When an · airplane is to be designed, there are certainimposed elements on the basis of which it is necessary toconduct the study of the other various elements of thedesign in order to obtain the best possible characteristics.Airplanes can be divided into two main war airplanesand mercantile airplanes.In the former, those qualities are essentially desired whichincrease their war efficiency, as for high speed,great climbing power, more or less great cruising radius,possibility of carrying given military loads (arms, munitions,bombs, etc.), good visibility, facility in installingarmament, etc.For mercantile airplanes, on the contrary, while the speedhas the same great importance a high climbing power isnot an essential condition; but the possibility of transportingheavy useful loads and great quantities of gasoline andoil, in order to effectuate long journeys without stops,assumes a capital importance.Whatever type is to be designed, the general criterionsdo not vary. Usually the designer can select the type ofengine from a more or less vast series; often though, thetype of motor is imposed and that naturally limits thefields of possibility.Rather than exposing the abstract criterion, it is moreinteresting to develop summarily in this and the followingchapters, the general outline of a project of a given typeof airplane, making general remarks which are applicableto each design as it appears. In order to fix this idea,let us suppose that we wish to study a fast airplane to beused for sport races.