Need some advice please,

I shoot stills and footage of model planes and sometimes I am controlled by the light available, using fill flash conquers issues on the ground but if it's a bright sky with the sun up high, or no sun even. I have trouble getting any decent detail as the models do low flybys unless the light/sun is behind me or there is ground/hills or trees in the background, I think I need to work manualy with aperture settings but just can't get it right. Any pointers would be appreciated, (have learned a lot through reading the posts on this site)