Category AIRCRAF DESIGN

The payload-range capability constitutes the two most important parameters to rep­resent commercial transport aircraft. It is the basic aircraft specification and require­ment as a result of market studies for new aircraft designs.

Figure 4.4 shows the payload-range capabilities for several subsonic-transport aircraft (i. e., turbofans and turboprops). The figure captures more than fifty differ­ent types of current designs. The trend shows that the range increases with payload increases, reflecting the market demand for the ability to fly longer distances. Long – range aircraft will have fewer sorties and will need to carry more passengers at one time. The classic debate on the A380 versus the B787 passenger capacity is captured within the envelope shown between the two straight lines in Figure 4.4. It is inter­esting that there are almost no products carrying a high passenger load for shorter ranges (i. e., < 2,000 nm). At the other extreme, the high-subsonic, long-distance executive jets, the Bombardier Global Express and Gulfstream V, are already on the market (not shown in Figure 4.4) carrying executives and a small number of passengers very long ranges (> 6,500 nm) at a considerably higher cost per pas­senger. It is obvious that because of considerably lower speeds, turboprop-powered
aircraft cater to the shorter-range market sector – they provide better fuel economy than turbofans. The author considers that the future may show potential markets in the less affluent areas. Major countries with substantial population centers could fly more passengers within their borders, such as in China, India, Indonesia, Russia, and the United States.

The points in Figure 4.4 include the following aircraft: Lear 31A, Lear 45, Lear 60, Cessna 525A, Cess 650, Cess 500, Cess 550, Cess 560, Cess 560XL, ERJ 135ER, ERJ 140, ERJ 145ER, CRJ 100, CRJ 700, ERJ 170, DC-9-10, CRJ 900, ERJ 190, 737-100, 717-200, A318-100, A319-100, A320-100, Tu204, A321-100, 757-200, A310-200, 767-200, A330-200, L1011, A340-200, A300-600, A300-100, DC-10-10, MD11, 777-200, 747-100, A380, Short 330 and 360, ATR 42 and 72, Jetstream 31, Saab 340A, Dash 7 and 8, Jetstream 41, EMB 120, EMB 120ER, Dornier 328-100, and Q400.

Commercial aircraft operation is singularly dependent on revenue earned from fare-paying passengers and cargo. In the operating sector, load factor is defined as the ratio of occupied seats to available seats. Typically, for aircraft of medium sizes and larger, operational costs break even at approximately one-third full capacity (this varies among airlines; fuel costs at 2000 level with regular fares) – that is, a load factor of about 0.33. Of course, the empty seats could be filled with reduced fares, thereby contributing to the revenue earned.

It is appropriate here to introduce the definition of the dictating parameter, seat – mile cost, which represents the unit of the aircraft DOC that determines airfare to meet operational costs and sustain profits. DOC is the total cost of operation for the mission sector (operational economics are discussed in detail in Chapter 16). The U. S. dollar is the international standard for aircraft cost estimation.

seat-mile cost = DOC———————– = (cents/seat/nm) (4.1)

number of passengers x range in nm

The higher the denominator in Equation 4.1, the lower is the seat-mile cost (i. e., DOC). The seat-mile cost is the aircraft operating cost per passenger per nm of the mission sector. Therefore, the longer an aircraft flies and/or the more it carries, the lower the seat-mile cost becomes. Until the 1960s, passenger fares were fixed under government regulation. Since the 1970s, the fare structure has been deregu­lated – an airline can determine its own airfare and vary as the market demands.

A careful market study could fine-tune an already overcrowded marketplace for a mission profile that offers economic gains with better designs. Section 2.6 addresses the market study so that readers understand its importance.

An aircraft can be classified based on its role, use, mission, power plants, and so forth, as shown in Chart 4.1. Here, the first level of classification is based on oper­ational role (i. e., civil or military discussion on military aircraft is given on Web site) – and this chapter is divided into these two classes. In the second level, the clas­sification is based on the generic mission role, which also would indicate size. The third level proceeds with classification based on the type of power plant used and so on. The examples worked out in this book are the types that cover a wide range of aircraft design, which provides an adequate selection for an aircraft design course.

Figure 4.2 indicates the speed-altitude regimes for the type of power plant used. Currently, low-speed-low-altitude aircraft are small and invariably powered by pis­ton engines of no more than 500 horsepower (HP) per engine (turboprop engines start to compete with piston engines above 400 HP). World War II had the Spit­fire aircraft powered by Rolls Royce Merlin piston engines (later by Griffon piston engines) that exceeded 1,000 HP; these are nearly extinct, surviving only in museum collections. Moreover, aviation gasoline (AVGAS) for piston engines is expensive and in short supply.

The next level in speed-altitude is by turboprops operating at shorter ranges (i. e., civil aircraft application) and not critical to time due to a slower speed (i. e., propeller limitation). Turboprop fuel economy is best in the gas turbine fam­ily of engines. Subsonic cargo aircraft and military transport aircraft may be more economical to run using turboprops because the question of time is less critical, unlike passenger operations that is more time critical with regard to reaching their destinations.

The next level is turbofans operating at higher subsonic speeds. Turbofans (i. e., bypass turbojets) begin to compete with turboprops at ranges of more than 1,000 nm due to time saved as a consequence of higher flight speed. Fuel is not the only factor contributing to cost – time is also money. A combat aircraft power plant

Figure 4.2. Engine selections for speed – altitude capabilities

uses lower bypass turbofans; in earlier days, there were straight-through (i. e., no bypass) turbojets. Engines are discussed in more detail in Chapter 10.

Figure 4.3a illustrates the thrust-to-weight ratio of various types of engines. Figure 4.3b illustrates the specific fuel consumption (sfc) at sea-level static takeoff thrust (TSLS) rating in an ISA day for various classes of current engines. At cruise speed, the sfc would be higher.

Design lessons learned so far on the current trend are summarized as follows: [4]

Figure 4.4. Range versus passengers

of passenger movement. Lower acquisition costs, lower operational costs, and improved safety and environmental issues would act as design drivers. The SST would attempt an entry and HST operations still could be several decades away.

• Military aircraft design: Very agile aircraft incorporating extensive micro­processor-based control and systems management operating below Mach 2.5, high altitude (> 60,000 ft), and BVR capabilities would be the performance demand. The issue of survivability is paramount – if required, aircraft could be operated unmanned. The military version of hypersonic combat aircraft could arrive sooner, paving the way to advance civil aircraft operations. Armament- and missile-development activities would continue at a high level and would act as one of the drivers for vehicle design.

Figure 4.1 shows the history of progress in speed and altitude capabilities. The impressive growth in one century is astounding – leaving the Earth’s surface in a heavier-than-air vehicle and returning from the Moon in fewer than 66 years!

It is interesting that for air-breathing engine powered aircraft, the speed – altitude record is still held by the more than 40-year-old design, the SR71 (Black­bird; see Figure 1.11), capable of operating at around Mach 3.0 and a 100,000-ft alti­tude. Aluminum-alloy properties would allow a flight speed up to Mach 2.5. Above Mach 2.5, a change in material and/or cooling would be required because the stagna­tion temperature would approach 600° K, exceeding the strength limit of aluminum alloys. Aircraft speed-altitude capabilities have remained stagnant since the 1960s. A recent breakthrough was the success of “Spaceship One” which took aircraft to
the atmosphere edge to 100 km altitude. In civil aviation, the SST aircraft “Con­corde” was designed nearly four decades ago and has not yet been supplanted. The Concorde’s speed-altitude capability is Mach 2.2 at around 60,000 ft.

In military aircraft scenarios, gone (almost) are the days of “dogfights” that demanded a high-speed chase to bring an adversary within machine-gun firing range (i. e., low projectile speed, low impact energy, and no homing); if the target was missed, the hunter became the hunted. In the post-World War II period, around the late 1960s, air-superiority combat required fast acceleration and speed (e. g., the Lockheed F104 Starfighter) to engage with infrared homing missiles firing at a relatively short distance from the target. As missile capabilities advanced, the cur­rent combat aircraft design trend showed a decrease in speed capabilities. Instead, high turning rates and acceleration, integrated with superior missile capabilities (i. e., guided, high speed, and high impact even when detonated in proximity of the tar­get), comprise the current trend. Target acquisition beyond visual range (BVR) – using an advance warning system from a separate platform – and rapid aiming com­prise the combat rules for mission accomplishment and survivability. Current mili­tary aircraft operate below Mach 2.5; hypersonic aircraft are in the offing.

4.1 Overview

This chapter presents important information on aircraft configuration that is required in Chapter 6 coursework. The current design and configuration para­meters from aircraft production and operations serve as a template for identifying considerations that could influence new designs with improvements.

During the last century, many aircraft configurations have appeared; today, most of those are not relevant to current practice. Older designs, no matter how good they were, cannot compete with today’s designs. This book addresses only those well-established designs as shown in the recent Jane’s All the World’s Air­craft Manual; however, references are made to interesting and unique older aircraft configurations. The chapter starts by examining growth patterns in the aircraft oper­ational envelope (e. g., speed-altitude capabilities). It continues with a classification of generic aircraft types that show distinct patterns within the class in order to nar­row down the wide variety of choices available. Statistics is a powerful tool for establishing design trends, and some pertinent statistical parameters are provided herein.

This chapter compiles the available choices for aircraft-component configura­tions, including types of wing planform, fuselage shape, intake shapes and position­ing, and empennage arrangements. These are the “building blocks” for shaping an aircraft, and as many configurations as possible are described. Artistic aesthetics are considered as long as they do not unduly penalize cost and performance – everyone appreciates the attractive streamline aircraft shapes. The new Boeing 787 Dream­liner (see Figure 1.8) shape is a good example of the company’s latest subsonic com­mercial transport aircraft. It is interesting that the Dreamliner configuration transi­tioned to the new B787 with more conventional aeroshaping. The B787’s advances in technology were not as radical an aerodynamic venture compared to Boeing’s earlier Sonic Cruiser proposal (see Figure 1.7), which was shelved. These decisions were made by one of the world’s biggest and best companies; the Sonic Cruiser was not a fantasy – it simply was not timed with market demand. It signifies the impor­tance of conducting a market study, as emphasized in Chapter 2.

Civil and military aircraft design are discussed separately because of the differ­ences in their mission roles (see Table 2.2).

4.1.1 What Is to Be Learned?

This chapter covers the following topics:

Chapter introduction

Evolutionary patterns in current aircraft design trends and their

classification into distinct categories

Civil aircraft mission (domain served, role of economics)

Civil aircraft statistics (template for new design)

Civil aircraft component geometries (possible options)

Fuselage group (e. g., statistics, options)

Wing group (e. g., statistics, options)

Empennage group (e. g., statistics, options)

Nacelle group (e. g., statistics, options)

Summary of civil aircraft design choices Military aircraft detailed classification Military aircraft mission (domain served)

Military aircraft statistics (template for new design)

Military aircraft component geometries (possible options) Fuselage group (e. g., statistics, options)

Wing group (e. g., statistics, options)

Empennage group (e. g., statistics, options)

Nacelle group (e. g., statistics, options)

Undercarriage

Miscellaneous

Summary of military aircraft design choices

4.1.2 Coursework Content

The author recommends that readers browse through this chapter even though there is no coursework involved yet. This information is essential for designers, and this chapter will be better understood after reading Chapter 11 on aircraft sizing and engine matching to finalize the design. Readers will use the information provided in this chapter in Chapter 6.

4.2 Introduction

Previous designs have a strong influence on future designs – real-life experience has no substitute and is dependable. It is therefore important that past informa­tion be properly synthesized by studying statistical trends and examining all aspects of any influencing parameters in shaping a new aircraft – this is one of the goals of this book. Many types of aircraft are in production serving different sector requirements – the civil and military missions differ substantially. It is important to classify aircraft categories in order to identify strong trends existing within each class.

Existing patterns of correlation (through regression analysis) within a class of aircraft indicate what may be expected from a new design. There are no surprise elements until new research establishes a radical change in technology or designers

Year Year

(a) Speed (b) Altitude

Figure 4.1. Aircraft operational envelope

introduce a new class of aircraft (e. g., Airbus 380). In civil aircraft design, a 10 to 15% improvement in the operating economics of current designs within the class is considered good; a 20% improvement is excellent. Of course, economic improve­ments must be supported by gains in safety, reliability, and maintainability, which in turn add to the cost.

Readers are encouraged to examine the potential emerging design trends within an aircraft class. In general, new commercial aircraft designs are extensions of existing designs that conservatively incorporate newer, proven technologies (some result from declassified military applications). Currently, the dominant aerodynamic design trends show diminishing returns on investment. Structure technologies seek suitable new materials (e. g., composites, metal alloys, and smart adaptive materi­als) if they can reduce cost, weight, or provide aerodynamic gains. Engine design still needs aerodynamic improvements to save on fuel consumption and/or weight. Chapter 1 highlights that the current challenge lies in manufacturing philosophy, better maintainability, and reliability incorporating vastly improved and miniatur­ized systems (including microprocessor-based avionics for control, navigation, com­munication, and monitoring). This book briefly addresses these topics, particularly from the weight-saving perspective. It also describes conventional aerodynamic and structural considerations and available types of power plants.

Speed brakes and dive brakes have the same definition. They are mounted specifi­cally on the fuselage for military aircraft and as spoilers on the wings for civil aircraft (Figure 3.51). However, there are civil aircraft that use this type of device mounted on the fuselage.

Speed brakes are specifically designed to reduce speed rapidly, typically on approach and in military combat maneuvers.

Figure 3.51. Speed brakes and brakes

Speed and dive brakes are primarily drag-producing devices positioned in those areas that will create the smallest change in moments (i. e., kept symmetrical to the aircraft axis with the least moment arm from the CG). Figure 3.51 shows fuselage – mounted devices.

The Boeing F22 does not have a separate dive brake. It uses the two rudders of the canted V-tail deflected in opposite directions along with spoilers and flaps deflected upward and downward, respectively.

Chapter 7 is devoted entirely to a discussion of undercarriage design.

1.24 Nacelle and Intake

A nacelle is the structural housing for an aircraft engine. In civil aircraft, nacelles are invariably externally pod-mounted, either slung under or mounted over the wing or attached to the fuselage (see Figure 4.28). The front part of the nacelle is the intake and the aft end is the nozzle. Military aircraft engines are invariably buried in the fuselage; the front is called the intake in the absence of a nacelle. Chapter 10 discusses the nacelle in detail.

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. For subsonic turbofans, the intake acts as a diffuser with an acoustic lining to abate noise gen­eration. The inhaled air-mass flow demanded by an engine varies considerably: At idle, just enough is required to sustain combustion, whereas at maximum thrust, the demand is many times higher. A rigid intake must be sized such that during critical operations (i. e., takeoff, climb, and cruise), the engine does not suffer and gener­ates adequate thrust. Supersonic intakes are even more complex and are designed to minimize loss resulting from shock waves.

For a noncircular cross-section, this is the average of the fuselage height and width at the constant cross-section barrel part (Dave = (H + W)/2). Sometimes this is defined as Deffective = V(H*W); another suitable definition is Dequivalent = perimeter/2^. This book uses the first definition.

3.23.12 Cabin Height, Hcab

This is the internal cabin height from the floor, as shown in Figure 3.50.

3.23.13 Cabin Width, Wcab

This is a the internal cabin width, as shown in Figure 3.50.

3.23.14 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.

A military aircraft fuselage is very different because it does not have passen­gers to carry and is more densely packed. Various types of fuselage cross-sections are shown in Figure 4.7 (see Web site). Their associated fineness ratios and other statistical data on fuselage parameters are provided in Section 4.7.

This starts from the end of the constant cross-section barrel of the mid-fuselage up to the tip of the tail cone (Figure 3.49a). It encloses the last few rows of passenger seat­ing, rear exit door, toilet, and – for a pressurized cabin – the aft pressure bulkhead, which is an important component from a structural design perspective (La> Lf).

1.23.5 Midfuselage Constant Cross-Section Length, Lm

This is the constant cross-section midbarrel of the fuselage, where passenger seat­ing and other facilities are accommodated (including windows and emergency exit doors, if required).

1.23.6 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 3.50).

1.23.7 Fuselage Width, W

This is the widest part of the fuselage in the horizontal plane. For a circular cross­section, it is the diameter shown in Figure 3.50.

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.

1.23.4 Front Fuselage Closure Length, Lf

This is the length of the front fuselage from the tip of the nose cone to the start of the constant cross-section barrel of the mid-fuselage (Figure 3.49a). It encloses the pilot cockpit/flight deck and the windscreen – most of which is associated with a kink in the mould lines to allow for a better vision polar (see Section 4.7.4) and to enable the use of flat windscreens to reduce cost. In general, it includes the front door and passenger amenities, and may have a row or two of passenger seating.

Figure 3.50. Fuselage cross-section geo­metrical parameters

This is along the fuselage axis, measuring the length of the fuselage from the tip of the nose cone to the tip of the tail cone (which is unlikely to be on the axis). This is not the same as the aircraft length, L, shown in Figure 3.49a.

1.23.2 Fineness Ratio, FR

This is the ratio of fuselage length to average diameter, FR = L/Dave. A good value for commercial transport aircraft design is from 8 to 10.

1.23.3 Fuselage Upsweep Angle

In general, the fuselage aft end incorporates an upsweep (Figure 3.49b) 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 to 16 deg; however, the slope of the bottom mould line 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.