The XV-15 experimental tiltrotor aircraft made its first flight on May 3, 1977. Only two were built, but the results yielded from extensive testing led to the development of the Bell Boeing V-22 Osprey in service today.

Westland started looking seriously at convertiplane projects the same decade, particularly after its takeover of Fairey Aviation Ltd, which had put great effort into the Rotodyne. Westland gravitated towards the “tilting powerpack and rotor”, going so far as to produce a full-scale mock-up of the nacelle assembly for the Westland WE-01 small transport in the early 1970s, pictured here. Much larger military and civil transports were proposed but none progressed to actual hardware.

The model after covering, with motors “tilted for take-off or landing”. A small crank handle for adjusting the nacelle tilt angle may just be seen on the wing centre section.

Otwell had a model built as a proof-of-concept machine, based on his 1934 patent. Of trimotor configuration, the model is seen in these photographs before the wings have been covered, the picture above apparently having been taken before the engine mountings or engines have been fitted. The image at left shows the model with the engines fitted and positioned horizontally for forward flight, with the picture below depicting the wing-mounted 0-6 b.h.p. Forster .99 spark-ignition motors in their vertical position for landing. The patent stated that any position between horizontal and vertical could be selected.

Another illustration of dubious quality apparently depicting a pair of trimotors fitted with Otwell’s tiltable engine arrangement. The machine furthest away is flying with the motors in the horizontal mode for forward flight, while the nearest appears to be preparing to alight on a somewhat jagged bed of very unfriendly-looking rocks!

Otwell promoted his concept with some illustrations of indifferent quality, this example showing a trimotor aircraft with the radial piston engines tilted in the vertical position and an inset of gulls alighting on water to demonstrate the effect.

Illustrations from Otwell’s patent granted in October 1932. Fig 1 shows a front view of the engine mountings attached to the wing; Fig 2 is a side elevation of one of the engines in its mounting in the horizontal position, with dashed lines showing the vertical position; Fig 3 is a plan view showing the sprung, sliding bolt that locates in notches in the guide rail; Fig 4 shows the nacelle angle-fixing mechanism.

An illustration from Otwell’s improved and simplified patent dated July 10, 1934. Fig 5 shows the flexible fuel pipe with springs marked as “36”.

FOLLOWING HIRAM OTWELL’S largely ignored innovation of changing the thrust angle by inclining the entire engine nacelle, a number of designers independently arrived at the same solution. The earliest after Otwell was British designer Leslie Baynes, who developed the strikingly modern-looking Baynes Heliplane in the late 1930s and early 1940s. Baynes’s patents in 1937 and 1938 sought to combine some advantages of both helicopter and aeroplane, with an aircraft mounting a rotor at each wingtip that could articulate through 90°, driven by shafts from a fuselage-mounted engine. Baynes also innovated a novel form of propulsion, using a free-piston gas-generator in the fuselage supplying hot gases to a turbine. In 1940 Baynes combined both ideas into a heliplane with turbines in wingtip-mounted nacelles, which could swivel from vertical to horizontal. Wartime pressures saw that the idea never progressed.

As the tiltrotor gradually approached practicality in the 1960s, more projects began to emerge. As predicted by Baynes, the development of small but powerful turbine engines made the pivoting nacelle a much more attractive proposition than complex shaft-driven rotors. In West Germany, Weserflug Flugzeugbau updated ideas it had developed in the 1940s to create the WFG P16/P23/P24 series (a model of the P16 is seen here). In Weser’s wartime designs, the outer portion of wing swivelled with nacelle attached. In its 1960s designs, only the nacelles pivoted. These were proposed as executive transports, passenger aircraft and even an anti-helicopter combat aircraft.