I am trying to make a flat coil/helix out of a cylinder like this:
I have tried playing with the screw modifier but couldn't get anything close to the image.
Is there a modifier to do this or will I have to do it manually?
A easy way is to use Extra Objects addon. And choose Spirals. You can get an expected curve with a few tweakings in the left-hand Tool bar. Then use either one of the following four methods to find out the best one for you:
Added a Demo File.
I personally recommend to use Solidify modifier here, since it is easy to control, and with caps on both ends.
I'm not sure you will get exactly what you are asking with this method. Maybe close enough.
Use the array modifier.
Start like this. It could also be a cylinder or anything.
Scale it up along the X axis 10%.
Add an array modifier. Uncheck relative offset. Check Object offset. Create an empty and select that as the object to offset.
Rotate the empty 90 degrees around z-axis. Scale it up 10%.
Increase the count in the array modifier.