No abstract

The compound helicopter has much to offer as a means of expanding the¯ight envelope of rotorcraft. There have been signi®cant amounts of research performed, albeit disjointed, and manȳ ight demonstrator compound helicopters produced and trialled since the end of the Second World WarÐmainly with the aim of increasing the helicopter's maximum forward speed. This paper aims to pool this knowledge, along with relevant developments in other ®elds, not only to state the current level of understanding of the compound helicopter con®guration but also to use it to predict likely con®gurations and technologies that would be best suited to future compound helicopter designs. From this basis it is suggested that the most likely con®guration will be a low wing of minimized area to alleviate retreating blade stall at a minimum hover blockage, with plain¯aps for blockage reduction and control of the rotor±wing load share. In terms of propulsion a ducted fan or propeller are deemed most suitable, driven by either an advanced mechanical drive or variable cycle engine. The areas of rotor±wing interaction and advanced rotor design are also noted as subjects of signi®cant interest in the optimal design of a compound helicopter.

No abstract

Blade sailing is an aeroelastic phenomenon affecting helicopter rotors when rotating at low speeds in high wind conditions. This is a potentially dangerous blade motion and the excessive flapwise tip deflections generated endanger the airframe, the flight crew and any personnel working close to the aircraft. This phenomenon is particularly applicable to naval helicopters or those operating off exposed sights such as oil rigs. The research covered an experimental investigation into the effect of an abeam wind flow over a simulated flight deck of a Rover Class Royal Fleet Auxiliary vessel. Blade flexibility and rotor hub mechanical features were introduced into the theory and the resulting method applied to the Westland Lynx and Sea King aircraft. The semi-rigid rotor of the Lynx is relatively well controlled, but the rotor hub construction of the articulated rotor of the Sea King and the interaction with the flexing blades allow blade tip deflections to be generated of an order to strike the fuselage. The research described in the paper has led to an improved understanding of the blade sailing phenomenon. The results concur with reports of blade sailing occurrences and the operational conditions that are likely to trigger this phenomenon. The development of theoretical models may be used to assess the importance or otherwise of this effect on future rotor designs or the operational limitations of the aircraft.

A computational¯uid dynamics model of a hovering helicopter main rotor is developed to examine air¯ow in the presence of ship structures and side winds. An illustration of the problem is given. The rotor is modelled by modifying the governing N avier±Stokes equations in the region of the disc. The extra terms added to the governing equations apply a downward force to the¯uid; these forces are independent of the¯ow around the rotor and are equal to the helicopter weight.The helicopter rotor model and the ship model are combined to yield one¯ow solution, which, due to the severe non-linearities of the problem, cannot be achieved by superpositio n. The resultant¯ow yields valuable data about the induced velocities at the rotor, which ultima tely determine the control pitch and power required to maintain the hover in a given location. Indeed, the interactions between the rotor downwash and ship air¯ow are known to produce unexpected and adverse¯ight dynamic behaviour of the aircraft.