For years, the development and testing of aero gas turbines has been hampered by the inability of development and test engineers to perceive and readily measure the positional changes of static and moving components within the engines due to the opaque nature of the engine casing walls. ‘Touch’ or abradable probes have been used but require special engine builds to employ them.

With the introduction of high strength materials and the thinning of engine structures and as the performance requirements have become more demanding, small component movements during engine handling have become more important. This is particularly true in areas where the airflow has the greatest work potential such as the tip and root seals of the high pressure turbine and the root seals of the final compressor stages.

The ideal of high speed centre-to-centre inter-city air transport remains unrealised with the problems of poor economics and excessive noise dominant and still largely unsolved.

Economics hit particularly hard at VTOL aircraft. Thrust/weight ratios in the order 1·15:1 are necessary with the result that total installed thrust is much larger than is required for the cruise condition and payload and range suffer. It is therefore tempting to compromise and go for a STOL solution and current trends are in this direction.

The fuel control system for a modern gas turbine engine has to meet far more stringent requirements than demanded by early engines. To achieve greater efficiency the controlling parameters such as pressure and temperature must be measured and acted upon with a greater degree of accuracy. A 5°C change in turbine entry temperature can cause a 200 hour decrease in the life of a turbine with an expected life of 2000 hours. With increased fuel efficiency, a saving of less than 1% in fuel consumption is a vital economic factor. These higher demands can be met in most instances by mechanical or hydromechanical fuel systems but the problems of these systems such as hysteresis, friction, fuel temperatures and coefficients of expansion increase rapidly with the search for higher performance. The complexity and number of moving parts becomes greater and so the system is commensurately less reliable.

The first item in this paper is a comparative review of the history of air and underwater achievements. We shall see that commercial underwater vehicles are a recent development and offer today a magnificent challenge to engineers with many opportunities and pitfalls. Small multi-disciplinary teams of engineers are able to advance rapidly, and the calibre of a few men matters, rather than vast organisations. In some ways this resembles the aeronautical world of the 1920s and 30s, during which period R. J. Mitchell contributed so greatly. Certainly it is closer to that period of aeronautics than to aerospace today, as I shall try to show.

It is a sign of greatness in a man that he leaves footprints like those of Good King Wenceslas to guide and invite others to follow. Sir Frederick left his boldly in the Aircraft Industry, SBAC, Cranfield, Royal Aeronautical Society, Air Registration Board and many other places. Not in every place but in all those I have named he has been my Good King Wenceslas and I his page.

In this lecture I could choose to follow any one of these tracks but they are not divergent but criss-cross and intertwine with the result that in starting along one I may well switch onto another. The track I shall start to follow is aeronautics at Cranfield.

A previous discussion of the expanding flow of a choked axi-symmetric jet into a stationary atmosphere drew attention to the differences between various existing sets of observations and between observation and analysis. One reason advanced for these discrepancies was the influence of the mixing region at the edge of the jet while another was the degree of uniformity and steadiness of the flow at exit from the nozzle.