Having–honoured me with an invitation to deliver a lecture this Session, the Society intimated that the most acceptable subject would be “Heavy–Oil Engines for Aircrat.” Unfortunately difficulties intervened which prevented the Society's choice from being adopted, and the outcome has been the hurried preparation of this lecture upon a subject which may seem remote from that mentioned.

As regards the broad title, this remoteness is more apparent than real, however, since the difficulties to be surmounted in evolving heavy–oil engines for aircraft relate quite as much to the mechanics of engine design as to problems of fuel injection and combustion. That this is so may be gathered from the paper Mr. Pye read before the British Association last summer, and from Mr. Chorlton's comments in the December issue of the Society's Journal. In Mr. Pye's paper, information is given which shows that much progress has been macfe in this country (and notably at the Royal Aircraft Establishment) in the direction of obtaining satisfactory combustion at high engine speeds with airless fuel injection.

In a paper in the number of this journal for November, 1925, it was shown that if a flying machine were fitted with wings of standard section and these were flapped in a rhythmical manner, the machine would be supported and propelled, its weight, dimensions and velocity being those of a typical bird; the degree of accuracy attempted in the analysis did not exceed five per cent.

A subsequent examination of the power involved established a high efficiency; but this was due to a large amount of negative work during up–beats; and although such conditions might be maintained in a mechanical model they were unlikely in a bird, which has a large muscle, the pectoralis minor, for lifting its wings. In a bird efficiency seemed to require that the angle of incidence of the outer portion during an up–beat should be negative (see §7, p. 593, of the previous paper); the angle was however made zero (see §9) in order to secure adequate lift, and it is this feature which gives rise to the negative work.

The method adopted by Mr. Fearn in dealing with the above subject in the February number of the Journal is open to many serious objections. To begin with, it is at best approximate and as Mr. Farren points out in his article in the March number, Mr. Fearn's approximation, although near enough for displacements, give on double differentiation less close approximation for acceleration. And for less regular motions than in the case under discussion the approximation to the acceleration by Mr. Fearn's method might be comparatively poor.

The greatest objection which the writer has to Mr. Fearn's method is that it is far too intricate and cumbersome. It unnecessarily involves mathematics of a high order when the problem can be solved by means within the compass of any first year student of engineering.

Mr. Farren's method, on the other hand, is of extreme simplicity and as a graphical method could hardly be improved upon. But there is a very simple-analytical solution and one moreover of the familiar type for the ordinary nonarticulated case.

Before asking the Lecturer to reply, I should like to ask him one or two questions.

What percentage of the mechanics now in the R.A.F. have been trained at Halton?

It would appear to me that two classes of mechanics are wanted. Firstly, those who are competent to do what might be termed running repairs; and secondly, those who are highly skilled and capable of undertaking major repairs generally. The scheme in force at present seems to fill neither of these requirements. General education is of course needed, but far too much attention would seem to be paid to this subject. The aim is surely to train out personnel who will be able to keep the equipment of the R.A.F. in efficient condition and this aim should not be lost sight of. It is presumed that but a small number of those trained will be able to get Commissions, and this being so, the present system would seem liable to engender dissatisfaction.