Figure 6.14. Three-view diagram and a CAD drawing of the preliminary aircraft configuration

6.8 Undercarriage Positioning

Chapter 7 provides details of the undercarriage (i.e., landing gear) design. There is little difference between civil and military aircraft design layouts in undercarriage positioning.

Undercarriage positioning is CG-dependent. At this design stage, the CG position is not established because aircraft component weights are not known. It is now evident that an iterative process is necessary. From experience, the undercarriage may be positioned after estimating the CG position and rotational tail clearances. Ensure that the aircraft does not tip in any direction for all possible weight distributions. (Tipping occurs in some homebuilt designs – especially the canards – when the pilot steps out of the aircraft.) This book addresses only the tricycle type – that is, a forward nose wheel followed by two main wheels behind the aftmost CG. The undercarriage position shown in Figure 6.13 is approximately 60% of the MAC. Readers should use the three views.

6.9 Worked-Out Example: Finalizing the Preliminary Civil Aircraft

Configuration

It is interesting to observe how the aircraft is gradually taking shape – it is still based on a designer’s past experience but soon will be formally sized to a satisfying rational configuration to offer the best characteristics for the design.

A preliminary three-view diagram of the civil aircraft can now be drawn (Figure 6.14). It will be revised after the remaining aircraft components are positioned and a revised CG location is established. The next iteration is after aircraft sizing in Chapter 11.

At this stage, all aircraft components are ready to be assembled using the building-block concept to generate a preliminary aircraft configuration, as shown in Figure 6.14. The three variants (see Figure 6.8) maintain the same wing, empennage,

and nacelle (some internal structures are lightened or reinforced without affecting manufacturing jigs and tools).

The configuration is similar to the Learjet 45 but it is not the same; there are considerable differences in configuration, component weights, and performance. Readers may compare the two using the Jane’s All the World’s Aircraft Manual.

Chapter 11 sizes the aircraft to its final dimensions and finalizes the configuration based on the aircraft and component mass worked out in Chapter 8. Following is a summary of the worked-out civil aircraft preliminary details (from statistics):

Bare Engine (each)

Takeoff Static Thrust at ISA Sea Level: 3,800 lb (17,235 N) per engine with BPR = 5

Engine Dry Weight: 379 kg (836 lb)

Fan Diameter: 0.716 m (28.2 in)

Length: 1.547 m (60.9 in)

Short Variant (all component dimensions except the fuselage length are invariant) Fuselage: Length: 13.47 m (44.2 ft) (see Figure 6.8).

Long Variant (all component dimensions except the fuselage are invariant) Fuselage: Length: 16.37 m (53.7 ft) (see Figure 6.8).

6.10 Miscellaneous Considerations in Civil Aircraft

Following are additional considerations that could enhance aircraft performance but are not addressed here. At this design stage, none of the additional surfaces described needs to be considered except the dorsal fin. All add to aircraft weight.

1.Winglets. It took some time to establish the merits of having winglets that can reduce or induce drag – some manufacturers claim a reduction as high as 5% of induced drag (i.e., approximately 1.5% in total drag reduction), which is substantial. Currently, almost all large-aircraft designs incorporate winglets. Learjet has been using them for some time and they have become a symbol of its design.

2.Dorsal Fin. A dorsal fin ahead of the V-tail could work like strakes on a wing, and they are incorporated in many aircraft – at least to a small degree. They prevent the loss of directional stability.

3.Ventral Fin. This is sometimes installed at the tail end as an additional surface to the V-tail. The ventral fin also serves as a skidding structure to protect the fuselage from damage at excessive early rotation, which causes tail-dragging.

4.Delta Fins. These fins come in pairs at the aft end of the lower fuselage. Not all designs have delta fins; they are used if an aircraft shows poor stability and/or control problems. Aircraft with a flat, rear-loading, raised fuselage upsweep demonstrate these problems and delta fins are deployed to resolve them. A good design should avoid incorporating delta fins; however, on some designs, drag reduction can be achieved with their installation.

Several external-surface perturbations on aircraft add to parasitic drag, including antennas, inspection-hatch covers, vent pipes, and lightning dischargers. Engine and system intake and exhaust ducts and vents also increase drag.

It is suggested that readers determine whether there are any innovative requirements that should be incorporated in the conceptual design. Trends should be investigated continually for ideas to improve on aircraft design.

6.11 Configuring Military Aircraft – Shaping and Laying Out

This extended section of the book can be found on the Web at www.cambridge

.org/Kundu and gives a brief overview of today’s military aircraft shapes and their layout arrangements, as shown in the following charts and figures.

Figure 6.15. Falcon F16 fuselage cross-section and layout Figure 6.16. Flight deck (cockpit) layout – military aircraft

Figure 6.17. USAF F18 details showing internal structural layout and armament load

Chart 6.2. Phase I, conceptual study: methodology to freezing military aircraft configuration

6.12 Worked-Out Example – Configuring Military Advanced Jet Trainer

This extended section of the book can be found on the Web at www.cambridge

.org/Kundu and presents details of worked-out examples of the Advanced Jet