The purpose of this note is to call attention to the convenience of glass cloth/polyester resin laminates as a means of constructing certain parts of experimental apparatus, such as small ducts or nozzles.

The advantages of the material are that it is strong, light in weight, easily worked in that heat and pressure are not required, and a high degree of accuracy and surface finish are easily achieved. For the purposes considered here, convenience in fabrication is the main consideration, since it is easier to shape the outside of a suitable male mould rather than the inside of a small duct.

The title of this paper is not strictly correct since it is not really concerned with “efficiency” in the accepted sense of the word. It has in the past been considered that the sole duty of an engine air intake is to deliver air to the engine at a high efficiency, that is at a mean total pressure as nearly as possible equal to the aircraft pitot pressure. More recently other considerations have arisen, such as the necessity of avoiding asymmetric flow in twin duct air intakes. These aspects of air intake design have frequently been discussed, for example in Dr. Seddon’s lecture to the Society in December 1951, but the particular subject of this paper has been rather neglected—the velocity distribution of the air delivered to the engine.

Two recent contributors to TechnicalNotes have expressed the view that, in rocketry, specific impulse should be defined as thrust/mass of propellants consumed per second, rather than as thrust/ weight of propellants consumed per second. The only reason given in support of this view is that the weight of a given mass varies with location. Our purpose here is to give reasons for taking the opposite view, namely that the second definition is preferable.

The design of the static and pitot static tubes which are still in common use—such as the National Physical Laboratory standard head or the Prandtl tube, dates back some forty years (see Ref. 1). Before these tubes can be used it is necessary to calibrate them, using a replica of the instrument which is commonly regarded as the standard. This procedure assumes a perfect replica. Furthermore, the shape of the instrument head used hitherto (conical or hemispherical) is known to be aerodynamically poor and, as a consequence, the calibration factor varies unpredictably with wind speed, stream turbulence and Mach number.

A general expression is obtained for the potential of sound or aerodynamic disturbance from a source moving with variable vector velocity, based on linear theory. Knowing the space time curve for the source, the Doppler frequency corrections and intensities can be determined. If in any part of its trajectory the aircraft exceeds the velocity of sound, the number of sonic bangs and the times of occurrence can be calculated. A numerical example is given. The following conclusions are deduced: (i) If at any point of its trajectory, the component of the aircraft velocity in the direction of the observer is equal to the velocity of sound, a sonic bang is propagated to the observer with the velocity of sound. This has been previously postulated by C. H. E. Warren. (ii) A bang as determined by conclusion (i) comprises two components and in certain circumstances the components in some of the bangs may be acoustically separable, thus giving rise to double bangs.

Regarding the recent Technical Note of Mr. RAsquith and Mr. Whitaker (May 1954 journal):

• 1. The dependence of the weight of a given mass on location is a sufficient reason for the abolition of the ” weight” definition of specific impulse. This is likely to become increasingly apparent as rockets penetrate further into interplanetary space.

• 2. It is convenient that the “weight” definition gives values of specific impulse which are independent of the units employed, provided that the units of thrust and weight are the same, for the unit of time employed in the definitions under discussion is the second. This raises the question of whether the unit of time should have been specified in the definitions. It seems to me that an improvement on fundamental grounds would be to define (S.I.)m as thrust per unit mass of propellants consumed per unit time and (S.I.)ω as thrust per unit weight of propellants consumed per unit time.

When a turbulent fluid flows past a solid surface whose temperature differs from that of the fluid, the shear stress at the surface and the heat flow from it can be related by means of the Reynolds analogy. This analogy has been improved by Prandtl, Taylor, von Kármán and others, and its validity has been tested for flow through tubes and past flat plates by several investigators. In this note the analogy is checked against shear stress data and heat transfer data for a cylinder rotating in “still” air, when the flow is turbulent.

In a recently published book, The Aerodynamics of Propulsion, by Küchemann and Weber, there is included a brief discussion of heat flow in an air-air heat exchanger. The treatment given rests on an assumption which appears to be generally accepted, but which seems to the present writer to be physically unsatisfactory and to yield at least one misleading result. It is the purpose of the present note to show that a solution of the heat transfer equations can be obtained using an alternative assumption which seems physically more acceptable.

The problems of noise on the ground coming from aircraft on the ground, while still not simple to resolve, are rather less difficult than those associated with noise coming from aircraft in flight. The source of noise—the aeroplane—is no longer travelling rapidly through the air with three degrees of freedom but is either stationary on the ground or taxying at a comparatively slow speed across an airport. In the first case, the aeroplane is at rest, either for a short period when the pre-take-off check is being performed near the end of the runway, or for a much longer time while a post-servicing engine check is being made in a maintenance area. During each check, up to maximum engine power may be used, although for jet aircraft the pre-take-off check is relatively short and appears to be becoming part of the take-off itself. In the maintenance area prolonged running is likely and will consist of operation at a medium engine power with occasional bursts of maximum power. Of the three ground cases, this is the one on which attention has to be focused. Indeed, it is not unreasonable to confine an examination of the problems of ground-to-ground noise to this and to this alone.

We gave two main reasons in favour of the “weight” definition—(1) it considerably reduces the possibility of confusion over units, (2) it “ legalises “ the almost universal practice of expressing S.I. in seconds. We do not consider that Mr. Greenwood's remarks invalidate our conclusion that the “weight” definition is preferable.

We agree with Mr. Greenwood that provided forces are everywhere measured in earth gravitational units, (S.I.)� is independent of location: this is because the universal use of earth gravitational units is tantamount to using absolute units. But if a true gravitational system of units is employed, (S.I.)m must vary with location

A new method of presenting the aerodynamic data for a compressor or turbine blade cascade is shown, with particular reference to flutter excitation or damping. The application of these results to wake excitation is demonstrated.

The basic equations for flutter excitation or damping are derived in the paper “The Aerodynamics of Compressor Blade Vibration” presented by H. Pearson at the 1953 Anglo-American Conference (London). No new equations are derived but the method of plotting the relevant data leads to a simpler appreciation of the criteria.

The largest air transport at present operating on air routes is the Douglas Globemaster which is able to lift up to 130 passengers and a considerable load of mail and freight. Experience of flying Globemasters many times on the military route between Seoul City Airport (Korea) and Tashekawa Air Force Base (near Tokyo) suggests the following observations. They may be of value to those aircraft manufacturers in Great Britain now building 130/150 seater jet air liners and to British Overseas Airways Corporation and other air lines who will be concerned with operating them, possibly by 1960.

Measurements were made of the pressures in flattened, blunt-nosed pitot tubes, in an air stream at Reynolds numbers from 13-1,000 (based on the external height of the tube). Three tubes were used, having cross sections with ratios of width to height (W/H) of 3, 7 and 11.

An electro-mechanical method of simultaneous component separation is suggested as an alternative to mechanical systems. Based on the commonly used weighbeam, employing a motor driven leadscrew, poise weight, selsyn repeater circuit and counter, component separation is proposed by means of the selsyn differential circuit.

The use of the selsyn motor as a direct or differential telemeter mechanism is well known, and as such, has many industrial applications. Such use, applied to the aerodynamic balance, offers the advantage of avoiding many of the difficulties associated with design and in particular, the construction of a mechanically independent and simultaneous measuring instrument.