One of the main problems in connection with the development of air-cooled engines appears to consist in securing adequate cooling without increasing the head resistance above that of corresponding water-cooled engines.

This problem particularly applies to large air-cooled engines which are limited in size by cooling difficulties. Air-cooled engines for windmill planes and helicopters for alternate slow and fast flying present a further problem, because such aircraft engines will be required to develop maximum horse-power when the speed of translation is lowest.

The airflow over the nose of a bulky body, such as that of an aeroplane, diverges radially in every direction from its axis. Such radial flow tends to overshoot laterally, at its core, the periphery of the engine or other obstruction and so depart from the contour thereof, with the result that a considerable turbulence is set up. Such turbulence has also the undesirable effect of causing a reversal of the pressure gradients, and a corresponding reversed or forward flow of air, in the boundary layer behind the engine, thereby creating a ” dead “ area or areas and so considerably reducing the cooling effect of the general air flow and increasing the turbulence.