The following extremely important table, compiled by Major Linton Hope, was inadvertently omitted from the reprint of his lecture in the August issue of the Journal. It should also be noted that the last set of Flying Boat lines, on page 470, is the design which Major Linton Hope made in 1916, after the trials of the 41ft. design, Fig. 2, and to which he refers in his reply to Mr. Baker. It should be noted that the angles of the step tangents to the foreplaning bottom have been increased and the back step reduced in size and moved aft, both of which features appear in C.E.I. designed some considerable time afterwards.

The object of this note is to draw attention to a misconception of the theory of screw propulsion which appears in much good work now being done.

This misconception lies in the combination of the Froude inflow theory and the Drzewiecki blade element theory used in conjunction with aerofoil characteristics obtained from tests in an air tunnel.

To explain further one must refer briefly to the various theories which are put forward to account, wholly or partially, for the behaviour of a screw propeller.

The theory and method of design in use for aircraft propellers is that due to Drzewiecki and modifications of it. The method in its simplest form consists in considering a tangential element of the blade as an aerofoil.

In the course of his lecture, he said:—Sometimes it was easy to find the way, but at other times it was extraordinarily difficult, and he would describe certain methods they had for getting over those difficulties; they were not perfect nor were the instruments used perfect, so he hoped that someone there might hit upon improvements arising out of what he was going to say, and, if so, he hoped they would write and tell him all about it.

Suppose we were in the middle of Hampstead Heath on a very dark night and that there were no lamps nor lanterns that one could recognise and we wanted to find our way. One would, of course, suggest that a compass would first be necessary. A night marching compass with a luminous dial was the next step. With these we could find how far one had gone in a certain direction.

The fundamentals of the blade-screw theory have been lately the subject of numerous discussions. Most of these discussions are the result of misunderstandings arising from the fact that the fundamental conceptions and quantities involved in the subject are left without sufficient definitions. The blade-screw problem is very complex. Its study requires the use of many new conceptions and physical laws in generalised form. For the last reason, special attention must be paid to their exact statements. The following remarks are intended to bring attention to some of the fundamentals of the subject which seem to be the main source of the misunderstandings which are the most widespread at the present time. All the considerations that follow are quite general; they take place for any blade-screw, but it is the propulsive blade-screw or propeller that we will chiefly have in view here.

This paper has been prepared as a continuation to the very interesting general airship paper read before the Royal Aëronautical Society by Wing-Commander T. R. Cave-Brown-Cave. An attempt has been made not so much to describe in detail the various parts of a rigid airship as to discuss along broad lines the underlying requirements and problems and the difficulties facing future development. It is hoped that this paper will also serve as an introduction to further papers dealing with specialised parts of the general subject “ Rigid Airships,” such as mooring, fabric, machinery installations, etc.

When considering problems connected with future technical development, it is in the first instance necessary to realise clearly the scope and functions of the operational requirements, and thus to provide a sound basis from which to start. The first problem to consider in this connection is the difference between military and commercial requirements.

At the end of 1915 I was invited to compile and deliver a course of lectures on the Theory of Ballooning at the Royal Naval Air Station at Roehampton; and for the following three years I attended regularly at Roehampton, delivering two lectures daily and setting examinations for the officers under instruction to pass into the Airship service and into the Kite-Balloon Service.

Every encouragement was given for questions to be asked on aerostatic problems after each lecture, my endeavour being to remove as many troubles in the minds of probationers as possible. The trouble which stood out above all others soon became apparent. What would happen to the observers in a kite-balloon in the event of the cable breaking owing to a high wind or to its being shot through? The questions put to me showed this to be the chief cause of anxiety, and it will be seen that the causes for this anxiety were well founded.

The resistance offered by bodies to motion through fluids has for many years been a problem of the first importance, and one which has received a correspondingly large degree of attention both from scientific investigators and those commercially interested in the subject. A considerable amount of mathematical investigation has been carried out; but a detailed study of fluid motion has only proved possible in the case of a perfect fluid, i.e., one in which viscosity is not present. Assumptions have been made as to the quantities upon which the motion of the fluid and the body depends, and formulæ have been prepared expressing the laws connecting force and speed for dynamically similar bodies.

In the present note attention will be directed chiefly to the case where the only properties of the fluid upon which the motion depends are the viscosity and density, though brief mention may be made of instances where the acceleration due to gravity is also important.