If you have a regular progression from natural aid to artificial aid, your horse will understand what it is that leads you to tap with a crop. I use soft leg, medium leg, hard leg, one cluck [i hate people who cluck constantly, obviously your horses doesnt care about your bird noises !], then tap with the crop. If you are consistant with your progression they will learn it, and learn what to do not to get hit. If you randomly hit them for no reason, ya they will probably fear you.
I also use a crop/bat, not a whip. Have you ever hit yourself with a crop ? It doesnt hurt, its more for the noise of leather slapping leather.