Having been delivered about 3 years ago of a paper for the Society's Centenary Journal on the subject of rotorcraft in retrospect—without, apparently, undue birth pains—I was led to think some months ago, when I was asked to write a similar paper—this time on the prospect for rotorcraft in the next 100 years—that the process would perhaps be equally painless. However, unwittingly, I had made one grave logical mistake: I did not realise that this time I was involved with an altogether different bairnie!

In looking backwards one is concerned with collecting and collating factual matter from various sources, comparing present views with the views held at the time, and with such hindsight arriving at fairly positive conclusions, a more or less routine logical process, the quality of which is mainly measured by the care taken in considering all important factors in sufficient detail to ensure valid conclusions.

Almost exactly ten years ago the National Research Development Corporation (NRDC) decided to support Christopher Cockerell's ideas for what soon became known as hovercraft. Since then there have been remarkably rapid developments in marine hovercraft and in other applications of the air cushion principle, and there is little need to remind this audience of what has been achieved during that time. Indeed, the large cross-Channel SRN4, and the smaller new Hovermarine sidewall craft, as well as the recently announced design by Vosper-Thornycroft, all speak for themselves and demonstrate vividly the major strides that have been made within a single decade.

During this period there has naturally been a strong emphasis on development work closely related to specific engineering and design projects, but experimental and theoretical investigations of a more general and basic nature have also been encouraged in this country. It is always difficult to separate clearly research from development and this is particularly so in the first decade of hovercraft.

The paper considers the possibilities of a single type of VTOL vehicle—the compound rotorcraft—in the context of 1968 technology. Past and current compound programmes are reviewed in establishing technical status and include the XV-1, Fairey Rotodyne, Kamov KA-22, US Army Aviation Material Laboratories experimental compounds and the AH-56A (AAFSS).

Qualitative characteristics of compound rotorcraft such as airframe vibration, gust response, landing and take-off characteristics, and safety are discussed and compared with both fixed- and pure rotary-wing aircraft.

Characteristics such as cruise speed, vehicle lift/drag ratio, empty weight fraction, and maintenance are examined quantitatively for a 30-passenger compound rotorcraft, and appropriate limitations of these characteristics are discussed. Sensitivity of direct operating costs to changes in these characteristics is considered.

The compound method of VTOL flight appears to offer great promise in both military and commercial fields, particularly as it can be applied to the inter-urban mass transport problem and to Army logistics and ground attack functions.

I Feel honoured to be asked to lecture here at my old university about the early days of British aeronautical research. I intend to mention only people and investigations relating to times that are more than fifty years ago, apart from incidental references which appear to be relevant. My first emphasis will be on persons, then conditions of work and later on some of the historic results relating to this early period, some of which have been neglected or forgotten in the intermediate period.

Calvert has considered alternative methods of estimating the blockage corrections for blunt-based bodies of revolution in closed wind tunnels at low Mach numbers. His models were all of maximum diameter 152 mm, with an ellipsoidal nose section 203 mm long followed by a cylindrical afterbody. The ratio of overall length L to maximum diameter d varied from about 1.5 to 5.5. For each model the base pressure was measured in wind tunnels of two different sizes, so that the blockage correction factor e for the smaller tunnel could be derived for each model from the experimental results. These experimental values of e were compared with alternative theoretical estimates, using the methods given by Evans, Maskell and Pank-hurst and Holder.

In this note it is suggested that the three-tube yaw probe be placed on the boundary and used as a Preston tube to measure the boundary shear stress when both its magnitude and direction are unknown. The calibration results necessary for this method are also presented herein.

Attitude dynamics of a spinning, axi-symmetric, rigid body undergoing central force motion has received considerable attention in recent times. Thomson et al studied the problem where the satellite was restricted to follow a circular trajectory. This stipulation reduced the system to an autonomous form which was then treated by linearised or Liapounov type of analysis. For the satellite in an elliptic orbit, Kane and Barba presented numerical solution to the linearised equations of motion. Recently Wallace and Meirovitch investigated the same problem by performing asymptotic analysis on linear and low order non-linear systems.

Two probes which have been developed at the University of Toronto's Institute for Aerospace Studies for the purpose of determining the complete velocity vector in subsonic flow are described. That is, both probes provide the magnitude of the velocity at a point plus the two angles which determine the vector's orientation. The first probe, which also measures static pressure, was developed by K. Dau to determine wake properties behind a blowing wing. It uses a multiple pressure-hole technique. The second probe was originated by K. Dau and D. Surry and experimentally developed by M. McLeod. It is basically a spinning hot-wire probe whose periodic waveform uniquely defines the complete velocity vector.

A novel technique of dealing with differential eigenvalue problems has recently been introduced by Wadsworth and Wilde . The differential equation is expressed as a set of simultaneous first-order differential equations, the eigenvalue λ being regarded as an additional variable by adding the equation to the basic set. The differential eigenvalue problem is thus reduced to a set of non-linear first-order differential equations with two-point boundary conditions. This treatment of the problem, although novel, suffers from two serious disadvantages. First, it introduces non-linearity into an otherwise linear set of equations. Thus, the solution can no longer be obtained by linear combinations of independent particular solutions. One method of solving the non-linear systems is by assigning arbitrary starting values at one boundary and performing a step-by-step integration to the other boundary where in general the boundary conditions are not satisfied. The problem can be solved by adjustment of the initial assigned arbitrary values until the given conditions at the other boundary are satisfied. A second method and the one used by Wadsworth and Wilde is to estimate the unknown boundary values at both boundaries and integrate inwards to a meeting point. Changes can then be made to the unknown boundary values to make the two branches of the curve fit together.