ANALYSIS AND OUTPUTS

As shown in Figure 38 the final activities in this conceptual design methodology are concerned with the analysis of performance attributes, preparation of specifications and plans, and an assessment of uncertainties and sensitivities associated with the concept. It should be noted however that all steps in this conceptual design process may feedback into earlier steps as an iterative process, with the main objective of this methodology is to finalise the preferred design concept. This is achieved through establishing the specification and associated plans for the next design phase.

5.4.1 Performance metrics and analysis – Step 8 5.4.1.1 Aircraft performance

Aircraft performance metrics associated with the natural gas and EP system modification case studies are related to the specific mission and role of each aircraft.

In the case of the commuter aircraft mission, the natural gas fuel modification range performance was rated as important, via the QFD matrix requirement relating to “low

drag and low weight”. In this instance a measure was adopted that encapsulated this performance requirement, which was expressed in a payload-range diagram. The payload-range diagram is a standard means of presenting this performance information and provides a means of comparing range of various aircraft. This payload-range attribute incorporates the weight impact of the modification and fuel load defining payload, and also reflects range performance as a function of the specific fuel consumption and lift to drag ratio. This payload-range diagram was developed in Appendix 2, and presented in Appendix 1 for the natural gas fuel modification case study. Cruise performance was also estimated as a result of the drag impact of the external natural gas fuel tank mounted beneath the fuselage. The estimate of drag and hence impact to aircraft cruise airspeed was developed in Appendix 2, and presented in Appendix 1. It is important to note that the metrics presented in the quantified morphological matrix are reflected either directly or indirectly in the range-payload and cruise speed performance attributes.

In the case of the skydiving aircraft mission, the EP system modification climb speed and useful load performance was rated important via the two QFD matrix requirements relating to “time of climb to altitude” and “useful load to altitude”.

Subsequent change options analysis confirmed that the provision of “useful load to altitude” was rated marginally higher than the “time of climb to altitude”, as this useful load attribute encapsulated the value proposition. Given this, climb performance measures were developed and modelled based on maximum useful load to jump altitude within the time of climb requirement. This modelling also supported the sizing of propulsion system to achieve these climb performance requirements. The initial sizing performance modelling also determined battery energy requirements, and hence battery and other system weights which impacted useful load. Climb performance modelling including initial sizing is presented in Appendix 3, via the quantified morphological matrix. This quantified morphological matrix incorporated the weight metrics corresponding to subsystems added using parametric relationships and initial sizing modelling; and predicted climb performance based on initial sizing climb performance modelling.

5.4.1.2 Aircraft costs

Costs have been incorporated into this methodology as part of the morphological analysis where these costs are based on parametric estimates of hardware costs. In the

case of the natural gas fuel modification commuter aircraft mission, cost metrics were based on natural gas fuel tank costs as this was determined to be the most expensive contributor to the natural gas segment costs. Apart from the construction costs associated with ground fuelling infrastructure, aircraft operational costs comprised significant contributor to Life-cycle Costs associated with the natural gas modification.

The aircraft annual operating costs were determined using a method described by Gudmundsson (2014), which is based on experiences associated with the actual ownership of a GA aircraft. The primary inputs are flight hours per year, cost of fuel, amount borrowed to fund the aircraft and the natural gas modification, and the associated insurance coverage. As described by Gudmundsson (2014), this can be presented as a flight hour cost as shown in Appendix 1.

In the case of the EP system modification skydiving mission, cost metrics were based on parametric estimates of subsystem costs given by motor, battery and electric controller hardware. Batteries were determined by inspection to be the most expensive contributor to the EP system costs, as this component is likely to be a bespoke solution.

The operating costs associated skydiving using an aircraft modified with an electric propulsion system is dependent on several parameters related to energy requirements of the flight, usage profile and the conditions associated with electricity supply. Given that there are currently no electric aircraft used for skydiving at this time, these operating costs were difficult to determine accurately. Given commercial electricity rates and the estimate of the climb energy and reserve energy requirements, an estimate of costs to complete a typical skydiving mission was estimated. However, this estimate did not include battery amortisation costs or demand charges for the reasons as discussed in Appendix 3.

5.4.1.3 Emissions

The emissions associated with each case study were determined from literature reviews or by inspection. In the case of the natural gas fuel modification commuter aircraft mission, six papers describing the results of tests comparing natural gas emissions with gasoline were analysed to provide the percentage reduction/increase in natural gas fuel emissions, as summarised in Appendix 2.

In the case of the EP system skydiving mission, emissions were not determined directly in the case study. Rather it was observed that an electric aircraft possesses zero emissions at the point of operation.

5.4.1.4 Ground infrastructure performance

The ground infrastructure performance metrics associated with each case study are incorporated into each respective morphological matrix shown in Appendices 1 and 3. In a similar way to aircraft performance measures, the ground infrastructure metrics were dependent on the aircraft mission and role, and the energy type and technologies involved.

In the case of the natural gas fuel modification commuter aircraft mission, the main metrics fell into three categories being cost, fuel storage and fill times. The costs were determined by natural gas fuelling station construction costs comprising buildings, equipment and facilities, and delivery/production costs associated with CNG or LNG fuels, which involved transportation of fuels to site, or the production of fuels onsite. The fuel storage and fill time metrics were determined by the storage time of fuel without degradation or evaporation for a period of 1 week. The fill time metric was based on a refill rate of at least 200 gallons per hour (760 litres per hour).

In the case of the EP system modification skydiving mission, metrics were divided into three broad areas being costs, battery recharge time and flight duty cycle.

The cost metric related to the ground charging system equipment costs, and the costs of additional battery units used for exchange. Given that the objective function was to minimise costs, then the number of additional battery sets were minimised in this analysis. The maximum battery recharge time metric was determined for corresponding charging station levels based on an assumption of average recharge times applicable to an average Electric Vehicle (EV), noting that actual recharging time was subject to a combination of factors such State-of-Charge (SOC), Charging Level and battery size. The flight duty cycle metric related to the number of flights achievable during an assumed 8-hour day. This was determined from the recharge time for each system charging level, and the spare battery sets available for exchange after each flight.

5.4.1.5 Uncertainties and sensitivity analysis

Conceptual design, as an early life-cycle activity, is subject to constraints in relation to the fidelity of models and the availability of accurate data. It is accepted that low fidelity models are applied in early design phases with increasing fidelity in later phases. Price et al. (2006) states that early design life-cycle models usually comprise low fidelity simple equations, look up tables with no associated geometry,

and data which is subject to some level of uncertainty. The natural gas fuel modification case study undertook a range sensitivity analyses on two parameters based on uncertainties in installed LNG fuel tank drag coefficient (CD) and LNG Specific Fuel Consumption (SFC). These uncertainties impacted the Breguet range either directly as a SFC term, or indirectly through the Lift/Drag ratio (CL/CD) term.

The sensitivity analysis decreased CL/CD ratio by 5% to account for variation in LNG fuel tank installation drag, and increased LNG SFC by 10% to account for uncertainties in LNG-related engine performance and SFC data. The results are shown in Appendix 2.

In the case of the EP system modification, a sensitivity analysis was undertaken to investigate the impact of battery specific energy density, motor peak specific power and propeller type on useful load and total battery weight. This EP system analysis has focused on useful load and battery weight as these are parameters that determine the viability of the EP system modification as described earlier. The results of this sensitivity analysis is shown in Appendix 3 where useful load and total battery weight as a function of battery specific energy density was determined for the preferred candidate configuration.

This analysis also presented useful load and total battery weight as a function of the motor peak specific power as determined for a range of motors considered in the corresponding morphological matrix. Also shown in Appendix 3 was the relationship of total battery weight with motor peak specific power where the variation in battery weight was proportional to motor peak power.

5.4.2 Risks safety and airworthiness

The safety and airworthiness requirements rated as one of the highest for both natural gas fuels and EP systems case studies. Airworthiness certification is dealt with in Step 7 of this design methodology providing a means of documenting certification risks as well as estimating costs. The certification risks were combined with change propagation severity risks to provide a consolidated project risk register as shown by Step 9.

5.4.3 Methodology outputs – Step 9

The conceptual design methodology as formulated in this thesis provides as outputs several documents comprising specifications, plans and registers. The

development of these documents is dependent on the data and information provided at various steps in the methodology. As described in Chapter 4 and illustrated in Figure 38, these documents comprise the:

• System Design Specification Document (DSD) – This specification document includes the technical, performance, operational and support details for the modified system. It also includes the allocation of functional requirements, and it defines the various functional interfaces as described in the engineering DMM.

• Initial Cost Breakdown Structure (CBS) – This CBS collates the estimated certification and engineering costs determined from the respective DMMs into a single document. This cost breakdown structure is described by Fabrycki and Blanchard (1991), as a means to facilitate the initial allocation of costs on a functional basis using a bottom up approach.

• Draft Certification Program Plan (CPP) – The draft CPP is the only dedicated certification-related output of this design methodology.

However, unlike other conceptual design methods, this draft CPP is key to the overall modification effort being the main artefact used by NAAs to determine the adequacy of the proposed approach to the certification of the modification. The key input to this document is the draft compliance summary, which is determined from the Certification DMM.

• Project risk register/matrix – The project risk register is the central record of those change severity risks determined from the change propagation matrices, engineering DMM and certification DMM. This risk matrix will become the central part of the formal risk register for the project in the subsequent design phases.

In order to maintain brevity, Step 9 design outputs are not formally presented or discussed in Appendix 1 or Appendix 3.