More Electric Aircraft Conversion to All-Electric During Ground Operations: Battery-Powered Landing Gear Drive System

Raising awareness about environmental issues moves the aerospace industry toward electrification, and the corresponding solutions are already present at some airports. However, commercial aircraft are the missing links in claiming all-electric ground operations. They rely on fossil fuels without any electric alternative due to the technological inability to store large amounts of energy while maintaining a low weight of batteries. The issue diminishes if an electric system uses only a fraction of the energy normally consumed by the engines and comprises kinetic energy recovery. Accordingly, this article demonstrates the landing gear drive system for a narrowbody airplane, which has the sustainable and economic means to replace all onboard engines throughout ground operations. The system is simulated in MATLAB/Simulink and leads to the kinematic results that are based on the real drive cycles. The kinematics are subsequently used to estimate the overall on-ground power and energy demand of a more electric aircraft (MEA). The impact is maximized with the components scaled according to performance metrics and two-speed gear ratio optimization. The net fuel advantage is demonstrated for different ground operation modes, taxi times, and flight path lengths.


More Electric Aircraft Conversion to All-Electric
During Ground Operations: Battery-Powered Landing Gear Drive System Jakub Deja , Iman Dayyani , Varun Nair, and Martin Skote Abstract-Raising awareness about environmental issues moves the aerospace industry toward electrification, and the corresponding solutions are already present at some airports.However, commercial aircraft are the missing links in claiming all-electric ground operations.They rely on fossil fuels without any electric alternative due to the technological inability to store large amounts of energy while maintaining a low weight of batteries.The issue diminishes if an electric system uses only a fraction of the energy normally consumed by the engines and comprises kinetic energy recovery.Accordingly, this article demonstrates the landing gear drive system for a narrowbody airplane, which has the sustainable and economic means to replace all onboard engines throughout ground operations.The system is simulated in MATLAB/Simulink and leads to the kinematic results that are based on the real drive cycles.The kinematics are subsequently used to estimate the overall on-ground power and energy demand of a more electric aircraft (MEA).The impact is maximized with the components scaled according to performance metrics and two-speed gear ratio optimization.The net fuel advantage is demonstrated for different ground operation modes, taxi times, and flight path lengths.
Index Terms-Aircraft, aircraft propulsion, energy recovery, energy storage.

I. INTRODUCTION
T HE aviation industry is subjected to growth, and the number of passenger aircraft in service is predicted to double by 2041 in comparison to the value from 2020; accordingly, there are increasing concerns about greenhouse gas emissions [1].Various bodies act to mitigate the harmful impacts, and for example, the European Union Aviation Safety Agency [2] prepared the Flightpath 2050 strategy that strives to limit greenhouse gas and noise emissions.The aircraft manufacturers address the sustainable goals by developing more fuel-efficient and environmentally friendly aircraft.This is being achieved with an MEA where the systems that were originally powered by pneumatics and hydraulics are replaced with electric alternatives [3].The ongoing electrification aligns well with the postulate of the Flightpath 2050, which requires emission-free taxi movement by 2050 [2].To meet this target, the widely used ICEs must be replaced including the APU, which burns approximately 2 kg/min [4] of jet fuel and the main engines that burn 7.7 kg/min per engine [5] during ground movement of short-haul aircraft.Both ground and onboard systems have been proposed for replacing the ICE of aircraft, of which the most impactful are summarized in the following.
The improved ground operations of aircraft receive increasing attention, and there are multiple publications that bring novelty to electric taxiing, APU removal, energy storage onboard commercial aircraft, and gear-aided takeoff.There are many electric taxi projects to date, including Safran's EGTS, TaxiBot, WheelTug, and the L-3's concepts [6].There is also an outside-of-the-box idea that integrates both ground-fit and onboard systems.Rohacs and Rohacs [7] led the GABRIEL project that uses a magnetic levitation ramp to electrically aid a takeoff and dissipate energy during landing.This ramp is combined with a separable tug device, which is used for taxiing.Nevertheless, reliance on the ground-fit deceleration systems without any backup led to safety concerns that must be addressed.Each of the systems listed above was brought to improve the economy and sustainability of aircraft.The ground movement of aircraft with the main engines off leads to reduced noise and jet blast, making the ground handling personnel safer.Simultaneously, the overhaul cost of main engines per flight is reduced, and the probability of unexpected FOD becomes smaller.The reduced brake wear rate occurs in contrast to the conventional ground movement, where the main engines produce excessive thrust for the taxi, and therefore, additional braking action is required.The unused brakes cool down much faster after a landing, and therefore, a faster turnaround can be executed.These aspects together with the fuel savings lead to as much as 18% of mission cost reduction, which is equal to 1470 euros per standard European flight [7].The list of advantages is longer for the onboard systems because reliance on the ground personnel, and the tugs is reduced.
Currently, the onboard ETSs obtain their electric power from an APU rather than the main engines during taxiing due to the APU's lower fuel consumption.However, to support this approach, a specially designed APU with a higher electric power output is necessary.Lukic et al. [8] estimated that an 80-ton single-aisle aircraft (Airbus A320 family size) requires as much as 240 kW, whereas the commonly fit Honeywell 131-9A APU provides only 90 kW of electric output meaning that additional weight penalty associated with electric taxiing is expected.The recent developments in the ETS focus solely on power utilization of the system itself when, in fact, the all-electric ground operations require energy storage that satisfies the power demand of multiple systems of an MEA.Pagonis [9] defined the electric loads for the key systems of a B787, including: 1) ECS [352 kW]; 2) hydraulics [40 kW]; 3) flight controls [14 kW]; 4) fuel pumps [32 kW]; and 5) forward cargo air conditioning [60 kW].A similar analysis was done by Wheeler et al. [10] who estimated that the power demand of the ECS equals 210 kW, and the fuel pumps drain 10 kW.In addition, the author stated that the operation of the landing gear takes from 5 to 70 kW of power, and the engine starters consume 200 kW.Herzog [11] approximated the electrical power consumption of the ECS for both a 100passenger aircraft and a 350-passenger aircraft with the values reaching 90 and 400 kW, respectively.
To fulfill the energy requirements, suitable energy storage is needed, and the best specific energy can be obtained with fuel cells.Stockford et al. [12] assessed the benefit and performance impact of the hydrogen-powered fuel cells and claimed reduced weight in comparison to batteries.The primary downside of hydrogen and fuel cells is their lack of technological maturity, which leads to a tremendous but justified amount of objections coming from aerospace bodies.In contrast, the Li-ion batteries are already present onboard the more electric Boeing 787 though their first years in service were badly eventful [13].The present advancement of battery technology permits specific energy up to approximately 300 Wh/kg [14], [15].
The issue of low specific energy can be minimized by adding the kinetic energy recovery capability.Heinrich et al. [16] were the first to merge the advantages of the KERS into the ETS and investigated the regenerative braking capability during taxi for short-haul aircraft.They stated that KERS coupled with ETS can recover as much as 15% of energy required for taxi cycle.Their analysis excluded the possibility of energy recovery during a landing roll, which would significantly increase the amount of harvested energy [17].Nonetheless, the maximum charge rate of Li-ion batteries is a limiting factor.At present, the battery can be charged up to 3 C [18], while, recently, the Li-ion battery that sustains 6 C has been developed [19].
Combining these aspects together and adding the capability of regenerative braking during landing potentially allow us to consider the battery-powered ground operations with the landing gear drive system for a short-haul aircraft as sustainably and economically feasible.We propose a system that involves energy storage onboard, and therefore, an APU becomes redundant.Removal of this device and related components allows to save as much as 630 kg [20].Furthermore, each aircraft has a Master Minimum Equipment List that defines what components and systems are critical for aircraft operation, and the APU is unnecessary unless the flight is subjected to the ETOPSs where increased redundancy of the electric system becomes safety critical [21].The main contributions of this article are given as follows: 1) literature review to discuss the state-of-the-art of electric taxi and demonstrate the possibility of a battery-powered version; 2) demonstration of the battery-powered landing gear drive system and assessment of its impact on an Airbus A320 aircraft architecture; 3) estimation of electric energy amount required to power the novel system together with the remaining MEA systems during ground operations; 4) analysis of energy recovery capability during landing phase and taxiing; and 5) definition of the fuel savings equations and simulations to provide a comprehensive benchmark for all-electric ground operations against the conventional methods.

II. LANDING GEAR DRIVE SYSTEM AND IMPACT ON AIRCRAFT
A high-power landing gear drive system is demonstrated in this article, which comprises the electric machines, the four-quadrant power converters, the drivetrain, and the large-capacity energy storage.These components are more powerful than in the EGTS and allow an aircraft to accelerate faster, while the high-capacity battery allows executing the ground operations without any ICE.In addition, the system aids the takeoff in parallel with the main engines.The landing gear drive system leads to further reduced noise and air pollution than the EGTS system, which sources power from the APU [22].Fig. 1 shows the system's demonstrator and the corresponding changes to the Airbus A320.
Multiple modifications to the aircraft are required and primarily; the redesigned landing gear is needed to accommodate the large electric machine and the drivetrain.Fig. 1(a) shows the electric machine located in the main landing gear, which is similar to Safran/Honeywell's EGTS [22].Nevertheless, the landing gear drive system differs from any solution proposed to date with much higher torque and power, and therefore, it takes more volume.The four-quadrant power converters are needed to control the electric machines during both propulsive and regenerative cycles.They are located in proximity to electric machines to minimize the wiring weight and allow better heat exchange with ambient air.The controller is located in the electrical cabinet to ensure a safe operation environment for this electric device.To further increase safety, a pilot is capable of rapidly disengaging the system during any malfunction.
The battery pack is located in the fuselage and can be charged by either a ground power source or the main engines.If the battery depletes during ground operations, the aircraft can continue to taxi with the main engines.The battery is modular where each module is connected in parallel, meaning that the ground crew can remove the battery cells that are redundant for shorter taxi cycles.This design objective allows for reducing the weight and, accordingly, maximizing fuel savings.
Another weight saving is achieved by completely removing the APU and corresponding components, which leads to 265 kg of weight reduced [20].In these circumstances, the battery can be considered a backup energy source for electrical systems during an ETOPS flight.

A. Dynamic Model
This section introduces the dynamic model, which is created to determine the energy transfer during ground operations of an Airbus A320 equipped with the landing gear drive system.It is based on the free-body diagram shown in Fig. 1(b) and takes into account the following factors: 1) the landing gear drive system; 3) the standard deceleration systems; 3) thrust of main engines; 4) drag; and 5) rolling resistance of aircraft.The incline of the runway is only used to validate the sufficiency of landing gear drive system torque during taxiing.Fig. 2 shows in the block diagram the relationship between each subsystem and the influence of each factor on the aircraft body.
The longitudinal model of the aircraft body is considered as a solid subjected to external forces a = ( F(t)/m), and the lower order kinematics are obtained by integration with respect to time.The following forces are applied (all the variables are defined in the nomenclature, and links are provided in an online version of this article): where each factor is described as and each system of aircraft has modeled saturation limits with the following saturation logic: The saturation logic for electric machines f sat is a scaled replica of Yasa 750 R experimental data [23].The two-stage gearbox denoted with "i" shifts when the aircraft reaches 12 m/s.This value is set to ensure that the whole taxi cycle is executed in the first gear, whereas, during a takeoff and a landing, the second gear is engaged The results' validation method relies on the law of conservation of energy.Therefore, the total energy in the system shall remain constant where the energy balance of each factor equals The fuel saving exerted during the aided takeoff is derived from the dynamic model by assuming that the amount of force applied by the landing gear drive system is subtracted from the main engines and multiplied by TSFC This assumption means that the pilots have to derate the thrust level during takeoff; however, this is a common practice in civilian aviation [24].To estimate how many kg of fuel can be saved, the TSFC metric is used for static conditions and is equal to 8 [25].

B. Initialization Parameters
The created model consists of many variables that should accurately represent a narrowbody aircraft, and therefore, the values for the Airbus A320 are used.An aircraft is subjected to drag during ground operations, and the drag coefficient differs depending on multiple environmental and operational factors, such as flaps and slats positions.Sun et al. [26] developed a stochastic hierarchical model to provide aerodynamic coefficients.They have provided the aerodynamic coefficients that reflect the landing gear and the flaps' extension.The empirical C d value based on trajectory data of an Airbus A320 equals 0.120.Their study assumes that a reference area is a wing surface area, which is equal to 122.4 m 2 for an A320 [27].
Besides drag, the aircraft is subjected to rolling resistance on the ground.The experimental data provided by Yager et al. [28] indicated that the coefficient of rolling resistance changes with velocity.They experimentally assessed the rolling resistance of 40 × 14-19 tires, which are dedicated to similar aircraft, a Boeing 737, and captured the values at 3, 51, and 82 m/s, which are equal to 0.01, 0.014, and 0.025, respectively.These values were used in the Simulink model as the breakpoints and were linearly interpolated for any velocity value between.Table I combines the parameters used in the simulation initialization.
The dynamic model reflects the angular speed changes, which are influenced by the gear ratio and wheel radius.The latter is equal to 0.58 m according to Dunlop Aircraft Tyres [33] tha supplies the tires to an A320.
The flight path distance influences the amount of fuel burned in-flight due to the weight penalty.ModernAirlines [31] defined the maximum range of an A320 aircraft as 6100 km; however, the linearization of block fuel intensity is accurate only for the flight paths considerably shorter than the maximum range, as described in Section VI, and consequently, the flight paths from 100 to 5000 km are taken into account in this study.

C. Drive Cycles
The realistic drive cycles that consist of taxiing, takeoff, and landing are needed to improve understanding of the Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

TABLE I DYNAMIC AND FUEL SAVING SIMULATION INITIALIZATION PARAMETERS FOR THE NARROWBODY AIRCRAFT
energy transfer during the ground operations, and therefore, the real data were captured for two commercial missions across Europe.In addition, the quasi-realistic model was made for the third mission by merging the taxi cycles captured by Heinrich et al. [16] with takeoff and landing phases from other flights.The recorded data are presented in Fig. 3 and are used as the velocity demand in the MATLAB/Simulink model.
The first recorded flight W65002 shown in Fig. 3(a)-(c) took place on June 25, 2022, from Luton, U.K. (LTN) to Krakow, Poland (KRK) and passed the great circle distance of 1425 km.The taxi-out cycle given in Fig. 3(a) took 280 s and was immediately followed by the takeoff.The landing presented in Fig. 3(b) was followed by taxi-in for approximately 3 min.Fig. 3(d)-(f) presents the flight FR7806 between Athens, Greece (ATH) and Luton that occurred on July 13, 2022.The taxi cycles were comparable in length to the first flight, but the flight path was much longer with a great circle distance equal to 2450 km.The first flight was executed with the Airbus A321NEO and the second with the B737-800 aircraft, which are designated for mid to long-haul operations but are often used on short flights as well.Each breakpoint consists of longitude, latitude, and timestamp, where each increment was equal to 1 s.Due to the poor GPS receiver reception, the signal was denoised by applying the moving mean with local sixpoint values.Fig. 3(g)-(i) presents the taxiing data captured by Heinrich et al. [16] merged together with takeoff and landing from W65002.This figure is an accurate representation of flight TS682 between Toronto (YYZ) and Calgary (YYC), where the great circle distance is equal to 2700 km.This flight is performed by, among others, the Airbus A320 family aircraft.

III. ELECTRIC POWER DEMAND OF MORE ELECTRIC
AIRCRAFT The systems of MEA require a continuous high power supply, and this section explains how much energy the onboard energy storage must provide to allow all-electric ground operations.The longest mean taxi cycles according to EUROCON-TROL [32] are used, where the taxi-out takes 26.3 min and the taxi-in occurs for 14.5 min.The taxi cycles from Fig. 3 were used in a loop to achieve these longer taxi times.Fig. 4 shows power against time for the electric power-consuming systems onboard.The warmup and cooldown cycles of the main engines are included in Fig. 4(a) and (b) and absent in Fig. 4(c) and (d).The power demand values for all but the landing gear drive system are traced from Section I and are taken as an average approximation.The consumption of the landing gear drive system is derived from the dynamic model explained in Section II-A, and it reflects the recorded taxi cycles.
The cycle in each outbound phase shown in Fig. 4(a) and (c) begins when the ground power is disengaged and electric energy is sourced solely from the battery.In parallel, the ECS is working to provide air conditioning to the passengers onboard.The average power demand is equal to 150 kW although this depends on weather conditions and climate zone [11].
More systems start to consume energy as the aircraft begins to move.The electric actuation drains 20 kW and increases for a short period of time when the pilot checks all control surfaces' preflight.Fig. 4(a) indicates that the highest power demand occurs when the main engine's startup procedure begins.The bleedless main engine starters consume 100 kW each together with the 20 kW from fuel pumps.Once running, the main engines supply electrical power to all systems and also charge the battery during the warmup, and consequently, the analysis stops.Similarly, the power usage during cooldown is neglected in Fig. 4(b).The energy transfer analysis continues up to the takeoff and landing phases in Fig. 4(c) and (d) because these graphs disregard the warmup and the cooldown periods for the main engines.The assumption of neglecting the warmup and cooldown is considered a potential future technology development, which would make energy recovery during the landing phase critical.The aircraft requires 194 kWh during the presented ground operations where 48 kWh is used for the landing gear drive system.The capability of kinetic energy recovery provides over 10% of the required propulsive energy.

IV. LANDING GEAR DRIVE SYSTEM COMPONENTS'
SIZING AND OPTIMIZATION The primary aim of the landing gear drive system is to reduce the environmental footprint of commercial aviation.Even if the ground operations are completely emission-free, sustainability can be maximized by wisely scaling and optimizing the components to reduce the weight penalty.Accordingly, the system-level sizing and optimization algorithm is proposed and shown in Fig. 5(a).The initial step consists of the power and energy analysis, which was accomplished in Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.Section III.This is followed by the battery size selection that matches the overall energy consumption during the all-electric ground operations.Section III explained that the maximum energy needed equals 194 kWh, and therefore, the maximum battery capacity needed is rounded to 200 kWh, which consists of four modules, 50 kWh each.
Once the battery capacity and the maximum C-rate are known, the electric machines, the drivetrain, and the power converters are scaled according to the maximum allowable charge rate of the battery leading to 600 kW of total power.Next, the weight for each component is estimated according to the performance metrics disclosed in Table II.The efficiency rates for the electric machine and the power converter are scaled from the experimental data provided by YASA [23], while the drivetrain is assumed to be 97% efficient.The total efficiency of the system is demonstrated in Fig. 5(b).The optimization of system performance is done by adjusting the gear ratios of the two-stage gearbox.To optimize taxiing, the taxi cycles from Section II-C were extracted and simulated for the multiple gear ratios.Fig. 5(c) shows that a gear ratio equal to 14 allows using the least energy to propel the aircraft.The second gear was optimized according to both takeoff fuel saving and landing energy recovery.Fig. 3 displays the results for both where the optimum ratio is equal to 3.2 for landing and 2.8 for takeoff.

V. INTERNAL COMBUSTION ENGINES' USAGE
The fuel combustion during ground operations begins as soon as a pilot turns on the APU at the gate and ends once it is turned off after a ground power unit is connected to the receptacle of the aircraft.Ultimately, more phases than the taxiing itself must be taken into account when trying to estimate the fuel burn by aircraft on the ground.Fig. 6 presents the usage of the onboard ICEs in a broader context and includes: 1) predeparture; 2) taxi-out; 3) landing; 4) taxiin; and 5) postarrival phases.
During the predeparture, the aircraft stands at the gate and the passengers' board.The electricity is supplied by the ground power unit, while the APU is started as the departure time gets closer.Airport operators limit how much in advance it can be launched and taking Heathrow as an example, and the device can be started no earlier than 15 min in advance for a narrowbody aircraft.This time is much greater for large aircraft; an Airbus A380 is allowed to start the APU 1 h in advance [29].There is often a mismatch between regulations and practice, especially when aircraft fly many missions in one day and a delay occurs.Then, the device may be kept on throughout the whole time at the gate.Padhra [30] captured the data for the single-day turnarounds around Europe and showed that the time between the APU switched ON and the aircraft departing is equal to 15 min or more for over half of the flights.

TABLE II SYSTEM-LEVEL PERFORMANCE METRICS
This is exceptionally important when considering a of delay where Zijadić et al. [51] measured that 64% of arrivals and 46% of departures were delayed at Sarajevo airport.
The taxi-out phase takes place after predeparture and begins with the pushback during which the main engines are started one after another unless the operation mode is single-engine taxiing (SET); thereafter, the APU is turned off.There is a small overlap with both types of engines running in parallel, and the data recorded by Winther et al. [34] showed that it equals 30 s on average at the Copenhagen airport.The timeline is different for EGTS, where an APU is kept on throughout the taxiing until the main engines have to be started for the warmup procedure, which typically takes 3 min [52].A similar overlap between the APU and the main engines still occurs with EGTS.The taxi-out ends at the runway and the aircraft begins to takeoff.The main engines are set to higher thrust levels and accordingly consume much more fuel.If present, the landing gear drive system works at full torque to aid the main engines in accelerating the aircraft until 70 m/s is reached.
After arrival, the multiple deceleration systems are engaged during the landing, and if weather conditions allow, pilots use the idle thrust reversers.The landing gear drive system also accounts for deceleration, and the electric machines work at peak torque rate in the regenerative quadrant leading to kinetic energy being harvested back into the battery.
The fuel consumption throughout the taxiing-in begins with the main engine's cooldown period that normally takes 3 min [52].The procedure begins to differ between the ground operation modes once the main engines can be switched off.One engine is switched off for the SET procedure and both for the EGTSs.If the EGTS is operated, the APU is turned on by the end of cooldown once again with a small overlap that is simulated as 30 s.
Once the aircraft arrives at the gate, the main engines are immediately switched off to enable the ground crew Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.to begin their procedures.Then, the APU is disengaged as soon as the ground power is plugged, which takes 3 min for 73% of interday flights [30], whereas the mean time for the Copenhagen airport equals more than 4 min [34].The maximum period of time for running the APU inbound is regulated by airport operators, and Heathrow allows 5 min maximum.We assume that the device is deactivated 3 min after arrival.

VI. NET FUEL SAVINGS
The fuel saved during ground operations is reduced by additional fuel burned due to weight penalty in-flight.The weight of the battery is estimated regarding the battery-specific energy W bat = E dem /S E bat , and the total weight of the electric machines, the power converters, and the drivetrain is estimated according to the specific power W pow = P dem /S P em + P /S P dr + P dem /S pcon .Finally, the total weight of the system is equal to Once the weight of the system is known, we estimate its relation to increased in-flight fuel consumption.This is achieved with the block fuel intensity metric that factors the weight of fuel burned per weight of payload per unit of flight distance, albeit this parameter is highly influenced by aerodynamics and remains nonlinear.Researchers often linearize it, which is accurate for flights up to approximately 75% of maximum aircraft range [35].Zheng and Rutherford [36] provided the block fuel intensity for the narrowbody aircraft manufactured between 1969 and 2019; the values varied between 200 and 390 grams of fuel per metric tonkilometer.These values were compared to Gao et al.'s [37] experimental aircraft data that comprised fuel usage against payload for the Airbus A321 flights from Beijing (PEK) to Chengdu (CTU) and the Boeing 737 flights from PEK to Shanghai (SZX).The values were between 220 and 350 for an A321 and 230 and 330 for a B737.Yanto and Liem [38] used a slightly different metric of kilograms of fuel per seatnautical mile.After assuming that a single seat equates to 104 kg of payload [39], the values were equal from 210 to 260.According to the gathered data, the conservative value of 315 g per metric ton-kilometer was used in the simulation.
The block intensity fuel among the parameters given in Table I allows benchmarking the net fuel savings of the landing gear drive system against the other ground movement methods.First, we calculate the fuel consumption exerted by both the APU and the main engines during ground movement Similarly, the fuel consumption for the inbound ground operations is equal to The division between outbound and inbound ground fuel consumption is important for the incurred weight penalty.If the landing gear drive system saves fuel during inbound ground operations, the mass of saved fuel can be subtracted from the total weight penalty of the system.The incurred fuel burn due to weight penalty is calculated Once the fuel consumption during the ground operations and the additional fuel burn due to weight penalty are known, the estimation of fuel saving is done Some flights at smaller airports have shorter taxiing times than the required warmup and cooldown periods.Consequently, an aircraft will turn on the main engines at the gate, and such a special case is considered in the calculus by assuming that t TXO equals t WUP , and T PD is corrected with the initial difference between t TXO and t WUP .A similar logic is applied to inbound ground movement if t CD exceeds t TXI .The presented equations also allow neglecting completely the warmup and cooldown cycles.In these circumstances, FU wup , FU cd , t WUP , and t CD are equal to zero.The recorded ground movement data from Section II-C are now used to estimate the fuel savings for each flight.Table III presents the expected fuel savings for the system with modular battery packs.
The results show that each mission augmented with the landing gear drive system has improved fuel economy.The greatest savings are calculated against the EGTS because the weight penalty with modular batteries is lower than the weight penalty of the EGTS, which also requires the APU.Therefore, the economy is improved during both ground movement and in-flight.
Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

VII. PARAMETRIC STUDY FOR FUEL SAVINGS
Each aircraft mission is unique when considering the duration of mission phases.In this section, the landing gear drive system is simulated with respect to varying flight distance and cumulative taxiing time.Different taxiing durations are considered although the postdeparture and postarrival times remain constant at 15 and 3 min, respectively.The cumulative taxi times are distributed with respect to the proportion between taxi-out and taxi-in times derived from the mean taxi duration for 555 airports provided by EUROCONTROL [32] and equal two-third and one-third, respectively.Fig. 7 presents the parametric study results for an Airbus A320, which performs standardized missions with varying cumulative taxi times and flight path lengths.
During short-haul flights, the landing gear drive system provides net fuel savings against each conventional ground operation mode.The greatest fuel savings are achieved in Fig. 7(a) and (d), where the twin-engine taxiing (TET) operation mode was replaced.Depending on the cumulative taxi time, the system can be advantageous for all simulated flight distances, including medium-haul flights.The net savings are lower in Fig. 7(b) and (e), which shows the comparison with SET.Nevertheless, the system remains advantageous for flights up to 3000 km during longer taxiing times.Fig. 7(c) and (f) depicts net fuel saving against the EGTS.Albeit the lower peak savings are indicated, the results are more consistent across all cumulative taxiing times.The savings are expected for every flight up to 1200 km, which is the case for nearly every domestic flight.Finally, the results from Fig. 7(d)-(f) show that modular batteries maximize the net fuel savings where the intermittent lines indicate that the additional 50-kWh battery modules are inserted when cumulative taxiing time exceeds 200, 950, and 1700 s.Furthermore, the benchmark with the EGTS in Fig. 7(f) shows the negative slope when the first and second modules are present.This is caused by a smaller weight penalty incurred by the landing gear driving system than by the EGTS.
The fuel saving is accrued at the price of electricity needed to charge the battery.Heathrow Airport Ltd. [53] meters the ground unit electricity supply and charges 19 pence per kWh.For the full, 200-kWh charge of the system at 90% charging efficiency, it would cost $42.This cost is offset by the fuel saving reaching as much as 500 kg.The assumption of 250-kg fuel saving, which costs $1.71 per kg [54], yields $373 saving per flight, and corresponds to 7% of a total fuel price reduction for a flight from London Heathrow to Glasgow [55].

VIII. LANDING GEAR DRIVE SYSTEM IN FUTURE
Any novel aircraft system implementation is more likely to succeed during the conceptual stage of aircraft development rather than during retrofitting to the air vehicles already in service.Retrospectively, the Airbus A350 program was launched in 2005, while the aircraft entered service ten years later, and similarly, eight years elapsed between the announcement of Boeing 787 and its entering into service in 2011 [56].In parallel, the electric vehicle components were significantly improved.Energy storage is a textbook example because the specific energy of battery cells available for aviation nearly doubled in the last decade [14], and this performance metric will unquestionably improve in the coming years.Accordingly, commercial aircraft concepts that are being discussed at present should include the landing gear drive system with performance metrics that will be available in near future.The predicted increase in the number of commercial aircraft in service will inevitably lead to increased taxi times unless the airport's operational efficiency is increased by, for example, fully autonomous taxiing.In the automotive industry, reduced congestion and traffic flow improvement are highlighted when describing autonomous cars [57], and the same conclusion is drawn for the fully automated taxiing of aircraft [8].The landing gear drive system is a large step toward autonomous taxiing for multiple reasons.Primarily, the operation of the electric taxiing system is much simpler compared to the main engines.Turbofans and turboprops are very complex, and pilots have to spend a considerable amount of time following the checklists to ensure a safe operation, whereas electric machines would require much less attention.Accordingly, the autonomous operation algorithm would be simplified as well.Another factor that currently hinders the autonomous taxiing idea is the jet blast, which is a serious threat to airports and ground crew.Implementation of the landing gear drive system allows for keeping the main engines off until an aircraft reaches a runway.Similarly, the risk of FOD would be inapplicable because the aircraft would be solely propelled by wheel drive.Finally, the high-power landing gear drive system offers better controllability because the torque response would be nearly instant compared to the main engines that take time to spool up and provide thrust.
The energy storage accounts for more than half of the total weight in the landing gear drive system; however, the upcoming technologies potentially allow reducing or completely removing the weight penalty incurred by the proposed system.This can be achieved by combining the wheel drive with large energy storage already available onboard future aircraft.
Xie et al. [58] reviewed the hybrid electric-powered aircraft concept that incorporates large batteries.They estimated as much as 30% fuel economy improvement with the hybrid technology.A further step is an all-electric aircraft, which receives increasing attention.This concept assumes that the ICEs are absent onboard, and the propulsive power is generated with electric machines only.Currently, the low specific energy of batteries is a restraining factor for this technology [59], [60], [61].The landing gear drive system would potentially reduce the severity of poor battery performance by increasing the energy efficiency of all-electric aircraft during ground movement.The energy transfer benchmark between propeller-driven taxiing and wheel-driven taxiing would provide the answer about feasibility.
Battery technology has seen significant improvements, such as fast charging, high efficiency, and lightweight designs.Furthermore, the current research areas include structural batteries that are also known as massless energy storage [62].They aim to significantly reduce the weight of battery-powered vehicles by integrating the battery into the structure, and consequently, the requirement for a separate battery is reduced or eliminated.
The discussed advantages of future technology are combined in Fig. 8, which demonstrates the projected increase in the net fuel savings of the landing gear drive system.
The system-level improvements will primarily impact the weight penalty.The benchmark in Fig. 8(a)-(c) is made with an assumption that the battery capacity and the power of the system remains unchanged, but the improved performance shown in Table II allows weight reduction.Fig. 8(a) shows that the net fuel savings against the SET will exceed 500 kg in 2030, and the growth will continue to 2040 [see Fig. 8(b)], while there is a stagnation toward 2050, as depicted in Fig. 8(c).This lack of improvement is caused by the warmup and cooldown periods of the main engines during which jet fuel is consumed.The future technology may allow skipping these cycles, and Fig. 8(d) depicts the fuel savings of the 2050 technology level with the warmup and cooldown cycles neglected.Further weight reduction can be achieved by considering an MEA with the large energy storage already fit onboard.Thus, the battery weight can be ignored when calculating the weight penalty of the landing gear drive system, and Fig. 8(e) indicates that the net fuel savings would exceed 1000 kg for the longest flights.The slope shows that the amount of fuel saved increases also with flight distance.In other words, the weight penalty without a battery will be lower than the mass of fuel needed for inbound ground operations in 2050.
Instead of reduced weight, the design objective may be set to maximize harvested energy during landing.Therefore, the weight of the system in the future would remain constant with the mechanical power increased.Fig. 8(f) shows the amount of energy recovered by the landing gear drive system during the W65002 landing for different technology levels.In 2050, the 1600-kW electric machines would be capable of harvesting over 4 kWh, which corresponds to about 16% of dissipated energy during landing by all deceleration systems fit onboard.

IX. CONCLUSION
The call for electrification of aviation was addressed in this article with a landing gear drive system for short-haul MEA.This work introduced the novel landing gear drive system that combines multiple academic developments and introduced regenerative braking during landing to maximize the energy efficiency of operation.Therefore, the primary obstacle for electrified aircraft, namely, the specific energy of the battery, was overcome to the extent that the system was proven to be both sustainably and economically feasible.
The landing gear drive system depleted approximately a quarter of the energy storage capacity during taxiing, while the rest was used by the ECS, actuation, fuel pumps, and main engine starters.The battery present onboard consisted of 50-kWh modules that stacked up to 200 kWh.This much energy allowed an Airbus A320 to execute the all-electric taxi cycles for as long as 26 min.To maximize the energy efficiency, the kinetic energy recovery was added during the landing, and it allowed the harvesting of approximately 10% of available energy.The anticipated quantity of fuel savings depended on flight distance and taxiing time.The calculation according to the present fuel and electricity prices demonstrated considerable cost reduction of short-haul flights with 7% of a total fuel price reduction for a flight from London Heathrow to Glasgow.The fuel saving projections were also made according to the future performance metrics, and in 2050, the landing gear drive system would reduce the overall consumption for nearly every mission.In the best cases, more than 500 kg of fuel would be saved.
The presence of the large battery fit onboard will further accelerate the electrification of the aerospace industry because it can be shared in-flight with multiple systems, such as hybridelectric propulsion.Consequently, researchers and aircraft manufacturers will be able to work on concepts that were otherwise inapplicable due to the limited specific energy of batteries.In the distant future, it is likely that the industry will move away from fossil fuels, and therefore, future work should focus on the landing gear drive system feasibility for all-electric aircraft.

Fig. 1 .
Fig. 1.Demonstrator of the landing gear drive system.(a) System-level presentation among with impact on aircraft.(b) Free-body diagram of the aircraft.

Fig. 2 .
Fig. 2. Dynamic model shown as block diagram disclosing the relation between multiple systems and the validation algorithm.

Fig. 5 .
Fig. 5. Scaling and optimization of the landing gear drive system.(a) Optimization algorithm.(b) Combined efficiency of electric machines, power converters, and drivetrain.(c) Gear ratio optimization for taxiing.(d) Gear ratio optimization for landing and takeoff.

Fig. 6 .
Fig. 6.ICE usage during ground and transition phases of the mission.

Fig. 7 .
Fig. 7. Fuel saving simulation results for technology level from 2022.Negative values indicate the excessive fuel consumed due to the weight penalty.The first row assumes that all battery modules are present, whereas the second row reflects the module removal for shorter taxiing cycles (indicated by the dashed lines).The landing gear drive system was benchmarked with (a) and (d) TET, (b) and (e) SET, and (c) and (f) EGTS and tugs.

Fig. 8 .
Fig. 8. Simulation results of fuel and energy saving for future technology.Negative values indicate the excessive fuel consumed due to the weight penalty.(a) 2030 performance metrics against SET.(b) 2040 performance metrics against SET.(c) 2050 performance metrics against SET.(d) 2050 performance metrics combined with the neglected warmup and cooldown cycles against SET.(e) 2050 performance metrics combined with the battery weight, warmup, and cooldown cycles neglected.(f) Energy recovery during W65002 landing for present and future performance metrics.
Date of publication 28 March 2023; date of current version 16 March 2024.This work was supported in part by the Engineering and Physical Sciences Research Council (EPSRC) under Grant 2517982 and in part by Airbus Operations Ltd. (Corresponding author: Jakub Deja.)The authors are with the School of Aerospace, Transport and Manufacturing, Cranfield University, MK43 0AL Cranfield, U.K. (e-mail: jakub.deja@cranfield.ac.uk).
dem Aircraft ground speed demand set by drive cycle in the longitudinal frame.

TABLE III SIMULATION
RESULTS FOR THE GROUND MOVEMENT DATA OF THREE DIFFERENT MISSIONS