Among the various experimental data which are required in the discussion of the theory and design of the aeroplane, there are none of greater importance than those relating to surface friction, while at the same time there are none about which greater uncertainty prevails.

Owing to the difficulty of separating skin friction from head resistance, very little can be learnt from observations on actual machines in flight, while on the other hand, if we turn to laboratory experiments on the resistance of planes, we are met with the most bewildering variations in the values obtained for it. Many of the most careful experimenters make out that it is quite inappreciable while others maintain that it is from 1—4 per cent. of the normal pressure, and therefore forms an important part of the drift at small angles of attack.