How To Have Fine Control Over Rotating Objects

Question

I recently downloaded this jet from BlendSwap which has dual propellers on the wings. The propellers do spin on their own when the animation is played.

But when I do animations, at some point I'd probably make a plane or a helicopter to slowly start up its propellers/rotors before they reach top speed and the vehicle lifts off.

Is there any way to make a spinning object slowly gain rotational speed, hold it, then slowly decrease (like in a landing)? I tried a Transformation constraint to a spinning emtpy and keyframing the influence, but the results didn't seem to go well. . .

Answer 1

This rig is over complicated: it has an IK bone that follows an empty, which rotates following a curve (circle); the animation is controlled by the evaluation time parameter of the curve, which has a cycle modifier in the graph editor, that you can see if you select the curve (and can be animated by hand using the 0-100 values, but it's difficult to achieve a smooth transition between different speeds due to its cyclic mode.

I think it would be better to delete the circle, the IK bone and the rotating empty, then select the bone called "prop rotate.L" in the "Armature prop L", set it as "Euler XYZ", and animate its Y rotation. So you can control the speed of the rotators by a simple use of ease in and ease out of a bezier curve in the graph editor.

Answer 2

Thanks a lot to Mr. Sanfelici for posting the first answer, but I think I just figured out the method that will work for me. It is all based on this YouTube tutorial which involves the Animation Nodes add-on. The installation is simple. Download the correct version and install from User Preferences>Addons.

First step is to create an empty and animate it. Set a keyframe where you want the non-moving rotors (or propellers) to start rotating. Go to the frame where you want your rotors to stop spinning (such as is a landing) and move the empty on the Z-axis a large distance (500 units) up or down.

The second step is to setup the nodes like the screenshot below.

That math node was a little tough for me to locate. It's under the category of Number. Then play with the values on the B input there until you find something you like. Making it a negative value will cause it to spin the opposite direction.