3.1.2 Coursework Content

The information in this chapter is essential for designers. Coursework is postponed until Chapter 6 (except for the mock market survey in Chapter 2). Readers should return to Chapters 2 through 5 to extract information necessary to configure the aircraft in Chapter 6.

3.2 Introduction

Aircraft conceptual design starts with shaping an aircraft, finalizing geometric details through aerodynamic considerations in a multidisciplinary manner (see Section 2.3) to arrive at the technology level to be adopted. In the early days, aerodynamic considerations dictated aircraft design; gradually, other branches of science and engineering gained equal importance.

All fluids have some form of viscosity (see Section 3.5). Air has a relatively low viscosity, but it is sufficiently high to account for its effects. Mathematical modeling of viscosity is considerably more difficult than if the flow is idealized to have no viscosity (i.e., inviscid); then, simplification can obtain rapid results for important information. For scientific and technological convenience, all matter can be classified as shown in Chart 3.1.

This book is concerned with air (gas) flow. Air is compressible and its effect is realized when it is flowing. Aircraft design requires an understanding of both incompressible and compressible fluids. Nature is conservative (other than nuclear physics) in which mass, momentum, and energy are conserved.

Aerodynamic forces of lift and drag (see Section 3.9) are the resultant components of the pressure field around an aircraft. Aircraft designers seek to obtain the maximum possible lift-to-drag ratio (i.e., a measure of minimum fuel burn) for an efficient design (this simple statement is complex enough to configure, as will be

Chart 3.1. All matter

observed throughout the coursework). Aircraft stability and control are the result of harnessing these aerodynamic forces. Aircraft control is applied through the use of aerodynamic forces modulated by the control surfaces (e.g., elevator, rudder, and aileron). In fact, the sizing of all aerodynamic surfaces should lead to meeting the requirements for the full flight envelope without sacrificing safety.

To continue with sustained flight, an aircraft requires a lifting surface in the form of a plane – hence, aeroplane (the term aircraft is used synonymously in this book). The secret of lift generation is in the sectional characteristics (i.e., aerofoil) of the lifting surface that serve as wings, similar to birds. This chapter explains how the differential pressure between the upper and lower surfaces of the wing is the lift that sustains the aircraft weight. Details of aerofoil characteristics and the role of empennage that comprises the lifting surfaces are explained as well. The stability and control of an aircraft are aerodynamic-dependent and discussed in Chapter 12.

Minimizing the drag of an aircraft is one of the main obligations of aerodynamicists. Viscosity contributes to approximately two-thirds of the total subsonic aircraft drag. The effect of viscosity is apparent in the wake of an aircraft as disturbed airflow behind the body. Its thickness and intensity are indications of the extent of drag and can be measured. One way to reduce aircraft drag is to shape the body such that it will result in a thinner wake. The general approach is to make the body in a teardrop shape with the aft end closing gradually, as compared to the blunter front-end shape for subsonic flow. (Behavior in a supersonic flow is different but it is still preferable for the aft end to close gradually.) The smooth contouring of teardrop shaping is called streamlining, which follows the natural airflow lines around the aircraft body – it is for this reason that aircraft have attractive smooth contour lines. Streamlining is synonymous with speed and its aerodynamic influence in shaping is revealed in any object in a relative moving airflow (e.g., boats and automobiles).

New aircraft designers need to know about the interacting media – that is, the air (i.e., atmosphere). The following sections address atmosphere and the behavior of air interacting with a moving body.

Figure 3.1. Atmosphere (see Appendix B for accurate values)

3.3 Atmosphere

Knowledge of the atmosphere is an integral part of design – the design of an aircraft is a result of interaction with the surrounding air. The atmosphere, in the classical definition up to 40-kilometer (km) altitude, is dense (continuum): Its homogeneous constituent gases are nitrogen (78%), oxygen (21%), and others (1%). After substantial data generation, a consensus was reached to obtain the ISA [1], which is in static condition and follows hydrostatic relations. Appendix B includes an ISA table with up to 20-km altitudes, which is sufficient for this book because all aircraft (except rocket-powered special-purpose aircraft – e.g., space plane) described would be flying below 20 km. Linear interpolation of properties may be carried out between low altitudes. At sea level, the standard condition gives the following properties:

pressure = 101,325 N/m2 (14.7 lb/in2) temperature = 288.16◦K (518.69◦R)

viscosity = 1.789 × 10−5 m/s (5.872 × 10−5 ft/s) acceleration due to gravity = 9.81 m/s2 (32.2 ft/s2)

In reality, an ISA day is difficult to find; nevertheless, it is used to standardize aircraft performance to a reference condition for assessment and comparison. With altitude gain, the pressure decreases, which can be expressed through the use of hydrostatic equations. However, temperature behaves strangely: It decreases linearly up to 11 km at a lapse rate of 6.5◦K/km, then holds constant at 216.66◦K until it reaches 20 km, at which it starts increasing linearly at a rate of 4.7◦K/km up to