Previous investigations on the effect of moisture on the elastic constants of spruce have been confined to its effect on Young's modulus in the direction of the grain. The most important work in this connection is probably that of H. D. Tiemann, who deduced curves for pine spruce and chestnut showing that Young's modulus along the grain decreased as the moisture increased up to the fibre saturation point, and that further addition of moisture had little or no effect on the value of the modulus. Tiemann thus found that the modulus decreased to a minimum at the fibre saturation point and then remained nearly constant and independent of any further addition of moisture. The reason for the independence of the modulus on the moisture beyond the fibre saturation point becomes evident if it is considered that when the fibres are saturated, any further moisture added is retained in the cells and in all probability cannot further affect the stiffness and strength of the wood.

One of the factors contributing to the rapid development of aviation during the last few years has been the testing of aircraft conducted on systematic and scientific lines. Even more so, perhaps, future development is dependent on this branch of aeronautics especially when one considers we are still in the early stages of experimental research. Theories are excellent in their own sphere, but when applied to aviation must be treated with more respect than usual, as failure to materialise in practice is a matter of personal risk apart from the financial aspect of the case. Designers and constructors cannot make real progress unless they are provided with accurate and reliable data from performance trials of aircraft. Model testing, intricate mathematical calculations and assumptions do much towards predetermination of results, but after all the main thing that matters is practice, whether concerned with military or civil aircraft. In fact, the performance testing of aircraft forms that connecting link in the union of theory and practice which is absolutely essential for the efficient success (efficient in all senses of the word) of any machine or engineering structure.

At the time when the question of the development of civil aviation is so much in the public mind, I am most grateful to the Royal Aeronautical Society for giving me this opportunity of summarising the technical position of the airship to-day.

It seems to me that if air transport is to take its place with other existing forms of transport the long distance routes of the world must be established, and my object in summarising the present technical position of the airship is to enable you to form an opinion as to whether the modern airship is capable of taking its place in establishing these routes.

I have confined my remarks to the rigid as it is the large airship which is the most suitable for this long distance work.

As this long distance work has a distinct bearing, in my opinion, on the value of the airship for naval purposes, I have made a brief reference to this aspect of the subject.

Since the driving torque exerted by the engine is a periodic function of the time, the speed of rotation must also be periodic, which implies that the airscrew must be subjected to inertia stresses as well as to the stresses due to air forces and centrifugal forces. In the case of engines having six or more cylinders these inertia stresses will not in general be important as the cyclic variations of torque will be comparatively small. But with engines having two or three cylinders these stresses must be considered, for.it will readily be seen that the bending moment due to inertia forces is such as to increase the stresses due to the air load.

There was perhaps nothing in aeronautics that developed so quickly during the Great War as the fighting aeroplane. At the beginning of the war the use of offensive weapons on aeroplanes was almost unknown. The idea of fighting in the air had hardly been discussed and certainly no country had made provision for carrying offensive weapons as part of the regular equipment of the aeroplane. Long before the end of the war every aeroplane was armed to a greater or lesser extent, while particular types of aeroplanes had been developed whose chief duty was the destruction of enemy aircraft and the protection of aeroplanes of its own country which were engaged in various specialised duties, such as reconnaissance and bombing.

In the truest sense of the word all aeroplanes used in warfare are fighting aeroplanes, but the scope of this paper is limited to those machines which are used primarily as weapons of offence against opposing aircraft.

Before dealing fully with the technical side of aviation in Canada it is advisable to discuss the different kinds of work that can be carried out by aircraft in this country. A few of these duties are described below.

Necessity for strength tests to supplement calculations and to check the assumptions on which the calculations are based. Historical outline. Examples of certain types of defect which are revealed by strength, test.

Assumed aerodynamic conditions. Approximations reproduced by test loading. Methods of support during test. Measurements, testing routine and technique.

The usual tests are described:—Flight test, C P . forward; Flight test, C P . back; Nose dive test; Tests of auxiliary and controlling surfaces; Ailerons, elevators and rudder; Tailplane and fins; Fuselage, down load, side load, torsion ; Undercarriage, discussion of stresses which may arise during landing, pointing out presence of frictional horizontal force causing rotational acceleration of the wheels; Static tests of undercarriages, dropping tests; General description of rib tests.

Description of shot bags and general principles of load application with a view to efficiency and safety.

The present article is an account of some calculations on helicopters.

Airscrews have been calculated for different conditions according to two theories: (1) the multiplane interference theory; (2) Glauert's vortex theor). According to both there should be no difficulty in designing a screw to give a good lift at a reasonable rate of climb, and the ceiling should also be quite good. When we consider the speed of falling, with the screws free-wheeling, the two theories give widely different results, and the practicability of the helicopter depends largely upon this question being settled. Simple airscrew theory shows that at least moderate speeds should be obtainable by inclining the airscrew axis. For many reasons it seems desirable that the screws should have at least four blades; gyroscopic couples on the whole machine are eliminated; the forces are widely fluctuating during forward motion with only two blades; the stability derivatives and equations are simplified; but the aerodynamic efficiency will be impaired. In general the stability equation is of the tenth degree, and the lateral and longitudinal stabilities are not separable when the machine, in a state of steady motion in a straight line, receives an asymmetrical disturbance.

The following notes are the results of an attempt to investigate the theoretical possibilities of the helicopter, and generally to develop some branches of the theory of helicopters. In this country extremely little work on the subject seems to have been published, and the only experiments I have been able to find are those given in the Report of the Advisory Committee for 1917-18 (Vol. II.). Several articles have appeared in “ L'Aérophile ” from time to time, notably by Lamé, Touissaint and Margoulis, and some experimental work has been done by Eiffel. But it is extremely difficult to find adequate experimental results with which to compare any theory ; for instance it is very rare to find results of tests on airscrews working under helicopteral conditions, and also the aerodynamic data of the aerofoil sections used. In the matter of stability I do not know of any experimental work at all.

I shall give first the results of my investigations into airscrews for helicopters, and then proceed to the consideration of the dynamics of helicopter flight and the development of the stability equations.

My subject to-night, specialised aircraft, is one capable of a wide variety of interpretations; so wide, in fact, that when I had to tackle the problem I found much difficulty in deciding which of the many possible lines I should take.

However, I realised that the congregation has no say in the sermon, and that your only alternatives were to stay or to walk out. I propose, therefore, to inflict upon those who choose to stay a short account of how aircraft (heavier than air) have been developed in England on specialised lines, followed by some remarks on the special requirements for commercial work, more particularly in respect to those points of design in which the fare-paying user is intimately concerned. In fact the passenger's comfort is really the main subject of this paper.

I would like here to emphasise the fact that I am addressing you to-night entirely in my personal capacity.

Before storing an aircraft, it has to be housed, and this in turn gives rise to various operations of handling, in which each type of aircraft calls for particular detail. It is now proposed to consider the questions of housing and handling for each of the types of aircraft given as typical examples for storage.

As for storage, there again appears general facts which may apply equally well to all types of aircraft, and then particular treatment for each type.

With a large type of aircraft a good deal of care is necessary in handling during the process of getting it into and out of the storage shed. The difficulties then experienced will be increased in windy weather, but these may be overcome by increasing the personnel of the handling party.

The high performance engine, whether military or commercial, is essentially the light one.

The motor which made flying possible was an internal combustion engine, and the basic reason of its success was the direct use of an easily volatile fuel of high heat value, namely, petrol.

The first engine developed to use this fuel was that of Daimler, and all motor-car, boat and aviation engines have sprung from this engine.

In order to more readily have in mind our problem, we can with advantage briefly run through the main aero engines so far constructed with the progressive results of tests.

The engine of Gottlieb Daimler is illustrated by Slide No. I. It had automatic inlet valves and exhaust governing, the latter being a point to be noted.

The next slide shows the engine of the Wright Brothers. This engine weighed 8.75 lbs. per B.H.P. inclusive; the most successful engine of to-day weighs under 3 lbs. inclusive.

At present, while we have but a vague conception of the form a practical helicopter is likely to take, it is useless to attempt to set out in detail the dynamics of helicopter flight or performance calculations. For the evolution of a practical machine of this type it seems to the author that it will be necessary first to study the stability equations neglecting everything but gravity and the screws themselves. This should show what stabilising surfaces are required, and it is with this object that the stability equations are given below. In the meanwhile it is perhaps worth while to obtain a broad view of the ground to be surveyed, and we, must expect the work to be more complicated than in the case of the aeroplane.

If the thought of another war troubles you, then don't read this article. If you would rather say to yourself as the Secretary of State said to the Air Conference, “ There won't be another war for ten years, so why worry? ” then no doubt you will think with him that it is better to let other nations have alk the bother and expense of trying to advance; after all, we are jolly fine fellows and can soon pick up. If, on the other hand, you have imagination which gives you a nasty queasy sensation when you think of what might be, then perhaps the following notes, albeit disjointed and mostly stale, may at least conjure up in you thoughts of your own on the subject. This is all that is needed to help, our advancement in the air–the stimulation of spoken and written thoughts by the British nation, for if every taxpayer in the British Empire says “ Air Force,” then the Press and Parliament will say it too.

It cannot fail to be a subject for remark that in spite of the very large amount of work which has been done in this country on the subject of formal stability calculations, these calculations do not form a part of routine design of the majority of aircraft firms, nor are they used by the Air Ministry as a standard process of checking the aerodynamical qualities of an aeroplane. I use the term “ formal ” stability calculations to cover the solutions of the equations of motion of an aeroplane in rectilinear flight developed from Bryan's work.