networks For GMES Operations), to carry out several experimental flight test with a small UAV and to demonstrate some critical technologies and applications. Electric brushless motor and LiPo batteries (for climbing phase missions), are used as propulsion system. The level flight power is being achieved from 2 m2 thin high efficiency (21%) mono-crystalline silicon arrays bonded over 7m wing span; during the level flight the needed power is being achieved from the solar cells system covering the wing, obtaining in such a way a far higher endurance up to 6 hours during June and July. The development of the MPPT (Maximum Point Power Tracking) electronic device was of particular importance for the success of the flight mission in order to optimize the maximum power that could be obtained by the solar cells and to improve endurance. The wing structure was realized using glass-fibre reinforced plastic for skin and carbon-fibre for main spar. The fuselage structure was designed for a better positioning of the masses inside the fuselage and for a better centre of gravity excursion.  The structures were designed to withstand the limit load of  3.8.  The structure was built by the “Archemide Advanced Composite” company using fibreglass for the fuse skin and carbon-fibre for a few frames, especially in the connection with the wing spar. The boom that connect the fuse and tails was manufactured in carbon-fibre. The new landing gear was also manufactured by CFRP.

Up to 12 servo-actuators could be remote controlled by a radio modem for the UAV flight. The elements controlled were: rudder and tail gear, 2 elevators, 2 ailerons, motor rpm. A telemetry system was also installed on board to transmit in real time all the most critical data for the safety of the flight to the ground control station. An autopilot system was installed on board for an autonomously flight up to 40km of distance by a highly integrated data acquisition, processing and control system which includes all necessary components for aircraft control. Satellite based Communication System was installedon-board for the Iridium© satellite communication network aiming to a BLOS flight. SESA command & control is mainly managed by an autopilot system providing a completely autonomous navigation based on spline-connected waypoints. That system operates with two separate data-link, one direct in Line Of Sight (LOS) with the GCS and the second satellite-based for a Beyond Line Of Sight (BLOS)  transmission. Both the links are able to transmit and receive re-planning information and telemetry data. In addition a manual control can operate by overriding the autopilot with a direct radio connection. This is required during the critical flight phases: take-off & landing or emergency. The 2.4GHz on-board R/C receiver is connected to the autopilot and a safety switching mechanism allow the autopilot to be overridden by the remote control at any time, as long as the remote control transmitter is within the aircraft range. That fact does not represent a limitation, the transmitting range in fact is about 2 km, which is a suitable range also for the visibility of the manual pilot on the ground. In this way, any possible malfunction of the autopilot or telemetry system is overcome and flight safety is increased to great extent. In order to show the opportunism of introducing such platforms in surveillance or monitoring systems, the model was equipped with a wireless colours camera (40x, resolution 720x576 pixel) and thermo-camera (160x120 pixel).