Aerodynamic Drag Loss Chargeability and Its Implications in the Vehicle Design Process

The purpose of this paper is to explore the nature of aerodynamic drag losses, relate them to aerothermodynamic losses in the aircraft as a whole, and develop methods for assessing drag chargeability. The concept of the aerodynamic “loss deck” is introduced and applied to the analysis of an F-5E fighter. It is shown that this loss deck can be used to integrate aero losses through the vehicle mission to obtain total losses attributable to each drag mechanism. This information is then used to facilitate design trades and estimate sensitivity factors useful in the design process. Introduction Aerodynamic configuration is one of the most important aspects of aircraft design. Ample proof of this fact is afforded by any of the hundreds of designs that failed because their creators ignored or were ignorant of good aerodynamic design practice. The reason for the importance of aerodynamics is simple: aerodynamic drag and engine inefficiency are the two primary sources of loss for an aircraft. As a result, they are strong drivers on fuel load required to complete a mission and by extension, strong drivers on overall vehicle size and cost. The definition of aerodynamic loss for most vehicle designs is relatively clear-cut, though some discussion is offered on this point. The primary contribution of this paper is the development and application of the concept of an aerodynamic “loss deck” that gives detailed knowledge of aerodynamic loss and facilitates its use in the larger context of vehicle design trades. The development of this loss deck is tailored such that aerodynamic losses can be partitioned for maximum usefulness when applied in a loss management model. However, it will be shown that the aerodynamic loss deck has value in and of itself when used as an analysis tool to understand the sources of aerodynamic loss. Since the best way to convey knowledge is often by example, this paper also includes an application of these methods to the F-5E fighter aircraft. This will start with a discussion on the aerodynamic analysis methods and tools used to model the F-5E and a validation of their accuracy. This will culminate in the development of an aerodynamic “loss deck” for the F-5E. * Research Engineer, Aerospace Systems Design Laboratory (ASDL), School of Aerospace, Georgia Tech. Member, AIAA. ©2001 by B.A. Roth. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Aerodynamic Loss Estimation As previously mentioned, the definition of aerodynamic loss is regarded by most to be well-defined and subject to little debate. It is generally thought of as the conversion of vehicle kinetic energy into useless forms of energy such as frictional heating. For the sake of precision, aerodynamic loss is taken in this discussion to mean the reduction in total vehicle work potential due to irreversible fluid-dynamic interactions between the vehicle and the atmosphere. Aerodynamic loss is typically due to the action of fluid dynamic drag upon the aircraft, which is the mechanism by which kinetic energy of the vehicle moving through the atmosphere is deposited into the atmosphere in the form of a momentum deficit, and ultimately, as heat. There are several physical transport mechanisms that are responsible for this movement of energy, but they all manifest themselves in the same manner: drag. Since drag is a force that acts on the vehicle as it moves through the atmosphere, drag work follows directly from the textbook definition of work as force acting through a distance: ( ) ( ) time * Velocity Flight * Drag Work Drag = . (1) In most situations, drag work is nearly equal to the reduction in vehicle kinetic energy (or propulsive work required to offset drag), so Eq. 1 is usually taken to be synonymous to aerodynamic loss. By this definition, calculation of aerodynamic loss is merely a matter of calculating total drag and multiplying by flight velocity and time. This is an entirely accurate and acceptable definition for aerodynamic loss in most situations, and will be used later in this paper for estimation of aerodynamic loss for the F-5E validation case. However, before proceeding into further descriptions of aerodynamic loss analysis methods, it is important to pause for a moment to point out that Eq. 1 is not the only model for aerodynamic loss available for use. In fact, for high Mach flight conditions, the definition of aerodynamic loss given in Eq. 1 yields results that are incorrect. This statement may at first seem to be highly dubious, but its merit can be understood through a simple thought experiment. To start, consider the simplest possible case of skin friction drag on a flat plate at zero angle of attack, as for instance, a wall in the test section of a wind tunnel. † Exclusive of those occurring inside the engine inlet capture stream tube. AIAA2001-5236