The diminutive Ryan X-13 Vertijet was designed to explore the feasibility of a pure-jet vertical takeoff and landing (VTOL) fighter aircraft. Using a Rolls Royce Avon turbojet engine, the aircraft easily made the transition between vertical and horizontal flight and enjoyed a highly successful career. Never seriously considered for further airframe development, it nevertheless paved the way for the modern Harrier fighter by showing that pure-jet VTOL flight was possible.
The experience of the two big tailsitting turbojets immediately raised the question: was it possible to design a jet aircraft to take off and land in the vertical mode? In theory, of course, there could be no doubt; all that would be required would be a jet engine capable of delivering more thrust to an aircraft than the combined weight of airframe and engine--i.e., a favorable thrust-to-weight ratio.
Commonplace today, this was not so easy to achieve in the mid-1950s. Until the present generation of high performance jet engines came along, delivering enormous thrust derived from high internal heat and pressures, aircraft depended upon light airframes, restricted payloads and superb streamlining in order to reach reasonable performance levels. With the modest engines then available, the only answer was to develop the smallest feasible airplane and, even then, to be content with moderate performance levels.
Based on the Ryan Aeronautical Company's successful combination jet-and-piston engine FR-1 Fireball fighter for the Navy, and impressed with the company's interest in developing higher thrust levels, the Air Force signed a contract for a pure-jet VTOL research aircraft on 28 July, 1954. The project expanded with the Navy and NACA joining as sponsors during various phases of the effort. The X-13 Vertijet which resulted was to be a proof-of-concept vehicle only and, in spite of overheated enthusiasm in some quarters, in no way was it ever a prototype fighter.
Minimalism was the key to the X-13's success. To begin with, it was tiny--24 ft long with a wingspan of only 21 ft, weighing a maximum of only 7,200 lbs--and numerous items found in conventional airplanes were omitted here: landing gear, flaps, dive brakes, no catapult or arresting gear, limited space for test instrumentation and absolutely none for armament. Fuel capacity was strictly limited, too, as one pilot found out to his discomfiture. During a demonstration flight from Andrews AFB to the Pentagon grounds in July, 1957, Peter Girard found that the plane carried barely enough fuel to do the job; a flubbed landing attempt would mean a prompt climb to altitude and ejecting. Nevertheless, he overcame Potomac boat traffic, blinding water spray and an unruly hedge in order to made history's first and only landing of a fixed-wing jet aircraft at the Pentagon.
For all its design limitations, the X-13 was a fully-functioning airplane with operational controls, a simple delta-wing, tailless design. Being jet-propelled, it had to deal with control problems which its tailsitting turboprop cousins never faced. The XFV-1's heavy prop blast washed over its wing and tail surfaces, making its rudder and elevators effective at zero airspeed. Whenever the X-13 perched atop its dynamic column of jet exhaust, however, its conventional flight controls were useless. In order to be able to maneuver while hovering, it used the first gimballed nozzle ever mounted in an aircraft to control pitch and yaw; wingtip nozzles provided roll control.
In place of landing gear, casters or a skid, the X-13 relied upon a simple hook mounted under its nose; instead of a launching pad or runway, it was mated to the flat bed of a truck. In preparation for a launch, the truck bed was hydraulically raised to a vertical position, like a dump truck, and the pilot would rotate his seat up to 45 degrees to compensate. At launch, the tiny jet would simply lift off vertically, rise to a few hundred feet, and arch over into horizontal flight, where its conventional controls would become effective. Its landing technique was equally simple and effective. Approaching the recovery trailer along a medium or low (usually low) profile, the pilot would point the aircraft's nose to the vertical, gradually slow to a zero airspeed a few feet above the ground, and then delicately "walk" his plane a few feet forward until the nose hook engaged a raised wire. With its engine cut, the X-13 would settle against the flatbed, ready to be lowered to the horizontal and trucked away.
The Vertijet was clearly a technical success. It proved its design concept and demonstrated that a VTOL flight with a jet aircraft was indeed possible. Practicality, however, was a different matter. The gallant little jet also proved to be a technological dead end; its feats were never replicated and no X-13 successors grace today's skies. Instead, aircraft designers took another and totally different approach to the problem of jet-propelled VTOL: directed thrust. Today's Harrier attack planes, widely used in several nations, take to the air on columns of vectored thrust, directed by gimballed nozzles. However little a burly Marine Corps Harrier may resemble the elegant and delicate X-13, it nevertheless can claim the Vertijet in its design ancestry.
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