PREFACE TO THE FOURTH EDITION The wide use of the first three editions of the book, both by the general public and by schools and colleges, has indicated to the authors that their hope of meeting a definite need in aeronautical literature has been fulfilled. Like its predecessors, this edition is intended primarily for the reader who desires a sound knowledge of the basic principles and a broad view of the present development of the airplane and its power plant, without giving to the subject the intensive study which is essential for the designing engineer or the expert mechanic. We have made every effort to introduce nothing which would require more than an elementary knowledge of physics or mathematics for its understanding. In preparing the fourth edition we have tried wherever possible to clarify the text, to amplify the explanations of fundamental principles, and to make the additions and changes needed to take account of the progress in aeronautics during the four years since the third edition was written. Much new material has accordingly been included, and descriptions and illustrations of the latest types of airplanes and engines which have proved themselves of genuine value or promise have replaced the examples of earlier types. The fourth edition includes discussions of the major advances in the airplane and its power plant which have been made since publication of the third edition in 1936. New illustrations are used for practically all the typical airplanes and engines and the descriptive matter, which accompanies them, has been revised accordingly. Major new developments are described in both principle and practice, including the latest sub-stratosphere transport airplanes and their equipment and the latest commercial and military designs. In the field of power plants, the general adoption of supercharging for large engines has led to revision of the text to include the principles and practice of this development. New American liquid-cooled, air-cooled, and light-plane engines, developed since 1936, are described, together with the most important new engines developed abroad. In the case of propellers, a description of the new full-feathering type has been added, and the chapter on instruments includes the latest developments in this important field, together with a discussion of the progress made to date in equipment for blind landing and sub-stratosphere flying.