Find a Security Clearance Job!

Military


XV-5 Vertifan

In November 1961, General Electric won a US Army contract to develop its fan-in-wing concept, the XV-5A, with design, construction and flight testing of the aircraft was sub-contracted to Ryan. General Electric retained responsibility for the propulsion system and integration with the aircraft. In the inboard portion of each wing a 5 ft diameter fan provided vertical lift. A smaller fan in the nose in front of the two person cockpit give pitch control and additional lift.

During 1958 the Army contracted with General Electric Co. to build a tip turbine lift fan powered by a gas producer in order to establish the potential value of this lift/thrust system. The results were so encouraging that in 1961 a competition was held for design and fabrication of two flight research vehicles incorporating the lift fan principle. From this then, Ryan Aeronautical Co. was selected to build theairframe of the XV-5A under sub-contract to General Elec-tric. Ryan had been interested in this means of VTOL for sometime, and had acquired a considerable in-house experience through studies and tests.

There were two large wingfans and a smaller nose fan. These are used to provide lift, control, and trim during fan supported flight. All three fans and the two J85 engines, similar to those used in the F-5 Freedom Fighter, were cross ducted to afford safety and control in the event of an engine failure. The location of the small fan in the nose is significant in that this fan is normally lifting and only produces a down load when required to overcome a nose-up pitching moment. Roll and yaw con-trol are accomplished by movement of exit vanes mountedunder each wing fan. A diverter value is positioned immediately to the rear of each gas generator. It is here that the hot gases are directed through the ducts to drive the fans, or aft through the tailpipes for forward thrust. An interesting point arising from cross-ducting provisions, is that the failure of one engine results in a loss of vertical lift of only 40% instead of 50%. This phenomenon is attributable to the power absorption characteristics of the fans. Under standard conditions and normal landing weights it is possible to make a satisfactory landing with one engine out.

The pilot was provided with conventional helicopter controls. Pitch attitude was controlled by longitudinal stick which actuated pitch-fan thrust-reverser doors. The collective stick provided height control during hover and slow flight by actuating the wing-fan exit louvers. Lateral stick provided roll control through differential actuation of the wing-fan exit louvers. Fan-mode (low) speed was controlled through a stick-mounted "beep"switch which changed the fan exit louver angle. Pedal movement provided yaw through differential actuation of fan exit louver angle.

One of the outstanding safety features of the gas-driven lift-fan concept was the robustness of the lift-fans themselves. The absence of drive shafts, shaft bearings, gearboxes and the attendant pressure lubrication systems resulted in relatively low maintenance headaches and high pilot confidence. The only indicators associated with the three lift-fans installed in the XV-5 were rpm and fan cavity temperature. The fans could take tremendous amounts of abuse including sand and pebble ingestion.

Two aircraft were built; the first one flew from 25 May 1964, the first hover was in June 1964, and first transition in November 1964. The 12,500lb. (maximum gross weight) XV-5A was evaluated in late 1966 by 15 test pilots (the"XV-5A Fan Club") during trials to evaluate the aircraft's potential as a strike escort/rescue aircraft for Viet Nam.

One aircraft was totally destroyed during an official flight demonstration in April 1965, killing the pilot during a transition attempt. One of the two XV-5As being flown at Edwards AFB during an official flight demonstrationon the morning of 27 April 1965, crashed onto the lakebed, killing Ryan's Chief Engineering Test Pilot, Lou Everett. The two aircraft were simultaneously demonstrating the high-and low-speed capabilities of the "Vertifan". During a high-speed pass, Everett's aircraft pushed over into a 30 degree dive and never recovered. The accident board concluded that the uncontrolled dive was a result of an accidental actuation of the conversions witch that took place when the aircraft's speed was far in excess of the safe jet-mode to fan-mode conversion speed limit.

The remaining XV-5A was rigged with a pilot-operated rescue hoist, located on the left side of the fuselage just ahead of the wing fan. An evaluation test pilot was fatally injured in October 1966 during the test program while performing a low-speed, steep-descent "pick-up" maneuver at Edwards AFB. The heavily-weighted rescue collar was ingested into the left wing fan as the pilot descended and simultaneously payed-out the collar. The damaged fan continued to rotate, but the resultant loss in fan lift caused the aircraft to roll-left and settle toward the ground. The pilot elected to eject from the aircraft as it approached the ground. As the pilot initiated the ejection, the aircraft rolled back to the left which caused the ejected seat trajectory to veer-off to a path parallel to the ground. The seat impacted the ground, and the pilot failed to survive the ejection. Although the pilot sustained fatal injuries, damage to the aircraft was moderate.

This second aircraft was rebuilt as the XV-5B. This had a wider landing gear, had an improved cockpit, and removed the thrust spoiler. During repair and rebuild into the XV-5B configuration, several modifications were incorporated as a result of lessons learned: 1) mechanical tie between the stabilizer and diverter valve actuators, 2) enlargement of main landing gear tread, 3) incorporation of an improved fuel supply and management system, and 4)an improved cockpit arrangement. The first flight of the XV-5B took place on 24 June 1968 [other sources say in July 1968], and the aircraft was turned over to NASA-Ames a month later. Flight tests which continued until January 1971 involved investigation of steep terminal area approaches and measurement of aircraft noise footprints.

The XV-5A evaluations demonstrated that the lift-fan concept was valid and that the operational procedures were relatively straight-forward and easy to adapt to in general. Handling qualities while hovering in ground effect (below i0 to 15 feet) were found to be unpleasant and noted to be more pronounced than that experienced with the X-14A jet-lift research aircraft. The XV-5A did not have an integrated powered-lift flight control system. Without an integrated powered-lift flight control system, the pilot could set himself up in potentially hazardous situations, and moderately high pilot workload was required to coordinate a smooth vertical take-off.

Conversions between jet and fan modes of flight had to be performed within a narrow air speed corridor in order to maintain safe longitudinal control during the rapid "bang-bang" conversion sequence operation. This corridor, was bounded by the overlap of maximum level-flight fan-mode airspeed (about 104 knots) and minimum jet-mode airspeed (about 89 knots). Ideally, jet to fan mode was performed at about 95 knots and fan to jet at about 88 knots. The requirements to perform conversions within this narrow operating envelope severely restricted the operational flexibility of the XV-5A and placed an unreasonable demand on the pilot's adherence to procedures.

Avoidance of the fan stall boundary placed significant operational limitations on the XV-5. Fan stall, like wing stall, must be avoided and requires the observance of during routine operations. Approach to the fan stall boundary particular problem in the XV-5B while performing steep terminal instrument approaches.

During conversions, two essential actions had to occur simultaneously for the aircraft to maintain longitudinal control) diverter valve movement, which controlled the flow of J85 gases to either the tailpipes for jet-mode or to the fans for fan-mode and) stabilizer nose-up or nose-down trim which compensated for the powerful changes in pitching moment as the fans were powered up or down. If either of the actions occurred without the other ("split-mode"), the aircraft became uncontrollable in pitch. Conversion between jet and fan modes of flight was the most exacting, interesting,and potentially hazardous operational aspect of the XV-5A. The conversion design and operational philosophy were felt to be completely unsatisfactory for everyday, operational use.

The drawbacks of the Vertifan were the large volume and weight occupied by the lift system, slow control response, and the narrow transition corridor. The XV-5 crashed twice, proved to generate a disappointing amount of lift, and was difficult to transition to horizontal flight. The lift fan system was heavy and required too much internal volume, and service pilots would have difficulty with the narrow transition zone.



NEWSLETTER
Join the GlobalSecurity.org mailing list