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To the Editor. DEAR SIR, In the June issue of AIRCRAFT ENGINEERING, Mr V. D. Naylor rightly asserts that, according to one‐dimensional theory, the velocity at the throat of a Laval nozzle is the local sonic velocity, whether friction is present or not. However his proof rests on an expansion law pvn=constant, when n≠y, and the throat velocity which he obtains differs according to the value of n. Both the assumption and the conclusion are false. The confusion which has existed on this point is, therefore, deepened.

WHEN dealing with questions on gas flow, a certain amount of visual simplification in the resulting equations is produced by using the reservoir conditions as our unitary system.

PROPELLERS, fans and axial compressors are all airscrews, the only difference being in the use to which they are put. Propeller theory and practice has been put on a firm basis by men like Glauert and the parameters used are now well established. The question therefore arises of whether or not it is advisable to hand over the parameters and results of propeller theory to the theory and practice of axial compressors.

THOSE engaged on airscrew work are constantly making use of the tables of experimental values of kT and kQ for different values of J given in R.M. 1673. In using these tables it is usually necessary to extrapolate. It is the object of this article to show that, where we have strict grounds for comparison, we can construct a single thrust curve, which covers a wide range of pitch and conditions of working, and so obviate the need for extrapolation. The curve found covers the range of pitch p/p=0 to 1 and the working range below stall.

THE problem of the expansion of a compressible fluid with friction and heat flow is one of great complexity. In this treatment it has been simplified to a one‐dimensional problem and the resulting relationships are thermodynamic. An expansion of this type cannot be described as reversible due to the presence of friction, and from this aspect the analysis may be suspect. However, any practical process is irreversible and it is common practice to apply to such processes an analysis which is theoretically confined to changes which occur reversibly. Thus, although the degree of irreversibility may be greater than usual in this case, there are many precedents for allowing the use of reversible thermodynamics in the analysis of irreversible changes. The relations for the expansion are well known, but the writer believes the analysis and discussion on the choking conditions of a nozzle are more general than anything yet published.

IN a recent article by the author, reference was made to the stalling of airscrew blades and to some of its effects. It seems worth while to examine more closely the kinematics of stalling. In some cases the thrust coefficient graph for a chosen value of p/D indicates a marked drop in kT at a certain value of J, this drop being attributed to stalling of the blade; point ? on ABC (Fig. 1) is such a point. It seems certain that the effect of stalling is complete at C when the kT graph is practically parallel to the J axis, but where the effects of stalling first come into play is not so clear. In other cases, especially for lower values of p/D, the kT graph has the shape DE, from which it is not clear whether or not any stalling at all has taken place.

THE following readings were found by experiment: mass flow (W) for different initial pressures, initial pressure (po) and temperature (To) pressure distribution along the nozzle, barometric height.

THERE are reasons for believing that in certain states of working, the resultant aerodynamic force on a blade element of an airscrew is normal to a helicoidal surface. The object of the present inquiry was to see to what extent this is borne out by experimental results. It is found that between no‐thrust and maximum efficiency, the hypothesis is confirmed by experimental results for a certain set of helicoidal airscrews. In the course of the inquiry, a method of evaluating the torque coefficient! over the same range, is indicated—the method giving good accuracy.

IT is usual in theory to treat questions on gas‐flow, as if the flow takes place isentropically, i.e. is subject to the law. This is equivalent to assuming the gas is frictionless or non‐viscous: it also implies that the gas is non‐conducting, for in writing down the equations of motion of a particle we must assume there is no transfer of heat to a neighbouring particle if the changes involved are to be even adiabatic. By ‘adiabatic’ we mean there is no heat transfer between the particle considered and neighbouring particles: by ‘isentropic’ we mean there is no change of entropy, so that an isentropic change implies while an adiabatic change does not.

To the Editor, DEAR SIR, In a letter, which has just come to my notice, published in your October 1951 issue, Mr A. V. Cleaver advocates the use of sub‐orbital refuelling techniques as a means of rendering interplanetary flight an economic proposition, and suggests that the importance of this factor was underestimated in the above paper which was reprinted in your August issue.