5.2.1 Generic Fuselages

The term ‘fuselage’ is derived from the French word fuselage meaning ‘spindle’ shape. All fuselages have a flight deck to serve as crew station. Fuselages of small utility, transport and military category aircraft differ according to their design specifications and mission roles. Typical differences between them are outlined in the following.

5.2.1.1 Civil Aircraft

Large transport aircraft have a long constant cross‐section, circular or close to circular in shape. However, small civil aircraft do not require a constant cross‐section. Single engine aircraft have an engine mounted at the extremities of the fuselage, jet engines are mounted at the rear. The dominant parameters in fuselage aerodynamic design are its maximum cross‐sectional area, and its front and aft‐end closure shapes. Since the cross‐section may not be exactly circular (Figure 5.1), for the main types of civil aircraft, the following definitions represent diameter (Figure 5.2 shows military fuselage cross‐sections for comparison).

The equivalent cross‐sectional area of the fuselage is defined as follows.

5.1

For an elliptical cross‐section the effective diameter, D_eff, is used that reduces Eq. 5.1 as follows.

5.2

Fuselages with close to circular constant cross‐sections can use the definition of average diameter, D_ave‐fus, as given here.

5.3

The BWB aircraft configuration has a fuselage merged with the wing, yet it can be delineated to be dealt with as required.

5.2.1.2 Transport Category

All transport type aircraft fuselages have a circular or near circular cross‐section with a constant midsection to house the payload (passenger/cargo) and equipment. In some cases they may accommodate part of the fuel load. These fuselages have closures at both ends, for subsonic types the front end is blunter than the gradual closure at the aft, that is, a tear‐drop streamline shape (Figure 5.3a). Aft‐loading fuselages with loading ramps have a blunt aft end (Figure 5.3b).

5.2.1.4 Military Category Fuselage

The role of military aircraft fuselage is very different (Figures 5.2 and 5.4) and configurations differ from design to design. The supersonic aircraft front end has to have a necessarily sharp nose cone to minimise shock wave drag. This is a tightly packed housing for engines (located at the aft end), fuel and most of the aircraft systems hardware.

5.2.2 Generic Nacelles Pods/Intakes/Auxiliary Bodies

Engines and their accessories/systems not buried inside fuselage need to be housed in specifically designed nacelle pods. The role of nacelle is solely meant to house the engine and its accessories and is dealt with separately. In this chapter only the external geometries are considered. The internal geometries depending on the aerodynamic considerations of air inhalation is discussed in Chapter 12 covering with aircraft propulsion.

This chapter deals with the various configuration options to consider for the choice of nacelle/intake. Military aircraft engines are fuselage mounted and hence do not have nacelles, unlike some in earlier bomber aircraft designs. Today's bomber designs, like the B2 Spirit, have engines buried into the BWB configuration.

There are other types of closed bodies of revolution, for example, drop tanks, armaments and so on that are not dealt with in this book. Their drag estimation is relatively simple. Enough information is given in the Chapter 11 on aircraft drag to evaluate auxiliary body drag.

5.3.1 Aircraft Zero‐Reference Plane/Fuselage Axis

The aircraft zero‐reference and the fuselage axis are used to position and locate aircraft components to facilitate computation and manufacturing processes. They are orthogonal to each other. The aircraft zero‐reference plane is a near vertical plane, typically passes through the farthest point of the nose cone, as shown in Figures 5.5a and 5.6, but designers can choose any station for their convenience, within or outside of the fuselage.

Given here are several ways to define fuselage axis as desired by the design bureau.

1. Fuselage axis as a mean line of the constant cross‐sectional part of the mid‐fuselage (Figure 5.6).
2. Fuselage axis passing through the nose cone if it is close to the line as defined before (Figure 5.5a).
3. Fuselage axis is close to the principal inertia axis of the aircraft.

Fuselage axis may not pass through the aft‐end closure point. The fuselage axis may or may not be parallel to the ground (tail dragger aircraft is an example). If the fuselage axis is parallel to the ground then the zero‐reference plane is vertical.

The overall fuselage length, l_fus consists of the (i) front‐fuselage nose cone (l_f), (ii) constant cross‐section midsection barrel (l_m), and (iii) aft‐end closure (l_a). The constant cross‐section mid‐fuselage length is established from the passenger seating capacity. The following geometrical definitions are extensively used in this book (see Figure 5.5).

5.3.2 Fuselage Length, l_fus

Fuselage length, l_fus, is the length along the fuselage axis, taken by measuring the length of the fuselage from the tip of the nose cone to the tip of the tail cone. This is not the same as the aircraft length, l_ac, as shown in Figure 5.5a. Aircraft length, l_ac, may not be equal to fuselage length, l_fus, if any other part of the aircraft extends beyond the fuselage extremities (e.g. the tail sweep may go beyond the tail cone of the fuselage). Depending on the rated passenger capacity, the fuselage length changes in discrete steps of rows and width changes in increments of one seat pitch and width at a time.

5.3.3 Front‐Fuselage Closure Length, l_f

This is the length of the front‐fuselage from the tip of the nose cone to the onset of the constant cross‐sectional barrel of the mid‐fuselage (Figure 5.5a). It encloses the pilot CFD and the windscreen, followed by the mid‐fuselage constant‐section barrel.

5.3.4 Aft‐Fuselage Closure Length, l_a

This starts from the end of the constant cross‐sectional barrel of the mid‐fuselage up to the tip of the tail cone (Figure 5.5a). It may enclose the last few rows of passenger seating, rear exit door, toilet, and – for a pressurised cabin – the aft pressure bulkhead, which is an important component from a structural design perspective (l_a > l_f).

5.3.5 Mid‐Fuselage Constant Cross‐Section Length, l_m

This is the constant cross‐sectional mid‐barrel of the fuselage, where passenger seating and other facilities are accommodated (including windows and emergency exit doors etc.).

5.4.1 Front (Nose Cone) and Aft‐End Closure

The front‐end closure of bigger aircraft appears to be blunter than on smaller aircraft because the nose cone is sufficiently spacious to accommodate pilot positioning and instrumentation. A kink appears in the windscreen mouldlines of the fuselage to fit flat glasses on a curved fuselage body; flat surfaces permit wiper installation and are less costly to manufacture. Some small aircraft have curved windscreens that permit smooth fuselage mouldlines.

The aft‐end closure is shallower to minimise airflow separation when the boundary layer becomes thicker. All fuselages have some upsweep for aircraft rotational clearances at takeoff. Designers must configure a satisfactory geometry with attention to all operation and structural requirements (e.g. pilot vision polar, pressure‐bulkhead positions and various doors).

5.4.2 Fuselage Upsweep Angle

In general, the fuselage aft end incorporates an upsweep (Figure 5.6) for ground clearance at rotation on takeoff. The upsweep angle is measured from the fuselage axis to the mean line of aft‐fuselage height. It may not be a straight line if the upsweep is curved like a banana; in that case, it is segmented to smaller straight lines. The rotation clearance angle is kept to 12–16°; however, the slope of the bottom mouldline depends on the undercarriage position and height. Rear‐loading aircraft have a high wing with the undercarriage located close to the fuselage belly. Therefore, the upsweep angle for this type of design is high. The upsweep angle can be seen in the elevation plane of a three‐view drawing. There is significant variation in the upsweep angle among designs. A higher upsweep angle leads to more separation and, hence, more drag.

5.4.3 Fuselage Plan View Closure Angle

The closure angle is the aft‐fuselage closure seen in a plan view of the three‐view drawing and it varies among designs. The higher the closure angle, the greater the pressure drag component offered by the fuselage. In rear‐loading aircraft, the fuselage closes at a blunt angle; combined with a large upsweep, this leads to a high degree of separation and, hence, increased pressure drag. A finer aft closure angle is desired, but for larger aircraft the angle increases and attempts are made to keep length L_f to an acceptable level to save weight and cost.

A finer aft closure angle is desired; however, for larger aircraft, the angle increases to keep the length (L_f) to an acceptable level to reduce weight and cost.

Figure 5.7 shows several examples of current types of commercial transport aircraft designs [1]. Statistical values for the front‐ and aft‐fuselage closure are summarised in Table 5.1. There are special designs that may not fall in this generalised table. Designers may exercise their own judgement in making a suitable streamline shape to allow for an upsweep to clear for aircraft rotation at takeoff.

Table 5.1 lists the front and aft‐fuselage closure statistics. The front‐fuselage closure ratio is F_cf ≈ 1–2 and aft‐fuselage closure ratio is F_ca ≈ 2–4.

Define:

5.4

Table 5.2 gives typical guidelines for the fuselage front and aft‐end closure ratios (Eq. 5.4) – the range represents the current statistical values.

5.6.1 Fuselage Height, H

This is the maximum distance of the fuselage from its underside (not from the ground) to the top in the vertical plane (Figure 5.9).

5.6.2 Fuselage Width, W

This is the widest part of the fuselage in the horizontal plane. For a circular cross‐section, this is the diameter shown in Figure 5.9.

5.6.3 Average Diameter, D_ave

For a non‐circular cross‐section, this is the average of the fuselage height and width at the constant cross‐section barrel part (D_ave = (H + W)/2). Sometimes this is defined as D_effective = √(H*W); another suitable definition is D_equivalent = perimeter/2π. This book uses the first definition.

5.6.4 Cabin Height, H_cab

This is the internal cabin height from the floor, as shown in Figure 5.9.

5.6.5 Cabin Width, W_cab

This is a the internal cabin width, as shown in Figure 5.9.

5.6.6 Pilot Cockpit/Flight Deck

This is a term used for the enclosed space for the flight crew in the front‐fuselage. Chapter 15 describes the flight deck in more detail.

5.6.7 Cabin Interior Details

Details of cabin interior details, such as the seat details/pitch, passenger amenities and so on.

5.7.1 Narrow Body

Single‐aisle passenger seating arrangements are called narrow‐bodied transport aircraft. A ‘3–3’ arrangement indicates that it is a narrow‐bodied aircraft, has one aisle and has total of six seats in a cluster of three seats at each window. Figure 5.10 shows the various options for an aircraft fuselage cross‐section to accommodate different cabin seating arrangements from two‐ to six‐abreast seating in a row.

5.7.2 Wide Body

When the seating number is increased to more than six abreast, the number of aisles is increased to two to alleviate congestion in passenger movement. More than one aisle (currently two, but possible may grow to three) is regarded as wide‐bodied transport aircraft. For example a ‘3‐4‐3’ arrangement indicates that it is a wide‐bodied aircraft has a total of 10 seats with two aisles and a cluster of three seats at each window side with a cluster of four seats in the centre flanked by two aisles.

Because of the current fuselage‐length limitation of 80 m, larger‐capacity aircraft have a double‐deck arrangement (e.g. the B747 and the A380). Figure 5.11 shows options for typical wide‐body and double‐deck aircraft fuselage cross‐sections to accommodate different cabin seating arrangements from seven‐ to 10‐abreast seating in a row.

It is interesting to study a six‐abreast wide‐body seating in 2‐2‐2 arrangement with only a 19 in. increase in fuselage diameter by having two 20 in. width aisles (Figure 5.11). This eliminates a centre seat, offering better comfort, easy movement, rapid evacuation and so on, possibly with a roughly 5% (not computed) increase in Direct Operating Cost (DOC): this should appeal to customers, both the operators and passengers.

5.7.3 More than Two Aisles ‐ Blended Wing Body

A three‐aisle arrangement with 10‐abreast seating would eliminate the cluster of four seats together. A BWB would have more than two aisles; there is no reason to not consider a triple‐deck arrangement. Figure 5.12 shows various options for futuristic aircraft fuselage cross‐sections to accommodate different cabin seating arrangements.

5.8.1 Narrow‐Body, Single‐Aisle Aircraft

Figure 5.13 shows a typical seating arrangement for a single‐aisle, narrow‐body aircraft with two to six passengers abreast and seating carrying up to about 220 passengers (all economy class). Sections 5.4 and 5.5 give the general considerations of closure angles and FR. Table 5.4 provides typical dimensions for establishing narrow‐body fuselage widths. Details of seat, internal facilities and doors, and so on for each type are given subsequently.

5.8.2 Wide‐Body, Double‐Aisle Aircraft

These aircraft are also known as wide‐bodied aircraft. Figure 5.14 shows a typical seating arrangement for a double‐aisle, wide‐body aircraft with 7–10 passengers abreast carrying up to 555 passengers; however, high‐density seating of all economy‐class passengers can exceed 800 (e.g. A380). These large passenger numbers require special attention to manage comfort, amenities and movement. Sections 5.4, and 5.5 give the general considerations for closure angles and FR. Table 5.5 provides typical dimensions for establishing wide‐body fuselage widths. Details of seats, internal facilities, doors and so on of each type are given subsequently.

5.9.1 Fuselage Configuration – Summary

Configuration for fuselage for civil aircraft is deterministic and does not rely on empirical relations. It is mainly decided by the number of passengers that has to be accommodated and at a specified comfort level (seat width and pitch – Section 7.5.2).

Given next is a generic step‐by‐step approach to configure a fuselage. It should be noted that for the same abreast seating there is about 5% variation fuselage width variation in different designs. Also, the cross‐section varies from a circle to a somewhat elongated height. Therefore, the readers may use their own discretion to make a design with difference that is better than the existing. This is a challenging task for all designers.

• Step 1. Decide the seat abreast arrangement for passenger capacity (Figure 5.8).
• Step 2. Determine fuselage width based on the abreast seating arrangement (Tables 5.4 and 5.5). Minimum aisle width is a certification requirement that is taken in account in the tables.
• Step 3. Develop a fuselage cross‐section based on fuselage width and under‐floor space provision (see Section 19.1.3). Container sizes are given in (Section 7.5.6).
• Step 4. Establish the constant cross‐section mid‐fuselage length, ‘l_m’ (Figures 5.5, 5.13 and 5.14) by computing the number of seat rows required. In case of a wide‐body layout, the last few rows may use the space in the converging section of the aft fuselage (Figures 5.14–5.16).
• Step 5. Generate the front‐fuselage length, ‘l_f’ and aft fuselage length, ‘l_a’ using the ratios given in Tables 5.1 and 5.2. Plan view closure angle is in line with the ratios used.
• Step 6. Fuselage upsweep angle has to clear the fuselage rotation angle at takeoff. At this stage, use the statistical values given in Table 5.1. Subsequently after configuring and positioning the undercarriage (Chapter 10), the upsweep angle may be revised.
• Step 7. Make sure that internal passenger/crew facilities (Section 7.5.4) are adequately catered for in Steps 4 and 5.
• Step 8. Finally, provide doors and windows as required, bearing in mind that the door requirements have to satisfy Federal Aviation Regulation (FAR) requirements (Section 7.5.3).

5.10.1 Fuselage Drag

Spindle‐like 3D fuselage shapes generate very little or no induced drag, C_Di. They mainly develop parasite drag, C_Dp (friction and pressure drag, see Chapter 9). Friction drag depends on surface skin friction (in coefficient form, C_F, at the Re) and the wetted surface area S_fus and pressure drag depends on the aft‐end closure shape. The minimum surface area for a given volume is a sphere, an impracticable shape that associates with high pressure drag (Section 3.4) as compared to a teardrop streamline shape with lower pressure drag. The optimum streamline shape offering minimum parasite drag has FR around 3.0, which is also not practical for fuselage design. Fuselage aft end closure shape also plays a role in pitching moment contribution. CFD optimisation can offer some solution to configure fuselage aft end closure complying with constraints of upsweep angle permitting the required takeoff rotation. This is a good example of how and where to apply optimisation process (see Section 2.5.1), an aspect beyond the scope of this book. This book examines aft‐end shapes of existing design to make the choice. Various types of fuselage design considerations are given next to assist to make the choice.

The characteristic length of fuselage is its length, l_fus. It should be long enough to have Re > R_crit, hence separation occurs at the aft end of fuselage, its severity depends on the fuselage length and its aft end closure shape. Separation contributes to drag increase. In addition, if there is any lateral instability then the aircraft may enter into certain kinds of unsteady harmonic oscillations. The aerodynamic quest is to shape the fuselage to force separation as far aft as possible, if required, use vortex generators such as those in wing or ventral fins to reduce if not eliminate these undesirable characteristics.

For high subsonic transport aircraft flying above Mach 0.75, the front‐fuselage closure ratio, F_cf is kept around 2.0. At lower speed, it can get blunter, rarely goes below 1.5. Aft‐end closure ratio, F_ca is kept between 2.5–3.75. To facilitate takeoff rotation, fuselage upsweep angle varies from 2 to 10° depending on the mission requirements/specifications. Table 5.1 lists the data for some existing designs that means the mission requirement can be compared to a particular aircraft in question.

5.10.2 Transonic Effects – Area Rule

For an aircraft configuration, it has been shown that the cross‐sectional area distribution along the body axis affects the wave drag associated with transonic flow. The bulk of this area distribution along the aircraft axis comes from the fuselage and the wing. The best cross‐sectional area distribution that minimises wave drag is a cigar‐like smooth distribution (i.e. uniform contour curvature; lowest wave drag) known as the Sears–Haack ideal body (Figure 5.15). The fuselage shape approximates it; however, when the wing is attached, there is a sudden jump in volume distribution (Figure 5.15). In the late 1950s, Whitcomb demonstrated through experiments that ‘waisting’ of the fuselage in a ‘coke‐bottle’ shape could accommodate wing volume, as shown in the last part of Figure 5.15b. This type of procedure for wing‐body shaping is known as the area rule. A smoother distribution of the cross‐sectional area reduces wave drag.

Whitcomb's finding was deployed on F102 Delta Dragger fighter aircraft (Figure 5.15a). The modified version with area ruling showed considerably reduced transonic drag. For current designs with wing‐body blending, it is less visible, but designers still study the volume distribution to make it as smooth as possible. Even the hump of a Boeing747 flying close to transonic speed helps with the area ruling. The following subsection considers wing (i.e. 3D body) aerodynamics.

In addition, the latest generation combat aircraft have leading edge root extension (LERX) (strakes), mostly above the engine intakes, where delineation between wings and body becomes difficult; normally taken at wing‐fuselage assembly joint line. These designs require developing fuselage mouldlines hugging the distributed internal contents, satisfying the CG location for a full flight envelope. Fuselage cross‐sections vary for the different types of combat aircraft configuration evolved by incorporating advanced technologies and mission requirement to encounter the potential adversaries with capabilities that can only be guessed. A BWB configuration like the B1 Spirit bomber aircraft design is seen as a flying wing, the fuselage merged and serves as part of the wing that also considers smooth changes in aircraft sectional area along its length.

5.10.3 Supersonic Effects

As a result of the presence of a shock wave, the flow physics of supersonic flow differs from subsonic flow. Nose cone of fuselage spearheads through air and develops a shock cone. Unlike thin aerofoils, the fuselage has conical angle of 2β (see Section 3.1) to displace air flow to produce a shock wave with and angle of shock β > μ.

5.12.1 Nacelle/Intake External Flow Aerodynamic Considerations

Nacelle pod is also like the fuselage 3D bodies of revolution as it has air flowing over it, but it differs as it also has internal airflow flowing through its intake duct facilitating air‐breathing into the engine. Nacelle aerodynamics need to be considered for both the external and internal flow characteristics. This section gives a brief introductory description of nacelle external aerodynamic considerations. Nacelle external flow aerodynamics have similar considerations as those discussed in Section 5.10.

In addition to housing the engine, the main purpose of the nacelle is to facilitate the internal airflow reaching the engine face (or the fan of gas turbines) with minimum distortion over a wide range of aircraft speeds and attitudes. Aircraft designers need to position the nacelle at an orientation to receive the incoming free stream tube into the intake duct with minimum distortion for the full flight envelope (see Section 5.8.1).

The nacelle aerodynamic design (external mouldline shaping and internal contouring) has progressed to a point of diminishing return on the efforts made and approaching to a generic shape. The characteristics length of nacelle is its length, l_nac. It is long enough to have Re > R_crit, hence separation can occur at the aft end of nacelle, its severity depends on its aft end closure shape. Separation contributes to drag increase. If required, vortex generators such as a few vanes are positioned at the critical positions. The entrainment effects of high energy engine exhaust flow helps to minimise separation of external flow over the aft end of fuselage. Nacelle pods are isolated bodies attached to aircraft with a pylon and have much lower length to diameter ratio (FR) compared to the fuselage.

Military aircraft with engines buried into the fuselage do not have nacelle pods; here, airflow to engine passes through a long intake duct. These are dealt with in better detail in Chapter 12.

5.12.2 Positioning Aircraft Nacelle Pod

In the vicinity of wing and/or fuselage, airflow past nacelle pod will be interfered with and that will give rise to interference drag. Positioning the nacelle pod with respect to the aircraft is an important design consideration.

The best position to place a nacelle pod is on the wing. The engine weight relieves some of the wing bending moment developed from wing lift force in flight. This also favours the aircraft CG to stay forward. However, low‐wing smaller aircraft do not have enough ground clearance for under‐slung nacelles, therefore they have to be mounted on the fuselage, always at the aft of it. Interestingly, Hondajet has defied the convention and positioned the nacelles over the wing. The authors believe that lessons from Hondajet's operational experiences will see more aircraft designs with over wing‐nacelle configuration. The considerations for a wing‐mounted nacelle (e.g. A320) and fuselage mounted nacelle (e.g. B727) differ as described in the following sub‐sections.

To minimise yaw moment at one‐engine inoperative situations, the engine thrust line should be as near to fuselage centre line as possible but at the same time not too close to cause high interference drag. As a compromise, typically, wing‐mounted nacelle pods are placed at about twice the maximum nacelle diameter ≈[±(2 × D_{nac_max})] away from fuselage and aft‐fuselage nacelles placed at ≈[±(1.5 × D_{nac_max})].

The closest an inboard engine should be kept to is at least 30° away from the nose wheel spray angle as shown in Figure 5.20 (B747 is a little widely spaced). It is found that the most inboard engines placed 30° from the nose wheel give the wing‐mounted nacelle position >[(2 × D_{nac_max})].

Aerodynamic, structural and CG locations influence the position of the nacelle with respect to the aircraft. This section discusses location of nacelle positions based on past experiences; those that are extracted from existing designs and those that are used during the conceptual design phase. Subsequently, in the next project design phase, the configuration is refined through CFD analyses.

5.12.3 Wing‐Mounted Nacelle Position

Structurally, outboard nacelle locations are desirable to reduce wing bending moments in flight but flutter requirements are complex and may show more inboard locations to be more favourable. The latter also favours directional control after engine failure. Finally, the lateral position of the engines affects ground clearance, an issue of special importance for large, four‐engine aircraft. Given next are some guidelines that may be used in classroom exercises. The following should be considered when positioning nacelles.

5.12.3.2 Vertical Position

Typically, wing‐mounted nacelles are under slung below the wing supported by a pylon. The dominating design consideration is to minimise wing‐nacelle interference drag. All nacelles are hung well over (Figure 5.20b) and ahead of the wing to keep interference drag low, almost to zero. Most high bypass ratio turbofans have short‐duct nacelles (Section 12.7.1), the cold secondary fan exhaust flow ejects over the primary duct case. The nacelle pod should be forward enough with a short fan cowl exit plane that stays ahead of the wing leading edge. A typical gap between the nacelle and wing may be taken as ≈15% of the local wing chord length.

The current design practice is to keep the nacelle position as far ahead of the wing with an orientation to capture the three stream tube diffused through a short intake duct length, normal to the fan face. A forward nacelle position favours adjusting the aircraft CG position in the conceptual design stage. There is no quick answer for the degree of incidence (Figure 5.20b), which is design specific and varies for the type of installation. It depends on the engine position with respect to the wing, for example, how much inboard on the wing, the flexure of the wing during flight and so on. However, the considerations for fuselage mounted nacelles, as given next, offer some idea of wing‐mounted nacelle positions. Post conceptual design studies using CFD, wind tunnel and flight tests would fine‐tune nacelle geometry and positional geometry to production standards.

Three typical relative positions of the nacelle with respect to wing are shown in Figure 5.21 [1] in detail: the top wing represents a B747, the middle represents an A300 and the bottom wing represents a DC10.

Other considerations are to keep adequate clearance for 15° roll during takeoff and landing (Figure 5.23).

5.12.4 Fuselage‐Mounted Nacelle Position

The wing‐mounted nacelle position, as discussed in Section 5.12.3, also offers some ideas for considerations of fuselage mounted nacelle positions (Figures 5.22). Fuselage mounted nacelle contours have similarity in design, but because there are no constraints of the wing, the interference drag is lower and can be brought closer to the fuselage, thus clearing the thicker boundary growth at the aft fuselage. A gap of roughly at least half the nacelle diameter may be left between fuselage and the nacelle. The vertical position is where it offers the least pitching moment developed by engine thrust about the CG for the whole aircraft, taking into account of contribution by the H‐tail. The position can be anywhere close to the fuselage centreline to high up on fuselage.

With an aft engine installations, the nacelles must be placed to be free of interference from wing wakes. The DC‐9 was investigated thoroughly for wing and spoiler wakes and the effects of yaw angles, which might cause the fuselage boundary layer to be ingested. Here, efficiency is not the concern because little flight time is spent yawed, with spoilers deflected or at a high angle of attack. However, the engine cannot tolerate excessive fan face flow distortion.

An aft fuselage mounted nacelle has many special problems. The pylons should be as short as possible to minimise drag but long enough to avoid aerodynamic interference between fuselage, pylon and nacelle. To minimise this interference without excessive pylon length, the nacelle cowl should be designed to minimise local velocities on the inboard size of the nacelle. On a DC‐9 a wind tunnel study compared cambered and symmetrical, long and short cowls and found the short banana shaped cambered cowl to be best and lightest in weight. The nacelles are cambered in both the plan and elevation views to compensate for the angle of attack at the nacelle (Figure 5.22).

5.12.5 Trijet Centre Engine

A trijet aircraft configuration has its odd engine at the centreline placed at the aft of the aircraft fuselage (Figure 5.23). The dominant choices are to have an S‐duct (B727) with the engine at the tail cone of the fuselage or straight through a duct (DC10) clear above the aft fuselage with a V‐tail on its top. This large nacelle pod over the fuselage also serves as part of a fin stabiliser for the aircraft.

The S‐duct configuration is lighter in construction but has lower recovery factor (RF – Section 13.2.1) with intake duct loss. The straight duct configuration is heavier but has a high RF in the order of a conventional nacelle pod. A tradeoff study (DOC comparison) is required to make the choice.

The S‐bend has a lower engine location and uses the engine exhaust to replace part of the fuselage boat‐tail (saves drag). It has more inlet loss, a distortion risk and higher inlet drag from the S‐duct bend. The straight‐through inlet with the engine mounted on the fin has an ideal aerodynamic inlet free of distortion, but does have a small inlet loss due to the length of the inlet and an increase in fin structural weight to support the engine.

5.12.6 Some Structural Considerations

Structurally, outboard nacelle locations are desirable to reduce wing bending moments in flight but flutter requirements are complex and may show more inboard locations to be more favourable. The latter also favours directional control after engine failure. Finally, the lateral position of the engines affects ground clearance, an issue of special importance for large, four‐engine aircraft.

Nacelle pod clearance from ground must ensure avoidance of debris ingestion. Also, crashworthiness of the pod in case of nose wheel collapse has to be catered for as shown in Figure 5.24.

Although rare, high heavy turbine discs at high rpm have disintegrated under high stress of centrifugal force operating at elevated temperature. The fragments have enough energy to cut through the fuselage shell causing catastrophic accidents. To contain fragments, the inner cowls around the rotor are reinforced by Kevlar, a material is also used as tank armour plates.

Another influence of wing‐mounted nacelles is the effect on flaps. The high temperature, high ‘q’ exhaust impinging on the flap increases flap loads and weight and may require a titanium structure (more expensive). The impingement also increases drag, a significant factor in takeoff climb performance after engine failure. Eliminating the flap behind the engine reduces C_Lmax.

In principle, the external contour lines of good nacelle designs are not necessarily symmetrical to its vertical plane. But to keep cost down by maintaining commonality, most nacelle designs are made symmetrical to the vertical plane. This would allow manufacturing jigs to produce interchangeable nacelles between port and starboard sides and be able to minimise the essential difference at the finishing end. The nacelle aerodynamic design (external mould‐line shaping and internal contouring) has progressed to a point of diminishing return on the efforts made and approaching to a generic shape.

Post‐conceptual design studies using CFD, wind tunnel and flight tests would fine‐tune the nacelle geometry and its positional geometry to production standards.

5.13.1 Civil Aircraft Thrust Reverser (TR)

TRs are not required by the regulatory authorities (Federal Aviation Administration , FAA/Civil Aviation Authority, CAA). They are expensive components, heavy and only applied on ground, yet their impact on aircraft operation is significant on account of having additional safety through better control, reduced time to stop and so on, especially at aborted takeoffs and other related emergencies. Airlines want to have TRs even at the cost of increased DOC.

Broadly speaking, there are two types of TRs: (i) those operating on a mixed fan and core flow and (ii) those operating on fan flow only. Their choice depends on BPR, nacelle location and customer specification.

The first type operating on the total flow (both fan and core) is shown the top of Figure 5.25. There are two types: (i) sliding port aft door type when the doors slides to the aft end as it opens up to deflect the exhaust flow and (ii) fixed pivot type when the doors rotates to position to deflect the exhaust flow.

The second type of TR operates on the fan flow only. There are two types: (i) the petal cowl type – its mechanism is the middle of Figure 5.25 – and (ii) the cascade cowl type shown at the bottom of Figure 5.25. There are two cascade types: the conventional type and the natural blockage type. Bombardier CRJ700/900 aircraft use their own patented cascade TR of the natural blockage type. The external cowls translate back blocking the fan flow when it escapes through fixed cascades that reverse the flow. This is an attractive design with low parts count, scalable, easier to maintain and offers relatively higher retarding energy. The petal type operating on fan flow is suitable to short‐duct nacelles as shown in the figure. The petal doors open on a hinge to block the secondary flow of the fan when it deflects to develop reverse thrust.

TRs are applied below 150 kts and are retracted back at around 50 kts when wheel brakes are effective. The choice for the type would depend on designer's comprise from the available technology at his/her disposal. Aircraft designers must ensure TR efflux is well controlled – there should be no adverse impingement of on aircraft surface nor there be re‐ingestion in the engine. Figure 5.26 gives a typical satisfactory TR efflux pattern.

5.14.1 Fuselage Axis/Zero‐Reference Plane – Military Aircraft

Fuselage axis is a line parallel to the centreline of the constant cross‐section part of the fuselage barrel. It typically passes through the farthest point of the nose cone, facilitating the start of reference planes normal to it. The fuselage axis may or may not be parallel to the ground. The principal inertia axis of the aircraft can be close to the fuselage axis. In general, the zero‐reference plane is at the nose cone, but designers can choose any station for their convenience, within or outside of the fuselage. This book considers the fuselage zero‐reference plane to be at the nose cone, as shown in Figure 5.27.

5.14.2 Fuselage Length, l_fus

This is along the fuselage axis, measuring the length of the fuselage from the tip of the nose cone to engine exit plane. Fuselages are further sectioned at the joints of the fabricated subassembly sections built in their dedicated jigs. As the manufacturing philosophies may differ, the fuselage sections subassemblies differ from design to design. Figure 5.27 gives a typical example.

5.14.3 Fuselage Nose Cone, l_cone

The pointed nose cone is the front closure is the aerodynamic requirement for supersonic flight and also serves as the housing for the radar, which is mounted at the front of the fuselage front section, l_front, bulkhead.

5.14.4 Front‐Fuselage Length, l_front

Following the nose cone is the front‐fuselage sub‐assembly with the length, l_front. This is the dedicated section to house the flight deck (crew station) for the crew, mostly single seated but can be dual (as required for the role), either in tandem or side‐by‐side arrangement. A canopy with streamlined windscreen mould lines covers pilot CFD.

5.14.5 Mid‐Fuselage Length, l_mid

The mid‐fuselage length, l_mid, of the subassembly follows the l_front up to the engine face carrying the intake ducts, cables, linkages and, depending on the design the fuel tanks, filling up the section cavity.

5.14.6 Aft‐Fuselage Closure Length, l_aft

Aft‐fuselage closure length, l_aft, is the last subassembly section following the l_mid housing the power plants. As mentioned earlier, combat aircraft with a blunt engine exhaust plane have no transport aircraft like pointed closure. Typically, this does not require an upsweep angle.

5.14.7 Fuselage Height, H_fus

Fuselage height, H_fus, is the maximum distance of the fuselage from its underside (not from the ground) to the top in the vertical plane (Figure 5.27). Combat aircraft fuselage cross‐section varies along the fuselage length, hence each cross‐section along the fuselage length has to be considered separately.

5.14.8 Fuselage Width, W_fus

Fuselage width, W_fus, is the widest part of the fuselage in the horizontal plane. Combat aircraft fuselage cross‐section varies along the fuselage length, hence each cross‐section along the fuselage length has to be considered separately.

5.14.9 Fuselage Cross‐Sectional Area, A_fus

Combat aircraft cross‐section area along fuselage stations is an important parameter to establish fuselage volume distribution required to estimate aircraft supersonic drag (Section 11.18). Being non‐circular, the concept of average or equivalent cross‐section diameter is not relevant. The term effective diameter, D_eff, as given in Eq. 5.2, exists for a non‐circular cross‐section as a reference value. It is not used in this book.

Because of complex and varying fuselage cross‐sections along the length, there is difficulty in interpreting military aircraft FR. The definition of FR with average or equivalent diameter serves little purpose, except as a reference number.

5.15.1 Flight Deck Height, H_fd

This is the internal cabin height from the flight deck floorboard to canopy top, as shown in Figure 5.28.

5.15.2 Flight Deck Width, W_fd

Flight deck width, W_fd (Figure 5.28), should be generous for accommodate the 99% percentile crew size. The W_fd should be more than 38 in. width. Fuselage width at the canopy interface should be adequate for ejection seat egress with its width close to 38 in. (trainer aircraft = 0.35–0.37 in.). Details of ejection process is given in Chapter 17.

5.15.3 Egress

See Section 22.8.1.

5.16.1 Military Aircraft Intake

While military aircraft do not have pods, their intake geometries have many configuration possibilities. Chart 5.1 gives the dominant types of military aircraft intake configurations: all have an engine buried in the fuselage.

Figures 5.30 and 5.31 give examples of the intake configurations given in the Chart 5.1. Earlier designs had an intake at the nose of aircraft for both subsonic (e.g. F86 with pitot intake) and supersonic (e.g. MIG21 with a movable centre body) operations. A centre body is required for aircraft speed capability above Mach 1.8, otherwise it can be pitot intake. The intake ducts were long and had bends to go below the pilot and then up again to the engine face, incurring flow energy loss resulting in low RF.

Later, with side intakes, bifurcated to both sides of fuselage considerably improved the RF by shortening the duct length. The loss from the bends on account of bifurcation of the intake duct is minimised through careful contouring, nowadays with CFD analyses. Other possibilities with a short intake duct is to have chin intake (e.g. F16) under the fuselage or intake over the fuselage (e.g. F107). Shorter intake ducts have the benefits of weight saving.

Intake design must also cater for flight at high incidence and yaw attitudes for the full flight envelope, so the flow distortion, separation, turbulence within the duct are at acceptable levels for engine to operate with compressor blade stall or engine flame outs.

Unlike the aircraft front intake, the side intakes are subjected to a breath of air flowing past the front section of the fuselage with the consequence of having boundary layer adhered to the fuselage surface in a favourable pressure gradient of the increasing fuselage cross‐section. Ingestion of this boundary layer through the intake will degrade engine performance. This problem is overcome by installing a splitter plate (Figure 5.32) acting as diverter to bleed the boundary layer to spill out from both sides the splitter/diverter plate. An adjustable ramp (F5) on splitter plate acts as shock diffuser. Front‐fuselage mounted gun is to be positioned to avoid ingestion of hot gas efflux from firing rounds.

Subsequently, further advancement have been achieved by using Diverterless Supersonic Inlet (DSI) is a type of jet engine air intake used by some modern combat aircraft to control air flow into their engines. It consists of a ‘bump’ (Figure 5.33) and a forward‐swept inlet cowl, which work together to divert boundary layer airflow away from the aircraft's engine. This eliminates the need for a splitter plate, while compressing the air to slow it down from supersonic to subsonic speeds. The DSI can be used to replace conventional methods of controlling supersonic and boundary‐layer airflow. This is a good example of using CFD to design DSI intake. It is practically impossible to find the shape of the bump for the DSI without using CFD.

The DSI bump functions as a compression surface and creates a pressure distribution that prevents the majority of the boundary layer air from entering the inlet at speeds up to Mach 2. There is no need for variable geometry supersonic intake for shock diffusion. There are considerable advantages with the DSI, as follows. Diverterless inlets lighten the aircraft weight.

1. Eliminates complex and heavy mechanical systems. There is a low part count.
2. Eliminates drag producing splitter plate through the boundary layer bleed passage.
3. No moving parts; for example, travelling centre body for shock diffusion.

Powerful gas turbine air intake at maximum power creates a strong suction vortex as shown in Figure 5.34a with ground water sucked in at the high‐wing mounted military transport aircraft. This is capable of ingesting solid debris damaging the engine that may lead in to engine failures. Foreign object debris (FOD) ingestion by engine is a real harmful issue and must be prevented. Airfields for civil operation keeps the metalled cleaned several times a day, also as and when required as observed from frequent inspections. Boeing737 pods are low slung too close to the ground, hence they have vortex dissipaters to reduce the airflow suction at the bottom part of the intake to reduce the chances of FOD ingestion. Personnel in front of operating engines is strictly forbidden – fatalities have occurred. Some turbofans do not have stationary fan guide vanes in the front of the engine.

High powered supersonic turbofan intakes in military aircraft are positioned much lower and may need to be operated from a rougher macadam airfield when gravel ingestion is certain to occur. As an adaptation to rough‐field operations, some designs, for example, the MIG 29 (Figure 5.34b), have the provision to close the front main air inlet, supported by properly sized louvre like the auxiliary air inlet on the upper side of the wing strakes under which the main inlet is positioned. The pilot operated auxiliary louvre intake is engaged during in field performance during takeoff and landing only. One of the other possibilities to prevent FOD ingestion is to have mesh screens in the main intake. For an S‐shaped duct, the side intake has a spring loaded flapped door suitably positioned at the end of the S‐bend so that the harmful heavier FOD failing to negotiate the bend hits a flap door to be thrown outside. S‐duct design incurs flow energy loss and is not desirable.

Last but not the least in nacelle/intake design considerations, is retaining engine accessibility for MRO activities.