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Our first topic: Attack helicopters. They are undoubtedly awesome, but in a large-scale conventional war, will they be resilient enough to have a presence on the battlefield?
During the 1960s, both the USSR and NATO countries had many programs for the development of VTOL aircraft. Most of these never reach flying status, though a few did fly at least in prototype form. One of these was the Ryan XV-5 Vertifan.
Development of the XV-5 began on November 10, 1961, when the US Army issued a contract for the development of an aircraft using the lift-fan propulsion system. Primary contractors were General Electric and Ryan Aeronautics. The lift-fan system was quite different from other VTOL systems of the time, such as the lift jets found in the original MiG-23 prototype or contemporary VZ-4, for instance. Rather than the jet exhaust directly providing the thrust for vertical flight, the exhaust drives several fans, which provide the thrust for vertical lift. This has the advantage of not projecting hot exhaust gases downward, however, there are losses in efficiency due to the extensive ducting needed.
The XV-5 (orignally designated VZ-11 at the start of the program) was powered by a pair of J85 turbojets, the same engines as found in the F-5. Maximum takeoff weight was 12,500 lb. Space for two crew members was provided. Two large lift fans were located in the wings, which provided most of the thrust for vertical takeoff. A smaller lift fan was located in the nose, which provided additional thrust as well as attitude control. The vanes on each fan could be pitched between -7 and 45 degrees to provide directional control while hovering. As the XV-5 would spend much of its time in hover, the test aircraft were fitted with helicopter style controls, to provide better handling while taking off and landing vertically.
Two XV-5A test aircraft were formally accepted by the US Army on January 26, 1965, and began flight testing shortly afterward, at Edwards AFB.
The XV-5A demonstrated the ability to land and take off vertically, as well as successfully transition to horizontal flight (transition took place at about 170 km/h). However, there were some issues. The aircraft's ground attitude meant that taking off perfectly vertically was quite difficult; it require the pilot to release the brakes, adjust pitch controls, and change engine power simultaneously. Additionally, the aircraft was found to be difficult to control during the transition period, as there was no integrated control system for both modes of flight. Often, the XV-5A would pitch up or down for a few seconds as the transition occurred. Numerous other small issues were noted; many instruments were poorly placed, and cockpit temperature control was ineffective. More importantly, visibility downward was very bad when hovering. Oddly, a parking brake was not fitted to the XV-5A, which caused issues during testing.
The XV-5A had decent conventional takeoff performance, with a takeoff run of about 800 meters needed. The aircraft also performed well during conventional landings. However, during vertical takeoffs and landings, severe turbulence was noted while in ground effect, making the aircraft difficult to control. This made it difficult to land in a precise spot (a major problem for an operational VTOL aircraft), and limited operations to when winds were less than about 10 km/h, obviously unacceptable for operational use. Another problem noted with vertical flight was that at high loads, the lift fans would reingest exhaust gases, leading to loss of power similar to vortex ring state. Despite these flaws, the XV-5A was judged adequate by the US Army as a research aircraft (however, it was recommended that these issues be fixed in follow-on research aircraft).
The first XV-5A aircraft was lost in an accident on April 27, 1965, which unfortunately killed the pilot testing the aircraft. Investigation showed this was likely due to the pilot accidentally switching the aircraft from horizontal to vertical flight mode (the switch was located on the collective control for convenient access, which made it easy to activate accidentally). Testing continued afterward with the second prototype. Later in the testing, the XV-5 was considered by the US Army for use as a close air support aircraft or as a rescue aircraft (the lack of hot exhaust gases meant that it could hover over people without inadvertently frying them). The second fatal accident in the XV-5 program occurred in 1966 during testing of this capability. A rescue harness was ingested into the lift fan on the left wing of the XV-5A, damaging it. The pilot ejected, but was killed as the seat deployed horizontally due to the attitude of the aircraft during ejection. Later investigation showed that the damaged fan was still capable of producing enough lift to slow the XV-5's descent to a survivable rate.
The XV-5A following the second crash.
Following the crash of the second XV-5A airframe, it was decided to rebuild it into the XV-5B, and continue the test program with that aircraft. Numerous improvements were made to the systems of the XV-5 (including improved control systems and cockpit layout), correcting some of the deficiencies of the XV-5A. The aircraft was also repainted in NASA colors (the XV-5A had been painted in US Army markings.)
In addition to being used for testing of the VTOL characteristics and the lift-fan concept, the XV-5B was used for testing of approach procedures for VTOL aircraft. Particularly, the XV-5B was flown at steep approach angles of up to 20 degrees.The aircraft was flown successfully in this role, but it was found to be somewhat difficult for the pilot, as engine throttle, lift fan controls, and conventional flight controls all had to be manipulated to stabilize that approach. Testing of the XV-5B in this role was continued until 1971, when the aircraft was retired. It is currently on display in Fort Rucker, Alabama.
Video footage of the XV-5A
PRELIMINARY PILOT QUALITATIVE EVALUATION OF THE XV-5A RESEARCH AIRCRAFT
Lift Fan Aircraft - Lessons from Pilot's Perspective