ESDU 81026 describes methods used to represent airframe drag for performance calculations for all flight conditions in terms of the general functional relationships between the variables. Several analytical variants of the familiar parabolic relationship between lift and drag are introduced, their limitations pointed out and some useful derived expressions presented and plotted. The Appendix gives a summary of the aircraft and component drag data available elsewhere in the ESDU Aerodynamics, Performance, and Transonic Aerodynamics Series. Examples of the drag characteristics of particular aircraft are also given in the Addenda to illustrate the methods of representation discussed. Each Addendum gives the relationship between lift and drag coefficients for a particular aircraft and in most cases curves are included to show the variation of drag with speed or Mach number at a typical weight and altitude. Brief descriptions of the aeroplane, particular features of the data and/or additional notation are also given.