"The methods of characteristics has been linearized by assuming that the flow field can be represented as a basic flow field determined by nonlinearized methods and a linearized superposed flow field that accounts for small changes of boundary conditions. The method has been applied to two-dimensional rotational flow where the basic flow is potential flow and to axially symmetric problems where conical flows have been used as the basic flows. In both cases the method allows the determination of the flow field to be simplified and the numerical work to be reduced to a few calculations. The calculations of axially symmetric flow can be simplified if tabulated values of some coefficients of the conical flow are obtained" (p. 933).

Combustion in a spark-ignition engine was investigated by means of the NACA high-speed motion-picture cameras. This camera is operated at a speed of 40,000 photographs a second and therefore makes possible the study of changes that take place in the intervals as short as 0.000025 second. When the motion pictures are projected at the normal speed of 16 frames a second, any rate of movement shown is slowed down 2500 times. Photographs are presented of normal combustion, of combustion from preignitions, and of knock both with and without preignition. The photographs of combustion show that knock may be preceded by a period of exothermic reaction in the end zone that persists for a time interval of as much as 0.0006 second. The knock takes place in 0.00005 second or less.