The field of viscosity forces in fluid currents of low viscosity—i.e., high Reynolds number—is frequently confined to a narrow zone in the vicinity of the surface; consequently, calculation can be simplified by the following assumptions :—

1. In the friction zone, customarily called the “ boundary layer,” only the most effective components of the viscosity force μΔω, are considered—e.g., for the x component of this force, μΔu (calling the direction of main flow parallel to the surface, the x direction, and that perpendicular thereto the y direction) the expressions μ (∂2u/∂x2) and μ (∂2u/∂z2) can be neglected for μ (∂2u/∂y2).

The performance of large aero-engines is limited by the weight of the charge that can be inspired at the high speeds and by the tendency to knock, which is due chiefly to the hot exhaust valve. The performance can be considerably increased when a valve control system is employed, which offers small resistances and large passages for the gas flow. Besides this, the walls of the combustion chamber must be kept at a low and uniform temperature, in order to ensure freedom from knocking even with high compression.

As tests show that the knock resistance and the piston pressure in engines with rotary valves are sensibly greater than in the same engines with poppet valves, measurement results are used in order to ascertain the possible limiting values obtainable for the compression ratio, the mean piston pressure and the specific fuel consumption of large aero-engines.

The superposition of the flexural instability of the plane form of a cantilever wing in its own plane, over that of the torsional elastic equilibrium, gives rise to a special phenomenon of mixed instability which has been translated analytically by Prof. C. Minelli into the problem of determining the first positive characteristic value of a certain system of two ordinary, linear and homogeneous equations, in two unknown functions, with given limiting conditions. The methods of calculation, both analytical and numerical, for the solution of this problem are described. The results confirm the intuitive inferences according to which, other conditions being equal, the backward displacement of the elastic axis worsens conditions by lowering the critical speed.

The article deals with the calculation of plane frames each bar of which is able to withstand flexion perpendicularly to the main plane. After having shown that the stress corresponding to the loads parallel to that plane is independent of the stress corresponding to the perpendicular loads, the author develops for this latter a method of resolution based on Cross’ principle. Fictitious supports are added to each knot and then successively removed, till the structure reaches the true form corresponding to the loading. A very important improvement of the original method—formerly not applied to a three-dimensional construction— consists in the use of adjustments, that is appropriate movements of parts of the structure as a whole.

The method proved to have many advantages: Difficulties independent of the degree of redundancy, no loss of accuracy between the beginning and the end of calculations, large number of verifications in the course of resolution, reliability of the results, easy understanding of the successive steps, possibility of simultaneous work for many contributors, and relative shortness of the amount of work required.