Sizing Methodology and Energy Management of an Air–Ground Aircraft with Turbo-Electric Hybrid Propulsion System

Round 1

Reviewer 1 Report

An interesting study on air-ground aircraft.

The original contributions are not very clear. It should be mentioned in the introduction and conclusion.

Power electronics part, especially bi-directional DC-DC could be explained a little bit more. Maybe some simulation results can be added.

No mention of standards. Many critical standards exist in aviation and automotive.

Is HAGA efficient enough? Can it be used in the future? These questions need clarification.

Author Response

Dear reviewer,

 

Thank you for the positive comments and suggestions concerning our manuscript entitled "Sizing Methodology and Energy Management of an Air-ground Aircraft with Turbo-electric Hybrid Propulsion System" (aerospace-2010164). We have carefully reviewed the comments and have revised the manuscript accordingly. Our responses are given in a point-by-point manner below. All these changes are highlighted in an additional copy of our manuscript.

 

Point 1: The original contributions are not very clear. It should be mentioned in the introduction and conclusion.


Response 1: Thanks for your question. With vertical take-off and landing (VTOL) capability, the hybrid air-ground aircraft (HAGA) is a promising solution for complex road conditions, high traffic pressure and harsh environments. The turbo-electric hybrid propulsion system (TEHPS) based on the high-power turboshaft engine, hybrid energy storage system (HESS) and distributed electric propulsion units has been designed and proposed for the large payload, high performance HAGA. Research on the HAGA sizing methodology was carried out based on genetic algorithm to obtain the power system parameters for optimum take-off weight and maximum cruise efficiency. A top-down energy management framework was creatively proposed to optimize the energy distribution between turbine generation system (TGS) and hybrid energy storage system, as well as the power allocation between batteries and supercapacitors. The software simulation results for the amphibious mission profile show that the deployment of turboshaft engine and HESS optimises the power performance and effectively reduces the mass and volume of energy storage system. It also demonstrates the superior fuel economy and emissions performance of the TEHPS system compared to conventional oil-powered air-ground aircraft. The introduction and conclusion sections with information about our contribution have been modified in the new manuscript.

 

Point 2: Power electronics part, especially bi-directional DC-DC could be explained a little bit more. Maybe some simulation results can be added.

Response 2: Thanks for your suggestion. The section on power electronics has been added and revised in the new manuscript. For the bidirectional DC/DC converter, only its efficiency is considered in the simulation, set to a constant value of 0.95, and its device losses are calculated according to Equation 12 for the different charging and discharging cases. Therefore, the simulation results for bidirectional DC-DC are not analyzed in this paper.

 

Point 3: No mention of standards. Many critical standards exist in aviation and automotive.

Response 3: Thanks for your question. Safety requirements in the aviation and automotive industries have served as a model for other industries to follow. Hybrid Air-Ground Aircrafts are a type of flying cars that combine the functions of flying in the air and driving on the ground. As defined in the DO-178 and ISO 26262 standards, exposure, controllability and severity are among the key indicators. In the first indicator, the HAGA with a turbo-electric hybrid propulsion system reduces the probability of system failure due to the presence of an auxiliary power source that allows emergency landings with stored energy in the event of engine failure. However, the probability of failure is slightly higher than for ground vehicles due to the airborne operating conditions. For the second indicator, the TEHPS consists of multiple power, energy and drive sources, with relatively high system complexity and slightly less controllability than ground vehicles. Compared to conventional aircrafts, however, they have better controllability due to their ability to vertical take-off and landing. Regarding the indicator of severity, like the previous results, HAGA is higher than ground vehicles and lower than the typical aircraft due to its low cruise altitude. These explanations have been condensed in the new manuscript.

 

Point 4: Is HAGA efficient enough? Can it be used in the future? These questions need clarification.

Response 4: Thanks for your question. Electric power outperforms hydrocarbon fuel in terms of reduced emissions, noise, hardware complexity, weight, and increased energy efficiency, confirming that electric power is more suitable for air-ground aircrafts. However, the durability and refueling speeds of hydrocarbon fuel are superior to electric power, making hydrocarbon fuel more suitable for long-distance transportation designs. Combining the advantages of both electric power and hydrocarbon fuel, hybrid AGAs has been proposed based on the TEHPS. In addition, the distributed electric propulsion units in the powertrain have excellent aerodynamic characteristics and propulsion efficiency. Therefore, hybrid air-ground aircrafts can offer better energy efficiency and propulsion efficiency than the conventional aircraft configurations.

In recent years, vertical take-off and landing air-ground aircrafts with various propulsion forms have been a hot topic for car and aircraft manufacturers, civil and military research institutes, including the Vahana, City Airbus and Joby Aviation Air Taxi with electric propulsion, Transition and TF-2 with hydrocarbon fuel, Nexus and WD-1 with hybrid power [Ref. 5-13]. As a result, HAGA based on high-power turboshaft engine and hybrid energy storage system has the potential for development and application.

 

We thank the reviewer for the patience and efforts to help us improve the technical content and the grammar in this manuscript. Your generous help is greatly appreciated.

 

Finally, we appreciate for your work earnestly, and hope that the correction will meet with approval.

 

Sincerely

Wenjiang Yang

Author Response File: Author Response.pdf

Reviewer 2 Report

This article presents a methodology to size the turboelectric hybrid propulsion system of an air-ground aircraft using genetic algorithm, as well as to optimize the power/energy management through a hierarchical energy management strategy, which involves a local optimization of fuel consumption and a fuzzy logic control to resolve the power split between the battery and the supercapacitor. The authors apply this modeling approach on an amphibious mission profile and demonstrate its effectiveness on tracking the power distribution and energy consumption through the mission. Comparison is made between the proposed hybrid powertrain with a hybrid energy storage system, a conventional oil-only powertrain, and a hybrid powertrain with a battery-only energy storage system, which shows weight and fuel advantages of the proposed architecture. Overall, this paper can be of great interest especially to researchers studying AGA system design.

 

General comments:

 

The proposed methodology uses operating conditions encountered during the nominal mission to size the TEHPS. The mission profile includes a level cruise segment in the air which apparently assumes no wind. The result would have been more robust if the impact of operational uncertainty such as headwind, gust, and surface inclination on the sizing of TEHPS can be quantified. The mission profile may remain the same, but the TGS may need to be upsized to satisfy the same point performance requirement in case of a gust. Perhaps this can be done as future work. 

 

It would be better to highlight the novelty of the proposed methodology. It is not very clear in the current state how the proposed methodology is superior compared to state-of-the-art.

 

Specific comments:

 

Figure 1: It is unclear from the images how the attitudes and modes differ in each subfigure. It appears that 1(a) shows a top view, 1(c) a side view, and 1(b) an isometric view or the vehicle is tilted. The authors may consider adding reference axes to indicate forward/backward, up/down, left/right directions, etc. Also, if the activity of components (turbogenerator, battery, supercapacitor, and motors) depends on the flight mode, the authors may consider highlighting the active components in each subfigure. 

 

Equation 3: Please clarify the units of P_eng and mdot_fuel in the text.

 

Equation 3 seems to be applicable only to a turboshaft engine of specific size (rated power), with peak fuel flow at P_eng = 4842. What is the engine rated power, and how are the coefficients determined?

 

Section 2.2.3 two lines above Equation 12: “bi-directional inverter efficiency” – Did the authors intend to say “DC/DC converter” instead of “inverter”?

 

Equation 15: In the first equation, the power of diameter should be 4 instead of 5.

 

Equation 15: Please provide some detail or references in the text regarding how relation between C_T and C_P are obtained, e.g., what kind of fan/propeller model is used.

 

Equation 17: In the third equation, what is beta? This equation seems to account for only the total rotor disc area. Do the authors implicitly neglect the vertical aerodynamic force acting on the center body due to nonzero vertical speed? If so, please state it clearly in the text. Otherwise, please show how this force is computed.

 

Equation 17: In the last equation, should the sign of the first term (R_climb/2) be positive?

 

Equation 17: This approach only computes the shaft power required using momentum theory. The actual shaft power required is higher due to viscous effects and other losses. The authors should consider using a figure of merit (ratio of the ideal power, as already computed in Equation 17, to the actual power) to obtain a more realistic estimation of the actual power required for VTOL.

 

Equation 18: How is the cruise lift-to-drag ratio determined? Based on Figure 1, the vehicle does not appear to be a lifting body or have lifting surfaces large enough to generate lift that is equal to the weight. In addition, Equation 18 only computes the drag power (drag x airspeed). For the AGA of interest, it appears that the thrust generated by the ducted fans should be decomposed such that the vertical and horizontal components balance weight and drag, respectively, assuming the body does not generate significant amount of lift. If this is the case, the cruise power requirement should be computed in a different approach, similar to helicopters or quadrotors. The authors should revisit the modeling approach here.

 

Equation 19: Why is the coefficient 1/3600 needed in every term? The power required on ground could have been obtained by simply multiplying V_g on both sides of Equation 13. Also please double check the numeric coefficient of the second term (wind resistance); it is recommended to keep the symbol for air density (rho) to maintain consistency of units.

 

Equation 17-19: The computed power required appear to be the shaft power output by the motors. A motor efficiency is needed in order to correctly estimate the energy flow within the propulsion system. Have the authors accounted for the motor efficiency?

 

Equation 21, 22, and 24: Please define eta_inv in the text. Is this supposed to be the efficiency of the AC/DC rectifier (instead of a DC/AC inverter) connected to the generator in Figure 2? If so, please consider changing the subscript to avoid confusion.

 

Equation 23: It is confusing to show two modes in one equation. The authors should consider splitting the equation into two (one with subscripts ADEPS and rm and the other with GDEDS and hm).

 

The paragraph before Equation 24: What is the logic to switch between the discharging mode (Equation 21) and the charging mode (Equation 24)? The phrase “when the required power is not significant” is vague, and what is the “upper limit” of the SOC of the HESS? These should be clarified in the text.

 

Equation 26: By definition, f_air is the binary air/ground mode flag and is time dependent. However, the energy is integrated over time, so it is unclear how this equation applies to the whole mission. Maybe the authors could replace energy (E) with power (P) so the whole equation applies to any moment in the mission. Alternatively, the authors may simply remove f_air and (1 – f_air) to describe the energy for the whole mission.

 

Equation 29: In the second equation, is the fuel mass computed based on mission analysis, or does it take the value (100 kg) from Table 1? Why are P_eng and eta_eng needed to compute the fuel mass?

 

Equation 29: In the third equation, please clarify the units of M_gen and P_gen in the context.

 

Equation 29: It is recommended to split the equations for inverter mass and converter mass to avoid confusion (similar to the comment for Equation 23 above).

 

Section 4.1: Are there particular reasons to use genetic algorithm (GA)? This should be explained in the text. The authors don’t have to compare GA to other specific algorithms (gradient descent, simulated annealing, etc.), but should convince the readers that using GA is advantageous for this problem. Also, please briefly describe the setup of GA such as the range of design variables, discretization (for continuous variables only), population size, mutation, cross-over, stopping criteria, etc.

 

Figure 8: There is a box “Calculate power and energy requirements”. Is the approach described in Section 4.2 used here? It appears that the power and energy requirements also depend on the sizing of ducted fans. Is there a feedback between the left and right iterations?

 

Equation 32: What are theta_bat and theta_sc? Is any reserve fuel considered during sizing and optimization? This could have an impact on the vehicle gross weight and rotor design.

 

Line 264: Consider replacing “which” with “as” and removing “the” before “Table 3”.

 

First line on page 14: Based on Figure 8, N_B has already been determined during the first optimization. In this case, can both n_s and n_p be design variables in the second optimization? Also, should either N_s and N_p be included as a design variable in the second optimization?

 

Equation 33: In the objective function, the two factors are the efficiencies of electric motors and ducted fans, respectively. It appears that this objective should be maximized. Also, should the subscript “m” be “rm”?

 

Equation 33: In the first and second inequality constraints, it appears that (4*R^4/pi^2) and (4*R^5/pi^3) should be replaced with d^4 and d^5 for consistency with Equation 15.

 

Equation 33: Please refer to the comment for Equation 18 above to see if the first inequality constraint should be modified accordingly.

 

Equation 33: In the second inequality constraint, should subscript “hm” be “rm” according to the paragraph below?

 

Line 277-278: Rephrase “applying the twice genetic algorithm...”

 

Table 4: The 1.5 m ducted fans will be huge especially when the vehicle is operating on the ground. Could the authors comment on the operational space constraint? Would the fans be folded during ground operation?

 

Table 4: The rotor motor speed is 8000 rpm. Assuming direct drive, with a fan diameter of 1.5 m, this gives a tip speed of 628 m/s which is about Mach 1.8. This would be infeasible for a typical rotor design. If it is not direct drive, please clarify the gearing ratio and any assumptions for gearbox efficiency and mass.

 

Table 4: The chord length of 1.18 m is even larger than the fan radius of 0.75 m. Is this realizable?

 

Equation 34: It is understood that the fuel flow depends on the motors and the HESS, and the operation of HESS may depend on SOC, but how is fuel flow *directly* related to SOC (i.e., what would be the partial derivative here)? This is not clearly explained in the context.

 

Equation 34 and 35: The symbol mdot_fuel is used here to denote the total aggregated fuel consumption, which can be confusing given its previous usage in Equation 3. Maybe the authors could consider changing one of the subscripts to avoid confusion.

 

Equation 34-36: The authors attempt to establish a single objective (equivalent fuel consumption) by aggregating the engine fuel flow and the output power of HESS. The factor to convert P_HESS to equivalent fuel flow is (s_eq/Q_fuel,low). How is the value of s_eq(t) determined? Also please define Q_fuel,low in the text.

 

Equation 36: P_HESS is the output power of HESS according to the second line of text after Equation 34. What happens when the HESS is being recharged?

 

Line 291: What are the constraints in “constraint ranges”?

 

Line 292: What are the elements of the “control vector”? It appears that P_HESS is a scalar from Equation 34.

 

Line 293-294: What is the optimization algorithm used to minimize total fuel consumption? Is this optimization applied at every time step during the mission?

 

Section 5.1: Please clarify in the text how the mission is discretized in the analysis.

 

Section 5.1 and Figure 12(b): During ground operation, is deceleration performed entirely by regenerative braking (without frictional brake)?

 

Line 324: Please provide a reference to NEDC, since not all readers are familiar with this mission profile. The text around line 329 does not describe the acceleration and deceleration requirements which are critical in ground operation.

 

Line 324: Given a steady level cruise segment as shown in Figure 12(a), the resulting sized vehicle may not necessarily satisfy the instantaneous power requirements for military combat which typically involves a lot of maneuvers that require more excess power and upsized TEHPS. The authors should consider removing “military combat” from the text.

 

Line 349: Is the duration of charging a result of optimization? Would the fuel consumption (or equivalent fuel consumption) be lower if the TGS runs at a power setting slightly higher than power required through the entire cruise such that the HESS is fully recharged right at the end of the cruise?

 

Line 349 and Figure 17: Recharging appears to stop (or the charging speed becomes very slow) before battery SOC reaches 100% for the HESS. Could the authors comment on this?

 

Figure 13 and 17: Does TGS have to run at 550 kW at the very beginning of the mission? It appears that such high power is only used to recharge the supercapacitors to 90% SOC in a few seconds. This leads to an impression that this initial high-speed charging would require the TGS to be sized for 550 kW, while the rest of the mission only requires a maximum of about 420 kW (beginning of takeoff, end of landing, and a few moments in ground mode). If the initial TGS power was reduced below 420 kW, the engine would be about (550-420)/3 = 43 kg lighter. Could the authors comment on this?

 

Figure 13: This figure appears to show a large fluctuation of TGS and HESS power whose magnitude is much higher than the required power during ground operation. The TGS power increases from about 20 kW to over 200 kW five times in the interval t = [2400,2800] to recharge the battery and keep the supercapacitor SOC above 83%, while the power required within this time interval is no more than 80 kW based on Figure 12(b). Is the computed TGS and HESS power physically optimal? This question arises since the TEHPS is supposed to “smooth out” the engine power. Would it be better to run the TGS at, say, about 50 kW with less fluctuation and charge the HESS at a different schedule?

 

Line 356: Need to rephrase “... between the electrical energy applied propulsion to the overall propulsion energy ...” 

 

Figure 15: It would be more informative to show two plots in this figure, where (a) contains the curve in the original Figure 15 and another curve showing the fuel consumption of the oil-powered AGA, and (b) contains three curves, the first two showing the engine fuel flow (slope of the two curves in (a)) vs time and the last one showing the equivalent fuel flow for the HAGA as defined in Equation 35.

 

Line 372: It is recommended to include a mass breakdown of the TEHPS and the conventional powertrain. According to Table 7, the total mass of HESS is 128.68 kg which is supposed to be removed in the conventional powertrain, but the masses of motors, engine (due to possible change of rated power), generator, and gearbox are unclear. It does not have to be very detailed, but should be sufficient for the readers to compare what is different between the two architectures.

 

Figure 17: The supercapacitor SOC drops about 5% at the end of recharging at cruise and another 10% at the beginning of descent. What happens at these two moments? 

 

Line 400-401: What is “redundancy interval”? 

 

Line 11 (abstract) and 432 (conclusion): The term “mass” is vague. The readers may not be certain that it refers to the mass of the whole vehicle (instead of the powertrain) until reading the results section.

 

Author Response

Dear reviewer,

 

Thank you for the positive comments and suggestions concerning our manuscript entitled "Sizing Methodology and Energy Management of an Air-ground Aircraft with Turbo-electric Hybrid Propulsion System" (aerospace-2010164). We have carefully reviewed the comments and have revised the manuscript accordingly. Our responses are given in a point-by-point manner below. All these changes are highlighted in an additional copy of our manuscript.

 

Point 1: The proposed methodology uses operating conditions encountered during the nominal mission to size the TEHPS. The mission profile includes a level cruise segment in the air which apparently assumes no wind. The result would have been more robust if the impact of operational uncertainty such as headwind, gust, and surface inclination on the sizing of TEHPS can be quantified. The mission profile may remain the same, but the TGS may need to be upsized to satisfy the same point performance requirement in case of a gust. Perhaps this can be done as future work.

Response 1: Thanks for your comments. The main contribution of this paper shows the exploration of a turboshaft-electric hybrid propulsion system (TEHPS) for hybrid air-ground aircraft (HAGA), incorporating a high-power turboshaft engine and a hybrid energy storage system. The sizing methodology and energy management framework are the means of optimization for TEHPS. In selecting the mission profile, the air-ground amphibious flight profile was picked from the functional perspective and did not take into account the complex environmental factors. As you mentioned, there is a significant necessity for research related to HAGA in complex operating conditions such as headwinds, gusts and surface inclinations, which we will refine in our subsequent work.

 

Point 2: It would be better to highlight the novelty of the proposed methodology. It is not very clear in the current state how the proposed methodology is superior compared to state-of-the-art.

Response 2: We feel sorry that we did not provide enough information about the novelty of the proposed methodology in the previous manuscript. The main contribution of our paper actually focused on the exploration of the application of turboshaft-electric hybrid propulsion system on HAGA, including turboshaft engine and hybrid energy storage system. Sizing design and energy management strategies were applied based on our previous research experience and migrated for the different characteristics and needs of the HAGA. Importantly, the simulation results indicate that the design of the powertrain is successful and the sizing methodology and energy management framework is available. Your thoughts on the proposed method and the state-of-the-art comparative analysis will be carried out in subsequent work and are in line with the next steps we expect to take in our research.

 

Point 3: Figure 1: It is unclear from the images how the attitudes and modes differ in each subfigure. It appears that 1(a) shows a top view, 1(c) a side view, and 1(b) an isometric view or the vehicle is tilted. The authors may consider adding reference axes to indicate forward/backward, up/down, left/right directions, etc. Also, if the activity of components (turbogenerator, battery, supercapacitor, and motors) depends on the flight mode, the authors may consider highlighting the active components in each subfigure.

Response 3: Thank you for your suggestion. Figure 1 shows the HAGA in top, oblique side and side views in different directions. Based on your suggestion, we have added reference axes to the updated picture which indicates forward/backward, up/down, left/right directions.

 

Point 4: Equation 3: Please clarify the units of P_eng and mdot_fuel in the text.

Response 4: We have corrected it and feel great thanks for your point out.

 

Point 5: Equation 3 seems to be applicable only to a turboshaft engine of specific size (rated power), with peak fuel flow at P_eng = 4842. What is the engine rated power, and how are the coefficients determined?

Response 5: Thanks for your question. Equation 3 describes the relationship between turboshaft engine power and fuel mass flow rate, with coefficients derived from experimental data from the NASA-Lewis experimental test engine [Ref. 52-53]. The maximum turboshaft engine power available to apply this equation is 4.84MW. Based on the required power analysis and the mixture of fuel and electricity, the rated power of turboshaft engine in this paper is set at 400kW.

 

Point 6: Section 2.2.3 two lines above Equation 12: “bi-directional inverter efficiency” – Did the authors intend to say “DC/DC converter” instead of “inverter”?

Response 6: Thanks for your careful examination, "bi-directional inverter efficiency " has been changed to " bi-directional converter efficiency" in the new manuscript.

 

Point 7: Equation 15: In the first equation, the power of diameter should be 4 instead of 5.

Response 7: Thank you for pointing out this problem, which we have revised in the new manuscript.

 

Point 8: Equation 15: Please provide some detail or references in the text regarding how relation between C_T and C_P are obtained, e.g., what kind of fan/propeller model is used.

Response 8: Thanks for your question. The ducted fans thrust coefficient and power coefficient can be calculated from the blade element-momentum theory and C_T and C_P are expressed as a one-dimensional quadratic function of the advance ratio respectively. The coefficients of this functions are determined according to the pitch, mean chord length of the blade, diameter and the number of blades, where the advance ratio can be expressed by the second formula in Equation 16. The blade profile is selected as NACA0016.

 

Point 9: Equation 17: In the third equation, what is beta? This equation seems to account for only the total rotor disc area. Do the authors implicitly neglect the vertical aerodynamic force acting on the center body due to nonzero vertical speed? If so, please state it clearly in the text. Otherwise, please show how this force is computed.

Response 9: Thanks for your question. We have changed the sign of the angle from β to α. And α represents the angle of attack, which has a value of 90º in vertical motion. The aircraft projection plane is perpendicular to the climb velocity vector. For this configuration of an aircraft, the flat plate drag assumption is valid. We neglected the vertical aerodynamic force acting on the center body ( ).

 

Point 10: Equation 17: In the last equation, should the sign of the first term (R_climb/2) be positive?

Response 10: Thanks for your question. V_f is the air-induced velocity through the rotor disc. From momentum theory, the induced velocity for axial climb is calculated as this Equation  [Ref. 57, 58].

 

Point 11: Equation 17: This approach only computes the shaft power required using momentum theory. The actual shaft power required is higher due to viscous effects and other losses. The authors should consider using a figure of merit (ratio of the ideal power, as already computed in Equation 17, to the actual power) to obtain a more realistic estimation of the actual power required for VTOL.

Response 11: Thanks for your question. The calculation of required power for vertical take-off and landing operation is shown in the first formula in Equation 17. For large diameter rotors, the rotor utility factor η_rot in the denominator takes values in the range 0.7-0.8. For conditions with gusts of wind, a theoretical increase in thrust of around 20% is required. However, the thrust calculations in this paper only consider the ideal situation.  

 

Point 12: Equation 18: How is the cruise lift-to-drag ratio determined? Based on Figure 1, the vehicle does not appear to be a lifting body or have lifting surfaces large enough to generate lift that is equal to the weight. In addition, Equation 18 only computes the drag power (drag x airspeed). For the AGA of interest, it appears that the thrust generated by the ducted fans should be decomposed such that the vertical and horizontal components balance weight and drag, respectively, assuming the body does not generate significant amount of lift. If this is the case, the cruise power requirement should be computed in a different approach, similar to helicopters or quadrotors. The authors should revisit the modelling approach here.

Response 12: Thanks for your question. We have revisited the HAGA power calculation method during the cruise phase, in particular the demand force. An equation that only considers the ratio of gravity to lift-to-drag ratio can only be used to calculate cruise drag, which is not sufficient. The value of cruising force demand is calculated by combining the horizontal and vertical forces. The horizontal force demand is added by the acceleration drag (δM_Ta_h) and air drag (0.5S_cC_adρV_h²) and the vertical force demand can be calculated by the second formula in Equation 17.

 

Point 13: Equation 19: Why is the coefficient 1/3600 needed in every term? The power required on ground could have been obtained by simply multiplying V_g on both sides of Equation 13. Also please double check the numeric coefficient of the second term (wind resistance); it is recommended to keep the symbol for air density (rho) to maintain consistency of units.

Response 13: Thank you for your suggestion, we have modified equation 19.

 

Point 14: Equation 17-19: The computed power required appear to be the shaft power output by the motors. A motor efficiency is needed in order to correctly estimate the energy flow within the propulsion system. Have the authors accounted for the motor efficiency?

Response 14: Thank you very much for your question, the efficiency of the motor does have to be considered and calculated. In order to facilitate the calculation, we have set the motor efficiency to a constant 0.95 in the simulation. However, in the future work, we will obtain the speed, torque and efficiency MAP curves of BLDC motor through parametric tests and feed the real-time efficiency of the motor at different speeds and torques back to the controller.

 

Point 15: Equation 21, 22, and 24: Please define eta_inv in the text. Is this supposed to be the efficiency of the AC/DC rectifier (instead of a DC/AC inverter) connected to the generator in Figure 2? If so, please consider changing the subscript to avoid confusion.

Response 15: We have defined η_inv in the new manuscript. η_inv indicates the inverter efficiency on the output side of generator, converting the alternating current output from the generator into direct current, which flows to the DC bus.

 

Point 16: Equation 23: It is confusing to show two modes in one equation. The authors should consider splitting the equation into two (one with subscripts ADEPS and rm and the other with GDEDS and hm).

Response 16: Thanks for your question, and we've changed it to two equations.

 

Point 17: The paragraph before Equation 24: What is the logic to switch between the discharging mode (Equation 21) and the charging mode (Equation 24)? The phrase “when the required power is not significant” is vague, and what is the “upper limit” of the SOC of the HESS? These should be clarified in the text.

Response 17: Thanks for your question. When the output power P_TGS on the generator side is greater than the motor required power [P_ADEPS*f_air+P_GDEDS*(1-f_air)] and the SOC value of the lithium battery in the HESS is less than the upper limit value 0.8, the turbine generation system adjusts the engine operating point to both power the distributed electric propulsion system and charge the remaining power to the HESS. The above switch logic has been expressed in the new manuscript.

 

Point 18: Equation 26: By definition, f_air is the binary air/ground mode flag and is time dependent. However, the energy is integrated over time, so it is unclear how this equation applies to the whole mission. Maybe the authors could replace energy (E) with power (P) so the whole equation applies to any moment in the mission. Alternatively, the authors may simply remove f_air and (1 – f_air) to describe the energy for the whole mission.

Response 18: Thank you very much for your suggestion, we have removed the mode flag f_air in the new Equation 26.

 

Point 19: Equation 29: In the second equation, is the fuel mass computed based on mission analysis, or does it take the value (100 kg) from Table 1? Why are P_eng and eta_eng needed to compute the fuel mass?

Response 19: Thanks for your question. The fuel mass in Table 1 represents the initial carried fuel mass of 100kg, while the fuel mass calculated in Equation 29 is the total fuel consumption of the turboshaft engine for a given flight time, which is used to calculate the total mass of the turbine generation system. The second equation for calculating fuel mass has been modified so that M_fuel can be obtained by summing the fuel consumption rates at each moment.

 

Point 20: Equation 29: In the third equation, please clarify the units of M_gen and P_gen in the context.

Response 20: Thanks for your question. The previous empirical formulae for calculating mass and power are applicable to small generators, so we have updated them in the new manuscript, with the units of P_gen and M_gen being kW and kg respectively.

 

Point 21: Equation 29: It is recommended to split the equations for inverter mass and converter mass to avoid confusion (similar to the comment for Equation 23 above).

Response 21: Thanks for your recommendation. We have split the Equation 29, which can be indicated in Equations 29 and 30 respectively.

 

Point 22: Section 4.1: Are there particular reasons to use genetic algorithm (GA)? This should be explained in the text. The authors don’t have to compare GA to other specific algorithms (gradient descent, simulated annealing, etc.), but should convince the readers that using GA is advantageous for this problem. Also, please briefly describe the setup of GA such as the range of design variables, discretization (for continuous variables only), population size, mutation, cross-over, stopping criteria, etc.

Response 22: Thanks for your question. Optimization algorithm is the main content in optimal sizing. There are currently three types of algorithms for the hybrid system design environment: gradient-based, derivative-free and metamodel, and two derivative-free optimization algorithms, DIRECT (Divided RECTangles) and Complex, are employed to solve the optimization problem. Both algorithms were able to converge to approximately the same solution; however, DIRECT was the most efficient for the problem. Genetic algorithm (GA) was widely applied in the optimal design of hybrid systems, especially in complex systems where a large number of parameters have to be considered. GA has a short computational time and stable convergence characteristics, effectively improving computational efficiency. GA has conquered the deficiencies of the gradient-based optimization algorithms that require calculating the derivative of the objective function and that are easily trapped into local optimal areas. In fact, GA offers a variety of hybrid systems of different sizes to meet the load requirements at a given location and evaluates them according to a well-defined objective function. Numerous studies confirmed that GA has been available and effective. In addition, we have added a description of the basic parameters setting of the genetic algorithm in a new manuscript.

 

Point 23: Figure 8: There is a box “Calculate power and energy requirements”. Is the approach described in Section 4.2 used here? It appears that the power and energy requirements also depend on the sizing of ducted fans. Is there a feedback between the left and right iterations?

Response 23: Thanks for your question. The box "Calculating power and energy requirements" in Figure 8 refers to the calculation of power and energy for the different flight phases in Section 3.1 and 3.2, with the initial take-off weight setting. It also calculates the power and energy required from the hybrid energy storage system to obtain the required number of batteries and supercapacitors. It should be noted that the method in Section 4.2 is applied in the overall simulation model under the premise of determining the individual design parameters.

 

Point 24: Equation 32: What are theta_bat and theta_sc? Is any reserve fuel considered during sizing and optimization? This could have an impact on the vehicle gross weight and rotor design.

Response 24: Thanks for your question. θ_bat and θ_sc represent the energy density of battery and supercapacitor, which can be shown in Table 3.

 

Point 25: Line 264: Consider replacing “which” with “as” and removing “the” before “Table 3”.

Response 25: Thanks for your suggestion, and we have made the modification.

 

Point 26: First line on page 14: Based on Figure 8, N_B has already been determined during the first optimization. In this case, can both n_s and n_p be design variables in the second optimization? Also, should either N_s and N_p be included as a design variable in the second optimization?

Response 26: Thanks for your question. The first optimization in section 4.1 focuses on the take-off weight parameter and must take into account the total number of batteries n_bat and supercapacitors N_sc, which determines the power and energy provided by the HESS. After determining the total number of batteries and supercapacitors, the number of series (n_s, N_s) and parallel (n_p, N_p) connections of the two energy storage units is then determined based on the second voltage optimization. So it is not a parameter that is optimized twice, but a mistake in our presentation, which has been corrected in the new manuscript.

 

Point 27: Equation 33: In the objective function, the two factors are the efficiencies of electric motors and ducted fans, respectively. It appears that this objective should be maximized. Also, should the subscript “m” be “rm”?

Response 27: Thanks for your question. We have modified it in the new Equation 33, the subscript of motor should be rm, and iteratively solve the HAGA for the maximum efficiency of the rotor motors and the ducted fans in the cruise phase.   

 

Point 28: Equation 33: In the first and second inequality constraints, it appears that (4*R^4/pi^2) and (4*R^5/pi^3) should be replaced with d^4 and d^5 for consistency with Equation 15.

Response 28: Thanks for your advice. We have made a modification to ensure consistency with the previous Equation 15. 

 

Point 29: Equation 33: Please refer to the comment for Equation 18 above to see if the first inequality constraint should be modified accordingly.

Response 29: Thank you for your advice. We have correspondingly modified the first inequality constraint of Equation 33 according to the new equation 18.

 

Point 30: Equation 33: In the second inequality constraint, should subscript “hm” be “rm” according to the paragraph below?

Response 30: We have modified these in the new Equation 33, the subscript should be rm.

 

Point 31: Line 277-278: Rephrase “applying the twice genetic algorithm...”

Response 31: Thanks for your question. We have rewritten the phrase in a new manuscript.

 

Point 32: Table 4: The 1.5 m ducted fans will be huge especially when the vehicle is operating on the ground. Could the authors comment on the operational space constraint? Would the fans be folded during ground operation?

Response 32: Thanks for your question. When the HAGA is driving on the ground, the ducted fans is folded and adducted from two different directions to reduce air resistance.

 

Point 33: Table 4: The rotor motor speed is 8000 rpm. Assuming direct drive, with a fan diameter of 1.5 m, this gives a tip speed of 628 m/s which is about Mach 1.8. This would be infeasible for a typical rotor design. If it is not direct drive, please clarify the gearing ratio and any assumptions for gearbox efficiency and mass.

Response 33: Thanks for your question. High rotor motor speed resulted in excessive fan blade tip speed, which we had not previously calibrated. Based on the power level of the rotor motors, we then reselected the motor's the back electromotive force constant k_E and torque constant k_T, and redesigned and recalibrated it according to Equation 33, finally determining the rotor motor speed to be 3200 rpm.

 

Point 34: Table 4: The chord length of 1.18 m is even larger than the fan radius of 0.75 m. Is this realizable?

Response 34: We are sorry that the results in the table are an error in our calculations. It is unrealistic for the chord to be larger than the radius. The propeller is designed according to the pitch angle  and mean chord length . The mean chord length can be expressed as a relationship between the blade diameter d and the fans disc solidity  ( ). At a design diameter of 1.5m, the disc solidity is taken to be 0.15 and the mean chord length is calculated to be 0.1125m. We have made corrections in the new manuscript.

 

Point 35: Equation 34: It is understood that the fuel flow depends on the motors and the HESS, and the operation of HESS may depend on SOC, but how is fuel flow *directly* related to SOC (i.e., what would be the partial derivative here)? This is not clearly explained in the context.

Response 35: Thanks for your question. The product function in Equation 34 treats the fuel mass flow rate as a function of the state variable x(t), the control variable u(t) and time t. The fuel mass flow rate in Equation 35 can be written as the sum of the current consumption of turboshaft engine and the equivalent consumption of HESS. Equations 3 and 36 show that the current fuel flow rate is a function of engine power P_eng, and the equivalent fuel flow rate is a function of the equivalence factor s_eq and HESS power P_HESS. Both equivalence factor s_eq and HESS power P_HESS are closely related to the state variable SOC.

 

Point 36: Equation 34 and 35: The symbol mdot_fuel is used here to denote the total aggregated fuel consumption, which can be confusing given its previous usage in Equation 3. Maybe the authors could consider changing one of the subscripts to avoid confusion.

Response 36: Thanks for your suggestion, we have revised the fuel consumption subscript in Equation 3 to mdot_fuel,cur to differentiate between the system fuel consumption and the current fuel consumption of turboshaft engine.

 

Point 37: Equation 34-36: The authors attempt to establish a single objective (equivalent fuel consumption) by aggregating the engine fuel flow and the output power of HESS. The factor to convert P_HESS to equivalent fuel flow is (s_eq/Q_fuel,low). How is the value of s_eq(t) determined? Also please define Q_fuel,low in the text.

Response 37: Thanks for your question. The equivalence factor is a vector value containing two elements for charging and discharging s_eq(t)=[s_eq,chg(t), s_eq,dis(t)], which assigns a cost to the use of electricity and converts it into equivalent fuel consumption. s_eq is a set of constants that can be understood as the average overall efficiency of the electrical path under specific operating conditions. Its upper and lower limits are determined from control experience and the optimal equivalent factor is selected by discretizing the interval. In addition, Q_fuel,low represents the low calorific value of aviation fuel, i.e. the available energy per unit mass of fuel, in this paper Q_fuel,low= 48140 KJ/kg.

 

Point 38: Equation 36: P_HESS is the output power of HESS according to the second line of text after Equation 34. What happens when the HESS is being recharged?

Response 38: Thanks for your question. Precisely, PHESS indicates the power of hybrid energy storage system with a positive output sign and a negative input sign. When P_HESS>0, the turbine power system (TGS) and the hybrid energy storage system (HESS) together supply propulsion power. When P_HESS<0, the excess power output from the TGS recharges the HESS, where the current flow to the lithium battery pack and supercapacitor pack is negative. The supercapacitor is intended for transient response to reduce the risk of high transient currents to battery pack, thus extending their life.

 

Point 39: Line 291: What are the constraints in “constraint ranges”?

Response 39: Thank you for your question. The constraint interval refers to the upper and lower values of the state and control variables for optimal control. It also contains parameters such as the maximum power of the turboshaft engine and generator, the maximum speed and maximum power of the motors.

 

Point 40: Line 292: What are the elements of the “control vector”? It appears that P_HESS is a scalar from Equation 34.

Response 40: Thank you for your question. As shown in Figure 9, when performing the optimal control of upper-level energy management, the control vectors are selected as the turboshaft engine power, the equivalent factor and the power of the hybrid energy storage system, i.e. u(t)=[P_eng, s_eq, P_HESS]. We apologize for the previous writing error and have revised it in the new manuscript.

 

Point 41: Line 293-294: What is the optimization algorithm used to minimize total fuel consumption? Is this optimization applied at every time step during the mission?

Response 41: Thank you for your question. The upper-level energy management controller adopts an equivalent consumption minimisation strategy (ECMS) based on optimization control, the algorithm steps can be seen in the yellow dashed box in Figure 9. Then the ECMS could find the control variables that minimize fuel consumption and input the P_HESS into the lower-level fuzzy logic controller. In addition, the simulation is discretized to determine the optimal solution at each discrete point within a set simulation time. The energy management framework based on ECMS and FLC methods forms a closed-loop feedback.

 

Point 42: Section 5.1: Please clarify in the text how the mission is discretized in the analysis.

Response 42: Thanks to your suggestion, we have clarified the discrete time, also known as simulation step, in the new manuscript.

 

Point 43: Section 5.1 and Figure 12(b): During ground operation, is deceleration performed entirely by regenerative braking (without frictional brake)?

Response 43: Thank you for your question. In this paper, when the HAGA is in NEDC operation, only regenerative braking is considered in the deceleration phase, not frictional brake. The conversion of HAGA kinetic energy and electric energy is realized by the reversible hub motors. During deceleration or braking, the reversible motors work in the form of generators, and the kinetic energy of HAGA travelling drives the generators to convert the kinetic energy into electricity, which is stored in the hybrid energy storage system.

 

Point 44: Line 324: Please provide a reference to NEDC, since not all readers are familiar with this mission profile. The text around line 329 does not describe the acceleration and deceleration requirements which are critical in ground operation.

Response 44: Thanks to your point, we have added the reference to NEDC in the new manuscript and a special textual, data-based explanation of its profile acceleration and deceleration characteristics.

 

Point 45: Line 324: Given a steady level cruise segment as shown in Figure 12(a), the resulting sized vehicle may not necessarily satisfy the instantaneous power requirements for military combat which typically involves a lot of maneuvers that require more excess power and upsized TEHPS. The authors should consider removing “military combat” from the text.

Response 45: The cruise phase of military missions certainly requires high mobility and sufficient power, and the smooth cruise flight profile in this paper is really not suitable for complex military combat missions, so we will delete the term "military combat" in the new manuscript.

 

Point 46: Line 349: Is the duration of charging a result of optimization? Would the fuel consumption (or equivalent fuel consumption) be lower if the TGS runs at a power setting slightly higher than power required through the entire cruise such that the HESS is fully recharged right at the end of the cruise?

Response 46: Thank you for your question. As can be seen from the power demand curve in Figure 12(b), the power values in the cruise phase decrease sharply relative to vertical take-off. During the vertical take-off phase, the hybrid energy storage system, in particular the lithium battery, provides more than 20% of the required energy and more than 30% of the required power. At the beginning of the cruise phase the battery SOC value has dropped below 30%, where the energy management controller issues a command to keep the turboshaft engine in rated operation and charge the HESS until the SOC of the battery and supercapacitor returns to normal.

Furthermore, the optimization results of the charging time are related to the lower-level fuzzy logic control strategy and depend on the setting of the affiliation functions. If the turboshaft engine power is set slightly higher than the power required for cruising, but the battery is recharged at the end of the cruise, whether the overall fuel consumption will increase will also depend on the charging time and the power value of the turboshaft engine at the charging time. This idea will be considered in our future work and the results will be given.

 

Point 47: Line 349 and Figure 17: Recharging appears to stop (or the charging speed becomes very slow) before battery SOC reaches 100% for the HESS. Could the authors comment on this?

Response 47: Thanks for your question. There was a problem with the phrase "…until SOC=100%" in the original article and we have amended it to "…until SOC recovers in the high charge range". The high charge interval can be interpreted as the affiliation function SOC_b={L} in the lower-level fuzzy logic controller. During the phase when the TGS is charging the energy storage system, the battery in the single energy storage system is charged alone and the SOC can be restored to 100%. In the hybrid ESS, however, the supercapacitor absorbs transient high currents and the SOC_sc tends to increase. The SOC_b of battery does not need to be fully restored to 100% in order to avoid higher fuel consumption due to longer high-power operation of TGS.

 

Point 48: Figure 13 and 17: Does TGS have to run at 550 kW at the very beginning of the mission? It appears that such high power is only used to recharge the supercapacitors to 90% SOC in a few seconds. This leads to an impression that this initial high-speed charging would require the TGS to be sized for 550 kW, while the rest of the mission only requires a maximum of about 420 kW (beginning of takeoff, end of landing, and a few moments in ground mode). If the initial TGS power was reduced below 420 kW, the engine would be about (550-420)/3 = 43 kg lighter. Could the authors comment on this?

Response 48: Thanks for your question. The maximum required power for the given mission profile is calculated to be 557kW based on the power analysis in Chapter 3.1, as shown in the Figure 12(b). The maximum required power is theoretically supplied by the TGS and HESS together, so there is no necessity to select a turboshaft engine with a rated power of 557kW. In this paper, the rated power of turboshaft engine is set at 400kW and the differential power is provided by the hybrid energy storage system, relying on the fast response characteristics of the supercapacitor at high transient currents. It should be noted that the blue line in Figure 13 indicates the power variation of the TGS, which operates at approximately 400kW during the vertical take-off phase, rather than 550kW. Hope our answer will satisfy you.

 

Point 49: Figure 13: This figure appears to show a large fluctuation of TGS and HESS power whose magnitude is much higher than the required power during ground operation. The TGS power increases from about 20 kW to over 200 kW five times in the interval t = [2400,2800] to recharge the battery and keep the supercapacitor SOC above 83%, while the power required within this time interval is no more than 80 kW based on Figure 12(b). Is the computed TGS and HESS power physically optimal? This question arises since the TEHPS is supposed to “smooth out” the engine power. Would it be better to run the TGS at, say, about 50 kW with less fluctuation and charge the HESS at a different schedule?

Response 49: Thanks for your question. As shown in Figure 13, when the HAGA enters the NEDC phase, the TGS and HESS power curves fluctuate more, especially during the time period t=[2400s,2800s]. The reason for this is that when the battery SOC is low, the energy management control strategy automatically monitors it and allows the engine to output more power to charge the battery. Meanwhile, the key problem is that the energy management strategy is too sensitive to low battery levels, as we have also set the control strategy to take into account the initial and end state charge maintenance of the HESS. Therefore, no additional charging operation is required at the start of the next cycle, so that the principle is similar to that of a plug-in hybrid electrical vehicle (PHEV). Subsequently we will consider continuing to optimize the power curve in the fluctuation interval and compare the EMSs to obtain the best fuel consumption according to the different mission profiles.

 

Point 50: Line 356: Need to rephrase “... between the electrical energy applied propulsion to the overall propulsion energy ...”

Response 50: Thanks for your question. We have rewritten the phrase in a new manuscript.

 

Point 51: Figure 15: It would be more informative to show two plots in this figure, where (a) contains the curve in the original Figure 15 and another curve showing the fuel consumption of the oil-powered AGA, and (b) contains three curves, the first two showing the engine fuel flow (slope of the two curves in (a)) vs time and the last one showing the equivalent fuel flow for the HAGA as defined in Equation 35.

Response 51: Thanks to your suggestions, we have appropriately supplemented and refined the curves in Figure 15 in the new manuscript.  

 

Point 52: Line 372: It is recommended to include a mass breakdown of the TEHPS and the conventional powertrain. According to Table 7, the total mass of HESS is 128.68 kg which is supposed to be removed in the conventional powertrain, but the masses of motors, engine (due to possible change of rated power), generator, and gearbox are unclear. It does not have to be very detailed, but should be sufficient for the readers to compare what is different between the two architectures.

Response 52: Thanks to your suggestions, we have clarified the difference between oil-powered AGA and hybrid AGA in the new manuscript, as well as explaining the mass composition of oil-powered AGA.

 

Point 53: Figure 17: The supercapacitor SOC drops about 5% at the end of recharging at cruise and another 10% at the beginning of descent. What happens at these two moments?

Response 53: Thanks for your question. At the end of the cruise phase charging and at the beginning of the vertical descent phase, the supercapacitor (SC) twice shows a drop in SOC_sc. At the start of the cruise phase, the turboshaft engine charges the batteries and supercapacitors in the HESS by maintaining a high-power output. When charging ends abruptly, there is a sudden and instantaneous change in HESS current (from positive to zero), resulting in a brief drop tendency for the DC bus voltage. The supercapacitor SOC_sc is calculated from the ratio of the terminal voltage to the maximum voltage and therefore has the same tendency to drop by approximately 5%. During the transition to the vertical descent phase, the battery and SC transform together from a silent state to an output state. The SC responds quickly, instantaneously discharging a high current, with a SOC_sc drop of around 10% at the moment of response. On the other hand, the battery responds slowly and gradually makes up for the lack of output current.

 

Point 54: Line 400-401: What is “redundancy interval”?

Response 54: Thanks for your question. The maximum fluctuation of the DC bus voltage is 25% above and below the nominal value, for a DC bus voltage of 800V the fluctuation range should be 600-1000V. The bus voltage fluctuation is higher for single energy storage and less for hybrid energy storage, but both are within the fluctuation tolerance.

 

Point 55: Line 11 (abstract) and 432 (conclusion): The term “mass” is vague. The readers may not be certain that it refers to the mass of the whole vehicle (instead of the powertrain) until reading the results section.

Response 55: The summary and conclusions express the total mass of hybrid air-ground aircraft, which has been modified in accordance with your suggestions.

 

We thank the reviewer for the patience and efforts to help us improve the technical content and the grammar in this manuscript. Your generous help is greatly appreciated.

 

Finally, we appreciate for your work earnestly, and hope that the correction will meet with approval.

 

Sincerely

Wenjiang Yang

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper addresses the problem of sizing and managing an air-ground aircraft equipped with a turbo-electric hybrid propulsion system. The novelty of the paper is not sufficiently put into evidence and the scientific rigor is limited by the unsatisfactory description and unsuitability of the models used in the investigation.

1)      The introduction does not explain the motivation of the paper. In particular, why the authors considered AGAs? Which are the advantages and disadvantages of this kind of vehicle?  Is it possible to apply the methodologies mentioned in the introduction for sizing and energy management to AGAs without particular modification or is it necessary to address the specifical issues of this application? It is clear from the plot of the required power of figure 13 that air operation requires a much larger power than land driving. Therefore, a powertrain sized for the air operation is largely oversized in the road operation. Moreover, one of the advantages of the hybridization for land vehicles is the possibility to decouple the engine working point from the dynamic request of power at the wheel. This is a key issue in this specific application because a turboshaft engine is not able to follow the fast dynamics of a typical driving cycle. Therefore, the results of table 6 are, in my opinion, meaningless and the whole energy management strategy should focus on smoothing transients for the turboshaft engine.  

2)      Figure 1 appear to represent different views of the vehicles instead of different operating mode. Please explain how the configuration of the vehicle changes in the different operating modes

3)      The model of the engine is not sufficiently explained and appears to be inadequate for the analysis performed in the paper. How was eq. 3 obtained? Is the model able to predict the fuel consumption of the engine when changing the load and the flight altitude? For the goal of the investigation, it is mandatory to account for the dynamic response of the engine that is not considered in the paper. In my experience, a turbogas engine is not able to follow the fast dynamic power request of the ground driving cycle (figure 13).

4)      The model of the battery is also not described in a suitable way, in particular the modeling of the thermal flows. Please explain what do you mean by “virtual experimental tests based on the Simcenter AMESsim”.

 

5)      Eq. 29. Please explain how the coefficients of this correlation are obtained. 

Author Response

Dear reviewer,

 

Thank you for the positive comments and suggestions concerning our manuscript entitled "Sizing Methodology and Energy Management of an Air-ground Aircraft with Turbo-electric Hybrid Propulsion System" (aerospace-2010164). We have carefully reviewed the comments and have revised the manuscript accordingly. Our responses are given in a point-by-point manner below. All these changes are highlighted in an additional copy of our manuscript.

 

Point 1: The introduction does not explain the motivation of the paper. In particular, why the authors considered AGAs? Which are the advantages and disadvantages of this kind of vehicle?  Is it possible to apply the methodologies mentioned in the introduction for sizing and energy management to AGAs without particular modification or is it necessary to address the specifical issues of this application? It is clear from the plot of the required power of figure 13 that air operation requires a much larger power than land driving. Therefore, a powertrain sized for the air operation is largely oversized in the road operation. Moreover, one of the advantages of the hybridization for land vehicles is the possibility to decouple the engine working point from the dynamic request of power at the wheel. This is a key issue in this specific application because a turboshaft engine is not able to follow the fast dynamics of a typical driving cycle. Therefore, the results of table 6 are, in my opinion, meaningless and the whole energy management strategy should focus on smoothing transients for the turboshaft engine.

Response 1: With the increasing burden of expanding populations and rapid urbanization in recent decades, public transportation systems and freight traffic are suffering huge pressure, plaguing local governments and straining economies. Engineers and researchers have started to re-examine, propose, and develop the underused near-ground spaces. Integrated air-ground mobility that can be operated both in the air with propulsion units and on the ground via wheels are a promising solution.

AGAs enable ground-level driving, vertical take-off and land, and near-ground flight, and feature great versatility, mobility, and environmental values. Due to the different characteristics of air-driving and ground-driving, the size of power supply is difficult to design. In addition, the control strategy should also be considered at the performance level, which has a significant impact on the sizing design. In this paper, the sizing methodology and energy management control strategy for the HAGA becomes intractable due to the introduction of hybrid energy storage system and turboshaft engine and the expansion of design space. Existing methods and strategies have to be adapted and optimized in the context of HAGA.

In the air-ground amphibious mission, HAGA really requires much less power when travelling on the ground than in the air. As can be seen from the flight profile configuration, HAGA is primarily intended for air flight in complex applications, with the simple ground drive only indicating that it is capable of amphibious driving.

In this paper, a series turbo-electric hybrid propulsion system (TEHPS) is considered precisely because it is possible to separate the engine operating points and regulate sudden required power changes by means of the hybrid energy storage system.

The results in Table 6 are presented to compare the advantages and disadvantages in terms of total HAGA weight, fuel consumption and pollutant emissions between only turboshaft engine powered and TEHPS. Meanwhile, the results show that the TEHPS has better fuel economy and emissions. However, this paper does not focus on the smooth transients of the turboshaft engine, which will be carried out in our future work.

 

Point 2: Figure 1 appear to represent different views of the vehicles instead of different operating mode. Please explain how the configuration of the vehicle changes in the different operating modes

Response 2: Thanks for your question. For the relevant textual representation in Fig. 1, we have corrected it in the new manuscript. When the HAGA is operating in vertical take-off and landing (VTOL) mode, the turboshaft engine and hybrid energy storage system supply electrical energy to drive the four electric ducted fans that generate lift. Figure 1(a) shows the top view of the HAGA's tail and ducted fans distribution during VTOL mode. It is more appropriate to represent the attitude of HAGA cruising phase by oblique front view, as shown in Figure 1(b). When driving on the ground, the ducted fans is folded and adducted from two different directions to reduce air resistance; The electric drive units are adjusted to the hub motors and wheels, which is represented by the left view.

 

Point 3: The model of the engine is not sufficiently explained and appears to be inadequate for the analysis performed in the paper. How was eq. 3 obtained? Is the model able to predict the fuel consumption of the engine when changing the load and the flight altitude? For the goal of the investigation, it is mandatory to account for the dynamic response of the engine that is not considered in the paper. In my experience, a turbogas engine is not able to follow the fast dynamic power request of the ground driving cycle (figure 13).

Response 3: Thanks for your comments. A lot of work has been carried out on modelling the recuperative turboshaft engine, but in this paper we only present part of the studied parameters that are relevant for subsequent calculations, such as engine output power, fuel mass flow rate, CO₂ emissions and temperature parameters such as turbine entry temperature (TET), exhaust gas temperature (EGT) and compressor discharge temperature (CDT). The above parameters are used to evaluate the power, fuel economy and emission performance of the turboshaft engine at TEHPS.

Equation 3 describes the relationship between turboshaft engine power and fuel mass flow rate, with coefficients derived from experimental data from the NASA-Lewis experimental test engine [Ref. 52-53].

In the new manuscript, as shown in Figure 16, we have added the dynamic response curve of the power turbine connected to the generator in a turboshaft engine, mainly its torque and speed. As for the fast power response of the turboshaft engine, the hybrid energy storage system in this paper provides one of the solutions. However, the problem of excess power in the ground drive cycle does exist, and we will continue to follow up on this research to find a better control method.

 

Point 4: The model of the battery is also not described in a suitable way, in particular the modeling of the thermal flows. Please explain what do you mean by “virtual experimental tests based on the Simcenter AMESim”.

Response 4: Thanks for your question. The thermal modelling of battery cell can be reflected in its parameters open circuit voltage V_cell and internal resistance R_cell both as a function of SOC and operating temperature T, and the two-dimensional look-up-table of equivalent parameters can be estimated from the experimental parameters of the battery at different temperatures. We have modified in the new manuscript.

The 2.3Ah battery cell selected for this paper is an existing battery product whose parameters can be obtained in the simulation software Simcenter AMEsim. To verify the validity of the internal resistance equivalent circuit modelling method, a software-based virtual comparison test was carried out. Given specific charge and discharge current inputs, the agreement between the actual voltage and temperature profiles of the product and the simulation results are compared.

 

Point 5: Eq. 29. Please explain how the coefficients of this correlation are obtained.

Response 5: Thanks for your question. The coefficients in the mass analysis are mostly determined based on historical data and expert experience. In Equation 29, a correction has been made to the formula for calculating the generator mass. The engine, generator and AC/DC inverter masses are obtained from the power density ξ_eng, ξ_gen, ξ_inv in the historical empirical database.

 

We thank the reviewer for the patience and efforts to help us improve the technical content in this manuscript. Your generous help is greatly appreciated.

 

Finally, we appreciate for your work earnestly, and hope that the correction will meet with approval.

 

Sincerely

Wenjiang Yang

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I appreciate the authors’ response and efforts to revise the manuscript. Most issues have been addressed adequately. However, there are still a few items that need attention; please see below for detail.

 

Point 3: Figure 1 is now clearly showing the viewing angles in the subfigure captions. However, the flight attitudes are still not clear. For example, is 1(a) showing a 90-deg nose-down attitude? If it is difficult to graphically depict the attitude, please remove “flight attitudes” from the caption. Also 1(c) should be a right view instead of a left view.

 

Point 15: An AC/DC power converter is known as a rectifier instead of an inverter, which does just the opposite and converts DC power into AC power. There seems to be no DC/AC inverter in your architecture based on Figure 2.

 

Point 19: In the second equation (M_fuel), consider replacing the summation with integration over time.

 

Point 22: The stopping criteria are still not clear. For example, is the algorithm considered to be converged after the lowest total mass among the 20 candidates does not change for a certain number of consecutive iterations? Simply taking the best-performing candidate from the 200th generation does not guarantee convergence. If a stopping criterion is used, please state it in the text and also provide the actual number of generations from your experiment (should be less than 200).

 

Point 24: To avoid confusion, please either define theta_bat and theta_sc in the text or add these symbols in Table 3.

 

Point 26: There are two new equality constraints in the revised Equation 33. The second one concerns the supercapacitor configuration involving N_s and N_p. Should these variables be also included as design variables (the paragraph before Equation 33)? If not, what effect does this equality constraint have on the solution? It seems that N_s and N_p are design variables based on Figure 8 but are missing in the text.

 

Point 34: The diameter is a design variable in the first optimization and the mean chord length is a design variable in the second optimization. In this case the solidity is not a free variable thus may not be simply “taken” to be some value. The authors need to clarify this if any detail is missing in the text.

 

Point 37: Since the conversion factor may have a significant impact on the optimal operation of engine and HESS (e.g., a very high s_eq tends to eliminate the HESS), please clearly state in the text how it is determined and what is the value. For example, include a plot showing s_eq vs time or a table showing s_eq at different mission segments, etc.

 

Point 38: The sign convention seems to be conflicting between Equation 24 and 38. If a negative P_HESS represents charging, then Equation 21 alone without Equation 24 is sufficient to describe the energy flow. Consider revising the relevant paragraphs to make them consistent.

 

Point 41: Perhaps I did not make myself very clear in the last review. I intended to ask whether the ECMS uses gradient descent/genetic algorithm/etc. and this needs to be stated in the text. The FLC alone is only calculating the output power of HESS given the SOC and the rules without any local optimization.

 

Point 48: The blue curve in Figure 13 is showing 557 kW in the first few seconds which corresponds to the fast charging of supercapacitor in Figure 18. Is this supposed to happen with a 400 kW engine? Also, the engine rated power of 400 kW should be clarified in the text, perhaps early around Equation 3 since it is not a design variable.

 

Point 54: Please clarify “redundancy interval” in the text as in the response so the readers will not get confused.

 

Point 55: Just a friendly reminder to also update the abstract that you typed in the online submission system.

Author Response

Dear reviewer,

 

Thank you for the positive comments and suggestions concerning our manuscript entitled "Sizing Methodology and Energy Management of an Air-ground Aircraft with Turbo-electric Hybrid Propulsion System" (aerospace-2010164). We have carefully reviewed the comments and have revised the manuscript accordingly. Our responses are given in a point-by-point manner below. All these changes are highlighted in an additional copy of our manuscript.

 

Point 3: Figure 1 is now clearly showing the viewing angles in the subfigure captions. However, the flight attitudes are still not clear. For example, is 1(a) showing a 90-deg nose-down attitude? If it is difficult to graphically depict the attitude, please remove “flight attitudes” from the caption. Also 1(c) should be a right view instead of a left view.

Response 3: Thank you for your suggestion. The flight attitude of the HAGA is not clearly shown in Figure 1, and we have amended the relevant caption. The view is defined by the aircraft head being forward and the tail being backward, so Figure 1 (c) would represent the left view of HAGA.

 

Point 15: An AC/DC power converter is known as a rectifier instead of an inverter, which does just the opposite and converts DC power into AC power. There seems to be no DC/AC inverter in your architecture based on Figure 2.

Response 15: Thank you for your comments. The back end of the generator outputs AC, which is output to the DC bus through the AC/DC rectifier. As the motors are the brushless DC motors, the TEHPS does not require the inverters. We have made changes to the subscripts for errors in representation.

 

Point 19: In the second equation (M_fuel), consider replacing the summation with integration over time.

Response 19: We have made changes in the new manuscript based on your suggestions.

 

Point 22: The stopping criteria are still not clear. For example, is the algorithm considered to be converged after the lowest total mass among the 20 candidates does not change for a certain number of consecutive iterations? Simply taking the best-performing candidate from the 200th generation does not guarantee convergence. If a stopping criterion is used, please state it in the text and also provide the actual number of generations from your experiment (should be less than 200).

Response 22: Thanks for your question. An approximate global optimum solution is considered to be obtained when the GA terminates. In this paper, the maximum fitness value or average fitness value tends to stabilize as the termination condition within the maximum number of iterations. The simulation results show that in the first optimization, when the number of iteration steps is 22, the fitness value tends to be stable and the approximate optimal solution is obtained. In the second optimization, the optimization stops to obtain the optimal solution when the number of iteration steps is 50. Relevant expressions have been illustrated in the new manuscript.

 

Point 24: To avoid confusion, please either define theta_bat and theta_sc in the text or add these symbols in Table 3.

Response 24: Thanks for your question. θ_bat and θ_sc represent the energy density of battery and supercapacitor, which can be shown in Table 3.

 

Point 26: There are two new equality constraints in the revised Equation 33. The second one concerns the supercapacitor configuration involving N_s and N_p. Should these variables be also included as design variables (the paragraph before Equation 33)? If not, what effect does this equality constraint have on the solution? It seems that N_s and N_p are design variables based on Figure 8 but are missing in the text.

Response 26: Thanks for your question. We have a problem with the representation of the optimization variables. The number of batteries n_s and supercapacitors N_s in series, the voltage U_hm and speed ω_hm of the rotor motors, etc. should theoretically be included in the variable matrix in the second optimization. Based on the total number of batteries n_bat and supercapacitors N_sc in the first optimization and the last two equation constraints in Equation 32, the number of batteries (n_s, N_s) and supercapacitors (n_p, N_p) in series and parallel can be calculated.

 

Point 34: The diameter is a design variable in the first optimization and the mean chord length is a design variable in the second optimization. In this case the solidity is not a free variable thus may not be simply “taken” to be some value. The authors need to clarify this if any detail is missing in the text.

Response 34: Thank you for your suggestion, we were imprecise in the presentation of our last response. In the first optimization, the number n_b and diameter d of the blades were considered as optimization variables for the initial selection of ducted fans. In the second optimization, the pitch  and mean chord length  were used as optimization variables, and the thrust coefficient C_T was determined according to blade element-momentum theory, combined with parameters such as number of blades n_b, diameter d and advance ratio J, and substituted into equation 15 to calculate the available thrust of the ducted fans in the cruise phase.

 

Point 37: Since the conversion factor may have a significant impact on the optimal operation of engine and HESS (e.g., a very high s_eq tends to eliminate the HESS), please clearly state in the text how it is determined and what is the value. For example, include a plot showing s_eq vs time or a table showing s_eq at different mission segments, etc.

Response 37: Thanks for your question. The equivalence factor s_eq represents the core of ECMS. This parameter influences the system behaviour as follows: if it is too large, the use of electrical energy tends to be penalized and the fuel consumption increases; if, on the contrary, it is too small, the use of electrical energy is overly favoured and the battery SOC decreases. The setting of the relevant parameters has been explained in text in our new manuscript.

 

Point 38: The sign convention seems to be conflicting between Equation 24 and 38. If a negative P_HESS represents charging, then Equation 21 alone without Equation 24 is sufficient to describe the energy flow. Consider revising the relevant paragraphs to make them consistent.

Response 38: The P_HESS symbol really has a positive and a negative sign, positive for discharging and negative for charging. Based on your suggestion, we have made changes to the relevant paragraphs in the new manuscript.

 

Point 41: Perhaps I did not make myself very clear in the last review. I intended to ask whether the ECMS uses gradient descent/genetic algorithm/etc. and this needs to be stated in the text. The FLC alone is only calculating the output power of HESS given the SOC and the rules without any local optimization.

Response 41: Thank you for your question. The implementation of the ECMS energy management strategy focuses on the application of equivalence factor and takes the minimum fuel target consumption as the mathematical problem of optimal control of a non-linear system. ECMS is a heuristic for optimal control and is essentially equivalent to the Pontryagin's Minimum Principle (PMP). In addition, the ECMS is computed by solving two-point side problems for differential or difference equations derived from the PMP, discretizing control variables and establishing equations and inequality relationships based on State Space Analysis. The relevant expressions have been added in a new manuscript.

 

Point 48: The blue curve in Figure 13 is showing 557 kW in the first few seconds which corresponds to the fast charging of supercapacitor in Figure 18. Is this supposed to happen with a 400 kW engine? Also, the engine rated power of 400 kW should be clarified in the text, perhaps early around Equation 3 since it is not a design variable.

Response 48: Thanks for your question. The power turbine is the main component for power output in the turboshaft engine, and the output power is calculated according to its speed and torque. As can be seen from Figure 16, there is a peak starting torque 257.6 N·m of the power turbine at the beginning and a constant speed of 20,600 rpm, which causes the peak power value shown in the blue curve in Figure 13. The duration is only 20s and the turboshaft engine is ready for normal operation. In addition, we believe that this phenomenon in the supercapacitor is caused by the control strategy. It is obvious from the power balance that the total TEHPS demand power is equal to the sum of the power of the TGS and the HESS. At the moment of transition to steady state in the turboshaft engine, there is a large negative value of HESS power, which causes a rise in SOC due to the fast response of supercapacitor. Subsequent work will therefore provide a detailed analysis of the transient response of the turboshaft engine. In the previous response there was a problem with the 400kW power of the turboshaft engine, which is the rated power of the turbine generation system. Since the efficiency of the AC/DC rectifier should be taken into account, the rated power of the turboshaft engine should be 420 kW, which we have corrected and added in the text of section 2.1.1.

 

Point 54: Please clarify “redundancy interval” in the text as in the response so the readers will not get confused.

Response 54: We are sorry for the lack of clear explanations in the text, which we have added to the new manuscript.

 

Point 55: Just a friendly reminder to also update the abstract that you typed in the online submission system.

Response 55: Thank you very much for your advice.

 

We thank the reviewer for the patience and efforts to help us improve the technical content and the grammar in this manuscript. Your generous help is greatly appreciated.

 

Finally, we appreciate for your work earnestly, and hope that the correction will meet with approval.

 

Sincerely

Wenjiang Yang

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors addressed all my previous comments. The paper can be accepted for publication.

Author Response

Dear reviewer,

Thank you for the positive comments and suggestions concerning our manuscript entitled "Sizing Methodology and Energy Management of an Air-ground Aircraft with Turbo-electric Hybrid Propulsion System".

We thank the reviewer for the patience and efforts to help us improve the technical content in this manuscript. Your generous help is greatly appreciated.

Finally, we appreciate for your work earnestly, and hope that the correction will meet with approval.

Sincerely

Wenjiang Yang