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.

Leave a reply

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>