Flying 40,000 km around the world with no fuel is no easy task. Pilot Bertrand Piccard at Solar Impulse did it, but to enable the rest of us to do so will require some engineering.

Imagine all the passenger and private fixed wing and rotary wing aircraft that fly each day in our world and the pollution it brings to our air. How can we improve this situation? Clean energy is coming.

I recently came across a really neat concept from a paper, SUAV:Q - A Hybrid Approach To Solar-Powered Flight,1 given at the 2016 IEEE International Conference on Robotics and Automation (ICRA) in Stockholm, Sweden, on May 16-21, 2016 by Ruben D’Sa, Devon Jenson, and Nikolaos Papanikolopoulos from the University of Minnesota. This paper looks at the concept of a small-scale hybrid unmanned aerial vehicle that can enhance the manoeuvrability of a quad-rotor with the energy collection and power supply of a solar-powered fixed-wing aircraft. They investigate the aircraft design, transforming mechanism, and energy management of the multi-state system using a quad-rotor demonstration.

Multi-rotor systems are typically used in applications that need high manoeuvrability and the ability to hold a fixed spatial position. The problem is that manoeuvrability and control come at the cost of high power consumption, resulting in short flight times. This is in contrast with the high efficiency, long-flight capable fixed-wing systems.

The paper presents the design of a re-configurable solar UAV that is able to change its flight capabilities between fixed-wing and quad-rotor states. A design architecture capable of both states removes individual limitations and combines the strengths of both systems. In the quad-rotor state, the aircraft cannot supply enough energy from solar power alone and has to rely on stored energy. Once stored energy drops to a low level, the aircraft transitions into a fixed-wing state where the on-board batteries will be able to recharge, allowing the process to repeat (figures 1 and 2).

EDNA plane 01 Figure 1: Shown above are the fixed-wing and quad-rotor positions of the proposed SUAV: Q hybrid aircraft design. In a ground state, the aircraft charges via sunlight in a fixed-wing orientation to maximize solar power intake.

EDNA plane 02 Figure 2: In this image we can see the transition from quad-rotor to fixed-wing states.

The power electronics design is capable of simultaneous battery charging and the power loading from a solar array has been validated. The experimental power system electronics were validated using 32 SunPower C60 solar cells and functionality in the quad-rotor state was demonstrated.

EDNA plane 03 Figure 3: The solar power system shown here consists of eight SunPower C60 solar cells per wing section; each wing is capable of producing 24 W. In fixed-wing configuration and with ideal solar conditions, the total power system of four wings is capable of supplying 96 watts from the solar array.

Due to the I-V characteristics of the SunPower solar cells, the impedance of the load needs to be well matched with the output impedance of the solar array. Given the varied angle of solar irradiance hitting each angled section of the aircraft, each section requires a maximum power point tracker (MPPT) like the Linear Technology LT8490, a high voltage, high current buck-boost battery charge controller with MPPT. Each of the MPPT will track and adjust the voltage operating point of its corresponding panel in order to maximise the amount of solar power available to the aircraft. In addition to the MPPT, each of the Panasonic NCR18650b lithium ion cells is monitored by a battery protection system such as an AA Portable Power Corp #PCM-LI22.2V20A to protect against over-voltage and unbalanced cells.

Power is monitored throughout the system using the Texas Instruments INA219 current shunt and power monitor. Each module communicates with an Atmel ATmega328 µC over an I2C bus. Voltage and current measurements received by the µC are stored locally to an SD card as well as sent in real-time using 915MHz radios to a ground station.

A Turnigy Plush 25A speed controller and Quantum MT2814 motor typically used by remote control vehicles were used in this experimental design as well. Additional work in optimisation of the propulsion system and airframe needs to be completed to maximize the performance of the hybrid system.