Category AIRCRAF DESIGN

With an increased demand in a pilot’s workload, it is important to understand the aircraft flight deck (i. e., cockpit) and the arrangement of systems. Readers must be aware of the nature of design features for the human-interface aspect, which can affect the aircraft weight, cost, and shape of the forward-fuselage-canopy area.

15.2.3 Systems Architecture

Aircraft subsystems consist of avionic, electrical, mechanical, hydraulic, and pneu­matic systems. The extent of an aircraft’s weight and cost depends on the subsystem design philosophy. Automation and microprocessor-based data management have advanced to a wider operational capability without a corresponding increase in a pilot’s workload. In this way, readers can see an aircraft as a system and sub­systems.

15.2.4 Military Aircraft Survivability Issues

Military aircraft design for combat survivability has been considered for some time, primarily as a consequence of potential damage occurring in combat. Crew-ejection capability is an integral part of survivability. In the last two decades, stealth technol­ogy – as a good survival measure – has advanced by minimizing an aircraft’s signa­ture. Electronic defense and countermeasures are other ways to thwart retaliation and increase survivability.

15.2.5 Emerging Scenarios

New topics are gaining importance and must be considered by the next generation of engineers. The emerging scenarios affecting civil aviation result from acts of terror­ism in recent years. Aircraft design must include damage limitation from an explo­sion in the cargo compartments and containment of terrorist activities within the cabin. Also, damage incurred from runway debris demands a new perspective on an old problem (i. e., the Concorde crash case). With greatly increased numbers of passengers crossing international boundaries, general health care and contagious infections are becoming important issues.

Since the advent of large commercial jets in the 1960s (e. g., the B707, DC8, Convair 990, and VC10), the noise profile has become an isssue for residents living in an airport vicinity. Litigation cases began to increase as a result of damages to property and health. First, the FAA and then the ICAO became involved in order to limit noise levels in a prescribed manner. Aircraft and engine designers strove to reduce or suppress noise during takeoffs and landings. Research is developing ways to reduce noise resulting from supersonic shock waves, also known as booms. Currently, there are no civil aircraft operating at supersonic speed – the Concorde was removed from service after a long operational period – partly on account of economic reasons. Supersonic aircraft are not addressed in this book; nevertheless, they fly subsonic during takeoffs and landings. References [1] through [3] may be consulted for more details on supersonic aircraft.

In the 1980s, concerns were raised on climate change to which engine emis­sions contribute. Again, regulatory agencies intervened to set achievable standards in order to limit pollution caused by engine exhaust gases.

The disposal of life-expired, grounded aircraft must be considered in an early design phase.

15.2.1 Materials and Structures

The choice of materials and the internal structural arrangement affect aircraft weight and cost. Materials used in the B787 made the aircraft lighter – it was a suc­cess with numerous sales even before its first flight. This demonstrates the impor­tance of selecting appropriate materials during the conceptual design phase. The IPPD environment is a good forum in which to make choices.

15.2.2 Safety Issues

With increased passenger capacity, a quick egress is required by the regulatory agen­cies to ensure safety in the rare event of a fire hazard on the ground or a postcrash evacuation. The regulatory requirements stipulate a minimum number of exit doors (i. e., both main and emergency) and slides and chutes that ensure egress within a specified time. For military aircraft, the extreme measure of ejection is incorporated into design.

15.1 Overview

Although the main tasks of the aircraft configuration are now completed, there are other topics of interest that require understanding of design. This chapter is an overview of the impact made by technological advances that must be considered at the conceptual stages to arrive at a “satisfying” design. It offers an understanding of specialized topics, some of which may appear out of context during the concep­tual phase, but they do contribute to aircraft design. The aircraft external geometry is not affected by these considerations (unless a radical approach is taken); how­ever, there could be weight and cost changes. The semi-empirical weight equations in Chapter 8 are sufficient and can be modified with improved data. In the industry, a more accurate weight estimation is required to reflect the changes affected by the topics discussed in this chapter.

A detailed study is beyond the scope of this book. Most academic institutions offer separate courses on some of the topics, such as aircraft structure, associated materials, and aircraft systems. Some of these topics sometimes escape attention because the undergraduate curriculum is already full with the main aeronautical subjects. Conceptual aircraft design is not only producing a geometry capable of meeting performance specifications; it also involves early thinking about environ­mental issues, safety issues, materials and structures, human interface with the flight deck, systems considerations, and military survivability issues that affect aircraft weight and cost. Therefore, whereas there is nothing to be altered on the final­ized and substantiated configuration thus far obtained, it is beneficial for readers to appreciate the role of these varied topics. Some of the emerging scenarios are new and have yet to be implemented. It is important that newly initiated readers have a sense for what is required by these topics, which may be developed during a second term of coursework.

Environmental and safety issues must comply with standards specified by the FAA (United States), the EASA (Europe), and the ICAO (international). The dif­ferences among the agencies are minor. Military aircraft requirements are governed by MILSPECS/DEF standards. Aircraft doors (including emergency types) and environment standards, regulated by the FAA and the ICAO, also are described in this chapter.

The topics concerning military aircraft design are complex issues and must be studied separately (refer to specialized textbooks). Previous chapters clearly indi­cate the complexity of military aircraft design, which makes advanced military – aircraft design more difficult for undergraduate students.

Future designs indicate changes in aircraft configurations that are currently undergoing research and development (e. g., the BWB; see Figure 15.10). The basics of the current type of aircraft design must be understood before advanced designs can be undertaken.

15.1.1 What Is to Be Learned?

This chapter covers the following topics:

Introduction to the topics discussed How noise emissions affect design Engine exhaust emissions Aircraft material selection Laying out internal structural arrangements Civil aircraft doors and emergency egress Aircraft flight deck (i. e., cockpit) layout Aircraft systems

Military aircraft survivability and stealth issues Emerging scenarios

15.1.2 Coursework Content

The coursework exercises pertain only to Section 15.7 in which readers verify the Bizjet emergency-door compliance with regulatory requirements. Also, the course – work discusses the choice of materials in Section 15.5. Otherwise, there is no other coursework unless a second term explores these topics. Readers may obtain equip­ment sales brochures supplied by various manufacturers in which dimensions and weights are listed; the Internet is also a good source of information.

15.2 Introduction

The following topics are chosen deliberately to broaden the design perspective of readers. Some are of relatively recent origin, gradually evolving since the 1970s:

• environmental issues (i. e., noise and engine emissions and end-of-life re­cycling)

• materials and structures (i. e., the advent of new materials impacting design)

• safety issues (e. g., emergency exits and chutes)

• human interface (i. e., flight-deck description and displays)

• system-architecture issues (e. g., avionics, electrical, and mechanical systems)

• military aircraft survivability issues (e. g., stealth and ejection)

• emerging scenarios (e. g., terrorism and health protection)

Euler equations are obtained when the viscous terms are omitted from Navier – Stokes equations, allowing faster predictions of pressure distributions. They can be usefully employed at the preliminary design stage. Viscous effects can be included by integrating boundary-layer methods and by displacing the surface of the aero­foil, wing, or aircraft by an amount equal to the local boundary-layer displacement thickness.

14.6.4 Full-Potential Flow Equations

The full-potential flow equations assume that the flow is irrotational. Compressible flows can be modeled but the “shocks” that are predicted are isentropic. The method is now quite dated but it can provide rapid information about pressure distributions and – like the Euler method – it can be integrated with a boundary-layer method.

14.6.5 Panel Method

This is simplest of all numerical methods for predicting flow around an aircraft. The surface of an aircraft is covered with panels, each one a source of sink, and some (e. g., those on lifting surfaces) are assigned a bound vortex (with its associated trailing vortex system). The strengths of the sources and bound vortices are initially unknown but can be determined through application of the boundary conditions (e. g., flow tangency at solid surfaces).

Descending through the hierarchy, the methods provide less physical fidelity but also require less computational effort. It is conceivable that the panel method, full-potential flow equations, Euler equations, and RANS method can be used in an undergraduate aircraft-design project (as a separate task), although not at the conceptual design stage. These methods provide a qualitative pressure-distribution pattern to help shape the geometrical details. Whichever method is used, the issue of grid generation must be addressed: More time is spent on the generation of a suitable mesh than on the prediction of flow.

A 3D model created in CAD is useful at this stage. The planning to prepare the 3D model in CAD should be done in such a way that Boolean operations can build it from isolated components, while still retaining the isolated components for a separate analysis. The wing-fuselage analysis provides the tail-less pitching moment data, which are useful in designing the aircraft H-tail and its setting relative to the fuselage to minimize trim drag.

CFD results can be compared with results obtained through use of the semi­empirical relationship (e. g., drag) (see Chapter 9). Generally, semi-empirical drag results are considered to provide good accuracy, validated on many aircraft consis­tent over a long period of use.

Figure 9.8 presents the wave drag, CDw, for the Mach number. CFD provides an opportunity to generate a more accurate viscous-independent wave drag versus the Mach number. When the CFD results are available, the data in Figure 9.9 may be replaced, thereby obtaining a further iteration on the drag polar of the aircraft. CFD is also a good place to generate ACDp values to be used for comparison. In general, CFD-generated ACDp values should provide good values if the CFD is set up properly.

If the CFD results are within 10% of the results obtained using semi-empirical relations, then they may be considered good. Some adjusting of the CFD runs should

improve the results – this is an area where experience is beneficial. Once the CFD is set up to yield good results, it is useful to improve and/or modify an aircraft configu­ration through extensive sensitivity studies. The spectrum plots in color show the hot spots that contribute to drag (e. g., local shocks and separation). These details can­not be seen as easily by any other means. Designers can follow through by repairing the hot spots to reduce drag. These opportunities are unique to CFD, making it an indispensable tool for optimizing a configuration for minimum drag. Any signifi­cant difference between the CFD and semi-empirical results should be investigated properly.

14.7 Summary

CFD simulation is a digital-numerical approach to design incorporated in the virtual-design process using computers. The current status is adequate for com­parative analyses at low cost and time; therefore, it must be applied early in the conceptual design phase as soon as a CAD 3D model drawing is available. The development of CFD is not necessarily driven solely by aerodynamic considerations but rather by the requirement to have a tool to design a better product at a low cost and in less time.

CFD continues to develop with greater computing power at lower cost and in less time along with advances made in the algorithms for resolving solutions, provid­ing considerable ease and automation to benefit users. Although researchers have achieved a degree of accuracy in drag prediction for a clean aircraft configuration, the generalized application by engineers is yet to achieve consistency in results. Ver­ification and validation of results from CFD analysis are essential for substantiation, and the state of the art is still being scrutinized and continuing to develop. Verifi­cation of new CFD software comes before validation; together, they involve a pro­tracted process in which research continues.

As a conservative user, the industry must ensure fidelity in a design. However, CFD is capable of comparison to recognize the better designs, even if the absolute values remain under scrutiny. This capability produces the best compromise in an early phase of the project at low cost and in less time, thereby avoiding subsequent costly modifications of an aircraft configuration – that is, it provides the opportunity to make the design right the first time. The design is subsequently tested in a wind tunnel for substantiation. Today, this approach requires few changes to the design after the final flight-test results.

The industrial effort in CFD is extensive and is not suitable for an undergrad­uate course. However, coursework can follow the industrial approach by solving smaller problems, such as those described in this chapter.

LES takes advantage of the fact that the smallest dissipative eddies are isotropic and can be efficiently modeled using simple subgrid scale models. Meanwhile, the dynamics of larger eddies, which are anisotropic, is simulated using a grid and time step sufficiently fine to resolve them accurately. The method, therefore, is applicable to flows at relatively high Re numbers but is still expensive for use as an engineering design tool.

14.6.2 Detached Eddy Simulation (DES) Technique

DES is considered halfway between the LES and the Reynolds Averaged Navier – Stokes (RANS) techniques. The method employs a RANS turbulence model for near-wall regions of the flow and a LES-like model away from the wall. The method was first proposed by Spalart et al. in 1997 and is still the subject of research. It may become a standard engineering tool, but it is currently unlikely to be an element of the conceptual and preliminary design toolkits.

14.6.3 RANS Equation Technique

The time-dependence of turbulent fluctuations is averaged to form the RANS equa­tions. This results in the appearance of the so-called Reynolds stresses in the equa­tions, and the modeling of these equations (i. e., turbulence modeling) becomes problematic. There are many turbulence models but each falls prey to the fact that turbulence is flow-dependent; consequently, no turbulence model can be generally applicable, and a CFD practitioner must be cognizant of the strengths and failings of the models employed. Nonetheless, RANS allows relatively inexpensive modeling of complex flows; when allied to a suitable optimization method, it can be a powerful tool for design synthesis.

A hierarchy of CFD simulation methods exists in which they are classified according to the physics they are capable of modeling. At the top of the hierarchy are direct

numerical simulation (DNS) techniques; at the bottom are panel methods. A rough guide to the members of the hierarchy and their respective abilities is provided in the following subsections.

14.6.1 DNS Simulation Technique

DNS is capable of simulating time-dependent turbulent flows, capturing the dynam­ics of the entire spectrum of eddy sizes. This requires grids and time steps that are finer than the length and time scales at which turbulent energy is dissipated. Low – diffusion numerical schemes are necessary. DNS is useful for supplementing experi­mental data and aiding the development of turbulence models, but it is prohibitively expensive at the flight Re numbers. It is not used in the design process. Currently, DNS is the most sophisticated method.

This section includes elementary examples of case studies beginning with 2D cases, as shown in Figure 14.5. The first diagram represents an aerofoil showing the grid layout.

The domain of analysis is large with the anisotropic adaptive grid (Figure 14.5a), which is more dense the closer it is to the LE and trailing edge matching the surface grids and where shocks are present. When the solver has been run, the results can be seen in the postprocessor showing the Mach number isolines (Figure 14.5b). In another run with a different setup, the results are shown in a color spectrum (i. e., the gray-scale version in Figure 14.5c).

The next example is a simple, isolated 3D wing, as shown in Figure 14.6a. Half is shown with a simple grid and the other half is shown in shaded geometry. The drag polar from the CFD analysis is compared with results in Figure 14.6b.

CFD analysis of an isolated fuselage should be easy but internal and external flow through the nacelle (Figure 14.7) can prove to be difficult.

Whereas CFD studies on aerofoils exist, flow-field analysis on nacelles is rare. Chen [17] et al. presented a flow-field analysis over a symmetric isolated nacelle

Figure 14.7. Nacelle grids for internal and external flow analysis

using a Euler solver (Figure 14.8). Subsequent studies by Uanishi [8] et al. showed confirmation of the velocity field obtained by Chen. No work has been found for velocity fields over the nacelle using Navier-Stokes solvers.

In a more recent analysis [12], it is stated that “…the observed scatter in the absolute CFD-based drag estimates is still larger than the desired single drag count error margin that is defined for drag prediction work. Yet, the majority of activi­ties conducted during an aircraft development program are incremental in nature, i. e., testing/computing a number of options and looking for the best relative perfor­mance.”

Step 4: The result thus obtained from the solver can now be viewed in the solver; select a display format.

Step 5: Analyze the results.

Step 6: For a new setup, verify and validate the results.

The results can be presented in many ways, such as Cp distribution, pressure con­tours, streamlines, velocity patterns, CL, CD, L/D, or parameters defined by a user. CFD can depict shock patterns, location, and separation similar to flow visualiza­tion with wind tunnels. These results provide insight for aerodynamic designers to improve the design for the best L/D, aerodynamic moments, and compromise shapes to facilitate production, for example.

The results may need adjustments for the iteration that is necessary beginning with Step 2 and/or Step 3.

CFD analysis requires preprocessing of the geometric model before computation can begin. It consists of creating an acceptable geometry (i. e., 2D or 3D) amenable to analysis (e. g., no hole through which fluid leaks). A preprocessing package comes with its own CAD program to create geometry, specifically suited for a seamless entry to the solver for computation. However, considerable labor can be saved if the aircraft geometry already created in CAD can be used in CFD. This is possible if care has been taken in creating a geometry that is transferable to a CFD prepro­cessing environment.

The next task is to lay a grid on the geometry in order for CFD to work on small grids/cells until the entire domain is achieved. The surface grids should be laid intelligently to capture details where there are major local geometric variations, typ­ically at the junction of two components and where there is steep curvature. The next task is to generate cells in the application domain that can encompass a large flow-field space around the aircraft body. At the far-field, variation in the flow field between the cells is small and therefore can be made larger. The preprocessor is menu-driven, providing options for various types of grid generation from which to select. Grids must meet the boundary conditions as the physics dictates. Fig­ure 14.2 is a good example of aircraft geometry (simplified by excluding the empen­nage and the nacelle) with structured grids and a section of the environment to be analyzed. Because it is symmetrical on the vertical plane, only half of the aircraft needs to be analyzed – the other half is the mirror image.

Figure 14.3 is another example of 3D meshing on a complete aircraft with the nacelle included. After grid generation in the preprocessor, the model is then

introduced into the flow solver, which is also menu-driven. The options in the solver are specific and a user must know which to apply. The solver then computes the results: The runtime depends on the geometry, type of grid, and solver options, as well as the computing power.

The results can be examined in many different ways in a postprocessor; the most important on an aircraft body include the Cp distribution, temperature distri­bution, streamlines, and velocity vectors. The Cp and temperature distributions are shown in grades of color representing bands of ranges. Figure 14.4a is a gray-scale

version of the color distribution and Figure 14.4b depicts the flow-field streamlines. The results also can be obtained numerically in tabular form. It is now understood that readers must have the background information and be familiar with the CFD software package. For newly initiated readers, this instruction should be conducted with supervision.

Following is a summary of the approach to CFD.

14.4.1 In the Preprocessor (Menu-Driven)

Step 1: Create the 3D geometry of an aircraft.

Step 2: Generate the grid on the body surface and in the application domain; match it to the boundary conditions.

14.4.2 In the Flow Solver (Menu-Driven)

Step 3: Bring the preprocessed geometry into the solver; set the boundary and initial conditions; make appropriate choices for the solver; run the solver; check results and refine (i. e., iterate) if necessary (including the grid pattern).

In his classic review, Chapman [1] (1979) advocated the indispensability of CFD, as computers began showing the promise of overtaking experiments as a princi­pal source of detailed design information. His view is now regarded as an overly optimistic estimate. CFD capabilities complement experiments. Chapman listed the following three reasons for his conclusion: [27]

Figure 14.1. CFD simulation of wing – body drag polar [7]

Chapman showed the chronology of progress as four stages, starting in the late 1960s by solving linear potential flow equations and then reaching a stage where the nonlinear Navier-Stokes equations could be solved.

In a later paper, Chapman [2] reviewed the rapid progress achieved in the 1990s. With a better understanding of turbulence and advances in computer technology in both hardware and software development, researchers successfully generated aero­dynamic results that had been impossible to obtain until then.

A more recent review by Roache [3] demonstrated that considerable progress has been achieved in CFD, but the promise is still far from being fulfilled in esti­mating complete aircraft drag. The AGARD AR256 report [4] is a technical status review of drag prediction and analysis from the CFD perspective. In the report, Schmidt [5] categorically stated that “consistent and accurate prediction of absolute

drag for aircraft configuration is currently beyond CFD reach____________________________ ” Ashill [ 5] was of

the same opinion, stating that the CFD flow modeling was found to be lacking in “certain respects.” Both agreed that the current state of the art in CFD is a useful tool at the conceptual design stages for comparison of shapes and diagnostic pur­poses.

An essential route to establish the robustness of CFD is through the success of the conceptual model code verification and validation. Roache [3] used the seman­tics of “verification” as solving the equations correctly and “validation” as solving the correct equations. The process of benchmarking (i. e., code-to-code comparison) results in the selection of software with the best economic value, although not nec­essarily the best software on the market.

Experimental results are used to validate and calibrate CFD codes. Various degrees of success have been achieved in the case studies. Melnik [5] showed that the CFD status in aircraft drag prediction of a subsonic-jet, transport-type aircraft wing on a simple circular cross-section fuselage had mixed success, as shown in Fig­ure 14.1. Some correlation was achieved after considerable “tweaking” of the results. The results using these methods are not certifiable because of considerable “gray” areas.

Currently, the industry uses CFD as a tool for flow-field analysis wherever it is possible to estimate drag in inviscid flow (e. g., induced drag and wave drag), but it is not used for complete aircraft drag estimation. In the industry, CFD is a general – purpose tool to simulate flow around objects for qualitative studies and diagnostic purposes.

It is difficult to capture the numerous “manufacturing defects” (e. g., steps, gaps, and waviness that result from surface-smoothness requirements) over an entire air­craft. CFD flow-field analysis of simple geometries for benchmark work has been conducted (e. g., on large backward-facing steps). An example is Thangam [6] et al., who described a detailed study of flow past backward steps to understand turbu­lence modeling (к-є). This type of work neither represents the problems associated with the small geometries of excrescence effects nor guarantees accuracy. Another example is Berman’s [9] work on a large rearward-facing step, but it is not represen­tative of the excrescence dimensions.

Assessment of excrescence drag using CFD requires a better understanding of the boundary-layer structure in turbulent flow. Although there is a voluminous lit­erature on CFD code generation and qualitative assessment of the pressure field, no work has been cited in estimating parasitic drag of excrescences. As modern CFD software becomes more capable, it may be possible to predict excrescence drag by simulating real cases of double curvature in compressible flow, with or without shocks or separation.

Reference [11] is a verification of excrescence drag on a flat plate in the absence of a pressure gradient to estimate the excrescence drag on a 2D aerofoil in the pres­sure gradient. The study of an aerofoil [11] may be seen as a precursor to examining the scope of CFD estimates of excrescence drag in the generic 3D aerofoil configu­ration.