If your shooting fast moving objects and you wish to freeze the image then exposure time is everything. You need to shoot fast, which mean you need more light to get proper exposure. Opening you diaphram a full stop (f stop) doubles the amount of light reaching your sensor. If you close the f stop a full stop, you cut the light in half.
Choosing speed has considerations:
1. speed of the moving object
2. direction of the moving object
3. distance of the moving object
4. focal length of your lens
We talked about speed, if your trying to freezez a running rabbit compared to a speeding bullet, the bullet will require a much fater speed. How about direction? Objects moving across your field of view, from left to right or right to left, appear to move much faster than an obect moving toward or away form you.
Distance plays a factor because the closer you are to an object, the larger the object will be when recorded. The larger the image, the faster it will move across your frame.
Focal length of lens? the longer the lens the larger the image, the faster it will move across the frame.
There are many factors at play, choosing a lens is a consideration not taken lightly by photographers. You must understand your needs and your budget. Capturing an airshow is very doable, you just need to understand your subject, your camera, and lens.
Zoom vs. Fixed Focal Length
Zooms are generally not as sharp at any given focal length as the best fixed lenses of that specific focal length.
The new age of Zooms are very, very good, but a fixed focal length of the same qaulity should out perform it in most cases.