**4. NASA X-57 Maxwell test vehicle**

Under the Scalable Convergent Electric Propulsion Technology Operational Research (SCEPTOR) project, the X-57 Maxwell test vehicle wing is presently being constructed at NASA Armstrong Flight Research Center. **Figure 1e** showed the NASA X-57 Maxwell experimental test aircraft concept [4] with a distributed electric propulsion system featuring 12 electric-motor-driven propellers on an innovative high-lift wing. The X-57 Maxwell vehicle will test the performance of this specially designed wing with distributed electric propulsion in order to evaluate mission benefits for this class of vehicle.

**Figure 11** shows the weight breakdown of the NASA X-57 Maxwell experimental test aircraft. The original wing of the Italian *Tecnam P2006T* aircraft will be replaced with a specially designed distributed electric propulsion wing with 12 electric-motor-driven propellers. The wing-tip propellers help reduce the induced drag from the tip vortex. The synchronized motors are powered by a 358 kg Nickel-Cobalt-Aluminum (NCA) battery pack. The electric power system is organized into eight battery modules, split into two packs with 4 battery modules and a control module each. Cooling is provided through 18,650 cells spaced evenly, 4 mm apart. The NCA cells provide sufficient energy density and the required discharge rate for the flight test mission. Each pack supplies 47 kWh of useful energy, with a peak discharge power of 132 kW. The total battery package weight is estimated to be 790 lb (358 kg), or 26% of the total aircraft takeoff gross weight of 3006 lb (1364 kg). The aluminum fuselage weight is 302 lb (136 kg), and the total estimated structure weight without the landing gear is 738 lb (335 kg).

**Figure 12** shows initial power requirement estimates for the standard mission of the X-57 Maxwell [6] flight test vehicle. The energy requirement for each phase of the mission is obtained by integrating the power requirement over time

*Environmental Impact of Aviation and Sustainable Solutions*

The performance goal for the M-SHELLS development was to demonstrate a specific power of 1000 W/kg at an energy density of 75 Wh/kg. The flight test goal was to augment the existing Li-Po battery with 33% of the required energy for 30 minutes of flight or, equivalently, to supply the full electrical energy for 10 minutes of level flight. The Li-Po battery capacity is 7600 mAh and it provides 7.4 volts with two 3.7 volt cells in series. With a gross weight of 2.3 lb (1.04 kg), the energy density of the Li-Po battery is 55 Wh/kg. The ideal power required by the aircraft at cruise is computed from weight × velocity/(L/D), where L/D is the lift-to-drag ratio. Considering the propeller and motor efficiencies, the total power required to be supplied to the electric motor spinning

*Comparison of component weights of the tempest test vehicle, initial two-spar wing model, and improved* 

*Wing deflection and strain of the improved finite-element model of the test vehicle in level flight.*

Power Required <sup>=</sup> weight <sup>×</sup> velocity/[L/D <sup>×</sup> (propeller efficiency) × (motor efficiency)] (1)

For the *Tempest* test vehicle, let us assume a baseline cruise weight of 20 lb (88 N), a cruise velocity of 40 mph (17.9 m/s), and a typical L/D of 20. Assuming a motor efficiency of 85% and a propeller efficiency of 80%, the power required = 88 × 17.9/ (20 × 0.85 × 0.80) = 116 W and the energy required for 10 minutes of level flight is (116 × 10/60) = 20 Wh. Hence, ideally, 0.58 lb (20/75 kg) of M-SHELLS material could provide full power for 10 minutes of level flight. The actual weight of the M-SHELLS power package would depend on the flight test voltages and current demand of the electric motor and the ability to package each unit in suitable series and parallel configurations to match the available power supply and required power demand.

**10**

the propeller is:

**Table 1.**

**Figure 9.**

*tempest FEM.*

**Figure 11.**

*Component weight fractions for the X-57 Maxwell electric distributed propulsion vehicle.*

#### **Figure 12.**

*X-57 Maxwell standard mission power requirement estimates.*

(area under the power requirement curve). For example, during the cruise time interval of 800 seconds (0.22 hours), at constant power the energy required is 120 × 0.22 = 26.4 kWh with the X-57 wing (blue line). Based on the current mission analysis utilizing the original *Tecnam* wing, 38 kWh is required to meet the peak power demand of 145 kW (red line).

Assuming M-SHELLS could produce 1000 W/kg specific power at a 75 Wh/ kg specific energy, a 120 kg M-SHELLS package would ideally provide 120 kW of power and 9 kWh of energy. Given the 120 kW of power required during cruise with the X-57 wing (blue line), the M-SHELLS package could supply energy for a duration of 0.075 hours, or 270 seconds, at level cruise.

A brief structural analysis of the fuselage was conducted, where a reinforced M-SHELLS multifunctional panel can be safely substituted to partially replace the lightly loaded aluminum floor structure. **Figure 13** shows an example of fuselage floor deflection and shear stress with the original floor replaced by a reinforced composite panel with the M-SHELLS core. The five-layer composite sandwich panel consisted of two 0.05 inch thermoplastic sheets for reinforcement and insulation on the outer faces, which were bonded to the two 0.002 inch aluminum sheets on the inner faces over the 1.0 inch deep M-SHELLS core. For this example, the total distributed floor load is 265 lb (120 kg) distributed over the forward fuselage floor area. The fuselage floor deflection is nominal and the majority of the shear stresses across all plies are generally within the allowable limits except at the end support areas, where local reinforcements will be needed.

**13**

*Structural Analysis of Electric Flight Vehicles for Application of Multifunctional Energy Storage…*

In the ARMD Advanced Air Transport Technology (AATT) project, several aircraft concepts are presently being studied to quantify the performance improvements and emissions reduction afforded by hybrid-electric propulsion. Jansen et al. [7] have conducted extensive systems analysis to evaluate the risks and benefits of a conversion from an all-fuel turbofan to a hybrid-electric turbofan engine concept. Among the propulsion options considered by this study, the "hFan" concept is a gas turbine-electric hybrid engine capable of operating in all-gas turbine, all-electric, or combined mode, depending on mission requirements. Conventional and trussbraced wing concepts with hybrid-electric propulsion were also investigated by

*X-57 floor deflection and shear stress analysis with 265 lb (120 kg) M-SHELLS distributed over the forward* 

Objectives of the NASA Electrified Aircraft Propulsion (EAP) research are to increase fuel efficiency and to reduce the emissions and noise levels of commercial transport aircraft. Primary EAP propulsion concepts include turboelectric, partially turboelectric, and hybrid-electric systems. Applications are presently being evaluated for regional jet and larger sized single-aisle aircraft. The overall goal is to demonstrate the viability of at least one of the EAP concepts. A hybrid-electric derivative of the N+3 technology conventional configuration (N3CC) is an ideal candidate for future applications of the M-SHELLS technology, by replacing lightly loaded portions of the fuselage structures where use of lightweight honeycomb panel is possible. The outer mold line (OML) of this aircraft concept [5] was developed using the Open Vehicle Sketch Pad tool [10, 11]. The internal structure of a fuselage segment of this vehicle was developed using SolidWorks [12] for finite element analysis. The structural analysis included a combination of aluminum and reinforced M-SHELLS composite panels for stress, deflection, and weight estimation. A block diagram of the FEM development and sizing process is presented in Appendix C. **Figure 14a** shows the N3CC vehicle model with internal structure, and the detailed FEM of a fuselage segment is shown in **Figure 14b**. The fuselage section design loads consist of an internal cabin pressure of 18.4 psi, passenger floor load of 1 psi, and cargo floor load of 2 psi. The weight analysis of the N3CC hybrid concept fuselage segment with Al 7075-T6 construction is shown in **Table 2**. The total FEM weight of this allaluminum fuselage segment is 4992 lb. This includes a passenger floor weight of 876 lb,

*DOI: http://dx.doi.org/10.5772/intechopen.86201*

**5. Hybrid-electric aircraft**

**Figure 13.**

*fuselage floor area.*

Bradley and Droney [8, 9] at the Boeing Company.

*Structural Analysis of Electric Flight Vehicles for Application of Multifunctional Energy Storage… DOI: http://dx.doi.org/10.5772/intechopen.86201*

**Figure 13.**

*Environmental Impact of Aviation and Sustainable Solutions*

(area under the power requirement curve). For example, during the cruise time interval of 800 seconds (0.22 hours), at constant power the energy required is 120 × 0.22 = 26.4 kWh with the X-57 wing (blue line). Based on the current mission analysis utilizing the original *Tecnam* wing, 38 kWh is required to meet the peak

*Component weight fractions for the X-57 Maxwell electric distributed propulsion vehicle.*

Assuming M-SHELLS could produce 1000 W/kg specific power at a 75 Wh/ kg specific energy, a 120 kg M-SHELLS package would ideally provide 120 kW of power and 9 kWh of energy. Given the 120 kW of power required during cruise with the X-57 wing (blue line), the M-SHELLS package could supply energy for a

A brief structural analysis of the fuselage was conducted, where a reinforced M-SHELLS multifunctional panel can be safely substituted to partially replace the lightly loaded aluminum floor structure. **Figure 13** shows an example of fuselage floor deflection and shear stress with the original floor replaced by a reinforced composite panel with the M-SHELLS core. The five-layer composite sandwich panel consisted of two 0.05 inch thermoplastic sheets for reinforcement and insulation on the outer faces, which were bonded to the two 0.002 inch aluminum sheets on the inner faces over the 1.0 inch deep M-SHELLS core. For this example, the total distributed floor load is 265 lb (120 kg) distributed over the forward fuselage floor area. The fuselage floor deflection is nominal and the majority of the shear stresses across all plies are generally within the allowable limits except at the end support areas, where local reinforcements will be

power demand of 145 kW (red line).

*X-57 Maxwell standard mission power requirement estimates.*

duration of 0.075 hours, or 270 seconds, at level cruise.

**12**

needed.

**Figure 12.**

**Figure 11.**

*X-57 floor deflection and shear stress analysis with 265 lb (120 kg) M-SHELLS distributed over the forward fuselage floor area.*
