12.1.2 The Function of the FuselageThe 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. Mewing commuters are practically unknown in modern times e the most recent one was theHFB-340 Hans jet, 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 hard points 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. 12. THE ANATOMY OF THE FUSELAGE522Tall people must lower their heads to avoid hitting the ceiling material that covers the spar. Aircraft with low wings may have the fuselage mountedontopofthesparinasimilarmanner,although they more often feature a reinforced spar box under the cabinfloor.Inthecaseoflow-wingaircraft,thereisnever aneedforthespartopenetratetheflooreitwouldinvite unacceptable risk to passengers, to have them maneuver around an elevation in a floor when moving about the cabin. Reg ardless of the wing attachment method employed, such aircraft usually feature external wing rootfairings toprevent separationof 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 config urations 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 pri mary structure should the engine catch on fire e 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 intheaircraftmustalso beredundante forinstance 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 inle ts 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 fuselagesallow 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 theelements. This chapter is intended to discuss a numberof topics that areimportant to the design of the fuselage.12.2 FUNDAMENTALS OF FUSELAGE SHAPESThis 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 FuselageThe 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.12.2.2 The Pressure Tube FuselageThe 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,12.2 FUNDAMENTALS OF FUSELAGE SHAPES 523pressurized 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. 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.12.2.3 The Tadpole FuselageThe 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 calledthe 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 theFIGURE 12-1 A Piper PA-28 Cherokee featuring a conventional frustum style fuselage. (Photo by Phil Rademacher)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.12. THE ANATOMY OF THE FUSELAGE524effect 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 30e40% 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 allmeridians 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 dia meter, whose fineness ratio (l/d) was 10. Both shapes were representative of those used for sailplanes (see Figure 12-4). TheFIGURE 12-3 A Rolladen-Schneider LS4 sailplane boasts a tadpole-style fuselage. (Photo by Phil Rademacher)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])12.2 FUNDAMENTALS OF FUSELAGE SHAPES 525study 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 7.1106 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. 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 theoverall 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 a ir 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.A flow-adaption design of this nature should only be performed using CFD technology or wind tunnel testing.。
