The problems which arise in the course of a study of aeroplane stability are of considerable complexity as compared with those confronting engineers in other branches of locomotion. Ocean–going vessels provide another instance of the motion of a solid body in a fluid and their stability is clearly one of vital importance. Naval architects have consequently studied the subject and calculations are made for the stability of each new design of ship. The process is so well known and so well founded that it surprises no one that a newly launched vessel remains on an even keel. One instance, not connected with British shipbuilding, serves to remind designers of the serious consequences of faulty calculation. The disaster relates to a ship which was launched fully engined, etc., and which turned turtle and sank within a few minutes of launching.

The type of stability which the naval architect needs to calculate most carefully is that which is concerned with the security of the ship when rolling.

Last year the Aëronautical Society kindly asked me to give my views on Military Aviation. I then tried to show the objects for which military aircraft would be used, the results likely to be gained, and the general lines upon which it was expected that progress would take place—in a word, the military aspect of aviation. Airships have now been handed over to the Naval Wing of the Royal Flying Corps and I propose to–night to deal almost entirely with aeroplane work, in the light of a year's progress. I make no excuse for reiterating the strong links which I feel bind soldier, sailor, designer and aircraft constructor in this matter. Rapidity of progress is essential and it can only be gained by cohesion of effort, lack of friction, and mutual understanding. The Service must remember that the aeroplane designer has to live and is not always merely chasing “X” with a slide rule; the constructor, that the soldier is not only pipe–clay and red tape and that there is sometimes method in his madness.

The material for this paper was collected for a written communication to the Research Committee for publication in The Aëronautical Journal, in the form of a brief annotated account of the work of a number of experimenters on the performance of the screw propeller in air and water; but, at the request of the Council, a month ago, the article has been re–cast in the form of a paper.

As can be readily understood, such a catalogue would be extremely boring to those who have so kindly come here to–night to listen, so that it was considered preferable to give only some general conclusions with sufficient experimental evidence to attempt to justify them.

The short résumé has, however, been retained in the form of an Appendix, for the use of those who desire to refer to it.

It was hoped that, by keeping the paper as short as possible in this manner, there would be plenty of time for the meeting to hear the opinions of the many able members who have had experience of this subject.

In dealing with the flow of fluids, and the reactions set up on moving surfaces, water and steam turbine engineers have to deal with definite masses of water or steam flowing along channels whose boundary walls are fixed.

Fig. 1 (reproduced from Stodola's Steam Turbines, 3rd edition) shows the nature of the assumption made for the so–called impulse turbine, assumptions which hold to an equal approximation for water turbines, showing nozzle and moving vanes

Reaction turbines present a more complicated state of affairs, which offers a less direct analogy with aeroplanes and airscrews, and which therefore need not be considered here.

In the figure (1) the mass of working fluid delivered per second is the cross section of the nozzle or channels between vanes, at any point, multiplied by the velocity of the fluid relatively to the walls of the channel at that point.

The following formulae have been devised for ascertaining the resistance and power of skimming boats, but they are equally applicable to the floats of hydroaeroplanes, although all the examples given are of skimming boats varying from 15 to 40ft. in length, the speeds varying from just under 28 knots up to 51 knots.

It will, of course, be readily understood that the power required to overcome the resistance caused by the floats on hydro-aeroplanes is the Effective Horse-Power (E.H.P.), which is the same as the H.P. required to tow a float at any given speed; while the formula for resistance only, does not take speed into account except in the ratio of

In the examples given it will be seen that the variation between R, as ascertained from actual trials by the ordinary ship formula of and r, ascertained by the formula of is very small; while, in the case of E.H.P. from trials, and e.h.p. by the formula the maximum error—if we except example No. 5 (which is a known bad boat)— is only 0.39%, and the average for the other nine examples is 0.2% for the A type, .04% for B, and 0% for C type.

The official document reproduced below, although in the nature of things destined soon to be out–of–date, is placed on record here, not because in a year or two it may read like Chaucer (to adapt Colonel Sykes’ happy expression, when speaking of the instructions issued in 1910, at the meeting of the 4th February), but because it is the first published attempt at official standardising of aircraft requirements. It is not proposed to subject it here to any detailed criticism, but it may be pointed out that the “minimum factor of safety of 6,” which is required, is not a factor of safety at all of anything like 6 in the ordinary engineering sense, but merely denotes the margin between the minimum and maximum stresses which are anticipated.