Item 0950

OTHER: Aerodynamics - General - Vortex Ring State (settling with power)

Flight test experience on a small tandem rotor helicopter showed that the characteristic vibration began at a rate of descent equal to about 23% of the hover-induced velocity and persisted until the rate of descent exceeded 125% of the hover-induced velocity. [Source ~ RWP1 p.102]

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Thread on Rotary Wing Forum:

From Jtravis1: The FAA Rotorcraft Handbook says the three conditions necessary are, rate of decent of greater then 300 ft/min, airspeed less then 30kts (below etl) and be producing lift. Truth be told ROD necessary for real entry is over 500ft/min and closer to 800ft/min before you really start to make it happen. It is different from ship to ship but no where near 300. Even in full-blown VRS you still have at least 50% lift.

From Hognose: That's one reason among the many that the lateral rotor layout went into the history books in the 1940s and only came back for tiltrotors, and then only when the engineers thought they had the stability problems licked with fly-by-wire.

From brent s: If the rotors overlap like the CH-46/47, it's almost impossible to enter VRS - they disturb each other's airflow enough to make it very unlikely. Knew a couple of IP's back in the day that used to demonstrate vertical powered descents that scared the crap out of transitioning Huey drivers... Like Justin pointed out, VRS isn't possible in autorotation.
From Jtravis1 on Rotary Wing Form: In the private world, we still demonstrate this.....as least I do. It is not much fun but very safe with altitude. Pull into a high OGE hover and lower the collective slightly, as your VSI starts getting to around 800-1000 fpm start raising collective. You can really feel it when you start to enter. At that point a smooth raise of the collective will send you plummeting.

From Jtravis1: I would imagine any rotorsystem can enter VRS. Some ships, not many, have the power to overcome it as some have stated.

From brent s: The descent rate required to enter VRS is a function of rotor downwash speed - lower disk loading means slower downwash speeds, and a lower descent rate required for VRS.

From Self: I am just hoping that the disks will be so big and the air velocities so small that the vortices can never seriously affect a significant portion of the disks.

From: Your only potential issue is how quick can you do a near zero-airspeed powered vertical descent (and that's not an area that gets used much) - it'll just be less than a design with higher disk loading.

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Thread on PPRuNe:

Leishman says "From a piloting perspective, entry into autorotative flight requires transition through the vortex ring state. The vortex ring state is known to exhibit considerable unsteadiness at the rotor, and this can lead to significant blade flapping and a loss of rotor control"

Quoting Leishman, again, he says "For traditional helicopters with low disk loading, the region of the flight envelope affected by the VRS is generally small, and has not overly constrained practical operations."

 

See also:- Paul Cantrell ~ Settling with Power

Many vortex ring state problems relate to tail rotors.

This is my crazy thinking; the vortex ring state may not exist, or be as noticeable, at the sides of the intermeshing helicopter, because the blade tips of the two rotors are in different positions laterally. In other words, the upwash from the upper blade may be interacting with the downwash from the lower blade. The reverse rotation of the two rotors might also be a positive feature.

 

'Helicopter wreckage analysis', presented at the 1979 Forum of the International Society of Air Safety Investigators, Jerry T. Dennis of the US National Transportation Safety Board

Settling with power can best be described as settling in the aircraft's own downwash. Technically it is called the 'Vortex Ring State', where the high rate of descent exceeds the normal downwash velocity on the inner blade sections and they stall. This then causes a secondary vortex, which results in turbulent flow over much of the rotor disk. It has been demonstrated that the stall starts at the hub and migrates outwards towards the tip as the rate of descent increases. Increased angle of attack (collective application) only increases the stalled area and resultant rate of descent. Descent rates exceeding 3,500 ft/min have been recorded.

 

According to FAA [US Federal Aviation Administration] Advisory Circular 61-13B, the pilot may get into this condition by:

  1. Attempting an Out of Ground Effect (OGE) hover above the hovering ceiling of the helicopter.
  2. Attempting to hover out of ground effect without maintaining precise altitude control.
  3. A steep powered approach in which the airspeed is permitted to drop nearly to zero.

Advisory Circular 61-13B further indicates that the following combinations of conditions are likely to cause settling with power:

  1. A vertical or near vertical descent of at least 300 ft/min. Actual critical rate depends on the gross weight, RPM, density altitude and other pertinent factors;
  2. The rotor system must be using some of the available engine power (20-100%).
  3. The horizontal velocity must be no greater then 10-mph. That velocity is not necessarily the velocity across the ground, but the transverse velocity through the rotor disc.

 

As a result a deceleration or approach can meet all the requirements, especially if downwind. Recovery can be accomplished by increasing the forward speed and flying out or lowering the collective to reduce the stalled area.

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Last Revised: September 17, 2011