The important thing in the land battle is to overpower your opponent by out-manoeuvring him and then hitting him with all the weapons at your disposal. Clubs, swords, spears, brickbats and arrows, or rifles, machine guns and artillery or nuclear, chemical and guided weapons, the principles are the same:

• (a) Seek the enemy out.

• (b) Move into position.

• (c) Destroy him in battle.

The first task is therefore surveillance and target acquisition. This is a task which requires a soldier to get into a position to observe and discover where your enemy is and what he is doing. Most sensible enemies hide behind hills to keep out of harm's way, in order to conceal their future intentions. Therefore one of our basic problems is to find out what goes on behind the hill.

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.

The attitude of a spinning satellite, specified in terms of the three Eulerian angles, is related to the angular velocity components in body-fixed co-ordinates by three coupled first-order non-linear differential equations. In general, these equations, being complex, are not amenable to analytical techniques. This study presents a new analytical approach to the analysis of these equations by first reducing the problem to the study of a single second-order non-linear differential equation and then establishing its equivalence with a second-order linear differential equation for the case of the satellite spinning with constant angular velocity components. The solution of this equivalent linear equation is then used to obtain the Eulerian angles.

The natural frequencies of a plate or a shell can be changed by altering its stiffness. This can be achieved by appropriately varying the mass or curvature, by introducing initial stresses or by attaching stiffeners to the system. The last method has obvious advantages over the other methods. The natural frequencies are thus raised, lowered or remain unchanged depending on (i) the dimensions of the stiffener relative to the plate or shell dimensions, and (ii) its position.