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settling with power


blueskies 54
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Ok, recently I have had some confusion on the specifics of settling with power. The three elements that are most likely to get into settling with power are:

 

1. 20% or greater power applied

2. slower than ETL

3. decent rate of greater than 300FPM

 

My question on is on the 20% or greater power applied. I had this explained to me today as 20% additional power applied. The example used:

 

You are in a schweizer 300c decending below ETL at 350FPM holding a constant 15" (25" available) of manilfold pressure all the way to the ground; hypothetically you dont pull any more power all the way to the ground. His answer was that you could not get into settling with power holding a constant power setting to the ground. He said if you were to pull 5" (5 divided by 25= 20%) additional inches of manifold pressure at the bottom ( i.e. bringing MP to 20") of the decent you would get into settling with power because you are now pulling 20% additional power???

 

My understanding was that the 20% power applied was power applied to the rotors by the engine at all times. For my understanding, 5" of total manifold pressure would be 20% power applied. And for all simplicities sake if the rotors are turning on the 300c and im not in an autorotation I have 20% or more power applied at all times. Therefore, my answer to his hypothetical situation of holding a constant power setting is that you would get into settling with power??

 

Thoughts????

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You'll be much better off if you can learn to "feel" when the aircraft is about to "settle with power". Getting too technical will just make you go nuts, and numbers aren't always right. :blink:

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Your understanding is correct, and the explanation you received today was wrong.

 

I wouldn't put a number to the "power applied" factor, and just say the ingredients are

 

1) lack of Airspeed

2) high descent rate

3) power applied, as in: not in an autorotation

 

Forget about the 20%, leave the calculator at home. Whenever you are using power, don't have ETL and are descending faster than you want to descent, you are settling with power, and risking vortex ring state (and yaw control issues, but that is another story).

 

 

If the explanation you received today was correct, what would happen if the person maintained the descent at the quoted 15" of MAP, but reduced airspeed below ETL, without touching the collective?

There would be no additional power applied, but the helicopter would definitely settle, the descent rate would increase well beyond 300fpm, VRS could develop. In any case it's not going to be a pretty landing.

Edited by lelebebbel
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Learn the book answers, then forget them, except for test situations. They define the conditions above which it's really hard to get into settling with power, but not impossible. In real life, any time you're slower than ETL, and too low to fly out of, and arrest a descent, fly to avoid settling with power- make all your descents slow, S-L-O-W! The problems with gettin' all calculator happy with this are:

Most helo airspeed indicators aren't accurate at slow speeds (use ETL).

Settling with power 'requires' 300 fpm descent, and won't happen at 299 fpm? Could you read the difference and is your VSI calibrated at that point?

"Some Power", yes, well that one's good. 20% applied power? Same issue as above, it won't happen at 19.9999%? And, how do you know what '20% applied power' is, exactly?

 

Have a well developed settling with power demonstrated to you. When you do, you'll see the aircraft come down FAST and sloppy, pitch and roll. If you allow this to happen to you at high hover, you won't be able to pitch out of it before the crash, so avoidance is the only answer.

Edited by Wally
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The 20% power requirement for settling with power is often misinterpreted. This does not deal with the engine, how could it? Settling with power is an aerodynamic phenomenon. The 20% power is referring to Collective pitch angle not the square root of 20% MP times the terminal velocity of the muffler bearing. At the end of the day you have to understand this promblem and stay far away as possible from the parameters that cause it. Read Principles of helicopter flight. By the way Great question

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As others have said, the explanation you got was wrong.

 

However, if you are below ETL, at a 350FPM descent, and pull an additional 5 inches of power you will induce settling with power. You may or may not be in SWP at the condition you listed, but if you start adding a bunch of power in that condition you will most likely end up in SWP.

 

Is it possible you misunderstood your instructor, and he/she was explaining how you would induce SWP in flight to practice recovery? All things equal you shouldn't need to add that much power to induce SWP, but it certainly helps make a more effective demonstration.

 

As for the "book stuff," you should keep this in mind: In theory, theory and practice are the same; in practice, they're not. Get too hung up on the book, and the reality will pass you by.

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What I was taught was 1-2-3. 10% power or greater, less than 20 kts and great than -300 FPM puts you into the window for settling with power. Also keep in mind that you can be outside this window and still have settling with power, as outside forces can put you into that region without the indicators. Like a gusty tailwinds, dropping below the tree tops, etc.

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  • 3 weeks later...

There is a lot of erroneous and misunderstood information above.

 

Take the term "settling with power" and throw it out the window, because it is confusing everybody.

 

Start using the term VORTEX RING STATE.

 

Now you can understand this "condition" better.

 

Because some people use the term "settling with power" interchangeably with vortex ring state, other people use it to describe the failure to execute a timely control application to arrest a rate of descent, and yet other people have know idea that there is a difference.

 

VORTEX RING STATE or VRS, can happen...

 

under the conditions of:

 

1. Using between 20% to 100% of your available power.

2. Little or no forward airspeed

3. 300 to 500 fpm Rate Of Descent (which is what I was taught) or otherwise a greater than 300fpm ROD.

 

Now, just because you put the helicopter under the conditions mentioned above, DOES NOT MEAN you are in VRS. YOU HAVE TO RECOGNIZE VRS.

 

How?

 

a. An increase in collective causes an INCREASE in the rate of descent. THIS IS YOUR BIGGEST DETERMINING FACTOR THAT YOU"VE ENTERED VRS.

b. Buffeting in the rotor system

c. Generally feeling of mushiness/unresponsiveness in the cyclic control.

 

It feels like somebody suddenly revoked the laws of aerodynamics..Like the blades were suddenly stopped or gone or something...at the touch of an increased collective.

 

VRS is an unstable condition...it comes and goes, according to the airflow around the rotor system. Remember the aircraft is descending into new pockets of air that will have different, random air currents than the parcel of air above it...But that may not matter when you wait until the point of no return to begin your termination of a poorly planned/executed approach.

 

There are 2 ways to get out of VRS...increase translational airflow over the rotor system, or enter autorotation. Now, if you guys are flying anything that has the possibility of MAST BUMPING, and understand what I just said about unresponsiveness in the cyclic, then hopefully you see the danger in executing one of the recovery methods i just listed...and then again, if you choose the autorotation recovery, you have to have the altitude left to execute it...hence you see the danger of operating in an envelope where VRS is possible, yet you don't have the airspace to execute a recovery...

 

So just because you are at the bottom of a poorly planned descent, and the ground is coming up fast, so you add collective, and the aircraft begins to slow it's descent, you have NOT entered VRS. If you overshoot your desired altitude of termination, i.e. you smack the ground, you just failed to properly execute your approach, if the aircraft began to slow its rate of descent and you just didn't have enough time to decelerate the vertical motion.

 

Imagine it this way, if you run your car at 60 mph straight at a brick wall, and only begin to apply the brakes at 25 feet, that is not a car problem that is a pilot error problem. But on the other hand, you drive your car (a late model TOYOTA perhaps?) at 60 mph at a wall, and something disconnects your brake pedal and connects it to the throttle, and begin to apply brakes at a proper 350 feet from the brick wall, and your car accelerates further.....now you have an idea of what VRS is like.

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So settling can occur when vortex ring state (VRS) is not present?

 

Yes, quite simply if your helicopter does not have enough power applied or available to maintain altitude.

 

It will then obviously begin to settle, even though you have "some (or all) power applied", and this is most likely to happen when you are below ETL, because that's when more power is required to stay afloat.

 

This happens far more frequently than actual VRS. The proper recovery is obviously to accelerate and fly away.

 

If you did nothing and continue to let the helicopter settle, it may or may not at some point enter actual vortex ring state. At the same time, you've set yourself up for an uncommanded yaw due to high torque and lack of tail rotor thrust (now someone will say "LTE" and we've got another discussion coming up)

 

I think it is appropriate to use the term "settling with power" for this condition, and "vortex ring state" for the aerodynamic trap described by archerm3.

Edited by lelebebbel
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yes - it is not an aerodynamic condition, it's simply a lack of power. If your helicopter does not have enough power to hover in ground effect, it will settle to the ground.

 

You can't get vortex ring state in ground effect, because you will be on the ground before it could develop.

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Does a vortical flow develop ahead of the helicopter when transitioning into forward flight? That is near the ground. Is it recirculated through the rotor changing power requirements – momentarily?

Edited by Tom22
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Does a vortical flow develop ahead of the helicopter when transitioning into forward flight? That is near the ground. Is it recirculated through the rotor changing power requirements – momentarily?

 

I know it is not vortex ring state but the helicopter will settle when transitioning into forward flight near the ground because of vortex phenomenon. Furthermore, I don’t think this situation would be considered in ground effect as the helicopter is moving out of ground effect.

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When you transition from a IGE hover to forward flight you leave the "ground cushion" of air that your helicopter produces below. When you accelerate the rotor disc starts to operate in cleaner air, translational lift sets in. At a certain point this translational lift becomes 'effective', i.e. you have more power without changing the power setting (ETL)

The trick is to trade the two effects. You want to get into ETL before you lose your ground cushion of air. If you accelerate too fast the air cushion won't catch up before you gain ETL, you will settle and have to add power.

In an ideal take off creep the hovering helicopter forward to stay over the air cushion until ETL is gained and you don't have to change the power setting at all.

Has nothing to do with vortex ring state or SWP

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Have you ever seen dust or snow thrown up in front of the rotor disk at slow forward speeds? It is caused by vortical flow region near the ground upstream of the helicopter. It is recirculated through the rotor as speed increases thus momentarily causing a settling situation. However, this vortex phenomenon will disappear as speed is further increased.

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The helicopter settles slightly during transition into forward flight because of a number of factors, but you can continue to fly at that low forward airspeed quite comfortably, with only a slight increase in power. Even though some air may be recirculating in the forward part of the rotor disc.

In VRS however, a increase in collective pitch / power will increase the rate of descent.

 

There is always some air recirculating through the rotor when the helicopter is in a hover or in slow forward flight, but that still doesn't have much to do with vortex ring state.

 

If it would, raising the collective while hover-taxiing would make the vortex situation you are describing worse, causing the helicopter to pitch nose down - in reality, the opposite happens.

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Man, this thread got convoluted, and I'm probably not going to help. The VRS/SWP issue has been beaten to death, and depending on what you were taught, you get the distinction or you don't.

 

There is the aerodynamic condition (best described as VRS), but also called SWP since the aircraft continues to descend despite having power available to arrest the descent.

 

There is also Settling, with Power Available. This is not aerodynamic--it's the end of a bad approach where power is applied too late to arrest the descent, or the rate of descent is too great for to be arrested with the available power. Say you can just barely HOGE at 4000 MSL. If you set up a shallow approach, you have the power available at the end and can make the landing b/c your RoD is low. If you set up a steep approach with a high RoD, you can't arrest the RoD and "settle" into the ground.

 

What Tom is talking about isn't covered in most of the basic texts. What most of us probably learned is that, during the transition to forward flight, you shift some of the lift from being vertical to being horizontal, and the helicopter descends some, depending on how much you nose forward. There is a "vortical thing" going on too. While hovering, you are sitting on a cushion of air that flows down through the rotor disc, outward along the ground, and then upward some distance from the helicopter (AIM 7-3-7). That's a vortex. As you nose forward to take off, many things happen, but one of them is that the forward part of the disc pushes into that vortex. Some aerodynamics happen and lift is decreased. You then fly through that disturbed air, get ETL and some undisturbed air, and don't give it another thought. This is sort of like recirculation in a confined area, which you can read about in the RFM, except that during take off the helicopter moves into the vortex, whereas in recirculation, the vortex moves into the helicopter.

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