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Flightline: 201 - US ARMY VTOL EXPERIMENTS # 3 - OTHER AVENUES

The US Army, despite limitations established after the USAF was established, continued to experiment with VTOL aircraft into the 1960s.

Background

Recognizing the potential of winged heavier-than-air flying machines first demonstrated by the Wright Brothers in 1903, the US Army Signal Corps established the Aeronautical Division, Signal Corps on 1 August 1907 to train pilots, and develop and operate aircraft. Operations expanded and the name changed several times between then and 1941, when the US Army Air Corps was renamed the US Army Air Forces just prior to America's entry into WWII. During the post-war reorganization of the military and intelligence agencies formally adopted under the National Security Act of 1947, the USAAF was established as a separate branch under the National Military Establishment (renamed the Defense Department in 1949). Further refined by the 1948 Key West Agreement and the 1952 Pace-Finletter Memorandum of Understanding (MOU), the USAF was established as the primary operator of fixed wing aircraft for both offensive (fighter, bomber, attack), defensive (interceptor, long range recon) and support/transport duties, while the US Army's fixed wing fleet was limited to theater-level transport and utility, observation/spotting, command and control of Army forces, and medivac. Additionally, the Army was freed from weight restrictions and was allowed to develop helicopters for attack, transport and other roles. Seeking ways to develop more capable fixed-wing craft within those restrictions (or to side-step them entirely...), the Army began looking into various vertical takeoff and landing (VTOL) technologies in the 1950s, requesting proposals for one-man platforms, flying jeeps, and larger aircraft that could be adapted for recon, transport or perhaps even attack missions.

Vertol VZ-2

On 15 April 1956 the Army and Navy signed a joint contract with Vertol (formerly Piasecki) to build a demonstrator to test tilltwing technology, that is, an aircraft built to allow the wing and engines to be tilted such that the thrust is pointing downwards, allowing the aircraft to take off vertically, then tilt the wing back down for forwards flight. The resulting Model 76 was a skeletal framework of aluminum tubing with a bubble cockpit at the front (both the frame and cockpit were adapted and built from components for the Bell Model 47 to reduce costs) with the high-mounted wing at mid point and a t-tail at the rear. A fixed taildragger landing gear was fitted, while a small wheel under the cockpit was also provided in case of difficulties. The Model 76 was 26' 5" long and 15' tall, with a wingspan of 24' 11". The three-bladed, variable-pitch props, each 9' 6" in diameter, were powered by a single Lycoming T53-L-1 turboshaft, rated at 700shp, which was mounted within the fuselage. A complex series of shafts and linkages connected the engine with the main rotors/propellers, as well as to a pair of smaller ducted fans, one each in the horizontal and vertical stabilizer, which were used to control yaw and pitch while in hover. The rotors, which were interconnected for safety, were also used to adjust pitch in hover, while more traditional aircraft flight surfaces would take over once in horizontal flight. Empty, the VZ-2 weighed 2,500lbs, while max weight was 3,700lbs.

Line drawing of the Model 76. | Illustration: Boeing Vertol

Line drawing of the Model 76. | Illustration: Boeing Vertol

Vertol delivered the aircraft, designated VZ-2 by the Army, in 1957, with ground testing beginning in April, followed by the first hover flight on 13 April. On 15 July 1958 the VZ-2 completed its first full flight, starting with a vertical takeoff, transitioning to wing-borne flight, then returning to hover mode for a vertical landing. The aircraft proved to be fairly agile, but was slow even in horizontal mode, setting a maximum speed of 210mph and a ceiling of 13,800 feet. Range, not particularly a concern of a research aircraft, was only 130 miles. In addition to the Army and Navy, the VZ-2 was flown by NASA for a time. The craft was retired in 1965, completing 450 flights, which included 34 full transitions. The VZ-2 is currently in storage at the Paul E. Garber Preservation, Restoration, and Storage Facility of the National Air and Space Museum.

The VZ-2, resplendent in white and day-glo orange scheme. | Photo: NASA

The VZ-2, resplendent in white and day-glo orange scheme. | Photo: NASA

Ryan VZ-3 Vertiplane

The Vertiplane as originally completed. | Photo: US Army

The Vertiplane as originally completed. | Photo: US Army

The Ryan Model 92 Vertiplane, designated the VZ-3RY by the Army, was designed to test the principle of deflected slipstream to enable a fixed-wing aircraft to achieve VSTOL without the complexity of a tilted wing or engine design. Originally built with a tailwheel design, the aircraft was rebuilt after a non-fatal accident in a slightly lengthened form with a tricycle arrangement. The high-mounted wing was fitted with full-width double-slotted flaps and down-turned wingtips that acted to contain and direct the airflow down, allowing the Vertiplane to take off after a 30-foot ground roll at a speed of just 25mph. Power was provided by an Avco Lycoming T53 turboshaft, mounted in the fuselage and shaft-linked to the two nine-foot diameter, three-bladed props. To provide control in a hover, the jet exhaust was fitted with a deflection nozzle. The aircraft could achieve a hover at altitudes up to 3,700 feet, but during testing it was discovered that a low-altitude hover would reingest air from its own slipstream (which possibly caused the crash). Construction of the Vertiplane began in 1957, with maiden flight taking place on 29 December 1958 and the Army completing 21 test flights before the VZ-3 crashed in 1959. Along with the rearranged landing gear, the rebuilt VZ-3 was also fitted with an open cockpit, bringing plane in its final configuration to 27 feet 8 inches long with a wingspan off 23 feet and a height of 10 feet 8 inches. After the rebuild the Vertiplane was handed over to NASA for further testing, and it was officially retired in 1961. In 2018, after a lengthy restoration, the VZ-3 was placed on display at the US Army Aviation Museum.

The rebuilt VZ-3 during NASA testing. The span and length of the flaps at full extension is apparent in this view. | Photo: San Diego Air and Space Museum Archives

The rebuilt VZ-3 during NASA testing. The span and length of the flaps at full extension is apparent in this view. | Photo: San Diego Air and Space Museum Archives

The Vertiplane in the Army Aviation Museum. | Photo: US Army

The Vertiplane in the Army Aviation Museum. | Photo: US Army

Doak VZ-4

Edmund R. Doak, Jr. was a self-taught aviation engineer and former VP of Douglas Aircraft who in 1940 formed Doak Aircraft. The company served as a subcontractor during WWII, eventually employing 4,000 workers. After the end of the war, Doak proposed a VTOL aircraft to the Army that would have the speed, range and firepower of a fixed-wing aircraft and the ability to take off vertically, hover and land vertically like a helicopter (which would also allow the Army to skirt the restrictions against operating fixed wing attack aircraft). On 10 April 1956 the Army awarded Doak a contract to build a single prototype of their Model 16 for use as a research aircraft. Designated the VZ-4DA and built with serial number 56,9642, the Model 16 was constructed from welded steel tube covered with fiberglass from the cockpit forward and with aluminum sheet over the remainder of the fuselage, which was 32 feet long. The mid-mounted wings were of all-metal construction and had two 5 foot diameter ducted fans on either wingtip, bringing the span to 25 feet 6 inches total. The fans could be moved through 92°, allowing the VZ-4 to take off vertically, fly horizontally, and hover (the fans pointed two degrees aft of vertical in hover to counteract the exhaust of the jet). Initially powered by a single Avco Lycoming YT53 turboshaft of 840shp, a more powerful T53-L-1 of 1,000shp was later substituted. As with the VZ-3, diverter vanes allowed the pilot to direct the thrust of the jet engine to control pitch and yaw in hover. In order to speed construction and cut costs, the VZ-4 utilized components from other aircraft, including the landing gear from a Cessna 182, the seats from a P-51 Mustang, and actuators from a T-33 Shooting Star. Despite being an R&D aircraft, the VZ-4 sat two, a pilot and observer, in tandem seats. The flight controls consisted of a standard stick and rudder, with mechanical linkages and interlocks adjusting the ducts, vanes, tabs and flaps as the aircraft went through all phases of flight. There was no artificial stabilization or power boost system.

Orthograph of the VZ-4. | Illustration: Doak Aircraft Company

Orthograph of the VZ-4. | Illustration: Doak Aircraft Company

Doak began ground tests in February 1958, with the Model 16 completing 32 hours of stand testing and 18 of tethered testing before the first free flight on 25 February 1958, then the first full transition flight on 5 May. Following a complete tear-down inspection of the craft, it was shipped to Edwards AFB in October for 50 hours of further acceptance testing. The Army accepted the VZ-4 in September 1959, after which it was transferred to NASA Langley to begin test work on its VSTOL performance.

The VZ-4 over Edwards in 1958. | Photo: US Army

The VZ-4 over Edwards in 1958. | Photo: US Army

The VZ-4 performed adequately during the Army and NASA tests; demonstrating the ability to hover and shift between horizontal and vertical flight at altitudes of up to 6,000 feet, A maximum speed of 230mph, a cruise speed of 175mph, a range of 250 miles with an an endurance of one hour, and a service ceiling of 12,000 feet. A few vices did crop up, the most serious of which was a tendency to nose-up when going from vertical to forward flight; additionally, STOL performance was less than anticipated. Doak's engineers were confident that they could correct these issues, with a swap to a more powerful T53 turbine being the first step. Before additional measures could be taken however, Doak, hit by financial problems due to a general recession in 1960, laid off 90% of their staff. The VZ-4 program was subsequently purchased by Douglas Aircraft, but by this point the Army had lost interest in the idea as improvements in aerodynamics and engines had produced helicopters which met their needs. The aircraft was transferred to NASA in 1963 and was flown sparingly until 1972, when it was grounded permanently. The VZ-4 was placed into storage at Fort Eustis in 1973 before being donated to the US Army Transportation Museum, where it is now on display.

The VZ-4 on display at the Army Transportation Museum. | Photo: Vladimir Yakubov

The VZ-4 on display at the Army Transportation Museum. | Photo: Vladimir Yakubov

Fairchild VZ-5 Fledgling

Like the Ryan Vertiplane, the Fairchild VZ-5FA was designed to test the principle of deflected slipstream to enable VSTOL flight. Known as the Model M-244-1 by Fairchild, the aircraft was of simple construction, with an open cockpit fuselage nearly thirty-four feet long mated to a high-mounted wing nearly thirty-three feet in span. A GE YT58 turboshaft of 1,032shp drove four small four-bladed props as well as two even smaller props in the tail for pitch and yaw control. In addition to conventional flaps and ailerons for control in normal flight, nearly two thirds of the wing would act as a full-span flap for VTOL flight. Extensive wind tunnel testing at NASA Langley cast doubt on the airworthiness of the Fledgling's design, with the aircraft producing almost enough lift to carry its empty weight aloft, but no fuel, pilot or cargo. The VZ-5 was also determined to be unstable in every flight regime, with violent pitching encountered and the available flying surfaces unable to compensate. Despite this bleak report, Fairchild pressed on and tethered tests began on 18 November 1959. The exact number of tests of the Fledgling are not known but suspected be in the low single digits, and the series only reinforced the conclusions drawn from the wind tunnel testing. The VZ-5 was abandoned before the end of 1959 and was apparently quietly scrapped.

The VZ-5 from in front and behind. The Fledgling likely never actually took flight, a victim of poor aerodynamics. | Photo: US Army

The VZ-5 from in front and behind. The Fledgling likely never actually took flight, a victim of poor aerodynamics. | Photo: US Army

Lockheed VZ-10 Hummingbird

In 1959 the Army awarded a contract to Lockheed for two examples of the VZ-10, an experimental STOVL observation aircraft based on Theodore von Karman's "augmented jet ejector" concept. Under von Karman's concept, jet exhaust was passed through a specially shaped chamber which drew in ambient air from the outside, which in lab tests had proven to increase thrust by two times. Lockheed's design resembled a conventional aircraft 32 feet long with narrow-chord, mid-mounted wings giving it a wingspan of just 25 feet. The interior volume of the fuselage was taken up by two P&W JT12A-3LH turbojets, along with the thrust diverter system. Fed by doors at the top of the fuselage, doors at the bottom would allow exhaust to lift the aircraft, while they could also be closed and allow the aircraft to take off and fly conventionally. Diverted exhaust could also be used to maneuver the aircraft during hover, while conventional flying surfaces were used during normal flight.

Orthograph of the VZ-10. | Illustration: airvectors.net

Orthograph of the VZ-10. | Illustration: airvectors.net

Prior to first flight, the aircraft was redesignated the XV-4, with the maiden flight (conventional) occurring on 7 July 1962. This was followed up by a hovering flight on 24 May 1963 and a transitional flight from hover to conventional on 8 November 1963. The thrust augmentation system proved to be heavier and less effective than anticipated, resulting in the Hummingbird having a thrust-to-weight ratio of on 1.04. Still, the XV-4A set a max speed of 518mph and a range of 600 miles. The first XV-4 crashed on 10 June 1964, killing the pilot and destroying the plane.

The XV-4A Hummingbird during testing. | Photo: Lockheed/US Army

The XV-4A Hummingbird during testing. | Photo: Lockheed/US Army

The second prototype was built to a different standard, designated the XV-4B. The two JT-12 engines and the diverter system were replaced by six GE J85 turbojets, four set vertically to act as lift jets and two set horizontally to provide forward thrust. The XV-4B was slightly larger than the A model, and much heavier, the A had an empty weight of just under 5,000lbs and a gross of 7,200lbs versus the B, which weighed 7,400lbs empty and almost 12,600lbs gross. The upgraded engines improved the thrust to weight ratio to 1.43, but the added weight reduced the top speed to 463mph. The XV-4B also had an improved stability system, giving the pilot a better feel for the controls and more authority in a hover. The aircraft was rolled out on 4 June 1968 and soon began a short builder's trial before being shipped to Edwards AFB for flight testing conducted by North American Rockwell on behalf of the USAF. The B model was lost in a crash on 14 March 1969, although the pilot was able to escape in the ejection seat. The Army canceled the program at this point, seeing no further development in the future,

The XV-4B, with a T-33 chase plane, at Edwards AFB. | Photo: USAF

The XV-4B, with a T-33 chase plane, at Edwards AFB. | Photo: USAF

Ryan VZ-11 Vertifan

In 1959 Ryan Aeronautical, who had been working on the X-13 Vertijet, a jet plane that could VTOL onto a dedicated platform, for the USAF, proposed an aircraft operating on a different principal, ducted fans, to the US Army. The Army in turn awarded a contract for two VZ-11-RY Vertifans, redesignated the XV-5A in 1962.

Orthograph of the XV-5. | Illustration: Aeroplane Monthly

Orthograph of the XV-5. | Illustration: Aeroplane Monthly

Labeled cut-away drawing of the Vertifan. | Illustration: Ryan Aeronautical

Labeled cut-away drawing of the Vertifan. | Illustration: Ryan Aeronautical

The Vertifans were 44 feet 6 inches long, with a wingspan of 29 feet 10 inches and a height of 14 feet 9 inches. Empty weight was just over 7,500lbs, while max takeoff weight was 13,600 conventionally and 12,300lbs while employing VTOL. Two General Electric J85s, rated at 2,600lbs each, propelled the aircraft forward, as well as driving the two GE X353-5 ducted fans in either wing, which measured 62.5 inches in diameter, as well as a smaller 36in fan in the nose. While operating in lift-fan mode, the engines produced 16,000lbs total thrust, three times that available from the turbojets alone. Cross-ducts provided a measure of safety, as either jet could dive the whole system in case of a failure. Louvered vanes underneath the fans provided yaw and roll control, while pitch control was effected via engine power settings. Traditional flying surfaces on the wing and T-tail provided control in conventional flight. The vanes underneath the fans as well as doors on top could be closed during normal flight to reduce drag. Additionally, the narrow-track landing gear was retractable. The wings of the XV-5 were vaguely based on the delta wing of the A-4 Skyhawk, and the intake duct for the engines was placed above and behind the two place cockpit.

This photo, from the front and underneath the XV-5A, shows the narrow track of the landing gear. | Photo: Ryan Aeronautical

This photo, from the front and underneath the XV-5A, shows the narrow track of the landing gear. | Photo: Ryan Aeronautical

While the XV-5s were under construction the 15 pilots chosen, known as the "XV-5A Fan Club", were trained on a flight simulator provided by Ryan. The first Vertifan was rolled out in early 1964, with the maiden flight taking place on 25 May. Testing progressed, but uncovered numerous issues with the aircraft. The J85s produced too little thrust, giving the XV-5 poor acceleration during conventional takeoffs. During hovers, the control system provided too little yaw authority, and opening and closing the louvers and doors caused abrupt changes in pitch. There were also issues with the opening mechanisms, which proved to have poor reliability. The first Vertifan crashed during a public flight demonstration on 27 April 1965, killing Ryan test pilot Lou Everett. The post-crash investigation determined that the Everett had inadvertently tripped the conventional-to-vertical conversion switch, which was mounted on the collective. The switch automatically triggered the horizontal stabilizer to force the nose down almost 45°, a maneuver intended to compensate for the lift generated by the nose fan. Everett initiated his low-altitude ejection seat, but it malfunctioned and he was killed.

The second Vertifan was modified, with the conversion switch, now a lift-lock toggle, being moved to the instrument panel. Trials resumed, with an emphasis on turning the plane into a search-and-rescue vehicle. A winch and rescue sling was added, with the idea being that a downed airman could be retrieved and winched into a compartment behind the pilot. During a test of the system on 5 October 1966, the sling was ingested into one of the wing fans. The pilot, Major David H. Tittle, ejected, but too late, and died of internal injuries he sustained when the seat impacted the ground. The plane was salvaged and rebuilt into the XV-5B, which featured fixed landing gear which was more widely spaced, improved controls, and improved lift and thrust capability. The Army, having lost interest in VTOL research in general and the Vertifan in particular, abandoned the project. The XV-5B was turned over to NASA for further testing, and was delivered in the agency's paint scheme of overall white with blue trim.

Composite photo of the XV-5B in flight and on the runway. | Photo: NASA

Composite photo of the XV-5B in flight and on the runway. | Photo: NASA

The XV-5B continued to be flow by NASA until 1971, providing much data on lift fan technology. General Electric took some of the lessons learned from the J85 engines and X353 fans and adapted them into the earliest high-bypass turbofan engines, the TF39, which powered the C-5A Galaxy. The Verifan was the last manned aircraft produced by Ryan, which pivoted into unmanned aircraft in the mid 1960s. The XV-5B, still in its white NASA paint scheme, was retired and placed on display at the US Army Aviation Museum at Ft Rucker.

The XV-5B Vertifan, on display outside the US Army Aviation Museum. | Photo: Alan Wilson.

The XV-5B Vertifan, on display outside the US Army Aviation Museum. | Photo: Alan Wilson.

Summary

The Army ceased supporting VTOL research during the late 1960s. Budgets were stretched by the war in Vietnam, and development of more capable helicopters like the UH-1, AH-1 and CH-47, resulted in a diminished interest in alternative technologies.

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Comments (2)

  • interesting!

      2 months ago
    • There was a lot of "Can we do this?" "Dunno, let's try it and see!" going on after WW2; much of it didn't work, but even the failures produce some fascinating stories.

        2 months ago
2