The Vanguard Omniplane was an American approach to vertical operation and high cruise speed. The ducts contained vertical thrust propellers that could be closed by venetian blind shutters to form small wings in forward flight. In forward flight, covers above the rotors and louvers below sealed the wing for aerodynamic lift.

The design, apparently not registered, used a 25-ft long Ercoupe light plane fuselage and weighed 2,600 lb. The round wings each housed a 6 ft diameter three-bladed propeller that was mechanically driven for vertical flight by a 265 hp Lycoming O-540-A1A six cylinder piston engine.

Thrust was produced by a 5′ shrouded prop in the tail, elevator and rudder behind the rear fan-controlled pitch and yaw, while differential propeller blade pitch affected roll in hover.

Ground tests, starting in Aug 1959 and including tethered hover trials, were followed by NASA full-scale wind tunnel testing.

Modifications in 1961, including a 860hp Lycoming YT53-L-1 turboshaft, improved control system, and 5′ nose extension to house a third lifting prop, led to the redesignation 2D. The nose prop improved control in pitch, as well as in yaw through the use of movable exit vanes. 2D completed tethered hover tests, but was damaged by a mechanical failure and discontinued in early 1962.

Engine: Lycoming O-540-A1A, 265hp
Wingspan: 22’0″
Length: 25’6″
Useful load: 500 lb
Max speed: 190 mph
Range: 150 mi
Seats: 2

The Transcendental Aircraft Corporation of Glen Riddle, Pennsylvania, was the first company to claim flying a successful Tilt Rotor aircraft. The Model 1-G was a small, high-wing experimental aircraft with a fuselage-mounted engine and fixed tricycle landing gear designed and built by Mario A. Guerierri. The single pilot sat forward of the wing in a semi-enclosed fuselage. The clear plastic nose gave the pilot helicopter-like visibility. A single Lycoming 0-290-A six cylinder engine produced 160 horsepower. The fuselage measured 7.9m long, and the aluminum wing measured 6.4m. The ailerons were fabric covered. Empty weight was 655kg and take-off weight was 790kg. The projected maximum speed in helicopter mode was 190km/h, and 260km/h in airplane mode.

A 5.2-meter diameter, three-bladed, fully articulated rotor was mounted at each wing tip. The rotor shafts tilted from pointing vertically for hover down to 6 degrees up from horizontal for forward flight. Electric motors controlled the tilt of the rotor shafts.

Interconnecting shafts ensured that both rotors maintained the same tilt angle. Each rotor was driven through a two-speed gearbox. This allowed the pilot to lower the rotor rotation speed for more efficient cruise in forward flight.

Although the Model 1-G was a private development, the Wright Air Development Center at Wright-Patterson AFB issued contracts to study many of the Tilt Rotor’s unique peculiarities.

The first was awarded in 1952 to investigate the dynamics and structural characteristics of the rotor system. The Air Force awarded a second contract in 1953 to investigate mechanical instability problems associated with tilting the rotors.

The first hover flight was either on June 15 or July 6, 1954 (references vary). The first forward flight in hover mode occurred on December 13, 1954, and the first forward flight with rotors tilted occurred only four days later. By April 1955, conversions with the rotors tilted up to 35 degrees from vertical were completed. Eventually, the Model 1-G completed numerous transitions up to 70 degrees of tilt with the wings sustaining over 90 percent of the weight.

The engine reduction was been replaced by a two-gear reduction box, at the exit of which a gearing system has been attached which enables the rotors to be put into auto-rotation. At the engine’s maximum output of 3000 r.p.m. the rotors rotate at 633 r.p.m. as airscrews, and at 240 r.p.m. as helicopter rotors.

Three concentric tubes start from the swivelling device, the first of which moves the rotor, the second controls cyclic variation and the third variation of collective pitch. The time needed to move from the helicopter to the aircraft position is roughly three minutes. In this change-over, the rotors swivel through an angle of 82 degrees.

The Model 1-G was destroyed during a test flight on July 20, 1955. After performing a virtually complete conversion, the friction lock on the collective pitch controller slipped, throwing the aircraft into an abrupt, steep dive. The pilot initiated a recovery, but there was not enough altitude to complete the pull-up before contacting the Delaware River. The aircraft flipped on its back, resulting in irreparable damage.

During the Model 1-G’s brief career, it made over 100 flights, achieved 90% conversion (about 70 degrees forward rotor tilt), and flew 60 hours. It demonstrated excellent controllability without vibration, and reached an altitude of 1060m and an airspeed of 185km/h in helicopter mode.

A second aircraft, called the Model 2, N546A, was to fly in late 1956. Transcendental received an Air Force contract in March 1956 and completed the Model 2 in October. Compared to the Model 1-G, it was stronger and more aerodynamic, but had the same basic configuration. The enclosed cockpit had side-by-side seating. Power was by one 250 horsepower Lycoming O-435-23 six cylinder engine. The wing was a 0.3m longer, but the fuselage was 1.2m shorter. Empty and gross weights were 708kg and 1015kg. Development ended when funding from Wright Air Development Center stopped. It could not be determined if the aircraft ever flew, and eventually it was dismantled.

The Model 3 of 1957 was also a convertible, and an extrapolation of Model 1G, it is intended to be powered by two gas turbines, and to accommodate a crew of two and also six passengers or alternatively four stretchers. It is expected that this model will have a cruising speed comparable to that of a traditional aircraft and also the special take-off and landing properties of a helicopter.

Model 1G
Engine: 1 x Lycoming G0-290-A, 160hp
Rotor diameter: 5.18m / 17 ft
Width (rotor tip to rotor tip): 11.58m
Rotors: 2 x 3-blade on outriggers
Max rotor speed: 633rpm
Wing span: 21 ft
Length: 7.93m / 26′ 0″
Height: 2.13m
Weight fully loaded: 794kg / 1,640 lb
Empty weight: 658kg
Maximum speed (as helicopter): 196km/h
Maximum speed (as aircraft): 256km/h / 150 mph
Absolute ceiling (as aircraft): 1525m
Endurance: 90 minutes
Seats: 1

Model 2
Engine: Lycoming O-435-23, 250hp
Wingspan: 22’9″
Length: 22’1″
Rotor dia: 18’0″
Useful load: 670 lb
Seats: 2