Now here's were I'm alittle confused and need some advise / guidance. I'm looking at visiting a few air shows and motor sport events over the coming year, and realise my current lens may not be he best for the job. Reading around it seems a lot of people recommend something in the 300mm range. My budget isn't a lot, under £200, so I realise I'm not going to get a very fast lens.
My goodness, with all these replies, you'd think you'd need to spend a fortune to get any shots at all and that every picture produced before image stabilization would look like it was painted by Vincent Van Gogh!
I've shot airshows with a
non-HSM Sigma 50-500mm (f/6.3 at the long end) and gotten great shots of jets (and non-jets) in flight- without much of a problem with AF either (on a Fuji FinePix S2Pro, known for being slow and slow to focus.)
You have to slow the shutter down for propeller-driven aircraft anyway to get some motion blur on the propellers.
I've also shot the corner at Indy during 500 qualifications with a lens that couldn't have been faster than f/3.5, and was likely at around f/5.6 (back in the old film-only days.)
Faster glass gives you a lot more in terms of options, especially if the skies are dark, or you're shooting late into the day or around dawn- but you can get along just fine without stabilization or fast glass if you have good technique and/or a monopod for longer stuff.
Get whichever lens produces a better picture, practice with it- especially panning to follow moving subjects (airports, highways, tracks...) Consider a monopod for added stability, and practice with that if you're going to shoot that way, otherwise just work on technique and bracing the camera well.
Even on a crop factor body, I'd rather be at 300+mm than 200mm, that 30% (or more) gives you options at airshows where you can't get out on the flight line, or where the controlled airspace is further away than you'd like because of housing areas or other flight restrictions.
WIth motorsports, I've always been trackside- but the last motorcycle race I shot was with a 400mm, and even with a press pass there were times when being at 400mm gave me better options for corner shots. The best things about tracks are that movement is relatively predictable, so you can prefocus, work out hyperfocal distances for your aperture and the like.