Flight that is unconventional now may not be unconventional in the future. In 1903 all flight by man was unconventional. Although the 1903 limitations in knowledge and convention have dropped away, certain kinds of flight remain unconventional for practical, if not good, reasons. Unconventional flight is flight outside the mainstreams of research and development. This implies that it includes flight modes for which little or no money is available and which, therefore, can attract only few knowledgeable and dedicated enthusiasts. Hence these modes lag behind the contemporarily directly useful aspects of flight.

It is a great privilege for me to give the Eighth Cierva Memorial Lecture and to pay tribute and honour to Juan de la Cierva who, more than any other inventor, was responsible for bringing into reality the principle of rotating wing flight. Cierva was an inventor of indefatigable industry, patience and genius. His faith in the Autogiro and the future of the rotating wing aircraft will be remembered for ever.

The invention by Cierva of the articulated rotor with flapping and drag hinges to relieve bending stresses at the blade root, and in the chord-wise plane, provided the solution to true helicopter flight which become a reality in 1936.

Everyone here will realise that I have to treat a vast subject in a short period of time, and those who are already familiar with it will be aware that I shall have to make a number of assertions, each of which really deserves separate treatment in its own right. I shall arrange the lecture as follows. I shall begin by clarifying the basic assumptions on which I intend to proceed. Next, I shall look at the state of the laws of aerial warfare at the end of the Second World War. Finally, I shall make a few observations on the present state of the law. As time is limited I shall devote more attention to the principles according to which, in my view, this subject should be broached than to detailed rules.

We are now developing two large, new aeroplanes in the Boeing Commercial Airplane Division. The 747 —(Fig. 1) is subsonic and is in the initial production stage. First deliveries are planned for the autumn of 1969. The other is the SST prototype—(Fig. 2) which is to fly in 1972, if all goes well, with production deliveries projected for the mid-seventies. Both aeroplanes are challenges in many ways—in a technical sense—from a production standpoint—from a financial standpoint—and from an overall resource standpoint.

The recently developed theory of modal control has so far been mainly applied to the design of process control systems. This theory is, however, capable of much wider application and it is the principal object of this paper to illustrate its use as a basis for the synthesis of aircraft control systems. The particular illustrative problem considered is that of synthesising a lateral autopilot for bank-angle and roll-rate mode control. Typical aerodynamic data for an aircraft flying at M=0·9 and at altitudes from sea level to 40 000 ft (12 km) are used in the calculations.

The analytical construction of displacement functions for plate elements subject to bending was discussed at some length in ref. 1. Attention was thereby drawn to both triangular and quadrilateral elements available in the ASKA system, which satisfy either all kinematic compatibility conditions or are deficient in the continuity of the gradient normal to the edge. The most rudimentary elements of the triangular set consist of the so-called fully compatible TRIB 3C and the TRIB 3, which satisfies the slope condition only at the vertices. The TRIB 3C corresponds in principle, but not in detail, to the element evolved by Bazeley et al but allows, however, also for linear taper.

The special structural problems of VTOL aircraft are examined, taking the Do 31 as a basis and it is shown how these problems can be solved without significant weight penalties.

After a short description of the aircraft and the systems associated with aircraft testing, the structural design of the Do 31 is described.

The effect of special VTOL load cases on the individual aircraft components are illustrated and it is shown where these are critical. The methods used in the calculations are explained briefly and theoretical results are compared with tests.

Special emphasis is given to the lift pod structure and its structural analysis.

There are special problems with VTOL aircraft which must be considered during the structural design phase. The effects of temperature on the engine pods, wing and fuselage are shown and the solution of the associated design problems are indicated. The results obtained are compared with a number of Do 31 test results.

If conventional British or US fire requirements are applied to the design of lift pods, there is a possibility of some weight penalties. Fire problems and fire requirements for lift pods are discussed with reference to test results. Recommendations for minimising the danger of explosions and for the use of fire detecting devices are also made.

A great advantage of VTOL aircraft is their ability to use unprepared sites. Debris may be thrown up by the downward deflected jet exhausts and strike the aircraft. Calculations and test results show the effect of impact velocities, stone shape, etc. Structural optimisation for withstanding debris impact is considered.

Noise problems have to be investigated during the aircraft design phase because of the high engine thrust installed in a VTOL aircraft. Results of the Do 31 tor near field noise levels are given. Particular attention is given to the proportion of overall noise level which affects the airframe.