I must begin with a confession which, but for the current high price of eggs and tomatoes, would probably have involved me in a bombardment of those agreeable, if messy, missiles. I approach this topic very much from the standpoint of one who served for two substantial terms at the Treasury.

In determining the airworthiness of a helicopter to fly in all weather conditions it is necessary to examine all the effects of such flying on the safety of the passengers, flight crew and third parties involved in such flying. Rule 24 of the Rules of the Air defines that “for VFR outside controlled airspace that an aircraft shall fly at least one nautical mile horizontally and 1000 ft vertically away from a cloud with a visibility range of five nautical miles, except that below 3000 ft the aircraft must be flown clear of cloud and in sight of the surface”. Inside controlled airspace the same rule applies except that the aircraft on Special VFR shall be flown in accordance with the specific instructions given by ATC.

Although this paper is entitled “Design Philosophy” it is my intention to show how this philosophy has influenced the design and construction of the Westland Lynx helicopter, with particular attention to those aspects which directly relate to all weather operation.

To set the scene I would like to give you a very brief description of the aircraft.

It is my very great honour to be asked as an operator to cover the Bell 212s part in the story of IFR operation of medium helicopters.

None of us should forget that two great British companies have for years been blazing a trail in the IFR operation of two helicopters, the S-61 and the Wessex 60, and I can only assume that it was because Irish Helicopters was, I understand, the first operator to put the CAA approved Bell 212/IFR into operation that the privilege of giving this talk has come my way, and I speak humbly and mindful of the invaluable efforts of others before us.

Before starting this discussion I must apologise to the many people here who have considerably more experience of civil instrument flying in helicopters than I. However, the major part of this talk is merely a history of the certification of the Bölkow BO. 105 for instrument flying. We all realise that flying under IFR is not the all weather flying quoted in the title of today's symposium however for the light helicopter it is a very significant step forward. My personal experience of this programme is very limited, and indeed the Civil Aviation Authority knows more of both the design philosophy behind the IF installation and the actual tests conducted than I. I intend to give a short historical description of the problems encountered and envisaged in getting the certification, followed by the equipment required in the aircraft and the use that an operator could make of a machine of this size when IF equipped.

Most of you will be familiar with the disclaimer which some people make before giving talks in these circumstances. Whilst what I have to say to you may not by any means be taken as an official statement of the policy of the Civil Aviation Authority it may be taken as an indication of the way the Authority is thinking about the introduction and operation of ratings for instrument flight in helicopters.

The theory of the self-excited oscillation of the hinged blades of a helicopter rotor has been treated by Coleman and Feingold and their paper, although written several years ago, remains a standard reference on the subject. Since then, the scope of theoretical studies and calculations has been broadened to include hingeless rotors, more degrees of freedom and blade aerodynamic forces. All this has led inevitably to a great complexity in the equations of motion, with the result that, although the dynamicist can perform adequate calculations, he loses the insight and physical appreciation that he needs, for example, to identify the important parameters in the system. In order to maintain these qualities a different mathematical model is required, one that retains only the most basic parameters and yet still exhibits the fundamental ground resonance instability.

This note attempts to obtain an approximate solution for a class of second-order systems represented by

where the well-known procedures, such as perturbation, variation of parameters, the WKBJ analysis, etc, may prove to be inadequate. The approach is illustrated through an example related to the planar librations (satellite oscillations about its mass centre) of gravity gradient cylindrical satellites in circular orbit, subjected to the solar radiation pressure damping (Fig. 1).

In an early investigation of gas flow through a spool valve model Stenning discovered the same bi-stable flow patterns as had been established for oil flow, but reported no measurement of quantitative similarity or of other compressibility effects which would be of importance if such valves were to be used in pneumatic systems. Since then there have been no reports of comparisons between compressible and incompressible flow patterns in control valves, and the present investigation has been carried out to determine the characteristics of compressible flow through a two-dimensional model of a control valve port and to try to derive these characteristics from established compressible flow behaviour.

For many years most transport aircraft throughout the world have used a common form of windshield construction with a single ply of tempered glass for pressure resistance, combined with a thick polyvinyl butyral interlayer. This interlayer is usually reinforced at the edges with a metal insert and acts as a diaphragm to provide bird impact resistance, and also serves as a secondary load path to contain pressurisation in the event of failure of the primary glass ply.

To anyone involved in aeronautics, Formula I cars must seem to be pretty weird devices and so, in order to explain some of the rather unorthodox aspects of FI aerodynamic design, I shall start by retracing the history of aerodynamics in FI a few years to 1968, when it was realised that greater gains could be made by increasing the tyre loads than by trying to reduce the drag. It was the tyre manufacturers who influenced this development more than anyone else, as the width of tyre being developed to generate cornering loads greater than 10 g was making any attempts to reduce drag pretty fruitless. On the average circuit only about one-fifth of the time is spent at around maximum speed and four-fifth is spent cornering, braking and accelerating, when adhesion is far more important than drag.