One of the factors contributing to the rapid development of aviation during the last few years has been the testing of aircraft conducted on systematic and scientific lines. Even more so, perhaps, future development is dependent on this branch of aeronautics especially when one considers we are still in the early stages of experimental research. Theories are excellent in their own sphere, but when applied to aviation must be treated with more respect than usual, as failure to materialise in practice is a matter of personal risk apart from the financial aspect of the case. Designers and constructors cannot make real progress unless they are provided with accurate and reliable data from performance trials of aircraft. Model testing, intricate mathematical calculations and assumptions do much towards predetermination of results, but after all the main thing that matters is practice, whether concerned with military or civil aircraft. In fact, the performance testing of aircraft forms that connecting link in the union of theory and practice which is absolutely essential for the efficient success (efficient in all senses of the word) of any machine or engineering structure.