Based on Bell’s experience with the XV-3 (see page 48), the Bell XV-15 tilt-rotor was designed and first flown in 1977. It has proven a reliable research aircraft, from which much knowledge has been gained about tilt-rotor technology. The much larger V-22 tilt – rotor (Fig. 1.40) flew in 1989 and in 2005, after a lengthy development phase, it was in low-rate production. The V-22 is designed to fullful many of the missions that are possible with a conventional helicopter, but with the added advantage of much higher flight speed capability. However, it must be borne in mind that the tilt-rotor design is a hybrid, and a performance compromise, between a helicopter and a conventional airplane. The Agusta – Bell Model 609 tilt-rotor was designed in the late 1990s for the civilian market.

The aerodynamic design of the tilt-rotor poses many additional challenges over that of the conventional helicopter. The smaller rotor diameter means that in hover the disk loadings of a tilt-rotor are much higher than those of a helicopter of the same gross weight. The efficiency of the rotor is also lowered because of the very high blade twist, a feature required to ensure good propulsive efficiency in airplane mode. In addition, when in hover the wings of the tilt-rotor operate in the downwash from the rotors, which produces a large download on the aircraft and decreases the payload it can carry. The fixed wings also influence rotor performance, and the combined effect degrades the hovering efficiency (figure of merit) compared to a helicopter of the same gross weight – see McVeigh et al.

(1991) . Various techniques are used to minimize these interactional aerodynamic effects, including 90-degree deflections of the wing flans — see Stepniewski & Keys (1984) and Wood & Peryea (1991). In fixed-wing mode, when the rotors act as propellers, they are rather less efficient because the disk loading is too low and profile losses are higher than would be achieved with a conventional propeller. Rosenstein (1986) and McVeigh et al.

(1997) give good overviews of the numerous aerodynamic design trade-offs for tilt-rotors.

The tilt-rotor has two sets of flight controls, one set for helicopter mode and another for fixed-wing mode. In helicopter mode, the rotors of a tilt-rotor are controlled with normal cyclic and collective pitch controls. Yaw is controlled by differential cyclic, just like a tandem rotor machine such as the CH-47. During transition from helicopter mode to fixed – wing mode, the aerodynamic environment encountered by the rotors becomes extremely complex and high airloads can be produced. The relationship between the airspeed and the rotor tilt angle must conform to a relatively narrow envelope for safe operation and this is controlled with the aid of the fight control system. In airplane mode the rotors act as conventional propellers, with all of the vertical lift forces being produced by the wing. In this mode, control is achieved with the use of conventional aerodynamic surfaces, namely the flaperons, elevator, and rudder. Despite the complexity and high costs of the tilt-rotor it has the potential to play a unique role in vertical flight aviation that a conventional helicopter cannot. It has not yet, however, advanced to the level of military or civil success enjoyed by the modern helicopter.

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