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Chapter 12

# The Anatomy of the Fuselage

## Abstract

The chapter focuses on the design of the fuselage by introducing three elementary fuselage shapes commonly used in aircraft design, the frustum, tubular, and tadpole. The help the designer with the selection of any specific geometry, a range of information on the pros and cons of each is presented. Then, a simple method to sketch a fuselage for a GA aircraft is introduced. While there are many ways to accomplish the initial fuselage sizing, this method forces the CG range and wing position to be considered from the beginning; an approach that quickly reveals unexpected problems with airplane passenger and payload combinations. This is followed by several methods to estimate geometric properties of typical fuselage shapes. Finally, practical information about the physical dimensions of typical dimensions for humans (male and female) is tabulated to help with the internal design of fuselages and cabin area. It also tabulates the internal dimensions of a number of aircraft.

### Keywords

Tubular; frustum; tadpole; pressurization; pressure-tube; sailplane; tail upsweep; cabin dimensions; cockpit dimensions; layout; volume; surface area; 95th percentile human

Outline

## 12.1 Introduction

The purpose of the fuselage is multifaceted. It does much more than just provide volume for occupants and freight to be transported by the aircraft or an aerodynamically efficient protection from the elements. Its design must be treated with a great deal of respect so it can truly serve the role for which it is intended. This section focuses on the design of the fuselage.

### 12.1.1 The Content of this Chapter

• Section 12.2 discusses three elementary fuselage shapes commonly used in aircraft design, the frustum, tubular, and tadpole. The section presents a range of information on the pros and cons of each, that will help with the selection although the tubular shape is usually default geometry for pressurized and non-pressurized passenger transport aircraft.

• Section 12.3 presents a simple method to sketch a fuselage for a GA aircraft. While there are many ways to accomplish initial fuselage sizing, this method forces the CG range and wing position to be considered from the beginning, an approach that quickly reveals unexpected problems with airplane passenger and payload combinations.

• Section 12.4 presents simple methods to estimate geometric properties of typical fuselage shapes.

• Section 12.5 presents typical dimensions for humans (male and female) that are helpful for the internal design of fuselages. It also tabulates the internal dimensions of a number of aircraft.

### 12.1.2 The Function of the Fuselage

The fuselage structure must allow components such as lifting surfaces, engines, and landing gear to be mounted and offer adequate load paths to react the large loads these generate. Among amenities that complicate the fuselage design are the various openings that are required for easy access into and out of the volume. The openings must be carefully laid out in order to keep the number of highly stressed regions to a minimum. Since doors are usually not intended to transfer axial and shear loads (except in the case of pressurized vessels, where doors must be capable of transferring the out-of-plane pressurization loads) the openings must be reinforced to relieve stress concentrations with minimum amount of deformation of the structure. It is inevitable that each such opening (door or window) will increase stress concentration, which calls for localized reinforcement. These, in turn, increase the empty weight of the vehicle. For this reason, the designer should evaluate objectively whether a given opening into the fuselage is justifiable: is it necessary or is it just desirable?

Some factors that will affect the design of the fuselage are:

(1) If the airplane transports people, sufficient internal space must be given to each person. Larger transport aircraft should offer ample space for the passengers and cabin crew members to move around (for instance, to go to a lavatory, or exit in case of an emergency).

(2) If the airplane is large, amenities (lavatories and galleys) must be provided for the occupants. Large passenger transport aircraft should have at least one lavatory per 50 passengers and one galley per 100 passengers. For instance, a typical 150-passenger Boeing 737 has two galleys (one in the front, the other in the back of the cabin) and three lavatories (one in the front, two in the back).

(3) The cockpit should be ergonomically laid out, regardless of airplane size. This means primary instruments and controls should all be within reach of the pilot and not require him or her to lean in order to access them.

(4) Windscreen shape and strength requirements will dictate the design of the forward part of the airplane and depend on airplane geometry and operational requirements (e.g. pressurization, bird strike, etc.).

(5) Layout of emergency exits: for instance 14 CFR Part 121.291 requires all operators of passenger aircraft with seating capacity greater than 44 to demonstrate it can be completely evacuated in less than 90 seconds.

(6) The layout of control, electrical, and other important systems. The fuselage structure should be expected to accommodate control cables, pushrods, pulleys, and wiring harnesses so they go around critical structural members and do not penetrate them.

(7) The fuselage should be designed with compartments intended to carry baggage and freight that are easily accessible. If the aircraft is large, such compartments must be accessible from the outside. The fuselage must provide structure to allow baggage to be tied down so it will not shift in flight, possibly altering the CG. This structure should be stout enough to react emergency landing loads as well.

If landing gear loads are reacted by the fuselage (in contrast to the wing) this will require hoop frames in the area of the landing gear to be substantially reinforced. Typically, the main landing gear will then retract into special aerodynamically shaped housings on the bottom of the fuselage. An opening should be provided in the front part of the airplane to house the nose landing gear. The author is not aware of any instance that features a nose landing gear that retracts into a separate housing unit and not the fuselage itself. It is good practice to examine existing aircraft of similar configuration and study how the landing gear housing is designed when evaluating the pros and cons of a design direction.

The fuselage must also provide structure to attach it to the wing. Commuters and similar passenger aircraft usually feature high or low wing configurations. Mid-wing commuters are practically unknown in modern times – the most recent one was the HFB-340 Hansajet, designed in the 1960s and operated until the early 1970s. There really is no good reason to mount the wing in the middle of the passenger cabin of such aircraft. High-wing aircraft typically feature hardpoints on the main and rear spars that allow the fuselage to be hung below them. Examples of such aircraft are the Shorts SD3-30; and Fokker F-27 and F50 twin-turboprop commuter aircraft. The second two have a part of the spar penetrate the ceiling inside the cabin; the protective spar cover in the cabin ceiling is clearly noticeable upon entry. Tall people must lower their heads to avoid hitting the ceiling material that covers the spar.

Aircraft with low wings may have the fuselage mounted on top of the spar in a similar manner, although they more often feature a reinforced spar box under the cabin floor. In the case of low-wing aircraft, there is never a need for the spar to penetrate the floor – it would invite unacceptable risk to passengers, to have them maneuver around an elevation in a floor when moving about the cabin. Regardless of the wing attachment method employed, such aircraft usually feature external wing root fairings to prevent separation of air flowing through the juncture. Depending on the aircraft, wing root fairings can be massive and require an internal support structure on their own.

Sometimes the fuselage is designed to carry engines. This is very common for small single-engine aircraft, but also for selected twin-engine aircraft. Most small propeller aircraft feature the engine in the front, while there are also a few pusher configurations as well. Among such aircraft are the Cessna 337 Skymaster and Adam A500, both of which feature two engines; a tractor and a pusher. Neither one is being produced at this time. Regardless of the location of the engine, such configurations always require a special fire protection to be placed between the engine and the cabin. This protective wall is called a firewall. Requirements for firewalls in GA aircraft are stipulated in 14 CFR Part 23.1191, Firewalls. Among requirements is that the firewall must withstand a flame temperature of 2000 ± 150°F for at least 15 minutes.

Jet engines mounted to fuselages are attached either internally or externally. An internally mounted engine must provide fireproofing and a fail-safe structure around the engine. The fireproofing must be capable of protecting the primary structure should the engine catch on fire – the heat of such a fire cannot compromise the structural integrity of the aircraft. Requirements for this fireproofing are given in 14 CFR Part 23.1195, Fire extinguishing systems. The failsafe structure is necessary in case fragments from a possible rotor-burst penetrate a primary structural member. Such an emergency may not cause the aircraft itself to disintegrate. Primary systems in the aircraft must also be redundant – for instance a rotor-burst cannot take out the primary flight control system and if it does, there should be a secondary control system to allow the aircraft to continue flying.

As stated earlier, the fuselage must often feature various openings, ranging from access to an avionics bay to jet engine inlets for buried engine configurations. There must also be access to baggage compartments. Some pressurized aircraft feature special pressurized baggage compartments that allow animals to be transported safely, whereas small aircraft always have unpressurized baggage compartments. Some fuselages allow the main landing gear to retract inside it, requiring cutouts in the fuselage.

The fuselage is equally as important as the other components of the aircraft. Serving to contain occupants, freight, and important systems, it has to be carefully designed to provide spaciousness and yet be light and not generate too much drag. Just as the wing must be properly sized to carry the airframe and useful load, the fuselage must be sized to carry payload and shield it from the elements. This chapter is intended to discuss a number of topics that are important to the design of the fuselage.

## 12.2 Fundamentals of Fuselage Shapes

This section discusses three fundamental shapes of the fuselage; frustum, tubular, and tadpole. Of these, tubular fuselage was brought up in the introduction to the structural layout of the fuselage in Section 5.3, Airframe structural layout. It is imperative for the designer to be aware of the pros and cons of each configuration.

### 12.2.1 The Frustum-shaped Fuselage

The frustum fuselage is used to describe a fuselage whose empennage is effectively shaped like a frustum or a trapezoidal prism. An example of such a fuselage is shown in Figure 12-1. It is common to manufacturers like Beechcraft, Cessna, and Piper. Such fuselages are easily recognizable by a tapered boxlike appearance, although the term frustum refers to a tapered cylinder (see Figure 12-23). They are inexpensive to produce because they can be made from folded sheet metal riveted to frames, which produces a light and stiff structure. It is a drawback that they generate far more drag than tadpole fuselages. The configuration is the right choice for roomy, inexpensive, stiff, and strong fuselages, where drag is not an issue, but internal volume is. The frustum fuselage is ideal for utility transport aircraft, for instance, feeder aircraft for package services. However, if the goal is an aerodynamically efficient aircraft, frustum-shaped fuselages are the wrong choice. Such fuselages are indicative of the aircraft design philosophy of yesteryear and, today, are primarily justified by reduced production costs.

FIGURE 12-1 A Piper PA-28 Cherokee featuring a conventional frustum style fuselage. (Photo by Phil Rademacher)

### 12.2.2 The Pressure Tube Fuselage

The pressure tube fuselage is used to describe a fuselage effectively shaped like a cigar, with a tubular main section and capped ends (see Figure 12-2). This fuselage shape is ideal for passenger aircraft, small and large, pressurized or not, of any airspeed range. If the airplane is pressurized, the cross-section is circular, as no geometry carries pressure loads more efficiently. The reaction of pressure loads is discussed in more detail in Section 5.3.4, Fundamental layout of the fuselage structure, so the presentation will be limited here.

FIGURE 12-2 A schematic of a pressure tube fuselage, showing typical lengths of the forward and aft sections in terms of the fuselage diameter.

The forward section typically ranges from 1.45 to 1.75 times the diameter of the fuselage. The length of the empennage ranges from 3 to 3.35 times the diameter for most airplanes. This fineness ratio has the least drag, as shown in Section 15.4.9, Form factors for a fuselage and a smooth canopy.

The tadpole fuselage is used to describe a fuselage whose empennage and forward portion resembles the shape of a tadpole, the larval stage of a frog. An example of such a fuselage is shown in Figure 12-3. Tadpole fuselages are more expensive to produce than frustum fuselages, in particular if made from aluminum as the geometry features compound surfaces that would call for expensive metal-forming processes. Their production can be achieved more economically using composites and this remains the primary method used for this purpose. All modern sailplanes feature a tadpole fuselage shape, as well as a number of modern propeller aircraft. Among those are the Cirrus SR20 and SR22, the Diamond DA-20 Katana, DA-40 Diamond Star, and DA-42 TwinStar. All are composite aircraft.

Galvao [1] pointed out the advantages of fuselages shaped like fish by citing examples from nature and used the superposition of sources and sinks to represent and evaluate the aerodynamic properties of such fuselages. He used what is called the three halves power law to determine the ratio between width and height of a fuselage and discussed means of converting an airfoil silhouette into a three-dimensional fuselage shape. While not a tadpole surface, the resulting fuselage resembles a short tadpole.

Althaus [2] was one of the first to investigate the properties of the tadpole fuselage. Dodbele et al. [3] used a surface singularity analysis method as a tool to help design such fuselages. Radespiel [4] presents the effect of proper contraction geometry (called waisting) on the transition region and, ultimately, the drag of the fuselage, supported by wind tunnel testing. The interested reader should be made aware that there is a plethora of literature on tadpole fuselages, presenting it in depth is beyond the scope of this book.

Tadpole fuselages generate far less drag than the frustum kind for two primary reasons: (1) their forward portion is shaped to sustain laminar boundary layer; and (2) their empennage shape results in as much as 30–40% less wetted area. In Ref. [2], Althaus says:

1. Similar to laminar airfoils, the front part of the body should produce favorable pressure gradients in all meridians even at incidences of about ±10°. At the same time, the whole surface should be smooth and leak-free in order to avoid any disturbances to the laminar flow.

2. Behind the transition front it is favorable to contract the cross-section. On one hand, this reduces the wetted surface; on the other, it shifts the unavoidable pressure rise to the thinner parts of the turbulent boundary layer, which is a well-known principle of favorable boundary layer control.

Althaus compared three tadpole-style fuselages to a frustum fuselage of equal length and diameter, whose fineness ratio (l/d) was 10. Both shapes were representative of those used for sailplanes (see Figure 12-4). The study revealed an important difference in the transition front (the curve along which laminar-to-turbulent transition takes place) and their drag characteristics. As shown in the figure, the minimum drag coefficient (based on the frontal area) of the fuselage at a Reynolds number of 7.1 × 106 is 0.034 for the tadpole and 0.0505 for the frustum (at an AOA of 0°). This means the drag of the frustum is 48.5% greater than that of the tadpole, explaining why such fuselages have become the norm in modern sailplane design.

FIGURE 12-4 Difference in transition and total fuselage drag of a tadpole and frustum fuselage. The drag of the frustum is almost 49% that of the tadpole fuselage. (Based on Ref. [2])

The tadpole design requires careful attention to the curvature of the geometry aft of the maximum thickness. Too sharp a contraction will result in a flow separation that will increase the overall drag of the fuselage. Too small a contraction will not reduce the wetted area rapidly enough to make a dent in the total drag. As can be seen in Figure 12-4, the contraction also reduces the local airspeeds slowly (relatively speaking) in the area where the turbulent boundary layer is still relatively thin. This helps prevent an early separation. This area is akin to the pressure-recovery region of an NLF airfoil. The goal is to maintain the laminar boundary layer as far aft along the fuselage as possible, but, once the maximum thickness has been exceeded, give air enough distance to slow down without flow separation.

Another aspect of tadpole fuselage design is the downward tilt of the fuselage, shown in Figure 12-5. This is a response to the upwash caused in the airflow ahead of the wing. A straight fuselage, as shown in the upper figure, will be at a higher AOA and this will increase its drag. To reduce this, the forward portion of the fuselage is tilted downward to align it with the oncoming airflow, reducing its drag. Some even tilt the tailboom down or reshape it to better match the flow field behind the wing as well.

FIGURE 12-5 Reduction of drag by aligning the fuselage with the oncoming airflow. (Adapted from Ref. [5])

A flow-adaption design of this nature should only be performed using CFD technology or wind tunnel testing.

## 12.3 Sizing the Fuselage

The initial design of the fuselage should be implemented once the designer has determined the fundamental dimensions of the wing and stabilizing surfaces and their representative locations in space. These should then be drawn to scale at the correct location. This is followed by the positioning of the most important components (such as engines and landing gear) and payload (passengers and freight), taking into account its dimensional restrictions. Finally, the outside mold line (OML) of the fuselage can be drawn such that it encompasses the entire layout. This process allows for modifications and shifting of components to ensure the CG will remain within reasonable margins.

### 12.3.1 Initial Design of the External Shape of the Fuselage

This author refrains from using statistical methodologies to estimate the physical dimensions of the fuselage design, as these are limited to specific shapes and do not prevent conflicts that may arise, for instance, due to the space claims of internal components. For example, what impact will the width and height of an engine have on the girth of the fuselage? What about the waterline adjacent to the head of the occupants? Will these be entirely on the inside of the airplane, or will parts thereof be exposed to the airstream? It is important to tackle space claim problems from the start and eliminate them head on. This can be done using the following process, which is outlined in more detail below using images where possible.

Step 1: Determine the general shape of the wing, as well as the HT and VT of the aircraft, using information gathered from Steps 1 through 9 in Table 1-3. This data should be detailed enough to provide the span and chords of the wing and stabilizing surfaces and also the proper location of the stabilizing surfaces with respect to the wing’s MGC/4 (see Figure 12-6). Enter this rudimentary data into a design drafting program or simply draw on a piece of paper. This image represents the aerodynamic requirement of the design.

FIGURE 12-6 In Step 1, the lifting surfaces are drawn to scale and separated by the correct tail arm lT.

Step 2: Indicate the desired CG envelope on the MGC (see Figure 12-7). For instance, the CG envelope for conventional aircraft may extend from 15% to 35% MGC. At this point in the game the final CG envelope remains unknown because far more detailed analysis remains to be done. However, we have to start somewhere and this is a reasonable first step.

FIGURE 12-7 In Step 2, the expected CG limit around the quarter-chord of the wing’s MGC is labeled.

Step 3: Estimate the weight of all known components making up the airplane. For instance, consider those presented in Section 6.4, Statistical weight estimation methods (omitting components that do not belong to the particular design, i.e. ignore pressurization or hydraulic systems for a small fixed-gear airplane). Tabulate these weights, ideally in a spreadsheet application, by entering them in a column format. The next column should be the x-location of the expected CG location of each of the components. For instance, estimate the CG location of the wing, HT, VT, and so on and enter in the second column. Summing these weights effectively yields the predicted empty weight of the airplane. Using these weights, estimate the empty weight CG location and indicate in the figure, as shown in Figure 12-8. Note that at this time we do not know if this CG location is viable – we will evaluate that in Step 4. Later, in Step 5, we may have to change the CG locations of selected components until the total CG can be “nudged” into the proper location.

FIGURE 12-8 In Step 3, all major components constituting the predicted empty weight of the aircraft are placed in their proper location. The empty weight CG is indicated on the drawing. Note that at this time we do not know if this CG is in the proper location.

Step 4: This step will require some trial and error, consisting of moving selected components and the pilot, passengers, and baggage around until the forward and aft CG limits mostly land inside the desired CG limits (see Figure 12-9). This process requires the designer to develop a loading cloud similar to the one described in Section 6.6.12, Creating the CG envelope, ideally using the spreadsheet tabulation from the previous step.

FIGURE 12-9 In Step 4, occupants, baggage, the engine, and other known “heavy” components are moved (or nudged) around until the CG loading cloud is mostly placed within the desired CG limits.

Using the components that make up the empty weight of the airplane, evaluate the CG locations of the empty aircraft plus as many combinations of occupants, fuel, and baggage as considered practical. Plot all the resulting CGs on the figure, as shown in Figure 12-9. If the bulk of the CG locations fall outside the limits, try to modify the CG locations of selected components, such as the landing gear, engine, avionics, and even the occupants. Remember that the location of the wing, HT, and VT should not be changed unless necessary.

Note that if the HT or VT are moved to a different location, the resulting tail arm (lT) will differ from the original one. This means that the horizontal and vertical tail volumes (denoted by VHT and VVT) have now changed. Therefore, the geometry of the HT and VT should be updated to ensure the selected VHT and VVT remain constant. This, in turn, will affect the weight of the said components, which affects the CG again. Welcome to the world of aircraft design. The process can be made much easier using a spreadsheet that automatically updates the necessary variables.

Some components will only require a small amount of nudging, while other may require a large relocation. While tedious, this process is necessary to prevent future “surprises” once the airplane has been developed further and making changes has become much harder. It is to be expected that components end up in “interesting” or “surprising” locations necessitated by placing the CG loading cloud in its proper location.

Step 5: Once the CG appears to be in a satisfactory location, indicate the occupants and component dimensions on the diagram. Trace a rudimentary fuselage that encloses occupants and all internal components, keeping in mind its aerodynamic and aesthetic appearance (see Figure 12-10). This effectively concludes the initial definition of the fuselage and it is now possible to begin refining it by considering other requirements it must satisfy.

FIGURE 12-10 Step 5 is the final step, taken once the proper range of the CG has been determined. With the approximate dimensions of internal parts known, trace the outline of the fuselage, preferably using a tadpole or other low-drag fuselage shape.

### 12.3.2 Refining the External Shape of the Fuselage

A number of factors must be considered when refining the fuselage.

(1) The fuselage should be made as streamlined as possible. It should feature the appropriate forward shape that will not result in an excessively large stagnation region. It should feature an aft shape that takes into account the effect of fineness ratio (empennage length/diameter) on drag. This usually means that the empennage should have a fineness ratio between 3 and 3.35 (see Figure 12-2).

(2) The tail upsweep angle must be considered to guarantee the airplane can rotate for T-O and flare for landing with the selected landing gear layout. Assuming compressed landing gear at gross weight, the upsweep should allow the airplane to be rotated to its clean stall AOA (see Figure 12-11). Also, designers of passenger aircraft always think about growth potential: is it possible this aircraft will require a larger passenger volume in future? If fuselage “plugs” are used to accomplish such a development, will additional engineering resources be required to modify the empennage for ground clearance? The value of such forethought is shown in Figure 12-11.

FIGURE 12-11 The fuselage should allow the airplane to rotate to an angle close to that of the AOA of its CLmax, although it is also prudent to think about growth.

(3) Designers of small composite airplanes should consider a tadpole shape as they generate less drag than conventional frustum-type fuselages. Airplanes made from aluminum almost always use the frustum shape, as a tadpole fuselage requires compound surfaces. Larger aircraft normally do not feature tadpole shapes as it reduces the available internal volume.

(4) The external dimensions of the cabin area of the fuselage must allow for the required internal dimensions plus the airframe around it. If the airplane is pressurized the designer should strongly consider a circular fuselage shape as this is structurally the most efficient configuration for pressure loads. An unpressurized fuselage is not limited by that shape.

(5) The designer should be cognizant of structural load paths when laying out openings for doors and windows. For instance, consider the aircraft of Figure 12-13. The frame (often called the A-pillar) to which the windscreen mounts should be placed as directly as possible where it is expected the wing’s main spar will be placed. This way, the A-pillar can be attached to the structure already required to react the wing loads. Window openings in the fuselage, besides having to provide convenient viewing angles for the passengers, should be placed so they fall between hoop frames. It is to be expected that hoop frames will be placed at even intervals – so, place the passenger windows at even intervals between them.

(6) The location of the wing (high, mid, low, parasol) must be taken into account when refining the external fuselage, as well as the possible shape of the wing root fairing. Typically, the maximum fuselage width is where the wing joins the fuselage, although this is not always the case. However, think about how the wing root fairing design will be affected by the region near the trailing edge of the root.

(7) Jet engines buried in the fuselage will have a great impact on the external geometry of the fuselage, and inlet and exhaust paths must be clearly defined for space-claim reasons. The volume reserved for the engine is out of reach for other systems. It is surprisingly easy to locate equipment near such a volume, later to discover it penetrates the engine volume.

(8) Placement of necessary equipment: avionics, battery, hydraulic system, control system, should take into account access for maintenance crews. Also, many aircraft use collector tanks that are placed inside the fuselage, typically between the main spar and aft shear web.

(9) And finally, this simple food-for-thought advice: many people find airplanes that “lean a little” on the nose-gear in the ground attitude have a more appealing look than if level or nose-up. Of course this is only in the eye of the beholder – it is akin to people’s views on the cosmetics of T-tails or winglets. However, even assuming the designing team agrees with this view, it is simply not always practical, as it is for many tractor propeller aircraft. Propeller aircraft must comply with prop-strike requirements and designing around this aesthetic view could call for excessively long main landing gear legs. However, if possible, consider giving the aircraft an approximately 1° nose-down ground attitude for that extra it-means-business look. Examples of aircraft that have an improved look (admittedly in the author’s opinion) are the Rockwell OV-10 Bronco, the Lockheed P-3C Orion, and the Grumman Gulfstream II. Examples of the opposite are the Piper Cherokee Six and the Beech A36 Bonanza. Not surprisingly, both are tractor propeller aircraft.

### 12.3.3 Internal Dimensions of the Fuselage

If the aircraft is designed to carry passengers, the designer must be aware of the constraints the human body places on the external shape. For instance, Figure 12-12 shows three cross sections intended for a side-by-side seating. It can be seen that for occupant comfort, the two on the left and right provide ample headroom, whereas the one in the center does not. For this reason, such a fuselage must be made larger, perhaps at least what is indicated by the dashed circle.

FIGURE 12-12 Cross sections of a fuselage for a small airplane reveal that different shapes result in different problems.

For GA aircraft a number of regulations apply to the design of the crew and passenger areas. These are:

Among factors that should be considered during the design of the fuselage are:

(1) Space amounting to 22 inches wide, 43 inches deep, and 65 inches high is typically given to each passenger.

(2) If the design features an aisle, it should be 76 inches high and 20 inches wide (if a stand-up cabin), to allow passengers to move around freely and for beverage carts to be able to fit comfortably. Note that the aisle widths given by 14 CFR 23.815, Width of aisle, are minimums.

(3) For weight estimation, the average weight of the modern passenger should be considered to be 200 lbf, even though federal regulations assume 170 lbf. The average weight of a person has increased in the last few decades and 170 lbf is simply too light.

(4) If the duration of a typical design flight mission exceeds one hour, a lavatory should be provided. The number of lavatories and galleys depends on the number of passengers. Typically, there is one lavatory per 1 to 50 passengers and one galley per 50–100 passengers. Thus a 50 passenger Fokker F-50 has a single lavatory and a (½-a) galley. A 150-passenger Boeing 737 typically has three lavatories (one front, two in the back) and two galleys (one front and one back).

(5) Size of the cockpit depends on the aircraft. Primary controls should be within reach, as should secondary, although these should be placed away from the primary. Details of equipment layout in cockpit design are beyond the scope of this text.

(6) Passenger and crew emergency exits must be provided in accordance with 14 CFR 23.

### 12.3.4 Cockpit Layout

The term accommodation refers to both the passenger cabin and the cockpit. In this section we will consider the human aspect of aircraft design – i.e. how to best fit people into a given volume and the dimensions of a typical fuselage. General dimensions for the cabin of a small, side-by-side (or single-) seat aircraft are shown in Figure 12-13. This is based on the general internal dimensions of the Cessna 150/152, which is reasonably sized for most people, although its width is a minimum by modern standards. Figure 12-14 shows such a cabin with a 95th percentile mannequin. It can be seen that for this design (which differs from that of the 152) the proximity of the head to the ceiling is not satisfactory. A design review should improve that detail.

FIGURE 12-13 General cabin layout for a small airplane. The dimensions are typical.

FIGURE 12-14 General accommodation of a large person (95th percentile) in the cabin of a small aircraft. A good fit that also accommodates smaller pilots is a challenge and should be studied in detail before the final loft for a proof-of-concept aircraft is released.

Minimum recommended width and height for a typical small aircraft are shown in Figure 12-15. The height assumes upright sitting. Sailplanes are exempted from the height requirement, as upright sitting results in a greater frontal area in a sailplane and, thus, greater drag. The cabin should allow at least 20′′ (50 cm) per person. Anything else will likely make a part of the population claustrophobic, and, frankly, will be too narrow for the stouter pilot. Cabin dimensions for a number of aircraft are provided in Table 12-5.

FIGURE 12-15 Minimum cabin/cockpit dimensions for upright seating.

#### Pilot Visibility

It should not come as a surprise that pilot visibility is of paramount importance in manned aircraft design. In fact, in certified GA aircraft, pilot visibility is subject to compliance to 14 CFR 23.773, Pilot compartment view. The recommended field-of-view (FOV) for a typical GA aircraft is shown in Figure 12-16. It is a drawback that a good FOV usually comes at the cost of reduced structural integrity, as transparencies do not provide acceptable structural integrity.

FIGURE 12-16 Recommended pilot’s field-of-view.

Transparencies are usually made from two kinds of plastics: acrylics or polycarbonate. Acrylic is a thermoplastic polymer and is both light and clear to look through, allowing more than 90% of external light to get through the material. It is the choice for non-pressurized aircraft. It allows complicate geometries to be formed with ease, by heating the material in molds. The primary drawback is cracking, which may start due to some imperfection and, if left to its own devices, will continue to propagate.

Stretched acrylics are a solution, in which the plastic is physically stretched using special machinery. This reorients the long polymer chains, giving the plastic much improved properties. Among those are improves tensile stress (of the order of 10,000 psi) and reduced crack propagation. This makes the material suitable for birdproof windshields and enables its use in pressurized aircraft.

Polycarbonate is a specialized plastic considered the strongest transparent plastic available. It offers outstanding toughness, temperature tolerance, and impact resistance, making it ideal for windscreens subjected to birdstrikes at high airspeeds, such as low-flying fighter aircraft. Its primary drawback is crazing (the formation of a network of small cracks), requiring it to have protective coating. Polycarbonate is generally not used for GA aircraft. Typical dimensions for various cabin layouts are shown in Figure 12-17.

FIGURE 12-17 Typical seating for cockpit layout.

#### Large Aircraft

It should go without saying that the design of the cockpit of a large aircraft is a monumental undertaking that would require volumes to present. While most of the cockpit design is really a task taken on during the detail design phase, it is imperative that the external shape of the aircraft allows for an internal geometry that will enable an ergonomically efficient cockpit to be designed. An example of a spacious cockpit is shown in Figure 12-18.

FIGURE 12-18 Cockpit layout in a large business jet, here the Dassault 7X. (Courtesy of Dassault Falcon Jet Corporation)

#### General Passenger Cabin Layout

Jenkinson et al. [6] cite common distances (or pitch) between seat rows in the passenger cabin of typical commuter or commercial aircraft. These are shown in Figure 12-19. An example of the luxurious spacing between passenger seats in a business jet is shown in Figure 12-20. Such comfort is of course out of the reach of most people, but is presented here to contrast the other extreme presented in Figure 12-15.

FIGURE 12-19 Typical pitch of seats in a commuter or commercial aircraft. (Based Ref. [6])

FIGURE 12-20 Cabin layout in a Dassault 7X business jet aircraft. (Courtesy of Dassault Falcon Jet Corporation)

#### Special Considerations: Galley Design

A galley is effectively a kitchen in an aircraft. It is where food and beverage carts are stored, where hot beverages are brewed and ice cubes are stored, as well as containing ovens that are used to reheat chilled meals for passengers. The galley contains cabinets for dry goods and duty-free goods. The aircraft manufacturer does not produce equipment for the galley, but rather purchases it from a vendor. The production of equipment for the galley is an industry in its own right and a number of engineering companies specialize in the manufacturing and installation of such equipment. A number of provisions must be made in the region of the fuselage where a galley is to be placed. First, the galley should always be next to a service door. Such doors allow a galley service truck to park outside the galley for a rapid removal and replenishing of food and beverage carts, as well as trash. Second, there should be plumbing for a sink, so that liquid waste can be disposed of. Third, there should be electrical outlets that are capable of powering a number of ovens simultaneously. Fourth, there should be structural provisions to anchor the galley equipment, but these have to be capable of reacting substantial loads. These loads are typically obtained per 14 CFR 23.561(b)(3) and amount to 3.0 g upward, 18.0 g forward, and 4.5 g sideward. A galley is only offered in larger multiengine passenger aircraft.

#### Special Considerations: Lavatory Design

Just like the galley, lavatory equipment is engineered and produced by specialized vendors. Such equipment consists of a sink and a faucet, a counter, mirror, lighting, soap dispensers, amenity racks, and of course the toilet itself (Figure 12-21). The lavatory is a fully functional module that can be installed anywhere in the aircraft, as long as some preparations have been made. It consists of a centralized waste tank that connects to the toilets using small pipes. This allows the waste tank to be emptied in a single place, rather than for each individual toilet. Modern toilets use vacuum flush technology that does away with the recirculation of the blue-chemical flush liquid of yesteryear. Such toilets are substantially lighter and environmentally friendlier than the old-generation toilet.

FIGURE 12-21 Space claims of a typical aircraft lavatory.

## 12.4 Estimating the Geometric Properties of the Fuselage

During the early stages of the design process, there simply is not yet enough detail available to calculate the surface areas and volumes with a high degree of accuracy. Often the design consists of nothing more than some preliminary dimensions. For instance, we may have a reasonable idea about how long the fuselage might be, or its average diameter. Then, it is often convenient to estimate the geometric properties of the airplane using some generic shapes that resemble the proposed form. This section will present a few such shapes and simple and handy formulas to estimate the areas of the surface and volume. We will start with the geometry of a few fundamental shapes that can be combined to form shapes that resemble that of a fuselage.

### 12.4.1 Simple Estimation of the Surface Area of a Body of Revolution

The fastest method to estimate the wetted area of a fuselage is to evaluate the areas of the side and top silhouettes (Figure 12-22). This is based on the notion that the surface area of an elliptic cylinder can be estimated using the average of the major and minor widths times its length.π

Surface area:

(12-1)

FIGURE 12-22 The least complicated method to estimate the wetted area of a body of revolution is to use the areas of its side and top silhouettes.

### 12.4.2 Fundamental Properties of Selected Solids

A more complex formulation for typical aircraft fuselage is based on segmenting it into simple geometric shapes such as the ones shown in Figure 12-23. This section presents formulas to determine the properties of various fuselage geometric types.

FIGURE 12-23 Elementary solids of equal height L.

FIGURE 12-24 An approximation of a jet transport fuselage using elementary solids.

FIGURE 12-25 An approximation of a tadpole fuselage using elementary solids.

FIGURE 12-26 An approximation of a pod-style fuselage using elementary solids.

The advantage of this method is that it provides reasonable accuracy for more fuselages that are more complicated than the typical pressure tube fuselage. However, it is a drawback that the formulation is somewhat complicated, but this is the cost of their accuracy.

Surface area:

(12-2)

Volume:

(12-3)

Surface area:

(12-4)

Volume:

(12-5)

Surface area:

(12-6)

Volume:

(12-7)

Surface area:

(12-8)

Volume:

(12-9)

Surface area:

(12-10)

Volume:

(12-11)

#### Test Values

In order to minimize the risk of an error when entering these in a spreadsheet or other computer codes, the validation values in Table 12-1 are provided for the reader to compare own calculations to.

### 12.4.3 Surface Areas and Volumes of a Typical Tubular Fuselage

The following geometry can be used to estimate the geometric properties of a generic passenger transport aircraft. The geometry consists of a paraboloid, cylinder, and a cone, featuring the dimensions shown in Figure 12-24. Each section height, which here is conveniently described using the component length, is denoted by an appropriate subscript. The fundamental dimension D (diameter) is the same for all sections.

Total length:

(12-12)

Surface area:

(12-13)

Volume:

(12-14)

#### Test Values

The following validation values have been tabulated for the reader. Consider a fuselage whose dimensions are given by: D = 2, L1 = L2 = L3 = 2. This way, we can simply add the values in Table 12-1 and compare them to the outcome of the above expressions. It is important to remember that the surface areas include the base and top, so these must be subtracted from the total surface area when combining solids, as is done below:

TABLE 12-1

Validation Sample

QED

QED

### 12.4.4 Surface Areas and Volumes of a Tadpole Fuselage

The following geometry can be used to estimate the geometric properties of the generic tadpole-shaped fuselage shown in Figure 12-25. This is handy when estimating the wetted area of sailplanes and the modern GA aircraft, such as the Cirrus SR20 and SR22. The geometry consists of a paraboloid, cylinder, and two sets of frustum, featuring the dimensions shown. Each section height, which here is conveniently described using the component length, is denoted by an appropriate subscript. The fundamental dimension D (diameter) is the same for all sections.

Total length:

(12-15)

Surface area:

(12-16)

Volume:

(12-17)

#### Derivation of Equation (12-16)

Begin by adding the contribution of all the elementary solids used, with tops and bases removed:

Then simplify:

QED

#### Test Values

In order to minimize the risk of an error when entering these in a spreadsheet or other computer codes, the following validation values have been tabulated for the reader to compare their own calculations to. Consider a fuselage whose dimensions are given by: D = 2, d1 = 1, d2 = 0.5, L1 = L2 = L3 = L4 = 2. This way we can simply add the individual values for each elementary component and compare the sum to the outcome of the above expressions. Thus, STAD = 5.9026 + 12.5664 + 10.2704 + 4.8216 = 33.5610 square units and VTAD = 3.1416 + 6.2832 + 3.6652 + 0.9163 = 14.0063 cubic units. It can be seen that both match well.

### 12.4.5 Surface Areas and Volumes of a Pod-style Fuselage

The following geometry can be used to estimate the geometric properties of a generic pod-style fuselage shown in Figure 12-26, such as that of airplanes like the P-38 Lightning. The geometry consists of a paraboloid, cylinder, and a cone, featuring the dimensions shown. This is effectively identical to the passenger transport fuselage, except the cylindrical segment is substantially shorter. Use Equations (12-12), (12-13), and (12-14).

A general mannequin is essential when trying to establish space claims of occupants during the layout of the cabin and cockpit. A simple one can be made using elementary geometry such as circles for joints and head, and sticks for torso, stomach, arms, and legs. Such a figure should feature joints to allow limbs to bend to help assess space claims when standing, sitting down, or reaching and so on. It should give a realistic representation of the size of buttocks and thighs so that when folded into a sitting position the height from the head to the buttock will be accurate. A common mistake is to size the geometry of smaller aircraft so a portion of the pilot’s head ends up penetrating the fuselage side. An example of such a mannequin is shown in Figure 12-27. The dimensions indicated by the letters A through L can be obtained using documents such as The Human Factors Design Guide (HFDG) [7], which provides a collection of dimensions of human physical data and is a great resource for this purpose. The dimensions have been listed in Tables 12-2 and 12-3 for both males and females, ranging from the 1st percentile to the 99th. The concept of percentile refers to the percentage of people who are included in that particular class. For instance, the 50th percentile means that 50% of the general population does not surpass the cited physical dimensions, and so on. For instance, the stature (A) of the 95th percentile is 186.7 cm. This means that 95% of all people are equal to or less tall than that. The modern aircraft should be designed to accommodate the 95th percentile male.

FIGURE 12-27 A standard erect and sitting mannequins used for space claims by occupants.

TABLE 12-2

Dimensions of the Erect Mannequin in Figure 12-27

Based on Ref. [7].

TABLE 12-3

Dimensions of the Sitting Mannequin in Figure 12-27

Based on Ref. [7].

Table 12-4 details typical occupant weights used in the design of GA aircraft. Table 12-5 shows the cabin dimensions for selected aircraft.

TABLE 12-4

Typical Occupant Weights Used in Aircraft Design

TABLE 12-5

Cabin Dimension for Selected Aircraft (Based on Manufacturers’ Information)

## Variables

 Symbol Description Units (UK and SI) Aside Side area of fuselage ft2 or m2 Atop Top area of fuselage ft2 or m2 CD Drag coefficient CLmax Maximum lift coefficient CMGC Mean geometric chord length ft or m D Diameter of geometric shape ft or m d1,d2 Diameters of frustum ends ft or m D1,D2 Major and minor diameters of an ellipsis ft or m Dfus Maximum diameter of the fuselage ft or m H Height of the fuselage ft or m L Length of geometric shape ft or m l/d Fineness ratio L1,L2,L3,L4 Fuselage segment lengths ft or m Lcabin Length of the cabin ft or m Lemp Length of the empennage ft or m Lfus Total length of the fuselage ft or m Lfwd Length of the forward section ft or m lT x-distance from wing MGC/4 to HT MGC/4 ft or m Re Reynolds number SC Surface area of a cone ft2 or m2 SEC Surface area of an elliptic cylinder ft2 or m2 SF Surface area of a frustum ft2 or m2 SFUS Fuselage surface area ft2 or m2 SP Surface area of a paraboloid ft2 or m2 STAD Surface area of a tadpole fuselage ft2 or m2 SUC Surface area of a uniform cylinder ft2 or m2 V Local airspeed ft/s or m/s V∞ Freestream airspeed ft/s or m/s VC Volume of a cone ft3 or m3 VEC Volume of an elliptic cylinder ft3 or m3 VF Volume of a frustum ft3 or m3 VHT Horizontal tail volume term VP Volume of a paraboloid ft3 or m3 VPAX Volume of aircraft (passenger cabin) ft3 or m3 VTAD Volume of a tadpole fuselage ft3 or m3 VUC Volume of a uniform cylinder ft3 or m3 VVT Vertical tail volume term W Width of the fuselage ft or m

## References

1. Galvao, Leme F. A Note on Low Drag Bodies. Poland: Paper presented at the XI OSTIV Congress, in Lesyno; 1968.

2. Althaus D. NASA CR-2315, Motorless Flight Research. In: James L, ed. Nash-Webber. 1972.

3. Dodbele SS, van Dam CP, Vijgent PMHW, Holmes BJ. Shaping of Airplane Fuselages for Minimum Drag. AIAA Journal of Aircraft. May 1987;24.

4. NASA TM-77014. Wind Tunnel Investigations of Glider Fuselages with Different Waisting and Wing Arrangements. Radespiel, R 1983.

5. Thomas F. Fundamentals of Sailplane Design. College Park Press 1999.

6. Jenkinson LR, Simpkin P, Rhodes D. Civil Jet Aircraft Design. Arnold 1999.

7. Human Factors Design Guide (HFDG). DOT/FAA/CT-96/1.

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