It would seem that Mr. Heughan (Journal, March 1954) has missed the basic design criterion of drag exerted for a given weight (and bulk) penalty. No mention is made of this fundamental point in the choice of a device for the reduction of the landing run of an aircraft. The choice of comparison of the “Wake brake” with a 7 ft. brake parachute has therefore been made somewhat arbitrarily. Brown does not state that “the usual size of a parachute used for braking an aeroplane is 7 ft.”; on page 271 of the reference concerned (“Parachutes,” Sir Isaac Pitman, 1951), a case for “a particular aircraft” was quoted as “ using a 7 ft. parachute.”

The only simple method of estimating the tip speed limitations imposed on a rotor by retreating blade stall on the one hand, and compressibility losses on the other, is that given by Hafner. It suffers from the lack of accuracy always inherent in a “standard radius” method, and for present-day use a more refined approach is obviously desirable, provided that it still enables rapid estimates to be made.

In the design and production of structures it is commonly assumed that the raw material supplied in the form of bar, sheet, forgings and castings is homogeneous, isotropic and stress-free. Some failures in service and more in the laboratory have drawn attention to the possible dangers arising from inhomogeneity, directional properties and residual stresses. It is the purpose of the present paper to review the situation with regard to the last, which the production engineer, who has to cope with distortion on machining, at least cannot ignore.

The flexural vibration of uniform plates on uniform elastic foundations is of interest in connection with fatigue and sound intensity studies.

The usual method of load measurement with strain gauge load cells in a bridge circuit involves the measurement of the out of balance voltage either by the height of a trace displayed on a cathode ray oscilloscope, or the width of a band produced by a tuned galvanometer. Both involve careful regulation of the input voltage or calibration during use. The method described provides for a null balance of the bridge at any instant in the cycle of a uniformly repeated cyclic load up to 10,000 cycles per minute. An accuracy of ±½ per cent, of the capacity of the load cell is obtainable.

I wish to support S. W. Greenwood's plea (December 1953 Journal) for defining “specific impulse” as Thrust/Mass flow of propellants.

Apparently some engineers are now using Thrust/weight flow of propellants as the definition of specific impulse, but it would appear that this innovation is merely due to inability to distinguish between mass and weight. The system used at the Royal Naval College, Greenwich, of using the capital letter to denote weight or force (e.g. Lb.) and the lower case to represent mass (e.g. lb.) is a very convenient way of distinguishing between the two.

Your contributors on Specific Impulse in the May journal say, “ The mass definition, on the other hand, gives a value of specific impulse which varies according to the units used.“

It must be realised that the value of a quantity cannot depend on the units chosen. To quote therefore the example on page 371 in gravitational units, namely: —

Stability criteria for systems governed by linear differential equations with constant coefficients are discussed in a general way. Detailed treatment of the sextic equation includes a simple and expeditious scheme for computing the test functions.

The Integral I occurs frequently in aerodynamics. To cite a particular case, Ward has shown that if S(x) is the cross-sectional area distribution of a slender body its wave drag D is given to a first approxition by D = qI/(2π), where q is the kinetic pressure. In this case, S(x) is often defined numerically, and the direct evaluation of I is then complicated by the presence of the logarithmic singularity. Several methods may be employed to avoid this complication. Legendre has recently suggested rewriting I in a non-singular form, which unfortunately is not well suited to numerical work. Here, a method for the evaluation of the unmodified integral is presented.

The collected comments of Messrs. Greenwood (December 1953 and October 1954 JOURNAL), Thorne (February and October 1954 JOURNAL), and Asquith and Whitaker (May and October 1954 JOURNAL) on the definition of specific impulse in the field of rocket propulsion argue certain traditional points of view, each of which has enough merit to gain proponents but apparently not enough merit to gain universal acceptance.

A method for obtaining high Mach numbers using compressed air of low pressures is described. The proposed system can be used to extend the working range of existing direct blow-down and induction facilities.

An investigation has been made to see whether a supply of compressed air of about 5 atmospheres could be used to achieve Mach numbers between 1.4 and 4.0.

The commonly used types of intermittent supersonic wind tunnels driven by compressed air are

1. (a) Direct blow-down tunnels.

2. (b) Induction tunnels.

At high Mach numbers the induction tunnel is known to be very inefficient while the direct blow-down tunnel requires high stagnation pressures.

The torsional stiffness of a thin-walled tube of given perimeter and thickness varies as the square of the enclosed area. The shape giving maximum torsional stiffness is therefore a circle, this being the curve with maximum area for given perimeter. We consider here the nature of the corresponding curve for maximum bending stiffness. The quantity to be made a maximum is now, not the area within the curve, but the second moment for the whole curve about the bending axis. This will give the greatest “moment of inertia” for a tube of uniform thickness. The theory is given in the Appendix and typical forms are shown in Fig. 3. A finite length of base has to be assumed; otherwise the curve reduces to a single straight line normal to the axis.

We can agree with Professor Thorne that no physical quantity can be altered by varying the system of measurement, in the sense that the length of a ruler, for example, is the same whether its length be recorded in inches or centimetres. It is of course the numerical value of the length that varies with the system of measurement, and perhaps we should have used the words “numerical value” instead of “value” in the paragraph to which Professor Thorne refers.

We maintain our assertion, however, that in recording specific impulses, there is more likelihood of confusion as to numerical value, and as to the nature of the unit, if the mass definition is employed