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SWP


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I posted this on Just helicopters and received the resposes that one should expect..... :( In my experience two bladed aircraft get in to SWP much easier than multi bladed, I just don't know why.

 

 

Not much to do with the number of blades as it has to do with the helicopters Disk load. Assuming you're talking about vortex-ring state.

 

It depends on the helicopters disk loading (disk-load). Disk-load is the helicopters gross weight divided by its rotor disk area. The higher the helicopters disk-load the higher the rotor downwash velocities (induced velocity). Therefore, descent rates needed to initiate the vortex-ring state must be higher to overcome those increased induced velocities.

 

NASA wind tunnel test found the characteristic vortex-ring state (vibrations) began at a rate of descent equal to about 25% of the induced hover velocity, peak at about 70% of the induced hover velocity, and persisted until the rate of descent exceeded 125% of the induced hover velocity. Passed the 125% point the rotor enters the autorotative boundary leading into the windmill brake state.

 

For most helicopters the range 300 to 600 fpm with some heavy lift helicopters ranging 600-850 fpm. Makes more sense when you look at the ranges of disk-load at gross weight.

 

The list below shows when the beginning stages start that lead to the possible onset of vortex-ring state in vertical descent, at sea level, no wind.

Itdependsonthehelicoptersdiskloading.jpg

 

The induced hover velocity at the rotor (v1) can be estimated: v1fpm = 870 X (square root of the Disk load)

 

Disk load = Gross wt of helicopter in lbs./ π x (rotor radius in ft. squared)

 

NOTE:

On the other hand, Settling With Power (SWP) is more a condition of the inability to arrest an inadvertent (unintended) descent rate brought about by the current power available not being able to meet the power required for a given flight condition. Should not be confused with vortex-ring state.

Edited by iChris
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The other phenom where the aircraft doesn't have enough power to hover or recover from a decent is what I would call power settling.

 

I always called that, "poor pre-flight planning"!

:D

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I posted this on Just helicopters and received the resposes that one should expect..... :( In my experience two bladed aircraft get in to SWP much easier than multi bladed, I just don't know why.

 

 

 

Sometimes its not the number of blades, vortex rings state, settling with power, power settling, LTE, or any of those other things we blame our mistakes on. Aircraft have to be flown within their design parameters. We all make misjudgments and sometime the pilot can be the weakest component in the aircraft.

 

The helicopter (two bladed) listed below had a settling issue, but what do you think was the real cause of this accident?

 

On July 25, 2007, at 1947 mountain daylight time, a Bell 214ST helicopter, N724HT, sustained substantial damage when it impacted terrain following a loss of control while conducting long-line operations in a remote area 22 miles northeast of McCall, Idaho. The airline transport pilot (ATP) sustained serious injuries and the passenger/crew chief, sustained minor injuries.

 

The helicopter was registered to US Leaseco, Inc of Baltimore, Maryland, and under the operational control of the United States Forest Service (USFS). Visual meteorological conditions prevailed and a flight plan was filed for the local public use flight. The purpose of the flight was to conduct external load operations in support of the Loon fire. 



 

In a written statement provided by USFS personnel, the pilot was making an approach to a lake in order to fill a 900-gallon Bambi bucket attached to a 150-foot long-line. As the approach continued, and just prior to the bucket entering the water, the pilot reported that "he was encountering the onset of settling with power." The pilot lowered the collective and initiated a 180-degree turn. During the turn, the bucket entered the water and the 150-foot long-line "pulled the helicopter" on "the right side of the aircraft" while the pilot was maneuvering about 20-40 feet above the water.

 

Subsequently, the helicopter collided with terrain in a marshy area adjacent to the shoreline resulting in substantial damage. 

The operator reported the estimated density altitude at the time of the accident was 10,221 feet. 

Examination of the helicopter revealed no mechanical anomalies with the engine or airframe.

 

NTSB Identification: SEA07TA214

Edited by iChris
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I always called that, "poor pre-flight planning"!

:D

 

Plan all you want, it still happens. There are too many variables to flying with a long line to know exactly how the aircraft will act. I am not saying that you don't look at the charts, its just a fact of flying. Good training and judgement is your safeguard.

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iChris,

 

I spoke to someone very close to that pilot the day after that happened, he said that the winds were very unpridictable in that dip site and ended up comming in with a tail wind at extremely high density altitude. Could have been either SWP or power settling depending on how much momentum he came in with.

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Could have been either SWP or power settling depending on how much momentum he came in with.

 

 

In either case it maybe academic in that the end result is power available does not meet power required. In the vortex ring state the magnitude of the vortex flow and thrust fluctuations for all practical purpose reduces power available. On the power settling side, available power is reduced because of environmental conditions and engine capabilities.

 

In my experience two bladed aircraft get in to SWP much easier than multi bladed, I just don't know why.

 

The one thing that did relate to the blades was “Blade Twist”. Vortex flows and thrust fluctuations were much larger on rotors with twisted blades. Blades with no twist behaved much better in the vortex ring state. They say the larger tip vortex on non-twisted blades reduced the overall vortex turbulence. See figure below.

 

fig28.jpg

 

NACA TECHNICAL NOTE 2474

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