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AERO2363 – Aerospace Structures Studio

Advancing Green Aviation through Composite Wing Design – Final Report – Group 21

Executive Summary

This report investigates the structure of the PILATUS PC-9 main spar, from Rib 9 through Rib 25. This report carries on from the previous “Progress Report”. The current PILATUS PC-9 wing structure is analysed with the intent of moving to a composite wing structure. By analysing the current structure, Margins of Safety (MOS’s), lifting loads, structural integrity, and fatigue life can be established. From this basis, a composite wing structure can be researched, with the aim of reducing emissions.

An aerodynamic analysis using XFOIL was performed to determine the maximum lift and drag generated by the wing planform. at cruise conditions, as well as the spanwise lift and drag distribution. The analysis found that the maximum lift produced by one wing at cruise is 19,887.5N, and the spanwise lift and drag distribution is nearly triangular.

The static analysis considers an unnotched and notched main spar and investigates fatigue.  Two load cases were considered: Case A assumes that the flanges of the main spar resist all direct stresses, while Case B assumes the web to be fully effective in resisting direct stress. Case B consistently yielded higher direct, shear, and von Mises stresses. The lowest MOS calculated was 4.56 for Case B.

The dynamic analysis analysed the dynamic loading and safe life of the main spar, assuming an Aluminium structure. Considering the mean stress effect, and using the SWT rule, the safe life was calculated to be 25.7 years.

To make transitioning to a composite structure feasible, the cost of retrofitting the PC-9 must not outweigh the cost-savings associated with reduced fuel use. By considering MOS, safe life, and aircraft loads, the feasibility of the composite structure can be later established.

Introduction

The introduction is too short and lacks key components such as the problem statement and the adopted methodology. You have only mentioned static analysis, while other essential components like dynamic and fracture analyses were overlooked. A more detailed introduction would set a clearer foundation for the report.

Everyday, new net zero targets are set for businesses, industries, and countries. Aviation is not immune to scrutiny, with net zero expected by 2050 (IEA 2023). Currently, aviation is not on track to achieve this objective (IEA 2023). Airframe. optimisation is one such way to meet net zero targets. The transition from an aluminium airframe. to a composite one could reduce emissions by up to 20% (Timmis et al. 2014).

This report seeks to optimise the PILATUS PC-9 wing planform. by replacing the existing aluminium wing structure with a composite structure.

Literature Review

Replacement of the PC-9 aluminium airframe. with composite has been investigated by Jost (1989), who highlighted the significant weight savings, which would enhance aircraft performance and improve fuel efficiency. Improved fuel efficiency means lesser emissions, the aim of this report. Composites are also slated to have improved durability and fatigue resistance when coMPared with aluminium (Jost 1989). The extended lifespan of the airframe. also reduces operating costs over the lifetime of the aircraft, and non-operational emissions associated with repairs.  Kopp (2003) also highlights the opportunity to replace ageing aluminium structure with composite ones. Kopp (2003) cites enhanced fatigue resistance, lower weight, and removal of threat of corrosion has benefits that make composite airframes attractive.

Methodology

This report provides a basis to showcase the potential benefits of composite airframes when compared with aluminium. First, an aerodynamic analysis is conducted using XFOIL to understand the loads the PC-9 undergoes in its operational cycle. A static analysis is undertaken on the existing aluminium airframe. to determine the overall stresses acting on the spar, and the MOS of the structure during use. A notched stress analysis is done using the Ramberg-Osgood relationship to determine the maximum and minimum stresses and strains around the edges of the holes in the flanges and the web. The residual stress of the main spar is examined. Goodman’s rule and Smith-Watson-Topper’s rule are utilised to determine the structural safe life of the aluminium structure. The mean stress effect is considered. Linear Elastic Fracture Mechanics (LEFM) is used to determine the structure's ability to sustain damage (such as cracks) without catastrophic failure.

Scope and Limitations

This analysis is limited by the assumptions used, and the lack of available proprietary data pertaining to PILATUS aircraft. It does consider a wide range of failure modes to ensure that a composite structure operating in the same conditions meets the requirements of existing customers to ensure transitioning to composites is an attractive prospect. It does not consider the costs associated with this transition, and whether the composite structure is feasible. This may be conducted at a later stage.

Aerodynamic Analysis

The aerodynamic analysis examines the lift and drag generated by the wing between Rib 9 and Rib 25. XFOIL was used to generate the aerodynamic characteristics of the wing at cruise conditions. The analysis found a maximum lift of 19,887.5 N per wing at cruise conditions, a uniform. spanwise drag distribution, and an almost perfectly triangular lift distribution. The discrepancy between calculated lift and expected lift at cruise is likely explained by trim conditions and overestimation of lift by XFOIL (Kallstrom 2022). These findings inform. the remainder of the report.

The PILATUS PC-9 uses a PIL15M825 airfoil at its root, and a PIL12M825 wing at its tip. These two airfoil geometries are proprietary, so no airfoil data is publicly accessible. The PILATUS airfoils are based on NACA64A415 and NACA64A612 airfoils, with modifications to reduce the pitching moment of the geometries (Dulio and Turi 1984).  By using the NACA derivatives, the calculated bending moments generated in the main spar are larger than reality, leading to a conservative calculation for the composite wing, and a higher Margin of Safety (MOS) for the composite wing.

The PC-9 has a cruising speed of 154.44m/s at an altitude of 7,620m (Jackson 2003). At this altitude, the speed of sound is 309m/s, and the Mach number for the PC-9 is 0.499 (NASA n.d.). These inputs yield the flow characteristics shown in Table 1.

Table 1. Fluid characteristics at PC-9 cruise (Engineering Toolbox n.d.)

Using these parameters, the Reynold’s Number per mean chord can be found according to Appendix B. The mean chord of each section is found using the taper ratio, shown in Appendix C. The mean chord of each section is shown in Appendix D. To calculate the coefficient of lift and drag for each section, the Mach number was defined in XFOIL. 100 iterations were allowed to ensure that XFOIL could converge on a result. The number of airfoil panels was set to 250 to strike a balance between computing time and accuracy. The calculated Reynold’s Number and corresponding AoA of the wing section was input into XFOIL, which then output a coefficient of lift, drag, and moments for that section. Varying Reynold’s number due to varying chord for each wing section is accounted for by varying the Reynold’s number in XFOIL. AoA was assumed to vary almost linearly along the wingspan. The airfoil profiles were interpolated between the root and the tip to give a more accurate representation of the wing structure. An XFOIL code excerpt is shown in Appendix E. From the data produce by XFOIL, the lift and drag of the section were calculated using the wing area. The table of drag, lift, and moment coefficients is shown in Appendix F. Table 2 shows the resulting lift and drag of the wing.

Table 2. PC-9 key aerodynamic properties

The calculated lift of the wing is slightly greater than half the MTOW of the entire aircraft. This discrepancy may be explained by the PC-9 requiring trim at cruise to reduce the lift generated by the wings. Furthermore, the real lift of the wing may be further reduced by turbulence induced by artefacts on the wing surface, such as rivets, sensors, and lights that are not accounted for in this simulation. The NACA64A415 and NACA64A612 are not identical to the PC-9 airfoil, which means that the actual PC-9 airfoil may not produce the same amount of lift at cruise conditions as the modelled airfoil. Additionally, XFOIL’s accuracy is limited above Mach 0.4, and the software tends to over-estimate lift, and under-estimate drag (Kallstrom 2022).

The calculated spanwise lift distribution is shown in Figure 1. The drag distribution has the same shape.

Figure 1. Spanwise lift distribution

The spanwise lift distribution is almost triangular, with only a minor deviation from linearly reducing lift with change in span. As such, a triangular lift distribution was assumed for the static analysis.

Static Analysis

The static analysis of the main spar requires analysis of loading applied to an aircraft during cruise conditions. Cruise conditions are used for the static analysis as this is when the wing will be under the highest static load. The analysis is conducted at ‘Rib 9’. Therefore, only loads from Rib 9 to 25 are considered for the analysis. Aircraft wing structure can be found in Appendix A.