Minimum trimmed drag and optimum C.G. Position

An analytical expression is developed which shows that wing/tail interference drag is determined by wing downwash at downstream infinity. With the use of this expression, relations for minimum trimmed drag and optimum e.g. position are presented in explicit form. From this it follows that minimum induced drag is less for the combination of wing-plus-tail than for the wing alone. It is shown that this is true even for the case when the optimum tail load is a download rather than an upload. Furthermore, it is shown which are the factors that have a decisive effect on optimum e.g. position.