Subsonic Wave Drag
Wave drag is caused by compressibility effects of air as an aircraft approaches high subsonic speed because local shock (i. e., supervelocity) appears on a curved surface as aircraft speed increases. This is in a transonic-flow regime, in which a small part of the flow over the body is supersonic while the remainder is subsonic. In some cases, a shock interacting with the boundary layer can cause premature flow separation, thus increasing pressure drag. Initially, it is gradual and then shows a rapid rise as it approaches the speed of sound. The industry practice is to tolerate a twenty-count (i. e., ACd = 0.002) increase due to compressibility at a speed identified as Mcrit (Figure 9.8b). At higher speeds, higher wave-drag penalties are incurred.
A typical wave drag (CDw) graph is shown in Figure 9.8b, which can be used for coursework (civil aircraft) described in Section 9.19. Wave-drag characteristics are design-specific; each aircraft has its own CDw, which depends on wing geometry (i. e., planform shape, quarter-chord sweep, taper ratio, and aspect ratio) and aerofoil characteristics (i. e., camber and t/c). Wind-tunnel testing and CFD can predict wave drag accurately but must be verified by flight tests. The industry has a large
databank to generate semi-empirically the CDw graph during the conceptual design phase. Today, CFD can generate wave drag accurately and is an indispensable tool (see Chapter 14), replacing the empirical/semi-empirical approach. CFD analysis is beyond the scope of this book. It is suggested that practitioners use data from tests or from CFD analysis in conjunction with an empirical approach.