That rudder is on the east side of the channel. From the imp's lair, fly at the height of the lower edge of that rock ledge back towards the start of the fjords. You will quickly (3 seconds) fly past a large tree on a ledge, with a wyvern nest containing the rudder a little down and to your left (you should be able to see it and the sparkle effect, if that is the one you are missing).
