Category AIRCRAF DESIGN

Following are general considerations important for configuring the empennage (see also Section 6.6):

The descriptions and definitions of the empennage are in Sections 3.22 and 4.9. The dominant civil aircraft empennage consists of the H-tail and V-tail placed symmet­rically about the fuselage axis. The H-tail could be positioned anywhere (see Fig­ure 4.24), going through the aft fuselage to the tip of the V-tail forming a T-tail. Some aircraft have twin booms, where the empennage has the same function; the V-tail is split over two booms.

It is important that the V-tail remains effective for the full flight envelope. Shielding of the V-tail, especially the control areas, may prove to be dangerous. A designer must ensure that the V-tail keeps at least 50% of the rudder unshielded (see Figure 4.26) at a high angle of attack. (The canard configuration is not worked out in this book). Also, at a high angle of attack, the H-tail should not remain within the wing wake; otherwise, it must be enlarged to be effective.

If a FBW control system is incorporated, the empennage sizes can be reduced because the aircraft would be able to fly safely under relaxed stabilities. However, this book is not concerned with control laws as design input in an introductory course. The FBW concept is introduced in Chapter 12 but not analyzed. It will not be long until tailless aircraft such as the B2 bomber appear in civil aircraft designs, especially for BWB aircraft.

Following are general considerations important for designing the wing:

Geometry

(1) wing reference area, S^

(2) span and aspect ratio

(3) aerofoil section, t/c ratio

(4) sweep, twist, dihedral, taper

(5) position (for the CG)

(6) glove/yehudi, if any

Structure (affecting weight and external geometry)

(1) spar and rib positions

(2) stiffness, aeroelasticity, and torsion stability

(3) fuel volume

(4) undercarriage and nacelle, if any

(5) weight

The first task for wing design is to select a suitable aerofoil. This book does not undertake aerofoil design; rather, it uses established 2D aerofoil data from the pub­lic domain (the aerofoil data in Appendix C are sufficient for this book). Industry takes an arduous route to extract as much benefit from its in-house research that is kept commercial in confidence. It is an established technology in which there is a diminishing return on investment. However, the differences between the best designs and those in the public domain are enough to encourage industrial compe­tition. The next task is to configure a wing planform with a reference area typically for the class of aircraft. It is not determined by the passenger number as in the fusel­age; the initial wing size is determined from statistics. Subsequently, the prelimi­nary wing reference area must be sized using the methodology described in Chap­ter 11.

Positioning of the wing relative to the fuselage is an important part of config­uring an aircraft. It requires knowledge of the CG position and its range of move­ment with weight variation (i. e., fuel and payload). Because the aircraft weight dis­tribution is not yet established, it is initially estimated based on experience and past statistics in the aircraft class. If nothing is known, then a designer may position the wing just behind the middle of the fuselage for rear-mounted engines or at the mid­dle of the fuselage for wing-mounted engines. Subsequently, the wing position must be iterated after the aircraft component weights are known and the wing is sized. This may not be easy because moving the wing will alter the CG position – an inex­perienced engineer could encounter what is called “wing chasing”; however, this is not a major concern. Here, the “zero reference plane” (typically at the nose of the fuselage) assists in tracking the aircraft-component positions.

A generous wing root fairing is used to reduce interference drag as well as vor­tex intensity at the aft-fuselage flow. A large aircraft BWB is an extreme example that eliminates wing root fairing problems. There is no analytical expression to spec­ify the fairing curvature – a designer should judge the geometry from past experi­ence and CFD analysis, considering the internal structural layout and the associ­ated weight growth. In principle, a trade-off study between weight growth and drag reduction is needed to establish the fairing curvature. At this stage, visual approx­imation from past experience is sufficient: Observe the current designs and make decisions.

When the seating arrangement is determined in the midfuselage section, it must be closed at the front and aft ends for a streamlined shape, maintaining a fineness ratio from 7 to 14 (see Table 4.3). Typical front – and aft-fuselage closure ratios are in Table 4.4.

The fuselage upsweep angle of the aft-end closure depends on the type of air­craft. If it has a rear-loading ramp as in a cargo version, then the upsweep angle is higher, as shown in Figure 6.4. The fuselage clearance angle, в, depends on the main-wheel position of the undercarriage relative to the aircraft CG position (see

Chapter 7). The typical angle for в is between 12 and 16 deg to approach CLmax at aircraft rotation.

The next step is to construct a fuselage axis and set the zero reference plane normal to the fuselage axis, as explained in Section 3.23. In Figure 6.4, the fuselage axis is shown passing through the tip of the nose cone, where the zero reference plane starts. In this book, the zero reference plane is at the nose of the aircraft (it could be ahead of the nose cone tip). The zero reference plane and the fuselage axis are data for measuring relative distances of various aircraft components and for aerodynamic geometries for use in calculations.

The fuselage axis is an arbitrary line but it must be in the plane of aircraft sym­metry. In general, for aircraft with a constant fuselage section, the fuselage axis is placed conveniently in the middle of the aircraft. The fuselage axis line could be the fuselage centerline. It is easier to assess if the reference lines are vertical and hori­zontal. If the aircraft’s normal position on the ground does not render the aircraft centerline horizontal, then the ground is tilted to show it with the associated angle. For simplification, this book keeps the centerline and ground horizontal, as shown in Figure 6.4. For military and smaller civil aircraft, there is no constant fuselage section, and the aircraft centerline must be conveniently chosen; it is the designer’s choice as long as the reference lines are clearly defined and adhered to for the entire life cycle of an aircraft that could encounter design modifications in its service life. The other possible choice is the fuselage axis as the principal inertia axis.

An interesting concept is to make variants of a modular fuselage – that is, with two types of aft ends easily interchangeable (see Figure 6.4). One type is for the conventional passenger version with a pointed aft-end closure, the other is for the cargo version with an increased upsweep to accommodate a rear-loading ramp. It can even be a “quick-change” version, swapping the type of fuselage needed for the mission; the changeover joint is located behind the main undercarriage.

Attaching the wing to the fuselage could have a local effect on the fuselage external shape. Following are the basic types of attachments:

1. Carry-through wing box. For larger aircraft, this is separately constructed and attached to the fuselage recess. Subsequently, wings are mated at each side in accurate assembly jigs. For smaller aircraft, it could be integral to the wing and then attached to the fuselage recess. In that case, the wing box is built into the wing, either in two halves or as a tip-to-tip assembly. A fairing at the junction reduces the interference drag. These wing boxes are primarily suited to civil aircraft designs. A central wing box is a part of the wing structure that integrates with the fuselage and is positioned high, low, or at a convenient mid-location (see Section 3.16).

2. Central beam and root attachments. These have a simpler construction and therefore are less costly, suited to smaller aircraft.

3. Wing roots (with multispar) joined to a series of fuselage frames. These are mostly suited to military aircraft designs. They are heavier and can be tailored to varying fuselage contours. The wing root is then secured to the fuselage struc­ture, sometimes outside the shell, with attachments.

4. Strut/braced wing support. This is suited to smaller, low-speed, high-wing air­craft. Some low-wing agricultural aircraft have braced wings. Struts add to drag

but for a low-speed operation, the increment can be tolerated when it is less costly to build and lighter in construction.

5. Swing wing. Attachment of a swing wing is conveniently outside the fuselage such that the pivots have space around them to allow wing rotation.

For smaller aircraft, the wing must not pass through the fuselage interior, which would obstruct passenger movement. If the wing is placed outside the fuselage (i. e., top or bottom), then a large streamlined fairing on the fuselage would accommodate the wing box. The example of the Cessna Excel shows a low-wing design; the DO328 includes a fairing for the high-wing design. The Dornier 328 (see Figure 3.33) con­ceals the fairing that merges with the fuselage mould lines. The extra volume could be beneficial; however, to arrive at such a configuration, a proper DOC analysis must demonstrate its merits. High-wing aircraft must house the undercarriage in a fuselage fairing, although some turboprop aircraft have the undercarriage tucked inside the engine nacelles positioned below the wing (see Figure 10.19).

Following are general considerations important for the fuselage layout. Section 3.23 provides definitions of associated fuselage geometries.

Geometry

(1) diameter (e. g., comfort level, appeal)

(2) abreast seating

(3) length, fineness ratio

(4) upsweep for rotation angle and rear door, if any

(5) cross-section to suit under floorboards and headroom volume and space

Structure (affecting weight

and external geometry)

(1) doors and windows

(2) wing and undercarriage attachments

(3) weight

Section 6.4 describes typical fuselage layouts from two-abreast seating to the current widest seating of ten abreast.

The important considerations for civil aircraft fuselage layout are as follow:

1. The current ICAO limit on fuselage length is 80 m. This is an artificial limit based on current airport infrastructure size and handling limitations.

2. The fuselage fineness ratio must be from 7 to 14 (see Table 4.3). Section 4.7.3 lists front – and aft-fuselage closure shapes. Section 6.3.1.2 describes how to make the fuselage closure. There is aft luggage space in front of the pressure bulkhead, especially in smaller aircraft.

3. Seat and aisle dimensions are obtained from Table 4.5.

4. For a fuselage with four-abreast seating or more, the cross-section could use space below the floorboards. If the bottom half is elongated (i. e., oval), then the space can be maximized. Full standing headroom is easily achievable for a fuselage with four-abreast seating or more. Cargo container sizes are described in Section 4.7.8.

5. Two aisles are provided for a fuselage with seven-abreast seating and more (the current maximum is ten). In the future, if a wider cabin is designed (e. g., with a BWB), then more than two aisles will be necessary.

6. The minimum number of cabin crew depends on the maximum passenger capac­ity that the airframe can accommodate. Although not required for up to 19 pas­sengers, a cabin crew is provided by some operators.

7. A pressurized fuselage is invariably circular or near circular to minimize weight. Unpressurized cabins for aircraft operating below 4,300 m (14,000 ft) need not be circular in the cross-section. Smaller utility aircraft demonstrate the ben­efits of a rectangular cross-section. A box-like rectangular cross-section (see Figure 4.14) would not only offer more leg space but also is considerably less costly to manufacture (e. g., the Short SD360).

8. The FAA and CAA have mandatory requirements on the minimum number of passenger doors, their types, and corresponding sizes dependent on the max­imum passenger capacity for which the fuselage is intended to accommodate. This requirement ensures passenger safety: certification authorities stipulate a time limit (e. g., 90 s for big jets) within which all passengers must egress if an unlikely event occurs (e. g., fire). The larger the passenger capacity, the more doors are to be installed. Not all doors are the same size – emergency doors are smaller. Passenger doors have several categories and are described in Sec­tion 15.7. All doors are kept locked while airborne.

9. The fuselage provision typically includes a toilet, a galley, and cabin crew seat­ing – the extent depends on the number of passengers and the duration of flight. Chapter 4 describes toilet and galley details. For smaller aircraft with a shorter duration of flight, it is desirable that at least a toilet be provided. To reduce cost, smaller aircraft with a low mission range do not have a toilet, but these aircraft can therefore be uncomfortable.

Section 2.6 stressed that the survival of the industry depends on finding a new and profitable product line with a competitive edge. A market study is the tool to estab­lish a product by addressing the fundamental questions of why, what, and how. It is like “crystal-ball gazing” to ascertain the feasibility of a (ad)venture, to assess whether the manufacturer is capable of producing such a product line.

Ideally, if cost were not an issue, an optimum design for each customer might be desirable, but that is not commercially viable. Readers can begin to appreciate the drivers of commercial aircraft designs: primarily, economic viability.

The product line should be offered in a family of variants to encompass a wide market area, at lower unit cost, by maintaining component commonalities. The first few baseline aircraft are seen as preproduction aircraft, which are flight-tested and subsequently sold to operators.

The final configuration is a “satisfying” design, which implies that the family of variants is best suited to satisfy as many customers as possible. Figure 6.1 shows how variants of the Boeing 737 family have evolved. Here, many of the fuselage, wing, and empennage components are retained for both the variants.

However, military aircraft designs are dictated by national requirements when superiority, safety, and survivability are dominant, of course, but without ignoring economic constraints. Today’s military aircraft designs start with technology demon­strators to prove the advanced concepts, which are considered prototype aircraft. Production versions could be larger, incorporating the lessons learned from the demonstrator aircraft.

Finalizing the aircraft configuration as a marketable product follows a formal methodology, as outlined in Chart 6.1; it is an iterative process.

Chapter 2 presents several aircraft specifications and performance requirements of civil and military aircraft classes. From these examples, the Learjet 45 class and RAF Hawk class – one each in the civil and the military categories, respectively – are worked out as coursework examples. These examples of civil aircraft family deriva­tives are shown in Figure 6.8; the baseline aircraft is in the middle of the figure and shown in Figure 6.2.

Initially, the conceptual study proposes several candidate aircraft configurations to search for the best choice. Comparative studies are carried out to confirm which choice provides the best economic gains. Although in practice there are poten­tially several candidate configurations for a specification, only one is addressed. Figure 6.3 shows four possible configurations (i. e., author-generated for coursework only). Comparative studies must follow in order to select one. The first configuration offers the best market potential (Figure 6.2).

Today, the industry uses CAD-generated aircraft configurations as an integral part of the conceptual design process, which must be implemented in classwork as soon in the process as possible. Most universities have introduced CAD training early enough so that students become proficient. If the use of CAD is not feasible, then accurate manual drawings are required; it is imperative that practitioners main­tain accuracy and control of manual drawings. CAD enables changes in drawings to be made easily and quickly; in manual drawings, the new shape may necessitate a total redraw.

A three-view diagram should show the conceptualized aircraft configuration. A preliminary configuration will change when it is sized; for experienced engineers, the change is relatively minor. Having CAD 3D parametric modeling allows changes to be easily, quickly, and accurately incorporated. Making 2D drawings (i. e., three – view) from 3D models is simple with a few keystrokes.

6.2 Shaping and Layout of a Civil Aircraft Configuration

The objective is to generate aircraft components, piece by piece in building-block fashion, and mate them as shown in the middle diagram of Figure 2.3. Section 4.11 summarizes civil aircraft design methodology.

Subsequently, in the next level, a more detailed breakdown (see first diagram in Figure 2.3) of the aircraft components in subassembly groups provides a better understanding of the preliminary layout of the internal structures and facilitates pre­liminary cost estimates. DFM/A consideration for subassembly components design is important in reducing production cost because the aircraft cost contributes signif­icantly to the DOC (see Chapter 16).

The general methodology is to start with the fuselage layout, which is deter­mined from the payload requirement (i. e., passenger capacity, number of seats abreast, and number of rows). The aerodynamic consideration is primarily deciding the front and aft closure shape for civil aircraft designs. The following section describes seating-layout schemes for 2- to 10-abreast arrangements encompassing passenger capacity from 4 to more than 600.

The next step is choosing a wing planform and an aerofoil section suitable for the desired aircraft performance characteristics. Initially, the wing reference area is estimated from statistics and is sized later in the process. The next step is to

Figure 6.3. The baseline version of the family concept of the classwork example

configure the empennage based on the current wing area. It will be resized (i. e., iteration) when the wing area is more accurately sized. Initially, the location of the wing relative to the fuselage and empennage is based on past experience and fine – tuned iteratively after establishing the aircraft CG and wing geometry. Finally, a matched engine is selected from what is available in the market. Engine matching is worked out simultaneously with wing sizing (see Chapter 11).

This chapter covers the following topics:

Introduction to configuring aircraft geometry: shaping and layout

Shaping and layout of civil aircraft

Civil aircraft fuselage shaping and layout with example

Configuration of civil aircraft wing design with example

Configuration of civil aircraft empennage design with example

Configuration of civil aircraft nacelle design with example

Undercarriage design considerations

Finalizing civil aircraft configuration

Miscellaneous considerations for civil aircraft

Shaping and layout of military aircraft

Configuration of military advanced jet trainer with example

Configuration of military aircraft CAS version with example

6.1.1 Coursework Content

Intensive classroom work starts with this chapter, one of the most important in the book. Readers begin with the layout of aircraft geometry derived from customer specifications. The information in Chapters 3 and 4 is used extensively to configure the aircraft.

6.1 Overview

The coursework now starts with this chapter. It follows the mock market study in Chapter 2, which generated customer-specified aircraft requirements. Civil and mil­itary aircraft configuration layouts are addressed separately because of the funda­mental differences in their approach, especially in the layout of the fuselage. A civil aircraft has “hollow” fuselages to carry passengers. Conversely, a combat-aircraft fuselage is densely packed with fixed equipments and crew members.

Industry uses its considerable experience and imagination to propose several candidate configurations that would satisfy customer (i. e., operator) requirements and be superior to existing designs. Finally, a design is chosen (in consultation with the operators) that would ensure the best sale. In the coursework, after a quick review of possible configurations with the instructor’s guidance, it is suggested that only one design be selected for classwork that would be promising in facing market competition. This chapter describes how an aircraft is conceived, first to a prelimi­nary configuration; that is, it presents a methodology for generating a preliminary aircraft shape, size, and weight. Finalizing the preliminary configuration is described in Chapter 11.

The market specification itself demands improvements, primarily in economic gains but also in performance. A 10 to 15% all-around gain over existing designs, delivered when required by the operators, would provide market leadership for the manufacturers. Historically, aircraft designers played a more dominant role in estab­lishing a product line; gradually, however, input by operators began to influence new designs. Major operators have engineers who are aware of the latest trends, and they competently generate realistic requirements for future operations in discussion with manufacturers. To encompass diverse demands by various operators, the manufac­turers offer a family of variants to maximize the market share.

The product has to be right the first time and a considerable amount of back­ground work is needed. This chapter describes how to arrive at an aircraft prelim­inary configuration that will be best suited to market specifications and could be feasibly manufactured. Finalizing the design comes later through an involved iter­ative process using aircraft sizing and engine matching (see Chapter 11). In the
coursework, one iteration is sufficient. An experienced chief designer could start with a preliminary configuration that is close to the final arrangement.

There is no mathematics in this chapter; rather, past designs and their reason­ing are important in configuring a new aircraft. Readers need to review Sections 4.11, 12.8, 12.9, and 13.7 on design considerations and discussion to gain insight from experience. Statistics is a powerful tool that should be used discriminately. Researchers and academics have worked on statistics to a great extent; however, in many cases, current market demands have stabilized statistics (Section 6.4).

Encountering unpredictable atmospheric disturbance is unavoidable. Weather warnings are helpful but full avoidance is not possible. A gust can hit an aircraft from any angle and the gust envelope is shown in a separate set of diagrams. The most serious type is a vertical gust (see Figure 5.1), which affects load factor n. The vertical gust increases the angle of attack, a, developing AL. Regulatory agencies have specified vertical gust rates that must be superimposed on the V-n diagrams to describe the operation limits. It is common practice to combine the maneuver and gust envelope in one diagram, as shown in Figure 5.4. The FAR provides a detailed description of required gust loads. To stay within the ultimate load, the limits of vertical gust speeds are reduced with increases in aircraft speed. Pilots should fly at a lower speed if high turbulence is encountered. The gust envelope crosses the limit load and its boundary varies with increases in speed. Equation 5.5 shows that

aircraft with low wing-loading (W/SW) and flying at high speed are affected more by gust load.

VB is the design speed for maximum gust intensity. This definition assumes that the aircraft is in steady-level flight at speed VB when it enters an idealized upward gust of air, which instantaneously increases the aircraft angle of attack and, hence, the load factor. The increase in the angle of attack must not stall the aircraft – that is, take it beyond the positive or negative stall boundaries.

From statistical observations, the regulatory agencies have established the max­imum gust load at 66 ft/s. Except for extreme weather conditions, this gust limit is essentially all-weather flying. In a gust, the aircraft load may cross the limit load but it must not exceed the ultimate load, as shown in Figure 5.4. If an aircraft crossed the limit load, then an appropriate action through inspection is taken.

Table 5.3 outlines the construction of a V-n diagram superimposed with a gust load. Flight speed, VB, is determined by the gust loads and can be summarized as shown in the table.

Linear interpolation is used to obtain appropriate velocities between 20,000 and 50,000 ft. The construction of V-n diagrams is relatively easy using aircraft specifi­cations, in which the corner points of V-n diagrams are specified. Computations to superimpose gust lines are more complex, for which FAR has provided the semi­empirical relations.

Vertical-gust velocity, Ug, on forward velocity, V, would result in an increase of the angle of attack, A a = Ug/ V, that would generate an increase in load fac­tor An = (Cbx Ug/V)/(W/S). Airspeed V is varied to obtain An versus speed. DATCOM and ESDU provide the expressions needed to obtain CLa. A typical V-n diagram with gust speeds intersecting the lines is illustrated in Figures 5.2 and 5.4.

VC is the design cruise speed. For transport aircraft, the VC must not be less than VB + 43 knots. The JARs contain more precise definitions as well as definitions for several other speeds.

In civil aviation, the maximum maneuver load factor is typically + 2.5 for air­craft weighing less than 50,000 lbs. The appropriate expression to calculate the load factor is as follows:

n = 2.1 + 24,000/(W + 10,000) up to a maximum of 3.8 (5.6)

This is the required maneuver-load factor at all speeds up to VC, unless the maxi­mum achievable load factor is limited by a stall.

Within the limit load, the negative value of n is -1.0 at speeds up to VC, decreas­ing linearly to 0 at VD. The maximum elevator deflection at VA and pitch rates from VA to VD also must be considered.

This is when an aircraft (and its occupants) experiences a force less than its weight. In an extreme maneuver in “bunt” (i. e., developing – negative g in a nose-down

curved trajectory), the centrifugal force pointing away from the center of the Earth can cancel the weight when the pilot feels weightless during the maneuver. The corner points follow the same logic of the positive load description except that the limit load of n is on the negative side, which is lower because it is not in the normal flight regime. It can occur in an aerobatic flight, in combat, or in an inadvertent situation caused by atmospheric gusts.

1. Negative High Angle of Attack (-NHA). This is the inverted scenario of +PHA explained previously. With -g, the aircraft must be in a maneuver.

2. Negative Intermediate Angle of Attack (-NIA). In +PLA, the possibility of – ve a was mentioned when the elevator is pushed down, called the “bunting” maneuver. Negative a classically occurs at inverted flight at the highest design speed, VC (coinciding with the PIA). When it reaches the maximum negative limit load of n, the aircraft takes the NIA.

3. Negative Low Angle of Attack (-NLA). At VD, an aircraft should not exceed zero g.

The corner points of the flight envelope (see Figure 5.2) is of interest for stress engi­neers. Enhancing structures would establish aircraft weight that must be predicted at the conceptual design phase.

Figure 5.3 shows various attitudes in pitch-plane maneuvers associated with the V-n diagram, each of which is explained herein. The maneuver is a transient situa­tion, and the various positions shown in Figure 5.3 can occur under more than one scenario. Only the attitudes associated with the predominant cases in pitch-plane maneuvers are addressed below. Negative g is when the maneuver force is directed in the opposite direction toward the pilot’s head, irrespective of his or her orienta­tion relative to the Earth.

Positive Loads

This is when an aircraft (and its occupants) experiences a force more than its normal weight. An aircraft stalls at a maneuver reaching amax; following are the various scenarios. In level flight at 1 g, the aircraft angle of attack, a, increases with slowing down of speed and reaches its maximum value, amax, at which the aircraft would stall at a speed VS.

1. Positive High Angle of Attack (+PHA). This occurs during a pull-up maneuver that raises the aircraft nose in a high pulling g-force, reaching the limit. The aircraft could stall if it is pulled harder. At the limit load of n, the aircraft reaches +PHA at aircraft speeds of VA.

2. Positive Intermediate Angle of Attack (+PIA). This occurs at a high-speed level flight when control is actuated to set the wing incidence at an angle of attack. The aircraft has a maximum operating speed limit of VC when +PIA reaches the maximum limit load of n, in maneuver; it is now in transition.

3. Positive Low Angle of Attack (+PLA). This occurs when an aircraft gains the maximum allowable speed, sometimes in a shallow dive (dive speed, VD). Then, at a very small elevator pull (i. e., low angle of attack), the aircraft would hit the maximum limit load of n. Some high-powered military aircraft can reach VD during level flight. The higher the speed, the lower is the angle of attack, a, to reach the limit load – at the highest speed, it would be +PLA.