I've always heard these terms used interchangeably. I figured that "settling with power" was what the pilot did to cause the vortex ring state (VRS). But comments on another question indicate that they are two entirely separate aerodynamic phenomena.
I understand VRS, so what is settling with power?
I knew that the debate would start :) This is probably the most contentious question in helicopter operations. it's right up there with the "equal transit time" theory of lift generation a few years back. The only agency I am aware of that defines them as different is Transport Canada. Instructors, pilots, WikiPedia, training videos, books and blogs all get this wrong often. It seems to be very common in the US for people to refer to vortex ring state as "settling with power".
Tom, I know you said you already know what vortex ring state but I think that the "why are they different" explanation is better when both are contrasted together. Please excuse me for repetition.
They are different.
Vortex ring state (VRS).
VRS ring state is a purely aerodynamic condition. In normal flight, whether climbing, descending or cruising, the relative airflow striking the blades is principally from ahead. In a developed VRS, the helicopter descends into it's own vortex which is "recirculated" as the helicopter descends faster than the vortex descends and results in the recirculated air being drawn up, then almost vertically down into the rotor.
This causes a critical increase in the angle of attack and a corresponding loss of lift causing the helicopter to descend faster and faster up to rates of 6000 feet per minute.
In order for the helicopter to descend into its own vortex, it must meet three conditions at the same time (typical values for most helicopters):
less than 30 knots airspeed; greater than 300 feet per minute descent; power applied to the rotor.
Removing anyone of these gets you out of VRS but there's a catch. If you try to reduce the rate of descent by increasing collective and therefore power, which would be the instinctive reaction of someone not trained to recognise and get out of the state, you increase the strength of the vortex and the recirculation and make the problem worse.
The only way to stop the rate of descent in VRS is to use the ground, which you really don't want to do at up to 6000 feet per minute and will probably spoil your plans for the weekend. However, you DO have power. You can use all of it until you hit the limits and still you won't stop that descent.
To recover, remove power by entering auto-rotation or increase airspeed to fly out of the vortex. Auto-rotating out of VRS can take a long time and use a lot of height in the recovery so the accepted and taught technique is to fly out of it, any direction will do, which takes a second or two to recover.
Settling With Power (SWP)
SWP is not an aerodynamic effect.
At it's simplest, it occurs when you need x amount of power to stop a descent but, for a lot of variable reasons, you have less than x power available. Increasing power results either in exceeding the engine or power train limits or stalling the rotor as drag increase exponentially and exceeds the power available to drive the rotor.
The slower a helicopter flies, the more power it requires:
When you get into the low speed regime, you can see that the power required rises rapidly.
Imagine you are coming into a hover. You are travelling slowly at about 20 knots but have a low rate of descent, not enough to enter VRS. If you keep going forward, you will be OK. But, our imaginary pilot makes a mistake and turns into a 15 knot tailwind. The airspeed drops to about 5 knots so the lift produced drops by the square of the speed change and the power required rises rapidly.
The helicopter will very quickly develop a high rate of descent. The pilot increases collective to compensate but finds that the engine cannot produce enough power to maintain lift and power the tail rotor which is also working harder since you increase the torque by increasing power. It is possible to simply run out of power. If the pilot is lucky, they will be unconscious and found with the collective lever buried in their armpit.
SWP nearly always happens close to the ground and once in it, there is no recovery (insert suitably large number like 99% of the time). You have no spare power to reduce the rate of descent or to increase airspeed to increase lift and fly away.
SWP is normally a result of pilot error (as above), mechanical problem leading to unexpected reduction in power available, wind shear causing unexpected reduction in airspeed or other environmental effects reducing lift available (coming into a hot and high hover for example).
Perhaps the easiest distinction to make is that you are already using all of your available power when you enter SWP. There is nothing left to give!
This answer is from the basis of being trained to fly helicopters by the US Navy, about 35 years ago, and spending a few decades as a helicopter pilot.
The usage has changed over the years. The point @Simon made about people not using standard terms doesn't help. In American circles, an original distinction between the two terms, even when both had the word "power" in them, is that each addressed a two different case of
"I am descending"
with the phrase difference indicating
"why I am descending"
usually applied to a case where I try to stop the descent and it doesn't stop. (Clear as mud, I know ... try having your check ride grade influenced by knowing the difference :p )
Power Settling meant -- "I am out of power, and thus I am descending (and RPM is drooping)" That term is now taught as power required exceeds power available.
Settling With Power meant -- "I have more power available, but I am still descending even if I add that power." This is the case of Vortex Ring State, and that is the term now used and taught in the Navy (reference is the same link as above).
As @Simon points out, the gotcha in VRS that makes it lethal is that adding power makes it worse. The corrective action is to reduce collective pitch and fly out of it.
When the Navy taught me how to fly helicopters (81/82) our text book term "power settling" for the situation where we ran out of power and our rotor RPM began to droop, and "Settling with Power" to describe the effects of Vortex Ring state.
This was faithfully and well described by @Simon. Between the time I earned my wings and ten years later, when I was instructing in Seahawks, the Navy changed to use the far more accurate usage: VRS. It was interesting to see the transition and how people got used to the new term. The manuals I just checked out for V-22 and MH-60R also use that term.
It's a confusing term, at best.
The continued use of "settling with power" is likely to do with where one learned how to fly, what country you are in, who your teacher was, and for how long someone used a term and may not want to change terminology.
Settling with power as a substitute for "power required excedes power available" is not universally accepted, although it appears to be a common usage in Canada. To support that point, I offer you:
Exhibit A
Exhibit B
Unfortunately, the ad hoc definition of "settling with power" as a purely power phenomena is not correct, in Canada or elsewhere. The idea that such a definition is needed for "an approach where insufficient power is available" is right on, but settling with power is not the term.
Check here: http://www.dynamicflight.com/aerodyn...ettling_power/
{snip} Evidence of the confusion caused by misuse of the terms is shown by the confusion about recirculation, which is the cause of VRS/Power settling/settling with power. It is the recirculation of the downwash that causes all the misery of VRS.
We should actually agree on a term for the falling through on an approach that is caused by insufficient power, and stick to it. ~ Nick Lappos
As regards expert advice, Nick Lappos is a well regarded helicopter aviation professional with over 4 decades of experience in the industry, and knows flying, test flying, rotary wing design, and much else. You can look him up. I find his take on most helicopter flying issues well grounded. (OK, lousy pun...).
"Settling With Power" is the same thing as "Vortex Ring State."
In the U.S.--where the term Settling With Power appears to have originated--it means the main rotor is in the Vortex Ring State. It describes what happens to the aircraft when it enters MR VRS: despite having plenty of engine power available, the helicopter makes an uncommanded descent, or "settles." In fully developed MR VRS, the available engine power is typically useless to stop the descent. (FAA-H-8083-21A, p11-9)
This distinguishes the emergency from a similar uncommanded descent that occurs when there is not enough engine power available to keep the rotors turning. This state could correctly be called "Settling Due To Insufficient Power," but is instead known as Overpitching. In this case, as well, there is a difference in terminology for the same phenomenon. In the U.S., overpitching is known as Low Rotor RPM. (FAA-H-8083-21A, p11-15)
In both cases, European terminology describes causes, while the U.S. terminology describes symptoms...but they refer to the same phenomena.
Cause: (Main Rotor) VRS
Symptom: Settling With Power
Cause: Overpitching (of the rotor blades, i.e. raising too much collective with not enough engine power available)
Symptom: Low Rotor RPM
It seems there's one more point of confusion, however. In Canada, the term Settling With Power has also come into use, but to mean something like hitting the brakes too late. When a pilot begins to decelerate too late during their approach, reaches the limits of their engine power available given the conditions, and cannot bring the helicopter to a stop, i.e. zero groundspeed, without an overtorque, and the result is either a hard landing or a go-around.
The real problem is a lack of standardized terminology across the industry.
