Conceptual Design and Control Schedule Optimization for Parallel Hybrid Propulsion System of Regional Turboprop Aircraft

. Current environmental policies for the aviation industry motivate the use of cleaner propulsion systems to reduce its CO2 and noise footprint in the coming years. In this context, hybrid propulsion systems have emerged as a potential solution as they have demonstrated a good trade-off between performance and low pollutant emissions. In the present work, the conceptual design and control schedule optimisation of a parallel hybrid propulsion system for the De Havilland Dash 8 regional turboprop aircraft is carried out. A parametric study of the turboprop engine with different degrees of hybridisation at design point and off-design performance analysis of the parallel hybrid propulsion system with different control schedules for an aircraft typical mission has been carried out. Overall aircraft performance parameters are used to evaluate hybrid propulsion systems with different degrees of hybridisation and control schedules. From the results, it was found that a control schedule with 10-15% hybridisation during the cruise phase is the most promising when considering the aircraft mission blocked fuel and fuel efficiency.


Introduction
Environmental impact is an increasingly important factor in the design of next-generation transport systems.The aerospace industry has begun the transition to zero-emission technologies, focusing primarily on electrification.Batteries and hydrogen have received the most attention as power sources.Companies such as magniX and Joby Aviation have successfully flown fully electric, battery-powered aircraft.Others, such as ZeroAvia, have flight-tested hydrogen fuel cell aircraft.
At low battery energy densities, which are expected in the near future, the fuel reduction potential is greater for short ranges than for longer ranges [1,2].As a result, hybrid propulsion is considered most suitable for short range market segments such as regional aviation [2,3,4,5,6].Following the successful flight of early CS-23 prototypes, regional turboprop applications are a likely first step towards the global electrification of commercial aviation [7].
In 2022, the European Union, under the Clean Aviation Programme, launched several projects focused on the design of hybrid electric regional aircraft -Hybrid-Electric Regional Aircraft (HERA), Multi Hybrid Electric propulsion system for regional AiRcrafT (HE-ART), Multi Power train InnovAtive for hyBrid-Electric Regional Application (AMBER), Thermal Management Solutions for Hybrid Electric Regional Aircraft (TheMa4HERA) etc.
In the present work, the conceptual design and control plan optimisation of a parallel hybrid propulsion system for the De Havilland Dash 8 regional turboprop aircraft is carried out.A parametric study of the turboprop engine with different degrees of hybridisation at design time and an off-design performance analysis of the parallel hybrid propulsion system with different control schedules for a typical aircraft mission have been carried out.Overall aircraft performance parameters are used to evaluate hybrid propulsion systems with different degrees of hybridisation and control schedules.

Baseline Regional Turboprop Aircraft
The aircraft studied is a De Havilland Canada Dash 8-100, a twin-engine, medium range turboprop aircraft.The De Havilland Canada Dash 8-100 is a platform for "Project 804", a regional aircraft with a parallel hybrid propulsion system developed by Pratt & Whitney Canada and Collins Aerospace.The goal of Project 804 is to replace one of Pratt & Whitney Canada's PW100 series turboprop engines with a 2-megawatt class hybrid propulsion system.
The parallel hybrid propulsion system combines an engine sized for cruise power with a similarly sized electric motor that provides additional power during take-off.The companies expect the hybrid electric propulsion system to deliver an average fuel savings of 30 per cent for a typical flight mission [8].
The architecture of a hybrid propulsion system is shown in Figure 1.For "Project 804", the aircraft with hybrid propulsion has a limited range due to the fixed weight of the system and is limited to a 250 nm (463 km) mission.The maximum range for the De Havilland Canada Dash 8-100 is 1900 km, but still more than half of the missions flown by this aircraft class are less than 463 km.Therefore, the mission fuel burn for the baseline aircraft and aircraft with parallel hybrid propulsion systems was evaluated for a "standard mission" range of 463 km.A typical 463 km mission for the De Havilland Canada Dash 8-100 is shown in Table 1.Normal cruise power for the Pratt & Whitney PW 121 is ~1000 hp, a flat rated combustion core ideally sized for this application is then ~1000 hp.Such a cruise sized engine will operate at peak efficiency at cruise.To provide power required for take-off and climb, two 950hp electric motors are used.The electrical power to drive the motors is supplied by an electric power train consisting of two motor drives, feeders and a battery pack that is charged on the ground.At the propulsors, a gearbox combines the power from the electric motor and the engine core to drive the propeller.At cruise speed, the gas turbine engine runs at peak power (1000 hp) and highest efficiency, providing all the power required (no electric motor power).The power profile for both the core engine and the electric motor is shown in Figure 2.

Parallel hybrid propulsion system design and performance analysis
A two-spool turboprop engine is considered in the GasTurb software for design point parametric analysis.The compressor pressure ratio is varied 9..12 and the turbine inlet temperature is varied 1300..1450K for cruise conditions.A reference turboprop engine model is shown in Figure 3.A hybrid propulsion system weight, mission blocked fuel and technoeconomical analysis is performed to evaluate the performance of an aircraft with different hybrid propulsion system configurations.
Propulsion system weight prediction model, developed by Kuz'michev et al. is used [9]: shows the effect of turbine inlet temperature,   shows engine sophistication impact (changes over the years),   shows engine life impact on weight, ̇ -engine mass flow rate,  -compressor overall pressure ratio, ,  1 ,  2 -empirical coefficients,   -gas turbine engine weight.
Mission blocked fuel estimation: -mission blocked fuel,   -averaged power specific fuel consumption for specific mission phase,   -averaged engine power for specific mission phase,  -duration of specific mission phase Fuel efficiency of aircraft is blocked fuel per 1km of flight and 1kg of commercial payload -fuel efficiency of aircraft,   -aircraft commercial payload,  -aircraft range.Electric motor weight model: -electric motor weight,   -electric motor power,   -electric motor efficiency,  -number of electric motors (2),   -electric motor power to weight ratio.
Motor drive weight model: -motor drive weight,   -motor drive efficiency,  -number of motor drives,   -motor drive power to weight ratio.
Feeders weight model: -feeder weight,   -feeder power,   -feeder efficiency, n -number of feeders,   -feeder power to weight ratio.
-battery weight,  -number of electric motors,   -battery specific capacity, С -battery capacity,   -battery efficiency. =    (9)  -battery capacity,   -electric motor power,  -flight duration.Hybrid propulsion system weighе model:   =   +   +   +   +   (10) Hybrid propulsion system and blocked fuel total weight:  + =   +   The fuel efficiency of the aircraft, hybrid propulsion system and blocked fuel total weight are considered as figures of merit for hybrid propulsion system design point parametric analysis and control schedule optimization.Each hybrid propulsion system control schedule is denoted АА-ВВ-СС-DD-EE, where AA is take-off hybridisation, BB-CC-DD is hybridisation for three consecutive climb phases, and DD is cruise hybridisation.For each hybrid propulsion system control schedule, a set of design points with different compressor pressure ratio and turbine inlet temperature is considered.The results of the design point parametric analysis and control schedule optimisation are shown in Table 2.
In table 2 ∆  is the difference the total weight of the hybrid propulsion system and the blocked fuel and PW121 and blocked fuel; and ∆ PW is the difference between the fuel efficiency of the hybrid propulsion system and the fuel efficiency of the PW121; ∆ 0% is the difference between the total weight of the hybrid propulsion system and the blocked fuel and the reference engine with 0% hybridisation; ∆ 0% is the difference between the fuel efficiency of the hybrid propulsion system and the reference engine with 0% hybridisation;   is the maximum range of the aircraft with the specific hybrid propulsion system; -PAX s the possible difference in PAX number due to the increased weight of the propulsion system.For each control schedule in Table 2, two rows can be seen indicating the engine with maximum and minimum compressor pressure ratio and turbine inlet temperature.
The effect of the hybrid propulsion system control schedule on the optimisation figures of merit is shown in Figure 5.In Project 804, the degree of hybridisation is 50% at take-off and decreases linearly as the aircraft climbs.In fact, Pratt&Whitney and Collins Aerospace are aiming to develop a technology demonstrator of a hybrid propulsion system with a MW-class electric motor.
State-of-the-art hybrid propulsion system component performance parameters are shown on fig. 4.

Fig. 5 .
Fig. 5. Effect of hybrid propulsion system control schedule on optimization figures of merit.

Fig. 6 .
Fig. 6.Effect of hybrid propulsion system control schedule on flight mission CO2.

Table 2 .
Hybrid propulsion system design parametric analysis and control schedule optimization results