# Aircraft Drag Estimation Methodology (Subsonic)

The semi-empirical formulation of aircraft drag estimation used in this book is a credible method based on [1], [3], and [7]. It follows the findings from NACA/NASA, RAE, and other research-establishment documents. This chapter provides an outline of the method used. It is clear from Equation 9.2 that the fol­lowing four components of aircraft drag are to be estimated:

1. Minimum parasite drag, CDpmi„ (see Section 9.7).

Parasite drag is composed of skin friction and pressure differences due to viscous effects that are dependent on the Re. To estimate the minimum parasite drag, CDpmin, the first task is to establish geometric parameters such as the characteris­tic lengths and wetted areas and the Res of the discrete aircraft components.

2. Incremental parasite drag, ACDp (see Section 9.10).

ACDp is characteristic of a particular aircraft design and includes the lift-dependent parasite drag variation, 3D effects, interference effects, and other spurious effects not easily accounted for. There is no theory to estimate ACDp; it is best obtained from wind-tunnel tests or the ACDp of similarly designed aircraft wings and bodies. CFD results are helpful in generating the ACDp-versus-CL variation.

3. Induced drag, Cm (see Section 3.12).

The pure induced drag, Cm, is computed from the expression Cm = CL2/пAR.

4. Wave drag, CDw (see Section 9.11).

The last component of subsonic aircraft drag is the wave drag, CDw, which accounts for compressibility effects. It depends on the thickness parameter of the body: for lifting surfaces, it is the t/c ratio, and for bodies, it is the diameter-to-length ratio. CFD can predict wave drag accurately but must be substantiated using wind-tunnel test data. Transport aircraft are designed so that HSC at Mcrit (e. g., for the Airbus 320 type, « 0.82 Mach) allows a twenty-count (ACDw = 0.002) drag increase. At LRC, wave-drag formation is kept at zero. Compressibility drag at supersonic speed is caused by shock waves.