Based on your question you’re either new to 3D or new to polygonal modeling. Those are not spesific to Blender. Blender use becomes easier fast but you still have many mountains to climb with the concepts of polygonal modeling, texturing etc.
Edge select mode (ctrl+tab), select the edge marked with cyan color, ctrl+alt+space to create a custom transform orientation (TO). This also automatically switches global/local TO to global/edge on the 3d view header. After that, vertex select mode, select the (scaling) target vertex from already made point, shift+S -> cursor to selected. Then switch pivot to 3D cursor (period key), extrude (E), cancel transformation (right mouse button), and scale along edge, excluding one axis: In the example pic I did S, shift+X+X.
S is to scale, shift+axis is to exclude an axis. Selecting an axis gives the global axis but pressing it the second time gives the custom transform orientation, that’s why shift+X+X.
You could also enable vertex snapping (ctrl+shift+tab) before scaling and if you then scale as mentioned and also hold down ctrl and point your mouse to the vertex where the 3D cursor is, you can snap to 90° angle (to the current transform orientation).
If that is not what you want, then you have to eyeball it because snapping functions doesn’t support other kinds of intersections between snapping element and transform orientation.
Maybe you could use temporary geometry to get face angles displayed (properties panel in edit mode (N) -> mesh display -> face angle) but the point is, you shouldn’t try to make it that precise. If pinpoint precision is what you need, then you’ve chosen the wrong modeling paradigm.
Yes, and that would be to start with a very simple approximation of the whole wing and then put in the detail.