UAV design process demonstrates that a higher level of integrated vehicle can be attained; identifying the requirements/functional/physical interfaces and the complimentary technical interactions which are promoted by this design process. The details of conceptual design phase, preliminary design phase, and detail design phase were introduced in Chapter 1. In this chapter, design activities in several underlying disciplines are provided.
2.2 AERODYNAMIC DESIGN
The primary aerodynamic function of the UAV components (e.g., wing) is to generate sufficient lift force or simply lift (L). However, they have two other productions, namely drag force or drag (D) and nose-down pitching moment (M). While a UAV designer is looking to maximize the lift, the other two (drag and pitching moment) must be minimized. In fact, a wing is considered as a lifting surface that lift is produced due to the pressure difference between lower and upper surfaces. Aerodynamics textbooks are a good source to consult for information about mathematical techniques for calculating the pressure distribution over the wing and for determining the flow variables.
During the aerodynamic design process, several parameters must be determined. For instance, for a wing, they are as follows: (1) wing reference (or planform) area, (2) number of the wings, (3) vertical position relative to the fuselage (high, mid, or low wing), (4) horizontal position relative to the fuselage, (5) cross-section (or airfoil), (6) aspect ratio (AR), (7) taper ratio (λ), (8) tip chord (Ct), (9) root chord (Cr), (10) mean Aerodynamic Chord (MAC or C), (11) span (b), (12) twist angle (or washout) (αt), (13) sweep angle (Λ), (14) dihedral angle (Γ), (15) incidence (iw) (or setting angle, αset), (16) high-lift devices such as flap, (17) aileron, and (18) other wing accessories.
One of the necessary tools in the wing design process is an aerodynamic technique to calculate wing lift, wing drag, and wing pitching moment. With the progress of the science of aerodynamics, there are variety of techniques and tools to accomplish this time consuming job. A variety of tools and software based on aerodynamics and numerical methods have been developed in the past decades. The CFD1 Software based on the solution of Navier-Stokes equations, vortex lattice method, thin airfoil theory, and circulation are available in the market. The application of such software packages, which is expensive and time-consuming, at this early stage of wing design seems unnecessary.
Wing is a three-dimensional component, while the airfoil is a two-dimensional section. Because of the airfoil section, two other outputs of the airfoil, and consequently the wing, are drag and pitching moment. The wing may have a constant or a non-constant cross-section across the wing. There are two ways to determine the wing airfoil section, the airfoil design and the airfoil selection. The design of the airfoil is a complex and time consuming process and needs expertise in fundamentals of aerodynamics at graduate level. Since the airfoil needs to be verified by testing it in a wind tunnel, it is expensive too.
Selecting an airfoil is a part of the overall wing design. Selection of an airfoil for a wing begins with the clear statement of the flight requirements. For instance, a subsonic flight design requirements are very much different from a supersonic flight design objectives. On the other hand, flight in the transonic region requires a special airfoil that meets Mach divergence requirements. The designer must also consider other requirements such as airworthiness, structural, manufacturability, and cost requirements. In general, the following are the criteria to select an airfoil for a wing with a collection of design requirements:
1. the airfoil with the highest maximum lift coefficient
2. the airfoil with the proper ideal or design lift coefficient
3. the airfoil with the lowest minimum drag coefficient
4. the airfoil with the highest lift-to-drag ratio
5. the airfoil with the highest lift curve slope
6. the airfoil with the lowest (closest to zero; negative or positive) pitching moment coefficient (Cm);
7. the proper stall quality in the stall region (the variation must be gentle, not sharp);
8. the airfoil must be structurally reinforceable. The airfoil should not that much thin that spars cannot be placed inside;
9. the airfoil must be such that the cross section is manufacturable;
10. the cost requirements must be considered; and
11. other design requirements must be considered. For instance, if the fuel tank has been designated to be places inside the wing inboard section, the airfoil must allow the sufficient space for this purpose.
12. If more than one airfoil is considered for a wing, the integration of two airfoils in one wing must be observed.
In designing the high lift device for a wing, the following parameters must be determined: (1) high lift device location along the span; (2) the type of high lift device; (3) high lift device chord (Cf); (4) high lift device span (bf); and (5) high lift device maximum deflection (down) (δfmax). For fundamentals of aerodynamics, please refer to references such as Anderson [44] and Shevell [45].
2.3 STRUCTURAL DESIGN
The structure of a conventional fixed-wing UAV consists of five principal units: fuselage, wings, horizontal tail, vertical tail, and control surfaces. The landing gear is also part of structure, but will be covered in Section 2.5. Engine pylon, engine inlet (for supersonic UAVs), fairings (and fillets), and landing gear bay doors are also assumed as part of aircraft structure. The primary functions of the structure is (1) to keep the aerodynamic shape of the UAV and (2) to carry the loads. Airframe structural components are constructed from a wide variety of materials. The earliest aircraft were constructed primarily of wood. Steel tubing and the most common material, aluminum, followed. Many newly certified aircraft are built from molded composite materials, such as glass/epoxy and carbon fiber.
Structural members of a fuselage mainly include stringers, longerons, bulkheads, and skin. The structural members in a wing/tail are spar, rib, stiffeners, and skin. The fuselage/wing/tails skin can be made from a variety of materials, ranging from impregnated fabric to plywood, aluminum, or composites. Under the skin and attached to the structural components are the many components that support airframe function. The entire airframe and its components are joined by rivets, bolts, screws, and other fasteners. Welding, adhesives, and special bonding techniques are also employed.
The most common form of UAV structure is semi-monologue (single shell) which implies that the skin is stressed/reinforced. The structural members are designed to carry the flight loads or to handle stress without failure. In designing the structure, every square inch of wing and fuselage, must be considered in relation to the physical characteristics of the material of which it is made. Every part of the structure be planned to carry the load which is applied on it.
The structural designer will determine flight loads, calculate stresses, and design structural elements such as to allow the UAV components to perform their aerodynamic functions efficiently. This goal will be considered simultaneously with the objective of the lowest structural weight. The most common tool in structural analysis is the use of finite element methods (FEM). One
