Rotating game object using UI Joystick

Hello,

I am learning unity and I want to know how can I rotate a game object (2D) using on screen joystick for touch devices.

I have a joystick setup and functioning, but I can't figure out how can I rotate a game object using the joystick.

As you can see in the image, the inner circle (grey) is a joystick which can be moved around in the outer circle (white). I want to rotate the Z component of the aeroplane when the grey circle is moved.

Thanks in advance