Simplistically, the only reason why you should change your apperture on cloudy or sunny days is because the apperture set effects the shutter speed the camera chooses. For any given level of light, smaller apperture equals slower shutter speed, wider apperture equals higher shutter speed. So, on dull days, you might find that F5.6 gives an unacceptably slow shutter speed and you might want to increase the apperture (set a lower number) whilst on bright days... Well on bright days you might want set a smaller apperture to get more depth of field (depth of field, what's that?).
So, aside from the relationship between appertue and shutter speed, the main effect of apperture is on depth of field. Depth of field is the area (distance) in front of and behind the focus point, which will also appear to be sharp. The smaller the apperture (a small aperture is a big number - i.e. F11 is smaller than F5.6), the greater the depth of field, and vice versa.
In aviation photography, when taking a side on
picture of an airplane, depth of field has relatively little impact on the result as essentially the majority of the airplane is at the same distance from the camera. So assuming you can focus correctly, the majority of the airplane should be in focus anyway. However, if you were to shoot a relative close up from the nose of an airplane, depth of field becomes more important because with a small/narrow depth of field the nose might be in focus whilst the tail isn't. Bottom line is that if you want a picture to be sharp across a wide distance, you need to select a smaller apperture.
However, the is one further consideration regarding apperture, and that is that all lenses perform better and give sharper results at smaller appertures (remember small apperture means big number) than they do at large appertures. Particularly, most lenses won't give overly sharp results at their largest apperture. It thus makes sense to try to use an apperture part way up the scale if possible, and try to avoid the maximum. In this respect, F5.6 should not be a problem (unless F5.6 is the maximum apperture of your lens), but F8.0 might give better sharper results. However, given the relationship between apperture and shutter speed, you may sometimes be forced to use a larger apperture than is ideal, just to keep the shutter speed up to an acceptable level.
Clear??? As mud!