An aircraft body is not smooth; located all over the body are probes, blisters, bumps, protrusions, surface-protection mats for steps, small ducts (e. g., for cooling), and exhausts (e. g., environmental control and cooling air) – these are unavoidable features. In addition, there are mismatches at subassembly joints – for example, steps, gaps, and waviness originating during manufacture and treated as discreet roughness. Pressurization also causes the fuselage-skin waviness (i. e., areas ballooning up).
In this book, excrescence drag is addressed separately as two types:  2
Excrescence drag due to surface-roughness drag is accounted for by using 2 to 3% of component parasite drag as roughness drag ( and ). As indicated in discussion after Equation 9.13, it is factored using 3% after computing all component parasite drag, as follows:
fcomp total — E°3( ff + + fn + fp + /othercomp}
The difficulty in understanding the physics of excrescence drag was summarized by Haines () in his review by stating “… one realises that the analysis of some of these early data seems somewhat confused, because three major factors controlling the level of drag were not immediately recognised as being separate effects.” These factors are as follows:
• how skin friction is affected by the position of the boundary-layer transition
• how surface roughness affects skin friction in a fully turbulent flow
• how geometric shape (nonplanar) affects skin friction
Haines’s study showed that a small but significant amount of excrescence drag results from manufacturing origin and was difficult to understand.