The Seventh Louis Bleriot Lecture was given in Paris on 10th March 1954, by Mr. Raoul Hafner, F.R.Ae.S., under the auspices of the Association Française des Ingenieurs et Tèchniciens de L'Aèronautique (A.F.I.T.A.)

The Lecture was attended by the President, Sir William S. Farren, C.B., M.B.E., F.R.S., F.R.Ae.S., and several members of Council of the Royal Aeronautical Society, by the Secretary, Dr. A. M. Ballantyne, T.D., A.F.R.Ae.S., members of the Society and of the British Aircraft Industry, and by a large and distinguished French audience.

Monsieur Jules Jarry, President of A.F.I.T.A., presided at the meeting and welcomed the guests before introducing Sir William Farren, President of the Society.

When accepting the invitation to present the Seventh Louis Blèriot Memorial lecture, the author was deeply conscious not only of the honour which was bestowed upon him by this invitation but, in an equal measure, of the responsibility which attached to it.

For the first time one of the eminent memorial lectures is devoted to the rotating wing; the helicopter has been raised to the level of discussion of the great aeronautical papers.

The natural impulse of the gentler sex when faced with a great occasion is invariably: “how shall one dress?” To the author, a somewhat similar problem became manifest: how can the helicopter, this new-comer, best be “ dressed?” How should it be presented, so as to make not only a good impression but also a full and, above all, a true impression?

Complicated oscillatory systems may be broken down into component “ sub-systems ” for the purpose of vibration analysis. These will generally submit more readily to analytical treatment. After an introduction to the concept of receptance, the principles underlying this form of analysis are reviewed.

The dynamical properties of simple systems (in the form of their receptances) may be tabulated. By this means the properties of a complicated system may be found by first analysing it into convenient sub-systems and then extracting the properties of the latter from a suitable table. A catalogue of this sort is given for the particular case of conservative torsional systems with finite freedom.

The properties of the composite system which may be readily found in this way are (i) its receptances and (ii) its frequency equation. Tables are given of expressions for these in terms of the receptances of the component sub-systems. All of the tables may easily be extended. The tabulated receptances may also be used for determining relative displacements during free vibration in any principal mode.

A method of presenting information on the vibration characteristics of machinery, which is effectively due to Carter, is illustrated by means of an example. More general adoption by manufacturers of this method (which requires no more computational effort than must normally be made) would lead to enormous savings of labour in calculating natural frequencies of composite systems.

It is currently believed in some parts of the world that the problem of structural fatigue in the case of aircraft having a modern multiple spar wing structure can be safely dealt with by allowing fatigue cracks to develop in service. The typical multiple spar wing has its bending tension area distributed between the “ caps “ of two or more spars and a large number of independent heavy stringers. It is believed in some quarters that complete failure in flight of any one spar boom or stringer due to fatigue will leave the wing with sufficient strength for the aircraft to land safely. This note does not discuss the validity of this belief, nor the level of safety so achieved, but attempts an objective assessment of a problem that arises as a consequence of this belief.

In the analytical presentation of curves the main requirements perhaps are those of accuracy and simplicity and the problem confronting the engineer is the efficient combination of the two. A further important consideration is that of the range over which the empirical formula (or formulae in some cases) is to be effective, for clearly the greater this range is, the more useful will be the formula.

It is obviously desirable that any equation that is chosen should be as close a fit as possible to the practical curve. It must not be sensitive to small parametric differences and it should be possible to determine these parameters with a fair degree of precision.

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.

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

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: —

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

Professor koiter, in his note (January 1954 Journal) states that I (April 1953 Journal) was “ apparently unaware” of Weber's two papers on the subject, published in 1924 and 1926. I was aware of the papers, having cited them in the bulletin by Seely, Putnam and Schwalbe on “The Torsional Effect of Transverse Bending Loads on Channel Beams.“

Some references in addition to those already cited may be of interest. The unorthodox behaviour of a channel beam in bending was observed and reported by C. Bach in 1909–1910. A. and L. Foppl wrote a section on the “Schubmittelpunkt” in their book “Drang und Zwang.“