Active Control Technology (ACT) is not a new subject; it has been utilised in engineering circles for over a century in various guises, the first application of any significance being the steam engine governor. The history of the subject is accordingly long and is well referenced, and we shall depart from the time-honoured rigmarole of providing a detailed survey of early developments by directing our audience instead to Ref. 1 where such a survey may be found. In similar mildly unconventional vein, we note that this platform, and many others like it in this country and elsewhere, has been used many times by eminent individuals to expound their own idiosyncratic views as to how ACT should be defined in its constituent functions and how, in turn, these might possibly be applied without much in the way of reference to what has been done or to the many practical limitations. While our own views presented here will also be idiosyncratic, we shall attempt to relate what we say to that which has been achieved in actual applications and, based on these achievements, to what might follow in the foreseeable future.

Aircraft lift and drag characteristics can be measured in flight by subtracting the components of engine net thrust along and perpendicular to airspeed from the corresponding components of the total aerodynamic force R.

In steady straight flight the flight path angle can easily be measured and consequently the components of R when aircraft weight is known. Measurements in steady-straight conditions, however, are time consuming and furthermore the resulting accuracies are limited due to unavoidable deviations from the nominal straight flight condition.

An abundance of cheap energy has been essential for the growth of civilisation during this century, and, like so many of our resources, has been taken for granted— almost for too long. The title is derived from American folklore and is self-evident. ‘Without energy, can’t travel’ is also obvious; perhaps for today’s lecture a more appropriate slogan could be ‘have alternative energy sources, continue to travel’.

Nowadays we enjoy a high level of safety in air transport. One generation ago, in the late 1940s, it was not so good. Two generations ago, when air transport began in the 1920s, safety was by today’s standards quite deplorable. For example, in UK passenger services, fatal accidents in the early 1920s occurred at a rate of one per half million aircraft-miles. In the first half of the present decade, the corresponding rate is around one per 500 million aircraft-miles. This represents an astonishing improvement of the order 1000-fold. To what extent this improvement came about to satisfy public demand, or as a result of pressure from the powers-that-be, or from a professional desire to do an increasingly good job, is hard to say. Probably all these motivating factors played a part.