The five-six seater single-engine helicopters represent a large share of the civil market. The market studies show that the main target to be reached in this field is to succeed in having a purchase price and operating costs significantly lower than those of current aircraft.

But the quality aspect of the helicopter should not be overlooked: the comfort and performance data should remain at a competitive level; the helicopter should have a high reliability level and offer easy maintenance characteristics.

Therefore the light advanced helicopter should represent an excellent cost efficiency compromise. This was the constant target kept in mind all along the design, development and production phases of the AS.350 (see Fig. 1).

A new generation of helicopters was born at the beginning of the sixties, when, following a US requirement for a light observation helicopter (LOH), a new breed of military machines, like the Bell 206 (OH-4 A, OH-58 A), the Hiller 1100 (OH-5 A) and the Hughes 500 (OH-6 A) was developed.

All the aerodynamic and structural progress resulting from the previous experience of the helicopter industry was incorporated in their designs. The development of these helicopters, demonstrating good performance and comfort levels, light and relatively inexpensive was made possible, however, only because of the availability of a new, light turboshaft engine, featuring acceptable levels of fuel consumption and reliability.

Bell's Model 222 is an 8-10 place helicopter powered by two Lycoming LTS101-650C2 engines. It is the result of extensive engineering design studies. These studies considered several configurations including fore and aft engines, laterally disposed engines, two across seating, fixed landing gear, skid landing gear and JetRanger growth versions. Two full-scale mockups were built and thoroughly evaluated prior to selecting the final configuration. The mockup selected was used extensively for evaluation by Bell and by commercial customers resulting in additional refinements. The final configuration is shown in flight in Fig. 1.

Largely as a result of frequent written reports by Pierre Des Noyers, secretary to the Queen of Poland, mid-seventeenth century scientists took a great deal of interest in the aeronautical work of an Italian engineer, Tito Livio Burattini. Born in Agordo, north of Venice, on 8th March 1617, Burattini early became a travelling scholar. By 1637 he had reached Egypt, where he helped John Greaves with his famous work on the pyramids. Despite his youth, he appears greatly to have impressed Greaves, who referred to him, with slight inaccuracy, as ‘Titus Livius Burretinus, a Venetian, an ingenious young man'. In 1641 Burattini returned to Europe, spending a little time in Germany and travelling on to Poland, where he lived until 1645. In 1647, after a visit to his native Italy and a second period in Egypt, he settled permanently in Poland, where he attempted, soon after his arrival, to establish himself at the court of Wladislaw IV (30th May 1595-20th May 1648).

There is a growing interest in the analysis and design of waffle cylinders because of the advantages of integrally stiffened shell wall construction. The usual approach to the stability analysis of a stiffened shell has been to replace it by an equivalent orthotropic shell. The eccentricity of the stiffener has a large effect on the critical load in cylindrical shells, and outside stiffening has been found to be stronger than inside stiffening. Inversion of the stiffener eccentricity effect has been observed in cylinders under hydrostatic pressure loading. Naturally, regardless of whether the stiffened cylinder is considered as an orthotropic continuum or a composite of discrete elements, the treatment cannot be complete if it fails to include the coupling between bending and extensional forces and deformations resulting from one-sidedness of the stiffeners.

Studies made of the flow about the wing-tip of a wind tunnel model showed that the vortex-type flow generally associated with the rolled-up vortex sheet well behind the tip existed just behind the trailing edge and seemed to be forming over the top of the wing-tip itself. As a result small cambered aerofoils were attached to the wing-tip to redirect the flow locally in such a way that each experienced a force with a significant thrust component. The effect is similar to that generated by the sail of a close-hauled yacht; consequently these small tip extensions were called sails. Reference 1 describes the original wind tunnel and the preliminary flight tests of sails mounted from the tip tanks of the Paris aircraft of the Cranfield Institute of Technology.