Turbulent motion in open air must be studied in a way differing from that followed in laboratory experiments. We cannot choose or arrange for the most suitable conditions; we have to make the best of what nature offers; it is nearly impossible to stain air masses sufficiently so that they show streamlines-we have to judge their possible form from complicated effects. The results arrived at will depend upon the means at one's disposal and the way pursued.

In section 8 the author's apparatus for the study of skin friction, based on direct application of the momentum theorem, has been described (Fig. 99). In a paper published in 1914, this idea was further developed and it was shown that it is possible to calculate approximately in this way the skin friction and the distribution of air velocities in different régime.

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.