To rotate an object, press the key 'r', and then press either 'x', 'y', or 'z' to get the rotation in a specific axis (This is not required. You can press 'r' and rotate freely.). The rotation will be based on the origin of the object, or the tiny yellow dot that shows up when you select it. In your case, if the origin is not at the base of the wing, follow these steps:
- 'Tab' to enter Edit Mode
- Select the bottom face of the wing
- Press 'Shift + S' and choose 'Cursor to selected'
- Exit Edit Mode
- Right click >> Set Origin >> Origin to 3D Cursor
Now, when you rotate in object mode, it should rotate and flip down.