Why are oblique wings not used?
The design was tested for transport aircraft mainly IIRC. The main problems with the oblique wing is the complexity demanded by the shearing forces on the main swivel mechanism and the inadequate roll controllability, which demands a complex flight control system. DARPA Oblique Flying-Wing (OFW) Project That program was eventually cancelled, citing difficulties with control systems. The program aimed at producing a technology demonstrator aircraft to explore the various challenges which the radical design entails.
What are the four types of wings?
There are four general wing shapes that are common in birds: Passive soaring, active soaring, elliptical wings, and high-speed wings. Birds are warm-blooded vertebrates (vertebrates have backbones) and are the only animals with feathers. Although all birds have wings, a few species can’t fly.