crab,
I fully agree with you - in the US the terms are used (improperly) interchangeably, even in texts. If you want proof, here is an entire Army that has the confusion:
http://www.dynamicflight.com/aerodyn...ettling_power/
The best definitions (IMHO):
Vortex Ring State [note - all ROD figures are for light training helos, turbines will have much higher VRS onset RODs]- when the aircraft forward speed is slow enough (below 12 knots) and descent is steep enough (faster than 700 fpm) that the rotor begins to consume its own downwash. In VRS, the torque oscillates and the aircraft bumps, pitches and rolls. In spite of pilot lore, VRS cannot be sustained at high rates of descent because beyond about -1200 fpm, the rotor is in autorotation, and not anywhere near VRS.
Settling with Power (SWP)- when the engine produces less power than the aircraft needs at the weight, altitude and hover condition. The aircraft bogs down, descends and is unrecoverable unless accelerated above translational lift. In bad cases the rotor rpm is pulled down by the enthusiastic pilot so that the tail rotor can also be swamped and a yaw can develop.
Why do pilot confuse the two? because virtually ALL VRS demos are began by descending slowly from an OGE hover, where the instructor is inducing settling with power by not raising the collective enough to stop the descent. In the typical training scenario, the instructor is demonstrating SWP at the onset, then as the ROD builds enough, he is showing VRS, then as the ROD goes to astronomical values (pilots love to wet their pants and brag about 4000 fpm) the instructor is actually demonstrating a vertical autorotation.
Myths about VRS:
Myth 1) VRS makes the aircraft descend so rapidly that the only way to recover is to dive out of it. Not true, if there is lots of altitude, the aircraft can simply be allowed to descend to vertical autorotation, where the rotor is eating clean air, and is powered by the descent. For light helos, autos are at about 1500 fpm, for heavier ones, 2 to 3000 fpm. For helos with lots of power, you can raise the collective while in VRS and motor up out of it. VRS typically raises the power required by something like 10 to 40% so this trick is not useful unless you are in a SkyCrane or a Black Hawk.
Myth 2) VRS causes the rotor to become dead aerodynamically, and if you don't do something, you will die. Only true low to the ground, the natural state of the helo in VRS is to accelerate downward until the rotor is eating clean air in autorotation.
Myth 3) If you descend in a hover at 300 fpm, you will get VRS. Not true, no helo will experience VRS at such low descents unless the pilot does nothing and lets the descent increase to real VRS speeds. At 300 fpm descent, the pilot is actually entering an artificially induced SWP regime, where the power is kept purposefully too low, and the descent is allowed to build.
Myth 4) High altitude or high gross weight makes VRS more likely - NOT true, the opposite is true, but since high weight and altitude make SWP more likely, it is easier to demonstrate the classic SWP becomes VRS becomes Autorotation demonstration that is labeled VRS by instructors and worshiped by students.
Myth 5) VRS occurs often and is a cause of many helo accidents. Not a chance, the conditions are very far away from where we operate (700 fpm downward, less than 12 knots). Most accidents where the VRS label is applied are mis-identified by the investigators, and are actually the much more common, much more dangerous SWP.
To recap: VRS is not what your instructor showed you, or at lease not all that he showed you. He demonstrated a complex SWP entry into VRS followed by a screaming vertical autorotation. In real life, the three distinct pieces of what he showed you are separate, controllable and in fact, very predictable.