The purpose of this note is:

1. (a) to provide a method for evaluating the fuel mass penalty incurred in flight after fitment of a system to an aircraft. The method relies only on a limited number of parameters and should prove particularly useful in the early design stages of a new project.

2. (b) to show the relative importance of the parameters involved, in particular the relationship between the fuel penalty due to system basic mass and that due to system drag (other than mass drag) which, depending on the flight conditions, could be significant or even greater.