The Invincible D-D was a prototype three/four seat touring aircraft, built in the US in 1929. One example flew but development was ended by the Great Depression.

The D-D^[1] was a mid-wing monoplane with a rectangular plan, blunt tipped, constant thickness wing. Canvas covered, it was built around a pair of wooden spars that continued through the fuselage with the pilot's seat between them. Dihedral was 1.25°. Constant chord ailerons filled the outer two-thirds of the trailing edge. The wings were braced with steel V-struts, individually enclosed in wooden fairings, from the two wing spars to a framework under the fuselage where their vertices were transversely joined by a horizontal steel bar.^[2]

The Invincible was designed to accept two engines of significantly different power and cost. These were the 67 kW (90 hp) LeBlond 90-7D, a seven-cylinder radial engine or the 130 kW (170 hp) Curtiss Challenger, a six-cylinder, two row radial. The Challenger-powered version could carry four, one more than that with the Leblond engine, but cost $7,800 rather than $5,500. The chosen engine was mounted in the nose with cylinders largely exposed for cooling; the mounting allowed easy interchange between the two types. Fuel tanks were in the wings.^[2]

Behind the engine the Invincible's fuselage had a steel tube, rectangular-section structure with fabric covering. The enclosed cabin had, in the four-seater, two rows of side-by-side seats, optional dual control and generous forward, side and upward glazing. Entry was via an oval port-side door.^[2]

Its tailplane, triangular in plan, was mounted on top of the fuselage and carried separate, parallel chord elevators. It was wire-braced from above and below to the rudder post of a quadrantal profile fin which carried a constant chord rudder, reaching down to the keel.^[2]

The Invincible's fixed, tailwheel landing gear had mainwheels independently mounted on cranked half-axles hinged at the centre of the fuselage underside. Their outer ends were joined to the fuselage sides by inward-leaning pneumatic shock absorber struts and to the ends of the horizontal bar of the wing struttage by short drag struts.^[2]

Its first flight was in late January-early February 1929 at Manitowoc, powered by the LeBlond engine and piloted by William Williams, with its designer, Irl Beach, as a passenger. It was scheduled to appear at the Detroit air show that April.^[3] Invincible, having entered into aviation in 1928 and produced three or four different prototypes, left the industry in 1929 because of the Great Depression.^[1]

Data from Les Ailes, July 1929^[2]

General characteristics

• Crew: One
• Capacity: Two passengers
• Length: 7.34 m (24 ft 1 in)
• Wingspan: 11.60 m (38 ft 1 in)
• Height: 2.21 m (7 ft 3 in)
• Wing area: 20.26 m² (218.1 sq ft)
• Aspect ratio: 6.7
• Airfoil: NACA M 15
• Empty weight: 585 kg (1,290 lb)
• Fuel capacity: 228 L (50 imp gal; 60 US gal)
• Powerplant: 1 × LeBlond 90-7D 7-cylinder radial, 67 kW (90 hp) at 1,975 rpm
• Propellers: 2-bladed

Performance

• Maximum speed: 175 km/h (109 mph, 94 kn)
• Cruise speed: 145 km/h (90 mph, 78 kn)
• Stall speed: 59 km/h (37 mph, 32 kn)