Aircraft Familiarity, Aircraft Design Process, Market Study
2.3 Aircraft Familiarisation
This section introduces generic civil and military aircraft. Geometric definitions relevant to aerodynamic considerations are addressed in Chapter 3 and detailed descriptions of various types of aircraft and their classification are provided in Chapter 4. A diagram of aircraft with major subassemblies as components is provided herein. Indeed, aircraft design has become highly modular in the interests of the ‘family’ concept, which facilitates low development cost by maintaining a high degree of parts commonality.
Aircraft span, length, and height are currently restricted by the International Civil Aviation Organisation (ICAO) to 80 m, 80 m and 80 ft, respectively, for ground handling and storage considerations. The height is in feet but the span and length are in metres; this restriction may change. Section 1.6 highlights the mix of SI and foot–pound–second (FPS) units in aerospace engineering.
2.3.1 Civil Aircraft and Its Component Configurations
In general, the civil aircraft category includes five types: (i) small club trainers, (ii) utility aircraft, (iii) business aircraft, (iv) single aisle narrow-body commercial transporters (regional aircraft to midsize) and (v) dou- ble aisle wide-body large transporters. The various types of available configuration options are described in Chapters 4–6.
2.3.1.1 Subsonic Jet Aircraft
The typical subsonic commercial jet aircraft structural components subsections are shown in Figure 2.1 (Lockheed L1011). These consist of typically, wing, fuselage, nacelle and empennage; others (e.g. winglets, strakes and auxiliary control surfaces) are less obvious but play vital roles – otherwise, they would not be included. Because there are many options, components are associated in groups for convenience, as described in the following subsections.
Figure 2.1 Lockheed 1011 blowout diagram. Source: Courtesy of Michael Niu – [24].
k k
Aircraft Familiarity, Aircraft Design Process, Market Study 49
1. Fuselage group. This group includes the nose cone, the constant midsection fuselage, the tapered aft fuse- lage and the tail cone. The fuselage belly fairing (shown in Figure 2.1 as several subassembly components below the fuselage) may be used to house equipment at the wing-fuselage junction, such as the undercar- riage wheels.
2. Wing group. This group consists of the main wing, high-lift devices, spoilers, control surfaces and tip devices. The example of Lockheed L1011 has low wing configuration with the structural wing box passing through the fuselage belly. High-lift devices include leading-edge slats or trailing edge flaps. The leading-edge slats are shown attached to the main wing and the trailing edge flaps and spoilers are shown detached from the port wing. Spoilers are used to decelerate aircraft on descent; as the name suggests, they ‘spoil’ lift over the wing and are useful as ‘lift dumpers’ on touchdown. This allows the undercarriage to more rapidly absorb the aircraft’s weight, enabling a more effective application of the brakes. In some aircraft, a small differential deflection of spoilers with or without the use of ailerons is used to stabilise an aircraft’s rolling tendencies during disturbances. Winglets (not in the figure) are one of a set of tip treatments that can reduce the induced drag of an aircraft.
3. Empennage group. The empennage is the set of stability and control surfaces at the back of an aircraft.
The Lockheed example, as shown in Figure 2.1, has vertical tail (known as theV-tail) split into a fin in the front and a rudder at trailing edge, with an end cap on the top. The horizontal tail (known as theH-tail) is attached to the fuselage as low tail configuration consisting of the fixed stabiliser and the movable elevator at the trailing edge.
4. Nacelle group. Podded nacelles are slung under the wings and one is mounted on the aft fuselage; pylons affect the attachment. Engines can be mounted on each side of the fuselage. The nacelle design is discussed in detail in Chapter 12. Turbofans are preferred for higher subsonic speed.
5. Undercarriage group. The undercarriage, or landing gear, usually consists of a nose-wheel assembly and two sets of main wheels that form a tricycle configuration. Tail-dragging, bicycle and even quad configurations are possible, depending on the application of an aircraft. Wheels are usually retracted in flight, and the retraction mechanism and stowage bay comprise part of the undercarriage group. Undercarriage design is discussed in Chapter 9.
Not shown in Figure 2.1 are the trimming surfaces used to reduce control forces experienced by the pilot.
During the conceptual phase, these surfaces generally are shown schematically, with size based on past expe- rience. The sizing of trim surfaces is more appropriate once the aircraft configuration is frozen (i.e. a Phase II activity). Trim-surface sizing is accomplished by using semi-empirical relations/computational fluid dynamics (CFD) analyses and is fine-tuned by tailoring the surfaces and areas or adjusting the mechanism during flight trials. In this book, trim surfaces are treated schematically – the main task is to size the aircraft and finalise the configuration in Phase I. On larger aircraft, powered controls are used; pitch trimmings in conjunction with moving tail planes.
2.3.2 Turboprop Aircraft
A cutaway diagram of a propeller-driven Bombardier Dash 8-300 turboprop aircraft is shown in Figure 2.2.
Propeller-driven aircraft speeds are below Mach 0.5. Some modern aircraft with advanced propeller design have the capability to cruise above Mach 0.6. Turboprop structural components subsections groups are similar to subsonic jet aircraft shown in Figure 2.1 and are associated in groups for convenience, as described in the following subsections.
1. Fuselage group. These have a near circular cross-section fuselage. This group includes the nose cone, the constant midsection fuselage, the tapered aft fuselage and the tail cone. It is also assembled from compo- nents in the similar manner to that shown in Figure 2.1.
2. Wing group. Figure 2.2 shows the high wing configuration with the structural wing box passing over fuselage and the external surface is faired to make it streamlined. This group consists of the main wing, high-lift devices, spoilers, control surfaces and tip devices.
k k Figure 2.2 ATR 72 turboprop aircraft cutaway diagram.
3. Empennage group. Figure 2.2 shows the empennage has the horizontal tail as a T-tail (see Section 6.4) set at the top of the V-tail, and consists of the stabiliser and the elevator.
4. Nacelle group. Podded nacelles are slung under the wings without a pylon. The nacelle houses the under- carriage.
5. Undercarriage group. These have a long undercarriage retracted into the turboprop nacelle pod. Undercar- riage design is discussed in Chapter 9.
2.3.3 Military Aircraft and Its Component Configurations
Table 1.3 in Section 1.9.1 compared the difference between civil and military aircraft design and operation. On account of its mission requirements military configurations are more diverse than civil designs. Figures 2.3 depicts a blowout diagram of the General Dynamics (now Boeing) F16 showing the internal structural layout.
Due to design differences on account of the mission role (Table 1.3), the combat aircraft structural subcom- ponent groups differ from the civil aircraft structural subgroups as described earlier. This is mainly evident in fuselage structure, which invariably houses the power plant (except large bombers and transport aircraft), unlike commercial civil aircraft designs. Combat aircraft structural components subsections are grouped as follows. The densely packed fuselage design typically has three subgroups as follows.
1. Front-fuselage sub-group(Military). The front fuselage starts with the nose cone, which has to be pointed for supersonic capability, and then houses a radar that could be of around 1 m diameter. The nose cone can be swung open to access for radar maintenance. The cone section is attached to flight deck module to house the flight crew, instruments, system black boxes and so on, all requiring relatively more frequent inspection and maintenance. The nose undercarriage is attached to this section. It may or may not have the intake, depending on the design.
The military aircraft pilot seat has more freedom to recline so as to shorten carotid artery height to reduce blood starvation to the brain at highgmanoeuvres that causes pilot blackouts.
2. Mid-fuselage group (Military). This is a complicated structural subgroup that bears the wing loads, air-brake loads and the main undercarriage loads. Wing are attached to this section and it houses the
k k
Aircraft Familiarity, Aircraft Design Process, Market Study 51
AFT fuselage module
Common integrated servo actuators Vertical stabilizer Horizontal stabilizer
Flaperons Wings
Center fuselage module
Canopy
Inlet module Nose landing gear
Main landing gear FWD fuselage module
Figure 2.3 Schematic blown-out diagram of Boeing F16 showing the internal structural layout. Source: Courtesy of Michael Niu – [24].
main undercarriage as well as having a hollow intake duct for the engines. Practically all the cable, fuel and oil pipes, pneumatic ducts and linkages go through this section. Narrowing of fuselage section for area-ruling in some designs is done. This makes military aircraft mid-fuselage design consider- ably more complex compared to hollow constant cross-section transport aircraft mid-fuselage design considerations.
The fuselage belly fairing would house accessories; in most cases, the undercarriage. Current tendencies for the wing-body fairing with considerable blending for superior aerodynamic considerations, for example, to improve lift to drag ratio and fly at higher angle of attack. In a blended fuselage it is hard to isolate the fuselage, possibly a convenient choice would be where the wing root is attached.
3. Aft-fuselage sub-group. The fighter aircraft fuselage would invariably house the power. Fuselage aft ends up as engine exhaust system and therefore will not have closure as in civil design. In a case where the engine dangles below the fuselage spine (F4 Phantom), then a pointed aft end closure follows. Power plants require periodic maintenance for which the aft fuselage needs to be spilt or the mid-fuselage to be opened to access the engine, especially if it has to be taken out.
4. Lifting surfaces group(Military). In military aircraft design wing, stability and control surfaces group are dealt with together as some designs have both empennage and canard all working together for extreme manoeuvres during combat, unlike civil aircraft designsWing. The evolution of fighter aircraft shows the dominant delta or short trapezoidal wing planform. This is for the obvious reasons of having high leading-edge sweep; a low aspect ratio to negotiate highg manoeuvres would generate a high wing root bending moment. It could restrict span growth but encourage a large wing root chord of delta or trapezoid shape with strake planform as in the cases of the F16, F18 and so on. For control reasons, it could have additional surfaces. The following are the configuration choices (strakes are taken as part of the wing).
Section 4.17 describes in detail with illustrations the following types of configurations.
k k i) One-surface configuration. Pure delta planform or its variation – the trailing edge of the delta like wing
can be made to work like H-tail as an integral part of the wing (Mirage 2000).
ii) Two-surface configuration. Two-surface configuration has two possibilities – tail in back or tail in front. Delta like wing or trapezoidal wing with conventional H-tail for pitch control (MIG 21). In some designs the H-tail is replaced by a canard surface for pitch control in relaxed stability (has desta- bilising effect). Two-surface configuration with strake is shown in. Variants are double delta (SAAB Viggen).
iii) Three-surface configuration. The ultimate kind is of three-surface configuration. It has wing, H-tail in aft end and canard in the front end (Sukhoi Su37).
5. Empennage group. Combat aircraft empennage shaping and sizing is a complex procedure (Section 6.15.6), primarily on account of short tail arm and the need to fly in relaxed stability to execute fast and hard manoeuvres. The B2 apparently appears to be without a tail. The F22 has a large canted V-tail. Options for control surface configuration are be shown along with wing options. The delta wing has an H-tail integrated with it. This book will adhere to the conventional configuration of H-tail and V-tail for the trainer aircraft example. Modern designs deploy tailerons (a stabilator – see Section 16.9.1) to initiate pitch and roll control by the H-tail.
6. Intake group. Instead of pod mounted engines, military aircraft have engines embedded into the fuselage with integrated intake. Military aircraft intake design is a complex procedure as the power plant is kept within the fuselage unlike the simpler pod mounted configuration of civil aircraft. Therefore, instead of having a nacelle, air-breathing intakes become an integral part of the fuselage.
These six groups of aircraft components offer the preliminary shape of candidate combat aircraft config- urations. Eventually, after the wing sizing and engine matching exercise, the choice for configuration has to be narrowed down to one that would offer the best choice for the mission. The family derivatives of military aircraft are quite different, again depending on the mission role, for example, use of additional crew, trainer version, carrier borne version, longer range version, improved variant version and so on. Undercarriage infor- mation is dealt with separately in Chapter 9.
Military configuration study would also require some iterations to position the empennage and under- carriage with respect to the wing, as initially the centre of gravity (CG) position is not known. Weights are estimated from a provisional positioning and then the positions fine-tuned through iterations when the CG is known. In a classroom exercise, one iteration suffices.
The role of the canard in military applications is quite different from that of the role in civil aircraft designs.
It has been found that strakes can also provide additional vortex lift and fast responses to pitch control with a conventional tail. The choice for strake or canard is still not properly researched in the public domain. It is interesting to note that US designs have strakes while the European ones have a canard. A detailed study of aircraft control laws and fly-by-wire (FBW) system architecture is required to make the choice. Up until the 1990s, flaws in FBW software caused several serious accidents.
Wing attachment to fuselage varies from case to case. The leading edge can have slats and the trailing edge would invariably have flaps. Centrally mounted large air brakes to decelerate have practically eliminated the role of spoilers. Landing in a shorter airfield may require deployment of a brake parachute.
Since military aircraft are expected to encounter transonic flight, aircraft cross-sectional area distribution becomes an important consideration. A seamless smooth distribution of cross-section (area-rule) is explained in Section 3.13.
Again, it is emphasised that this book is introductory in nature. Due to not having enough information on modern fighter design considerations, the author restricts military aircraft design exercises to a trainer class of aircraft. In this book, a military trainer in the class of Royal Air Force (RAF) Hawk is dealt with. An example of an Advanced Jet Trainer (AJT) with a Close Air Support (CAS) variant is described and worked on as a military trainer aircraft design, greatly simplifying the objective for military aircraft design. The read- ers will find that there is a lot to learn from this class of aircraft to have a feel for military aircraft design considerations.
k k
Aircraft Familiarity, Aircraft Design Process, Market Study 53