The aviation sector and its associated activities contribute to climate change, damage to the environment, community noise, and local air pollution. As a consequence it affects the health and quality of life of citizens that live close to airports, and through climate change every person on earth. To reduce the impact of air travel and thereby accelerate growth, new technologies are being investigated. One area of research looks into the opportunities of using batteries and electric motors as additional energy source and power converter. Besides solving some of the current problems, these new technologies could enable radical new forms of aviation and create new business models such as on-demand personalized air travel.
Because the design rules of novel hybrid-electric aircraft differ significantly compared to that of conventional aircraft, new methods must be developed tailored for this new technology. Due to the additional variables introduced by using hybrid-electric propulsion systems, a simple sizing study quickly becomes a multi-disciplinary optimization problem. Furthermore, these new propulsion systems should not be benchmarked in an isolated way due to synergistic benefits with other disciplines. To close the gap between clean sheet designs and retrofits, it has been concluded that research focusing on feasible concepts in the near future is required.
The objective of this research is to define the implications of using hybrid electric propulsion systems in general aviation aircraft. The influence of design, operational and mission choices on optimal performance are identified through optimization studies. The design space includes the geometry of the main wing and the operational variables of the propulsion system. The performance is either measured in terms of fuel or total energy consumption per kilometer for design ranges between 50 and 1000 km.
To perform the quantitative analyses, a conceptual design framework has been developed. Models of all propulsion systems have been developed that simulate their performance. Every part of the tool is either verified with experimental data or has already been verified in literature. A quasi-three-dimensional aerodynamic solver has been modified to determine the aerodynamic characteristics with minimal computational time. The battery performance is modeled as function of time and rapidly sized such that the required power can be delivered by the smallest battery possible. A multi-disciplinary optimization approach has been used to integrate all modules and converge to an optimal design as quickly as possible.
First of all, a difference is found in terms of optimal configuration and operation of an aircraft when designed specifically for minimal fuel or for minimal total energy, leading to a maximum difference of 5% in terms of energy consumption. A trade-off between designing for aerodynamic efficiency versus the ability to carry batteries is part of the optimization routine. It has been shown that aircraft optimized for fuel consumption carry systematically more batteries compared to aircraft optimized for total energy consumption, always leading to a heavier aircraft with larger wings. Regardless of objective, the aerodynamic efficiency becomes more important for an increasing range while the ability to carry batteries decreases.
Three variables have been defined that determine the amount of hybridization in terms of power and energy without the need for constraint functions, as any combination of these variables lead an inherently feasible design: a climb coefficient that determines the rate of climb as a fraction of the maximum rate of climb, the fraction of the cruise that needs to be performed full electric, and a hybridization factor that determines the throttle of the internal combustion engine.
Given a certain required power at the power management system, there exist one specific throttle that leads to the maximum effective efficiency of the internal combustion engine. This throttle depends on the round trip efficiency of the battery charge-discharge cycle. When the internal combustion engine is scalable, the optimizer always makes sure that the most efficient throttle matches the power requirements during cruise, i.e. it avoids recharging.
The maximum possible range is highly determined by the required cruise velocity, given a fixed internal combustion engine. By scaling the internal combustion engine such that it is powerful enough to deliver the required power for cruise, the maximum range can be significantly increased. Furthermore, the optimal cruise altitude is found to be increasing with range and cruise velocity. At a cruise speed of 60 m/s the optimal cruise altitude coincides with the lower bound of 2 km whereas the optimal altitude at a cruise speed of 90 m/s increases from 2 km to 4 km for design ranges from 350 km to 700 km. In the latter case, the cruise altitude settles at 4 km for longer ranges.
The maximum range at which full electric cruise is possible is found to be the most efficient range in terms of energy consumption. This range increases linearly with the battery specific energy, providing designers a good initial estimation tool.
This research adds to the scientific body of knowledge as it presents a method that solves the multi-disciplinary optimization problem associated with the design of hybrid electric aircraft. Furthermore, through multiple optimization studies it provides insight in the influence of the design choices, operational choices, constraints, and mission profiles on the optimal performance of general aviation hybrid electric aircraft.