I will describe in this paper some features of current Boeing-Vertol production helicopters that I have selected as those which strike me personally as being of particular engineering interest. But first just to provide some perspective on the subject I would like to summarise the background of the present Vertol Division of The Boeing Company. The Company was formed in 1944 as the Piasecki Aircraft Corporation and its first product was the HRP Tandem Rotor Helicopter, which first flew in 1945. This was followed by the HUP in 1948, the H-21 in 1952 and the H-16 in 1953. In 1956 the Company was renamed Vertol and the next aircraft to fly was the Model 107 Prototype in 1958. In 1960 Vertol was acquired by Boeing and became the Vertol Division. The currently manufactured models about which I am going to talk, are the 107 series (now mostly in production as the US Marine Corps Model CH-46, Sea Knight) and the 114 series (in service with the US Army as the CH-47, Chinook) of helicopters.

I have been asked to talk today about international co-operation with respect to helicopter airworthiness regulations. If I confined my talk to this subject only it would be very brief for, to my knowledge, this co-operation is at the present time very limited, if not non-existent.

The airworthiness committee of the ICAO which is the officially appointed organisation for international co-operation does not often deal with helicopter problems. As to the military, except for a number of points of a very limited scope (such as location of the cargo sling control on the cyclic stick) they have never really approached important problems specific to the helicopter from an international stand-point. The trend they are following today is rather to go by the civil regulations in force in their respective countries and there is an increasing tendency in the various manufacturing countries to consider conformity to civil regulations as the prime requirement to be met for the development of a prototype intended for military usage, except of course for a number of specific points related to battlefield applications which impose requirements different from those to be met for civil applications.

We continue our fundamental discourse on the natural modes in an element and extend our considerations to large displacements. First, we present a general procedure for establishing the so-called geometrical stiffness kG, when the natural modes of the complete element are given. The theory is a substantial generalisation and clarification of the method initially given in refs. 2 and 3 and shows that in establishing the modification to the transformation matrix aN it is necessary to ignore the contribution of the natural modes, which cause rotation at the nodal points but no displacement there. The point is subtle and was not made in ref. 2, although the applications given there are correct. As an example, the geometrical stiffness of a straight beam in space with all degrees of freedom is established. There follows the extension of the geometrical stiffness concept to a sub-element. This is of great practical significance for two reasons. First, it allows to derive the geometrical stiffness of elements of complex shape and behaviour, e.g. curved beams in space subjected to normal forces, bending moments and torque.