DURING the past year the increase in performance shown by certain American and Continental aircraft has merited the attention of all aircraft technicians. The performance is, of course, partly due to increase in horse‐power, but this is a small factor compared with the advance shown in the reduction of drag. Now that it has been shown that it is possible to build aeroplanes of low form‐drag and with little interference, the drag due to skin friction is becoming of great importance, and it will probably be of advantage to outline the methods of estimating skin friction, and discuss what information is now available on the effect of surface finish and protuberances.