I need some help with a propellor animating. I tried and the result was the propellor moving forth and back. Thanks for answers.
If you want to animate it rotating, you move your time to specific frame.
I to set a key frame, select rotation from the pop-up window.
Move to another frame (15 frames after the previous frame), rotate the object around it's axis, press
I again to set another key frame (select rotation again).
This will cause it to 'spin' for 15 frames. Without more details regarding your goal, this is the simplest way to animate.
You've tried pressing 'I' and you didn't like it because it was speeding up and slowing down. This is because blender tries to make everything smooth. If you want to make it the way you want, follow these steps.
Another way is to set up a driver
spin * frame
where frame is the current frame, and spin is a custom property, which can be keyframed.
This way you can fiddle the speed, or key in speed changes to get differing strobing effects. (If you rotated 360 degrees each frame you would see no rotation)
There are a couple of small sample files here http://blenderartists.org/forum/showthread.php?229688-Helicopter-Rotor&p=1937512&viewfull=1#post1937512