Settling With Power on Top of Osama

Courtesy: Reuters

The cause of the crash of the U.S. military helicopter during the assault on Osama bin Laden’s compound in Pakistan was reportedly a unique helicopter aerodynamic phenomenon that pilots call “settling with power.” The more the pilot pulled up on the collective handle in his left hand asking for more lift, the more lift degraded and the helicopter continued down.

The official term for settling with power is vortex ring state, and the term pretty well describes what is happening.

The main rotor blades of helicopters are actually wings, not propeller blades. Like any wing, some of the high-pressure air under the blade escapes at the tip. The air moves rapidly upward and twists itself into a vortex. It is the wing tip vortex trailing behind an airplane that can be powerful enough to upset smaller airplanes that fly through it. The vortex is often called a wake, but it is the rotating air in the vortex that is the danger to following airplanes.

In the case of a hovering helicopter, the vortex can rotate up and over and be drawn back down into the air flowing into the main rotor disk. In other words, the vortex forms a ring and flows out from under the blade tip and back down into the rotor.

The airflow created by the “rotating wings” of a helicopter. Like fixed wings, they create wing tip vortices, which in the right situations can create vortex ring state. Courtesy: Wikipedia

As you can imagine, the air in the ring vortex that is rapidly moving downward as it enters the main rotor disk, which dramatically degrades the lift production of the rotor. The air in the vortex ring is disturbed and jumbled, robbing the main rotor blades of lift generating efficiency. Because the air in the vortex ring is moving down more rapidly, the angle of attack of the main rotor blade is also changed.

 

The reason pilots call vortex ring state “settling with power” is because the more you pull up on the collective – which collectively increases the angle of all blades in the main rotor at every point on their rotation – the more lift you are requesting. Lift takes power so helicopter pilots use the term “power” as a general way to describe the need for more lift. But in the vortex ring state situation, asking for more lift only makes the vortex more powerful.

The stronger vortex created by the main rotor blades creating more lift actually is more disruptive, so the rotor actually loses additional lift. It is a vicious circle of the need for more lift – power – creating an ever stronger vortex that causes the main rotor to lose lift instead of gain it.

The V-22 Osprey tilt-rotor with its smaller diameter rotors could be susceptible to vortex ring state. However, it also has an advantage in that its tilting pylons can help it escape the situation much faster than a helicopter. Courtesy: Helicopterpage.com

Recovery from settling with power is very simple: Get the helicopter moving. As soon as the helicopter begins moving forward the ring vortex is left behind just like the vortex trails the wingtip of an airplane. With the helicopter moving, clean undisturbed air is drawn into the top of the main rotor disk and lift is fully restored.

 

But the pilots hovering over bin Laden’s compound had nowhere to go with the buildings and high walls. It’s even likely that the walls contributed to formation of the vortex ring state by changing the normal outflow of the rotor downwash. Hovering in a confined area has many risks, and encountering vortex ring state is one of them.

Lower air density is also a contributing factor, so if it was warm in Pakistan that night the risk was higher. And the weight of the helicopter is a factor. The higher the weight, the more lift each main rotor blade must produce, and the stronger the vortex created by the rotor tip. Fixed-wing airplane pilots know this because separation between heavy airplanes must be greater because the vortex produced by the heavy airplane is always more powerful.

The pilots over bin Laden’s compound clearly did a great job of making the best of a bad situation, because nobody was killed or seriously injured as the helicopter settled under power. Helicopter pilots train for settling under power at a safe altitude. The normal training procedure is to bring the helicopter to a hover, then begin a slow descent in the hover.

A portion of Osama bin Laden’s walled compound. The high walls may have helped corral the rotor downwash and feed it back into the airflow over the rotors, possibly exacerbating the vortex ring state. Courtesy: Aamir Qureshi/AFP/Getty Images

With the helicopter settling you pull power on the collective and, if all conditions are right, vortex ring state will develop and the sink rate increases. Once you have established that the vertical speed is increasing as you add power, you dump the nose down to get moving forward and escape the vortex ring state. Easy at 500 feet, but close to the ground, at night, and surrounded by obstructions, all you can do is manage the sink rate the best you can and hope not to hit too hard.

 

The magic of helicopters is their ability to hover, and to move in any direction including backward. But the mechanisms and aerodynamics of hovering are almost unbelievably complicated. When a helicopter is moving forward there are many analogies to a fixed-wing airplane. When the helicopter is hovering, it’s a unique aircraft with both the machine and the pilot performing the most demanding task in flying that I can think of.

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23 Responses to Settling With Power on Top of Osama

  1. Duane Beland says:

    Nice explanation.

  2. Bill Berson says:

    Actually, the terms “settling with power” and “vortex ring state” are not the same.
    It’s true that decades ago they were taught as the same, but times have changed.

    A modern advanced helicopter book such as Cyclic and Collective by Shawn Coyle can explain this thoroughly.
    I will quote just one sentence from Coyle’s book about this: “The term settling with power is very misleading and won’t be used. I’d ask you to remove it from your vocabulary.”

    And for an explanation of vortex ring state, I will paste a comment from retired Sikorsky test pilot Nick Lappos:
    “The issue with VRS is that the name is often applied to all those many more accidents where the power is simply not enough to both hover the aircraft where the pilot desires, and stop its descent prior to the hover. That temporary burst of energy needed on a normal steep, fast approach is often 10 or 15% more than that needed to perform a still hover. When a pilot makes a poor approach, drops through, droops the rotor, and whacks the ground, he says “Vortex Ring” and we all nod knowingly. The helicopter got the poor chap. If he spins around at the bottom (that extra 15% torque, remember?) we call it “LTE!”

    I do try to keep our folly in the properly labeled boxes in the hope that when it is all said, we all at least know WHAT caused the accident, as a slight first step to actually not having one.

    As a little refresher – you cannot experience vortex ring state in any helicopter at less than about 1000 feet per minute vertical descent, and at more than about 8 knots forward speed.”

  3. Bob Briggs says:

    Mac has it right.
    From “Rotorcraft Flying Handbook”, 2000, FAA-H-8083-21, page 11-5:
    Vortex ring state describes an aerodynamic condition where a helicpoter may be in a vertical descent with up to maximum power applied, and little or no cyclic authority. The term “settling with power” comes from thne fact that the helicopter keeps settling even though full engine power is applied.

    The following combination of conditions are (sic) likely to cause settling in a vortex ring state:
    1. A vertical or nearly vertical descent of at least 300 feet per minute. (Actual critical rate depends on gross weight, rpm, density altitude, and other pertinent factors).
    2. The rotor system must be using some of the available engine power (from 20 to 100 percent).
    3. The horizontal velocity must be slower than effective translational lift.

    Recovery is accomplished by increasing forward speed, and/or prrtially lowering collective pitch. In a fully developed vortex ring state, the only recovery may be to enter autorotation.

    page 3-5, as the helicopter accelerates through effective translational lift (ETL) speed, the rotor moves out of its vortices and is in relatively undisturbed air.

  4. Bill Berson says:

    Bob,
    The FAA Helicopter Manual* is out of date. That was my point. The 2000 edition repeats some of the errors of the previous 1978 edition.
    You would have to read an advanced manual such as that from Shawn Coyle to get the facts. The FAA manual doesn’t have the room for such complications.
    Rather than give a complete definition of the two different flight conditions, the FAA manual simply lumps them into one.

    Some pilots use the words “settling with power” to describe the common settling to the ground with insufficient power to stop the descent. Some pilots use “settling with power” to mean vortex ring state. These are two different things.
    I would urge Mac to consult with Shawn Coyle from the National Test Pilot School or some other expert, to clear this up.
    Bill

    *http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf

  5. Jeff says:

    I am not a rotor wing pilot, so I thought the explenation was very interesting. I could not imagine flying anything in the conditions those pilots had to that night.

  6. Gordon Arnaut says:

    Bill I think the author you refer just is expressing his own opinion about not liking the phrase “settling with power.”

    But this phrase is used interchangeably with Vortex Ring State. This is the case in my helicopter engineering book, “Principles of Helicopter Aerodynamics,” by Leishman. Typically pilots will use the phrase “settling with power,” while engineers prefer to use the term vortex ring state, which is more descriptive aerodynamically.

    I don’t agree however that it is unusually difficult to land a helicopter in a trough-like area where you have enclosed walls. This may contribute somewhat to the effect of reingesting the rotor’s wake, but Vortex ring state happens when you try to come in at too fast a descent rate or too steep a flight path—and then figure you are going to apply power at the end and drop her in nice and easy.

    There is a very good illustration in the above book which shows the streamlines. At the rotor tips you have vortices just like wingtip vortices. So the air entrained in those vortices is swirling down and back up and into the rotor. This makes no lift because the net result of this circular motion is zero force in the upward direction.

    So the helicopter only has the lift available from the inside part of the rotor, which may not be enough. That is why you have to descend more slowly and not as steeply, or make sure your helo is not loaded down too much.

    The crew pranged this helo because they were trying to bring it down quickly and they pushed the envelope. It was a mistake plain and simple. Given the nerves under this situation it is not surprising that somebody blinked.

  7. Bill Berson says:

    I don’t think we will ever know what happened on a secret mission.
    However, it is unlikely that the helicopter encountered vortex ring state, in my opinion, because a high vertical descent rate is required, more than 1000fpm for the Blackhawk. It is unlikely that any pilot would descend vertically at 1000fpm at night near the ground. It is also unlikely anyone would have survived true VRS encounter falling something like 6000fpm.
    But VRS continues to be blamed when the cause was probably just something familiar like hitting a wire or fence or something.
    This business of calling settling with power and vortex ring state as the same has caused confusion for decades. Not much that I can do about it now.
    Aviation is unforgiving, pilots should seek the facts.

  8. Frank Hugelman says:

    J Mac,

    I have to disagree with your diagnosis of the helicopter crash during the Bin Laden raid. As a former Army helicopter pilot we were trained on the conditions which cause an aircraft to enter a vortex ring state and those that don’t. A helicopter can not enter into vortex ring state when it is ground effect. Ground effect directs the rotor wash out and away from the rotor system. The only possible way it could have entered a vortex ring state was if they were hovering 1000 feet AGL. But would not be a tactically viable concept of employment given the nature of the operation.
    My theory is the aircraft probably suffered a loss of power at a hover due to enemy ground fire. Without one of the engines developing full power the aircraft probably could not maintain a hover and executed a controlled crash.

    Frank Hugelman
    Major, Aviation USA (Ret)

    • Jim says:

      “Ground effect directs the rotor wash out and away from the rotor system.” But they were in a walled compound so the wash could not go “out and away”.

  9. Ted Hammond says:

    I have to disagree with the description of VRS resulting from the blade tip vorticies as attributed to Wikipedia and stated in the article. VRS occurs when the entire rotor downwash field is recirculated under the specific conditions of power applied, lateral speed and vertical speed, not the much smaller blade tip vorticies which become disrupted very quickly. As Frank Hugelman pointed out, VRS does not happen with a helo in ground effect even though the tip vorticies are still present.

    Back in my days as an Army helicopter pilot (66-72), the term “settling with power” was as described by Bill Berson. In a nutshell not having sufficient power available to arrest a descent. This is different than what we termed “power settling” which is the VRS and which is now in my opinion incorrectly called “settling with power”.

    Whether this mishap was a result of “settling with power” (insufficient power available) or “power settling” (VRS) or from some other cause, the public may never know. What is important is that helo pilots understand the difference between the two conditions, know how to avoid and/or escape from each, and to use correct and standard terminology whatever that may be.

    • L Q says:

      Well, finally someone said something along the lines of my thoughts. Back in the day, we called it power settling, and the escape is to fly out of it, with foward being the most likely choice, although sideways works also.The other cases of hard landings were usually called PPPP. Another cause can be another helo near-by that blows away your ground coushion (probally not the case inside the compound). My experience is as a Marine Pilot, Viet Vet ( in the old UH34D) Aviation Saftey Officer for years in the Reserves( CH-53), various and assorted fixed wing and helo time, lots of t-28 time(fun),Hughes-500 in general aviation, and a career with a major airline. with that said, back to the origional discussion about fixed-wing tip vorticies, all that Mac said is correct, however there is at least one exception; the B-757 creates a “wake’ equal to that of a much heavier aircraft.

  10. Wes Bunker says:

    It always gets interesting when the “I have read a book” crowd bumps into the “I have experience” crowd!
    I tend to follow the “experienced” bunch unless I too have “experience”. Then I follow me. Which explains why I am where I am!

  11. Martin Randall says:

    Vortex ring state (VRS) and settling with power are still used in Army manual FM 3-04.203. They both mean the same thing. Sure VRS is a more technical term but settling with power isn’t removed from most popular teachings. Yes this aircraft could have gotten into VRS. I’ve gotten into VRS in a Blackhawk from a 1200 ft vertical descent before. You have to also realize that the conditions for VRS aren’t simply a text book definition. Factors of a possible tailwind, formation, and a confined area could aggravate the standard contributing factors. In Afghanistan one of our Blackhawks got into VRS while landing into a soccer field and did extensive damage to its landing gear. Trying to fly formation at night to a confined area is tricky business and it can bite you in the butt if you let the situation get out of control.

  12. Mac says:

    I didn’t fly helicopters in the Army, but I was taught to fly them the Army way. My helicopter instructors at FlightSafety International about 25 years ago were all actively flying in the Guard, and had been activity duty helicopter pilots in the Army. They taught me the way they were taught, including at that time autorotations to the ground, something I hear the Army has stopped because of too frequent damage to the helicopter.

    In any case, I was taught to call vortex ring state “setteling with power” so I’m glad to hear the Army still uses that term as well as VRS.

    Mac Mc

  13. John Colman says:

    Has pilot error been ruled out? Could he have just bumped the wall with the tail boom when he misjudged the size of the LZ in that courtyard? “Settling with Power” may go down as the official explanation of why a superstar Seal Team pilot crashed, but the pix clearly shows the tail rotor section laying against a wall. Hitting that wall will pitch the nose down really quick!!

  14. Jack is Back! says:

    Did the special “stealth” fuselage shaping and coating have anything to do with the VRS effect and hover v. descent control?

  15. anthony says:

    No one has mentioned the possible presence of a tailwind. If the pilot was descending quickly, with a tailwind, even with forward airspeed, he could have easily gotten into settling with power.

  16. david huckabee says:

    i think i do not want to fly in helos. they even look like they should not fly, but do very well. some jobs require a helo to get the job done, because a fixed wing can not hover, 99% of the time. then along came the OSPREY V-22. The rules of the game have been modified.

  17. Marcus Moon says:

    Very good debate, all good food for thought. Hey Mac, I have followed your writing for years in Flying mag., Glad you are with the EAA now. Marc

    • Mac says:

      Thanks for the kind words, Marc. It’s great to be working with EAA. I have admired the association for many years and hope to help EAA grow.

      Mac Mc

  18. kyle devenyns says:

    has any one questioned that an engine my have rolled back or flamed out?

  19. Mac says:

    Hi Kyle,

    The only public information is that the military said the helicopter experienced vortex ring state. I expect that is the last and only thing we will learn about what happened.

    Mac Mc

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