Thanks for all the replies. I'm using a Hitec Flash 5 transmitter, and I don't think there's a way to trim the servo centered without limiting the maximum throw. Like Lownslo, I found that all of the Hitec servo arms are different, I just couldn't find any that were quite right. It's too small a problem to take the servo apart, so I'll live with the bias on the ailerons.

7ECflyer