It is well-known that, at large distances from the flight path, the waveform of the sonic bang from an aircraft, without lift at any rate, has the characteristic N-wave shape, as illustrated in Fig. 1. When we depict a waveform such as this we are stating what would occur at large distances in the absence of viscosity and other diffusive effects, such as turbulence. We know, of course, that in reality, because of these effects, the bow and stern shocks will be “softened”, i.e. the pressure jumps associated with them will not be instantaneous, but will have a finite, but small, “rise time”. However, this is not the matter under discussion. The essential feature of the N-wave illustrated in Fig. 1 is that the pressure jumps associated with the bow and stern shocks are equal, and that the positive and negative phases are of equal duration. The arguments for the waveform to take up this shape at large distances are put in a classical paper by Whitham.

In a technical note in the January 1963 Journal J. Dunham shows a method to determine the choked condition in a cylindrical ejector nozzle which in a simpler way leads to the same expression that has been derived by H. Pearson et al. of Rolls-Royce Ltd. Although Mr. Dunham states it is unnecessary to use the “maximum-discharge” principle, his derivation in the case of two streams in a common passage is, by the assumptions he makes, based on just this principle.

For uniform-thickness, solid circular plates made of isotropic elastic material and elastically restrained at the edge, expressions are derived for the optimum support stiffness to minimise the following quantities:

1. 1. The largest effective stress based on several different strength theories.

2. 2. The largest effective stress per unit of maximum deflection or per unit volume displaced.

The toepler schlieren system of wind tunnels is usually designed in such a way that an image of the flow in the test section can be formed in a camera for taking flow pictures. For visual observation, a mirror is inserted in the light beam and the image is projected on a screen. When a photograph or film record of the flow is desired, the mirror is removed by some quick-acting mechanism to allow the light beam to enter the camera. Especially in the case of non-steady flows, the time lag involved in removing the mirror can prove to be a drawback. An interesting pattern observed upon the screen may have vanished by the time the light beam is switched over to the camera. At the same time the image disappears from the screen, leaving the operator in doubt whether or not the pattern lasted long enough to be recorded by the camera. In this note a very simple arrangement of the schlieren system is described, which gives simultaneously an image on the viewing screen and in the camera. So far as the author knows, it is not applied elsewhere. The method consists of the use of a mirror knife edge instead of the usual knife edge.

The Agricultural pilot looks for the following qualities in his aircraft, and I suggest that he looks for them in this order of importance:

Lively performance

Docile handling

Personal protection in an accident

Reliability of both aircraft and agricultural gear.

I said that performance should be lively. Perhaps it should be not far short of exhilarating. Certainly in many cases in current operations it can barely be described as adequate. Of course, some conditions of work call for a higher performance than others, and it is not possible to talk about ideal figures without relating them to a specific operation. Compromise is inevitable, but the most exacting types of work must be kept always in mind. Admittedly, most suggestions for performance minima made by a pilot may call for engines and power loadings which will seem quite uneconomical to the operator, particularly as engine overhaul costs often appear to increase as the square of the engine power.

The agricultural aircraft is a special vehicle with special needs for simplicity in design coupled with interchangeability and accessibility of components and equipment. Cheap price, commercial availability and easily maintained equipment of high reliability, field replacement capability, freedom from the need of special tools and rigs, together with lack of complication, reflect markedly in the economics of operation and provide improved safety in operation.

To establish that these parameters are appreciated and allowed for, it is profitable to consider whether operators themselves give adequate and logical thought to engineering problems in their forward planning, and whether aviation legislation permits an adequate projection of this philosophy rather than reducing true airworthiness, true safety levels, and reasonable economics.

Cockrell and Markland have found that the static-pressure recovery coefficient of a conical diffuser, which is well known to decrease as the (turbulent) boundary layer thickness at entry increases, actually rises again when the inlet boundary layer is so thick as to approximate to fully-developed pipe flow. It may be noted that this is not a consequence of the definition of the pressure coefficient because, for typical pipe flow velocity profiles, the pressure coefficient based on mean velocity, , is very nearly the same as that based on momentum flux .

A Matrix Eigenvalue Calculation is rich in information content but expensive in computer time. This is why, in a typical vibration problem, there are many more elements than masses, or rather, many more elementary deflections than degrees of freedom for vibrations.

To an increasing extent, aero gas turbine manufacturers are supporting high speed rotors on spring bearings, in order to escape the consequences of lightly damped and inconveniently placed critical speeds. While experience has been generally good, engineering doubts periodically arise.

(a) By introducing the flexible bearings, the critical speed is reduced, say, from 9,000 r.p.m. to 3,000 r.p.m. At 3,000 r.p.m. a peak amplitude response is experienced, although the bearing load is comparatively low. (Some spring bearing designs incorporate damping as an accidental feature, many do not, and very few have damping designed into them.) The peak amplitude at 3,000 r.p.m. can rub the seals or overstress the spring, and to prevent this the bearing amplitude is restricted by a circular stop known here as the “snubber.”

The notched fatigue strength of the aluminium alloy HE 15, under stress conditions similar to those occurring in aircraft structures, is greater when the material is naturally aged than when it is fully heat-treated. This is attributed to the lower proof stress of the naturally aged material, which enables the high stresses in regions of stress concentration to relax more readily. However, no further improvement was obtained by re-solution-treating the alloy to reduce the proof stress further.