**5. Hybrid-electric aircraft**

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

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,

**Figure 14.**

*N3CC fuselage segment analysis with aluminum 7075-T6 material construction.*


#### **Table 2.**

*Weight analysis of N3CC fuselage segment with aluminum 7075-T6 construction.*

an outer shell weight of 3461 lb, a cargo floor weight of 342 lb, and the total keel-beam and cross-beam weight of 313 lb. **Figure 14c** shows the all-aluminum fuselage deflection and **Figure 14d** shows the von Mises stress distribution.

**Figure 15** shows the modified fuselage section in which the passenger and cargo subfloor cross-beams were replaced with the five-layer reinforced composite panels with honeycomb core (5LCHC). The sandwich panels consisted of 1 inch deep M-SHELLS honeycomb core and 0.002 inch aluminum ply and 0.05 inch thermoplastic ply on each side. **Figure 15a** shows the N3CC fuselage model and design load. As before, the fuselage section design loads consisted of an internal cabin pressure of 18.4 psi, passenger floor load of 1 psi, and cargo floor load of 2 psi. The passenger subfloor and cargo subfloor cross-beams are now replaced with this five-layer bonded composite panel with M-SHELLS honeycomb core (**Figure 15b**). **Figure 15c** shows a significant increase in the maximum floor deflection compared to the all-aluminum construction shown in **Figure 14c**. **Figure 15d** shows maximum von

**15**

**Figure 15.**

**Table 3.**

*panel.*

*composite panels with M-SHELLS core.*

passenger sub-floor cross-beam.

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

Mises stress distribution across all ply, which are significantly higher locally in the

*Weight analysis of N3CC fuselage segment with aluminum 7075-T6 and M-SHELLS honeycomb composite* 

*N3CC fuselage segment analysis with passenger and cargo subfloor cross-beams replaced by reinforced* 

without risking the structural integrity (**Figure 15c** and **d**).

The weight analysis of the N3CC hybrid concept fuselage segment with aluminum and M-SHELLS composite panels is shown in **Table 3**. The total FEM weight of this fuselage segment is 4830 lb. The passenger floor weight is reduced to 728 lb from 876 lb for the previous case. The aluminum outer shell weight remains 3461 lb. The cargo floor weight is reduced to 328 lb from 342 lb. The total keel-beam and cross-beam weight remains 313 lb. Thus, the weight reduction for one fuselage segment is 162 lb or 3.2%, at the cost of higher fuselage deflection and stress, but

Since this substitution resulted in large increases in deflection and stress in the passenger floor (**Figure 15c** and **d**), additional sub-floor support in the cargo hold area was examined as shown in **Figure 16a** and **b**. The corresponding structural deflection and stress distribution are shown in **Figure 16c** and **d**. The maximum

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

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

#### **Figure 15.**

*Environmental Impact of Aviation and Sustainable Solutions*

an outer shell weight of 3461 lb, a cargo floor weight of 342 lb, and the total keel-beam and cross-beam weight of 313 lb. **Figure 14c** shows the all-aluminum fuselage deflec-

**Figure 15** shows the modified fuselage section in which the passenger and cargo subfloor cross-beams were replaced with the five-layer reinforced composite panels with honeycomb core (5LCHC). The sandwich panels consisted of 1 inch deep M-SHELLS honeycomb core and 0.002 inch aluminum ply and 0.05 inch thermoplastic ply on each side. **Figure 15a** shows the N3CC fuselage model and design load. As before, the fuselage section design loads consisted of an internal cabin pressure of 18.4 psi, passenger floor load of 1 psi, and cargo floor load of 2 psi. The passenger subfloor and cargo subfloor cross-beams are now replaced with this five-layer bonded composite panel with M-SHELLS honeycomb core (**Figure 15b**). **Figure 15c** shows a significant increase in the maximum floor deflection compared to the all-aluminum construction shown in **Figure 14c**. **Figure 15d** shows maximum von

tion and **Figure 14d** shows the von Mises stress distribution.

*Weight analysis of N3CC fuselage segment with aluminum 7075-T6 construction.*

*N3CC fuselage segment analysis with aluminum 7075-T6 material construction.*

**14**

**Table 2.**

**Figure 14.**

*N3CC fuselage segment analysis with passenger and cargo subfloor cross-beams replaced by reinforced composite panels with M-SHELLS core.*


#### **Table 3.**

*Weight analysis of N3CC fuselage segment with aluminum 7075-T6 and M-SHELLS honeycomb composite panel.*

Mises stress distribution across all ply, which are significantly higher locally in the passenger sub-floor cross-beam.

The weight analysis of the N3CC hybrid concept fuselage segment with aluminum and M-SHELLS composite panels is shown in **Table 3**. The total FEM weight of this fuselage segment is 4830 lb. The passenger floor weight is reduced to 728 lb from 876 lb for the previous case. The aluminum outer shell weight remains 3461 lb. The cargo floor weight is reduced to 328 lb from 342 lb. The total keel-beam and cross-beam weight remains 313 lb. Thus, the weight reduction for one fuselage segment is 162 lb or 3.2%, at the cost of higher fuselage deflection and stress, but without risking the structural integrity (**Figure 15c** and **d**).

Since this substitution resulted in large increases in deflection and stress in the passenger floor (**Figure 15c** and **d**), additional sub-floor support in the cargo hold area was examined as shown in **Figure 16a** and **b**. The corresponding structural deflection and stress distribution are shown in **Figure 16c** and **d**. The maximum

### *Environmental Impact of Aviation and Sustainable Solutions*

deflection was reduced significantly and the von Mises stress distributions were within the allowable limits. The additional M-SHELLS weight was 173.5 lb. Hence, the net weight increase was 11.5 lb (0.3%) per segment, compared to all aluminum construction, while adding 56 cubic foot of M-SHELLS storage volume. The fuselage section weight comparison summary from the three designs is presented in **Figure 17**.

These weight calculations with the reinforced M-SHELLS panel did not include copper current collectors, separator layers, and electrolyte that are required to complete the energy storage functionality but do not add to the structural strength. Appendix B shows the M-SHELLS panel density and properties. A full vehicle structural and systems analysis for the N3CC derivative with hybrid-electric propulsion was presented by Olson and Ozoroski [2] to predict the multifunctional performance and weight benefits with higher specific energy M-SHELLS replacing major primary structure. Their study showed that by offsetting the weight of some of the vehicle's primary batteries or mission fuel, an overall weight savings can be achieved through multifunctionality. An initial version of the paper was proposed for presentation in [13].

#### **Figure 16.**

*N3CC fuselage segment analysis with additional reinforced M-SHELLS panel added to the subfloor cargo area.*


**17**

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

The Multifunctional Structures for High Energy Lightweight Load-bearing Storage (M-SHELLS) research project is described. The proposed project goals were to develop M-SHELLS in the form of honeycomb coupons and subcomponents, integrate them into the structure, and conduct low-risk flight tests onboard a remotely piloted small aircraft. The M-SHELLS sample units were scheduled for flight testing onboard a remotely piloted small aircraft named *Tempest*. Detailed finite element models of this small test aircraft were developed for basic structural strength and accurate weight analysis. The *Tempest* wing FEM was refined to include the unique wing construction and provide a closer match with the wing deflection results from a bench test. The component weight analysis from the finite element analysis and load test data were correlated. Finite element analysis results of *Tempest* with a reinforced five-layer M-SHELLS composite panel replacing the mid-fuselage floor were presented. Approximately,

2.2 lb of M-SHELLS would provide power for 10 minutes of cruise flight.

Although the planned flight test was cancelled due to the project constraints, the analysis results indicate that the mid-fuselage floor composite multifunctional panel could provide both structural integrity and electrical energy to supplement

The NASA X-57 Maxwell distributed electric propulsion test vehicle was used as an example for potential application of the M-SHELLS technology. The fuselage floor structure was selected for substituting a reinforced composite panel with M-SHELLS core. A structural analysis of the fuselage floor indicated that it could self-support a 265 lb (120 kg) M-SHELLS system, providing sufficient power and energy for 270 seconds of cruise flight. The fuselage floor deflection is nominal and the majority of the shear stresses are generally within the allowable limits. For future applications of M-SHELLS, structural analysis of an advanced transport aircraft fuselage segment is presented. Secondary aluminum structure in the fuselage sub-floor and cargo area were replaced with reinforced composite panels with M-SHELLS honeycomb core. Fuselage structural analyses associated with three cases were described. The weight estimation with the reinforced composite M-SHELLS panels replacing the passenger sub-floor indicated a 3.2% reduction in fuselage weight, at the cost of higher deflection and stresses, but without risking the structural integrity. With additional M-SHELLS panels in the cargo hold area, the deflection and stresses were reduced. But, the net weight of the fuselage segment increased by 11.5 lb (0.3%) compared to all aluminum construction, while adding 56 cubic foot of M-SHELLS volume and ~22 kWh of energy capacity/segment. These weight calculations were with the

reactive materials that are required to complete the energy storage functionality.

The authors thank the NASA Transformative Aeronautics Concepts Program (TACP), directed by Dr. John Cavolowsky for funding this research. The M-SHELLS sub-project is part of the Convergent Aeronautical Solutions (CAS) project under the TACP Program. CAS is directed by Isaac Lopez, Project Manager, Marty Waszak, Deputy Project Manager and Daniel Williams, CAS Liaison Officer at Langley. We thank Dr. Nicholas Borer, Jeff Viken, and the X-57 Maxwell Systems Analysis and Integration team for their guidance. Thanks are also due to Kevin Roscoe, Greg Howland, David A. Hare, Luke Laub, David Klassman, and Frank Leone for performing the *Tempest* structural weight

density. This calculation did not include

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

**6. Concluding remarks**

the existing battery.

reinforced M-SHELLS panel with 11.9 lb/ft3

**Acknowledgements**

#### **Figure 17.**

*Summary of weight comparison from the three fuselage segment design.*

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