Many thousands of words have already been spoken and numerous papers written extolling the virtues of fly-by-wire and the control configured vehicle. Most of these issue from control system specialists and avionics engineers anxious to sell their new technology. Aircraft designers and development teams, including test pilots, must adopt a cautious approach to these new developments and ask the questions “What has it to offer?” and “What's the catch?”.

We must always balance performance rewards against development risks, ensuring that the risks are carefully evaluated and that we do not suffer from what the Americans call “Second Degree Ignorance” (we don't know what we don't know). If we can define the development problems, we are well on the way to solving them; if they remain obscure then the benefits will elude us. If the risk outweighs the reward a conservative solution must be adopted; it is this consideration that so often makes us, like Hamlet, “rather bear those ills we have, than fly to others that we know not of”.

Sir George Cayley was a great man, many years in advance of his time and with a grasp of fundamentals which must be the envy of many people, even today. Consider the mental prowess of a man who in the technical environment of 170 years ago, where a 12 hp engine was as shown in Fig. 1, could make the statement that, “The whole problem is confined within these limits, namely, to make a surface support a given weight by the application of power to the resistance of air”; it is truly remarkable. It seems to me that the phrase I have extracted from that statement as a title to this lecture epitomises both the man and the subject.

The two philosophies for the attitude control of flexible space vehicles that appear practicable are: firstly, the complete control of all the plant states, in the sense that all states tend to some bounded value as time tends to infinity; and secondly, to control the rigid body to lie within some bounded region and allow the vibrations within the structure to behave in an uncontrolled manner. If the former strategy is implemented, not only is the attitude of some part of the structure, for example a radio antenna, controlled to lie within some bounded region, but the modes of structural vibration are also explicitly controlled. The second method requires less state information than the first and is the less complex of the two.

In many design procedures for calculating supersonic isentropic flow the problem arises of calculating the change in Mach number which will occur due to a specified change in flow direction, e.g. considering simple Prandtl-Meyer corner flow, it is required to calculate the final Mach number M to which the initial sonic stream will be accelerated by turning through a specified angle v.