I want to animate a propeller so that it starts from a stopped position and then rotates and accelerates so that it becomes a blur (like an engine starting on an airplane). Suggestions? Thanks for the help to a newbie.
I did this kind of effect a few years back with only some minor adjustments to an expression from either Dan Ehbert's site or AE Enhancers ... I don't recall the details so I'm sorry I can't be more help in that way. However, I'll make one simple suggestion that I unfortunately remembered the hard way so that it might save you the grief I went through ... if you plan to have a sound track, you might want to mix it first, then build your animation to match it ... sound goes a long way in selling some effects, but sound that doesn't sync well with the visuals can also ruin otherwise good visuals.
Would you be able to use the graph editor to get a decent acceleration curve?
Also, I'd highly recommend having a propeller element that's already blurred. Many times, motion blur in AE doesn't exactly yield the results you might want.
What I would do is use the graph editor to get a nice acceleration curve, then possibly add a light wiggle expression to give it a little variance. Then I'd transition that into a blurred propeller with some rotation (both directions, look at some reference footage) and a high-frequency wiggle so you have more control over what the blurred propeller looks like instead of relying on the sub-frame samples in AE.
Adobe CS3, Apple FCS2
[Disclaimer: Sometimes I am an idiot and misinterpret people's posts. I'm sorry.]