Other machines followed. The German Dornier Do-31, another airplane look-alike that utilized both lift engines and vector-thrust, and the Ryan tail-sitting X-13 Vertijet taught engineers valuable lessons. During the same period, Bell Aircraft conducted test of its own with its Model 65. This innovative machine resembled a high-wing airplane with a turbine engine mounted on each side of the fuselage. To provide vertical thrust, both engines were swiveled downward. Then, in 1956, Bell constructed the X-14. This design resembled a conventional airplane but mounted two Armstrong Siddeley Viper engines side by side in the nose. It also differed in that the engines were fixed in place and the thrust was vectored downward by way of a cascade vane system under the aircraft’s center of gravity. The X-14 was so successful that it flew in text programs from 1957 until 1981, when it was finally retired from service at the National Aeronautics and Space Administration (NASA), which had taken it over from Bell.

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As these developments were taking place in the United States, the British were also busily pursuing experiments in vectored thrust on their side of the Atlantic. The Short Brothers and the Harland Company, teaming up with Rolls Royce, constructed the SC-1, a stubby bumblebee-shaped airplane powered by five Rolls Royce RB 108 turbine engines. Four were mounted vertically in a compartment at the fuselage center to provide lift, with the fifth in the tail to provide thrust. To control the aircraft’s pitch and roll in hover configuration when there was inadequate airflow over the normal aerodynamic control surfaces, the lift engines could be partially swiveled while high pressure air was directed to reaction control valves that controlled the air ejector nozzles at the nose, tail and wing tips. In 1960 the SC-1 dramatically demonstrated the ability to hover, shift to forward flight and return to hover – all under the power of vectored thrust. The concept of extremity-placed ejector nozzles, or puffers, to control roll and pitch would become the standard for future British V/STOL (Vertical and Short Takeoff and Landing) developments.

The French Dassault engineers, following the idea of separate engines for lift and forward flight, constructed a single-test machine known as the Balzac. Resembling a delta-wing fighter, the airplane seemed to hold promise for the French when, in 1965, it did manage to hover and attain forward flight. However, the Balzac design was involved in two crashes and no more were built.

The Soviet Yakovlev design bureau watched the American and West European developers carefully. Noting the successful efforts that combined both lift and forward-thrust engines, the Soviet engineers retained this line of thinking in their development of the Yak-38 Forger. This machine, powered by no less than three engines – two to provide hover thrust and one for forward flight – proved successful and entered operational service with the Soviet Navy. First seen by the West in 1976 on the flight deck of the Kiev when she entered the Mediterranean, the Yak-38 holds the distinction of being one of only two V/STOL fighter aircraft to enter military operational service following three decades of intense development. The other is the British Aerospace Harrier.

Breaking away from the idea of separate engines to operate in two environments – hover and forward flight – the technical director of Bristol Engine Company, Sir Stanley Hooker, took a new tack. He reasoned that if a single engine was powerful enough, and if the thrust provided by that engine could be directed or ducted where needed, then one power plant would not only be sufficient but also much easier to design an aircraft around.

His design, a large turbofan engine that could duct cool fan air to two swiveling nozzles at the front of an aircraft and hot exhaust air to like nozzles at the rear, became the Bristol Siddeley Pegasus. The accompanying airframe, designed by Hawker Siddeley, consisted of a negative dihedral, swept wing mounted high on the fuselage to keep clear of the side-mounted nozzles, two oversized fan inlets, and a ductwork of tubes that distributed high-pressure air to the puffers at the wing tips, nose and tail. Because the central section of fuselage contained the engine and thrust nozzles, two main landing gear were placed in a fore-and-aft tandem configuration along the fuselage center-line in front of, and behind, the engine. The wings were supported by two small wing-tip-mounted outrigger wheels. This combination of power plant and airframe was designated the XP-831.

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