The title of this paper is not strictly correct since it is not really concerned with “efficiency” in the accepted sense of the word. It has in the past been considered that the sole duty of an engine air intake is to deliver air to the engine at a high efficiency, that is at a mean total pressure as nearly as possible equal to the aircraft pitot pressure. More recently other considerations have arisen, such as the necessity of avoiding asymmetric flow in twin duct air intakes. These aspects of air intake design have frequently been discussed, for example in Dr. Seddon’s lecture to the Society in December 1951, but the particular subject of this paper has been rather neglected—the velocity distribution of the air delivered to the engine.