A GENERAL DESIGN METHODOLOGY FOR YEAR- ROUND SOLAR-POWERED STRATOSPHERIC UAVS FROM LOW TO MIDDLE LATITUDES Min Chang1,2, Zhou Zhou1, Rui Wang1, Xiaoping Xu1 1 Science and Technology on UAV Laboratory, Northwestern Polytechnical University 2 Xi’an Institute of Modern Control Technology Keywords: Solar-powred UAV, latitude, design methodology, energy performance Abstract A design methodology for configuration sizing 1 General Introduction of solar-powered UAVs is established, which could be equally applied to all configurations The solar energy has attracted many researchers capable of year-round operations in the in the last 40 years due to its clearness and stratosphere at low and middle latitudes. In eternity for high altitude long endurance (HALE) general, the configurations are classified into unmanned aerial vehicles (UAV). Solar- two representative types—the conventional and powered UAVs show their superiorities in the the wing-sail. The wing-sail configuration civil or military fields. Higher altitude means employs sail tails that can rotate around wider covering area of interest and higher individual roll axes to maximize solar energy survivability, and longer endurance means more absorption, and photovoltaic (PV) modules are timely intelligence and information. coupled to the wing and only one side of each References[1-3] mainly focused on sail tail. The configuration sizes are treated as traditional design processes of solar-powered key design variables, including wingspan, airplanes and validated the feasibility of solar- aspect ratio of wing, and area ratio of sail tails powered flight. Bailey[4] generally discussed to the wing. component parameters determination, presented The established methodology mainly and analyzed a high-altitude solar-powered contains two parts. The first part parameterizes platform for a proposed mission. Steven[5] energy absorption and energy consumption, studied the energy characteristics for solar- mass components and aerodynamic efficiency. powered flying wing, tandem wing and airship The second part employs an optimal approach under the constraint of energy balance. to obtain a group of optimized solutions. Then, Romeo[6, 7] carried out research activities on the methodology is applied to analyze HALE platforms to achieve persistent conceptual parameters at different latitudes for operations for several months in the northern both configurations. Finally, a solar powered latitudes of 36°~45° at the altitudes of 15~20 stratospheric UAV concept of wing-sail km in Europe. Noth[8] developed a conceptual configuration, PoXiao, is proposed for year- design methodology, and successfully achieved round operation at middle latitudes. Its energy continuous flight of 27 h by solar-powered Sky- performance is investigated to validate the Sailor near the summer solstice at the latitude of operational altitude and latitude capabilities 44 °N in 2008. Rizzo[9] proposed a throughout a whole year and demonstrate the mathematical model for conceptual design and utility of the design methodology. The compared 4 representative configurations based characteristics of stability and control for the on energy characteristics. Comprehensively wing-sail configuration is also preliminary speaking, these design methodologies above are analyzed. merely focused on continuous operation near 1 Min Chang, Zhou Zhou, Rui Wang, Xiaoping Xu the equator or continuous flight for a single day proposed the configurations which incorporated or several weeks at higher latitudes when the rotatable sun-trackers into solar-powered solar radiation is rich, and are scarcely focused airplanes of conventional configuration to on year-round operation at higher latitudes. In achieve longer flight duration at higher latitudes. the history of solar-powered flight, solar- However, they did not systematically present powered Helios and Zephyr respectively the methods for sizing the wing and non- produced records of absolute altitude of 29,531 horizontal sun-trackers. Ref. [15] also proves m for 4 hours on August 13, 2001 in Hawaii the superiorities of sun-trackers with variable (18 °N) [10] and endurance of 336 hours from orientations when applied to high-altitude solar- 15 km to 18 km in July 23, 2010 in Yuma powered UAVs at wide latitudes. (32 °N) verified by Fédération Aéronautique In general, the configurations for solar- Internationale. Their PV modules are both powered airplanes can be categorized into two horizontally mounted only on their wings representative types—the conventional and the conventionally. With conceptual parameters of wing-sail, as shown in Fig.2. For the flying-wing Helios prototype (HP01) from Ref. conventional, PV modules are mainly [10], Fig. 1 shows its achievable persistent horizontally disposed on the wing. For the altitudes from low to middle latitudes wing-sail, a portion of PV modules is throughout a whole year. horizontally disposed as the conventional, and 20 the rest are mounted on the sail tails that can 16 rotate around body axes. The conventional is a 0° special case of the wing-sail when the total area 12 10°N of sail tails is zero, and the all-wing design like 20°N 8 30°N Helios with no fuselages and tails belongs to the 40°N conventional. 4 60°N y=0 Achievable Persistent Altitude (km) Altitude Persistent Achievable 0 0 50 100 150 200 250 300 350 Day from Jan. 1 Fig. 1 Persistent altitude capability of the Helios prototype (HP01) The HP01 could fly continuously within the region bounded by each curve and the sea level(y = 0). It is obvious that the HP01 could not maintain station keeping at high altitudes and higher latitudes during the winter months. It is due to the fact that both the solar elevation angle on average and the day length near winter decrease with increasing latitude, leading to the decreasing of the solar flux in the daytime projected on a surface disposed horizontally, i.e. Fig. 2 Perspective views of exemplary embodiments of on the wing[11, 12]. It is also the key reason conventional and wing-sail configurations why to date solar-powered UAVs with PV Thus, this paper aims to define a modules only mounted on the wing have limited mathematical model of design methodology for operational values, especially at high altitudes configuration sizing of solar-powered and higher latitudes near winter. stratospheric UAVs for year-round mission Inspired by the solar collector “tracking” requirements from low to middle latitudes. This the Sun to minimize the angle of incidence of design methodology is under the constraints that beam radiation on the surfaces with PV modules, energy consumption is balanced by energy Keidel[13] from Germany and Gerald[14] from absorption within the daytime, the nighttime and the Boeing Company both innovatively a whole day of 24 hours. As to energy 2 A GENERAL DESIGN METHODOLOGY FOR YEAR-ROUND SOLAR POWERED STRATOSPHERIC UAVS FROM LOW TO MIDDLE LATITUDES absorption, power area density of PV modules is airplane. The solar-powered airplane does not modeled, considerations including PV modules’ change its weight in operation. In general, the orientation, thermodynamics of PV modules, total weight can be divided into eight parts. propagation of solar radiation, flight direction, Equation (1) summaries the total-weight buildup: operational altitude, operational latitude, mtot m pld m af m pm m mppt different seasons, etc. As to energy consumption, (1) m m m m each mass component is parameterized and bat p av lg aerodynamic efficiency model based on Here mpld, maf, mpm, mmppt, mbat, mp, mav and mlg configuration parameters is built. Besides, represent the mass of payload, airframe Quantum-Behaved Particle Swarm (QPSO) structure, PV modules, MPPT, secondary algorithm and Kriging surrogate model are batteries, propulsion systems, avionics and employed to achieve an optimal group of landing gear, respectively. configuration parameters efficiently. A fitness Secondly, the energy balance means that function of AR of wing, cruise velocity and the total energy collected from onboard PV payload weight fraction is defined to link the modules must be equal to or higher than the optimization and the QPSO algorithm. Finally, electrical energy consumed for mission the established design methodology is put into execution in a whole day of 24 hours. The applications. Firstly, the key conceptual relationship is defined by: parameters obtained by the design methodology SKHHPHHw pm d n tot d n c dc (2) for both configurations are compared from the Here Sw donates the reference area of wing, Kpm equator to the latitude of 50°N. Secondly, a donates daily-averaged total power of solar powered stratospheric UAV concept of photovoltaic modules per Sw, Hd and Hn donate wing-sail configuration, PoXiao, is designed at day period and night period, c and dc donate the latitude of 45°N. Then, the flight the efficiencies of charging and discharging of performance, power characteristics, and secondary batteries, and Ptot donates total power tracking angles of PoXiao concept on the consumption of solar-powered aircraft. summer solstice and the winter solstice are During the nighttime period, there is explored, and operational persistent altitude and another balance between the energy stored in payload power capabilities are further secondary batteries and the total power investigated at middle latitudes throughout a consumption multiplied by night duration. whole year. In addition, the impacts of the m P H (3) installation and the rotation of sail tails of the bat bat dc tot n wing-sail configuration from a stability and Here bat donates gravimetric energy density of control point of view are preliminary analyzed. secondary batteries. Equations (1) to (3) are the basic equations for determining configuration parameters, which 2 Basic Equations: Energy Balance and distinguish continuous-operation solar-powered Mass Balance airplanes from conventional-powered HALE Based on the potential application listed at the airplanes. In level flight, the total power beginning, steady level flight acts as the single consumption contains three parts. design point for solar-powered stratospheric PPPPPPPtot pld p av pld lev p av (4) UAVs as other HALE airplanes.