URMV-3 IS-4

The IS-4 high-performance glider followed the layout of the earlier IS-3d, constructed largely of wood with plywood and fabric skinning. The high-set cantilever wings with moderate dihedral (2º 30') were built with a single main spar with a plywood covered leading egdge torsion box. The ovoid section fuselage was of monocoque construction with plywood skin with a streamlined full-length cockpit canopyforward of the wings. The undercarriage consisted of a balloon main-wheel with nose and tail skids. Flying controls were largely conventional with plywood skinned fixed portions and fabric covered wooden built up movable surfaces. Pitch trim was achieved by a trim tab on the elevators and adverse yaw was alleviated through the use of differential ailerons.[1]

Very little is known of the IS-4s development or operational history.

Data from The World's Sailplanes:Die Segelflugzeuge der Welt:Les Planeurs du Monde Volume II[2]

General characteristics

• Crew: 1
• Length: 7.1 m (23 ft 4 in)
• Wingspan: 15 m (49 ft 3 in)
• Height: 0.9 m (2 ft 11 in) at cockpit
• Wing area: 14 m² (150 sq ft)
• Aspect ratio: 16
• Airfoil: Root:Göttingen 549, Mid Göttingen 549, Tip Göttingen 693
• Empty weight: 210 kg (463 lb)
• Gross weight: 320 kg (705 lb)

Performance

• Stall speed: 60 km/h (37 mph, 32 kn)
• Never exceed speed: 200 km/h (120 mph, 110 kn) *Rough air speed max: 150 km/h (93 mph; 81 kn)
• Aerotow speed: 120 km/h (75 mph; 65 kn)
• Winch launch speed: 90 km/h (56 mph; 49 kn)
• Terminal velocity: with full air-brakes at max all-up weight 185 km/h (115 mph; 100 kn)
• g limits: +5 -3 at 250 km/h (135.0 kn)
• Maximum glide ratio: 30 at 80 km/h (50 mph; 43 kn)
• Rate of sink: 0.64 m/s (126 ft/min) at 41.6 mph; 36.2 kn (67 km/h)
• Wing loading: 22.9 kg/m² (4.7 lb/sq ft)

Aircraft of comparable role, configuration, and era Grunau Baby Related lists List of gliders