斯威夫特 - 物理体系弯曲得体，无法获得正确的曲率

Question

我正在构建一个2D游戏，我正在寻找一个特定顺时针曲线的节点，从天空到地面（就像一只鸟会俯冲到地面）以顺时针方向运动，并产生一些摩擦力鸟类降落的影响。

我一直在玩'physicsBody'，但我似乎无法获得正确的曲线，任何人都可以帮助......这是我想要实现的曲线图。 http://www.theluxuryteam.com/uploads/shared/images/curved_arrow-black.png

谢谢

    func addCrow() {
        var crow = SKSpriteNode(imageNamed: "crow")
        crow.physicsBody = SKPhysicsBody(rectangleOfSize: crow.size)
        crow.physicsBody?.dynamic = true
        crow.physicsBody?.collisionBitMask = 0
        crow.physicsBody?.velocity = CGVector(dx: -200, dy: 0)
        crow.physicsBody?.angularVelocity = 100
        crow.physicsBody?.linearDamping = 0
        crow.physicsBody?.angularDamping = 100
        var random : CGFloat = CGFloat(arc4random_uniform(300))
        crow.position = CGPoint(x: self.frame.size.width / 1.4, y: self.frame.size.height / 1.2 ) //  1.4  0.8
        self.addChild(crow)

    }