Vehicle Aerodynamics Terminology

This document has been revised to correct numerous errors and omissions in the previous (1994) revision. That revision, whose sole purpose was to place it into the new SAE Technical Standards Board format, was the only revision to the original (1987) issue. The current (2010) revision has also been used as an opportunity to update applicable references, delete those that are no longer readily available, improve the organization of the document, and modify the directional sense of the axes system as indicated below. The following is the rationale for selection of specific terminologies, conventions, and definitions. Axes System – The SAE Road Vehicle Aerodynamics Committee agreed to modify the axes system in the original SAE J1594 issued in 1987, to have x positive rearward and z positive upward, to correspond with the positive directions of drag and lift, respectively. This change does not affect the positive sense of the aerodynamic forces and moments as defined in the previous version of SAE J1594, only their directional sense (specifically for drag, lift, yawing moment, and rolling moment) relative to the signs of the x and z axes in the new axes system. Resolving Center – Center of gravity (c.g.) and body geometry-defined resolving centers used in vehicle dynamics (Reference 2.1.1.1) and aeronautics, respectively, are not satisfactory for road vehicle aerodynamics applications. A large portion of automotive aerodynamics development testing is performed before the vehicle c.g. is known. The c.g. location can also vary significantly with vehicle option content and loading. Relating the axis center to the body geometry is also problematic when major body geometry changes are explored during wind tunnel tests. These situations are avoided by placing the resolving center at ground level, positioned at mid-wheelbase and mid-track. An added advantage of this location is the direct translation of aerodynamic loading to tire contact patch ground reactions. Forces and Moments – The primary terminology for aerodynamic force and moment components (drag, lift, side force, pitching moment, yawing moment, and rolling moment) were adopted from aeronautical usage. The symbols for drag and lift (D & L) were also taken from aeronautics. To maintain consistency with the symbols for drag and lift, and to provide a mnemonic aid, the other component symbols (S,PM,YM and RM) were based on terminology. Attitude Angles – Vehicle attitude angle definitions and symbols also correspond to existing aerodynamics terminology as used for aircraft development. Force and Moment Coefficients – Aerodynamic coefficient definitions were chosen consistent with aeronautical terminology, with one exception. Unlike typical aerodynamics convention, the wheelbase is used to compute moment coefficients. Although it makes more aerodynamic sense to use a body length dimension, this is more likely to change during wind tunnel development than wheelbase. Using wheelbase (WB) provides an additional advantage with the chosen axes system in simplifying the computation of axle loadings. For example, the lift coefficient for the front axle is then equal to CLF = CL/2 + CPM. However, if CPM were based on an overall length (OAL), a ratio of WB and OAL would have to be included in the computation. Vehicle Parameters – The wheelbase designator (L) used in vehicle dynamics (Reference 2.1.1.1) was not adopted, since it is used for the aerodynamic lift force. Frontal area and scale factor symbols are consistent with aerodynamic usage. Flow Parameters – Symbols and definitions for air flow parameters were chosen consistent with aerodynamics terminology. The definition of equivalent full scale velocity (VEQ) is included to provide a simple means of relating reduced scale model flow conditions to full scale. Standard day conditions were chosen to correspond to those defined at sea level conditions for the U.S. Standard Atmosphere adopted by NASA, NOAA and USAF in 1976 (Reference 2.1.3.1). For high-speed (motorsports) and high-humidity (thermal) applications, references are cited to account for the effects of compressibility on dynamic pressure and relative humidity on air density, if deemed necessary. Yaw-Weighted Drag Coefficient – Ambient wind magnitude, heading angle and vehicle path directions have an effect on the overall average aerodynamic drag of a vehicle during a particular duty cycle. The yaw-weighted drag coefficient is defined as the average drag coefficient during a particular driving schedule and ambient wind input. The wind and driving schedule factors affecting the wind-averaged drag coefficient have not been standardized. Some examples of yaw-weighted drag coefficient computations are given in References 2.1.1.2 - 2.1.1.4.