I once taught an introductory propulsion course - and I made it clear that I was comparing
Propulsion systems, and not merely comparing engines.
As a result, every system from a pure jet to a helo had, by definition, a bypass ratio. And any application using variable-pitch cold stream blades indeed had a variable bypass ratio.
And the chart I used was propulsion efficiency (y-axis) vs. airspeed (x-axis). A helo could create huge thrust (ie lift) at low speed, but ran out of steam quickly as IAS increased. A pure jet was relatively inefficient until reaching high speed; and the turboprop and turbofan lay somewhere in between.
