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best approach speed and techniques to avoid vortex ring condition

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best approach speed and techniques to avoid vortex ring condition

Old 5th Jun 2017, 07:21
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best approach speed and techniques to avoid vortex ring condition

hi all
i just having a doubt know what is the best rate of descent and safe speed for bell 412 to void vortex ring state and tail rotor ineffectiveness
also i would like to know what are the operation limitation which pertaining to Bell 412 avoid vortex ring state Ex : min speed, rate of descent and pwr
thanks
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Old 5th Jun 2017, 11:51
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If you look into the limitations and/or performance section of your helicopter, you will find the wind azimuth chart that will answer yojr question in regafds to wind affecting tailrotor effectiveness.


To avoid VRS, simply stay out of the areas where VRS can build up:
No/slow forward speed and RoD above 300 ft/min.
 
Old 5th Jun 2017, 19:04
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to avoid vortex ring you need to have a rate of descent less than the downwash speed. I would suggest that 300 ft a min is not right. Most downwash speeds are in the region of 800 ft a min plus but that does depend on heli type and how heavy it is.
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Old 5th Jun 2017, 21:29
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Hughes500 is quite right
There is far too much worry about Vortex Ring State and not nearly enough about avoiding entering OGE conditions with too little power margin. The number of real VRS accidents is insignificant, but the number of accidents due to "falling through" while trying to enter an OGE hover is very real and happens very often. In fact, many of those accidents are labeled "Vortex Ring State" accidents. Many experts try to differentiate these two by calling the Power limiting problem "Power Settling".

To encounter true VRS you must be descending nearly vertically at somewhere near your downwash velocity (at least 75% of your downwash velocity), which is at least 700 feet per minute in a Robbie and something like 2000 feet per minute in a large helicopter. If you need tips on how to avoid VRS, try this tip:
"Avoid 700 foot per minute near vertical descents".
Here is a plot, where 1.0 on the left axis is about 1000 FPM for a Robinson, 2000 for a 412 and 2500 for an S-76


How to avoid the other power problem? Power Settling occurs when you try to Hover OGE with too little power margin, typically 5% or less extra power above that needed for HOGE. If you try to hover with no margin, any descent will require a bit more power, so any rate of descent will start an inexorable fall. The US Army requires at least 8 to 10% more power than the OGE chart to operate with relative comfort while continuously OGE.
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Old 5th Jun 2017, 23:25
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Might be worth a little caveat on that chart. The OP is a professional pilot asking a specific question about a particular machine.

Lots of pilots read this forum though and many are in training or low hours. The reason instructors teach much lower limits is to keep them well away from the danger zone for both vortex ring and power settling.

300fpm and 30kts marks the boundary in the light helicopters in which we teach where significantly different handling characteristics begin to emerge. It is those changes that we try to instil as warnings to sort things out before problems occur.
 
Old 6th Jun 2017, 00:42
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but the number of accidents due to "falling through" while trying to enter an OGE hover is very real and happens very often
My first turbine was the 204, and we were taught a very simple means to establish what the capabilities of the 204 were when it came to landing. Meant primarily for mountain type work. Worked like this, though forget the exact figures.

1. Establish level flight at the intended altitude of landing at 60 knots
2. Note TQ
3. Pull power till reaching rotor droop and note TQ
4. If the difference between the two TQ readings is in excess of X, an OGE is possible
5. If the difference is between X and Y an IGE is possible
6. If below Y a run on is required

Often wished all types had a similar gouge.
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Old 6th Jun 2017, 02:28
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megan,
That is a good system, I used to teach a similar one, what you are doing is estimating the shape of the power required curve from the Vx to the OGE power. The ratio is probably about 2x, with the OGE power as close to twice the "bucket" speed level flight power.



Another rule is to perform the max power pull at the proper conditions of altitude and temperature near the landing site and note the maximum torque that is available. Subtract 10% and use that just below max power as your never exceed. Make a slow approach bleeding airspeed deliberately and increasing power deliberately and watch the torque. Do not keep your speed up and rely on a hairy cyclic flare at the bottom, or you might be surprised by the power suck-in and bottom falling out. Instead, keep slowing and progressively watching the power rise. If you can get close and slow and still maintain that margin you observed, then good on you. If not and the power is clearly going to be close to the limit, just lower your nose and accelerate away from the landing, all under full control.
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Old 6th Jun 2017, 05:41
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As an add-on to that, if you need to achieve OGE hover close to your power margins, make a level approach to the hover rather than a descending one, again without a harsh cyclic flare.

Then you are using what power you have to replace ETL instead of doing that as well as reducing rate of descent. Any tendency to 'settle with (not enough) power' is immediately obvious and a go-around can be initiated before the situation gets worse.

Brit mil teaching for power checks was within 5 miles and 500 feet of your intended LS/hover point.

Punto - the only validity of the 30 kts figure is that most ASIs don't register accurately below that figure - you can see from Nick's graph that you have to be a great deal slower than that to even get close to VRS.

The problem with 'demonising' VRS with such a huge safety margin is that pilots don't ever learn to recognise the real onset - you don't see any changes in handling at 30 kts and 300'/min RoD other than a slight vibration which is not VRS or even IVRS.
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Old 6th Jun 2017, 06:23
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A field expedient method I used in the Rockies was to shoot the approach to 100-150 ft above the highest obstacle...if successful, the landing was continued by reducing power...if it looked marginal, there was sufficient altitude to fly out.
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Old 6th Jun 2017, 07:55
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If you don't happen to have the RFM on your lap when you start your approach you can remember what I was taught many years ago in basic training. If you have a ROD of more than 400 fpm and your IAS is less than 40 kts then you should be aware. 400 fpm and 40 kts. Easy to remember, will keep you safe and has worked for me in all types that I have flown.
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Old 6th Jun 2017, 08:32
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Which RFM will give you a graph for VRS speeds and RoD???

40kts and 400' is far too conservative a set of figures - see Nick's graph.

The Brit Mil for many years have taught 30 kts and 500'/min as a 'beware' condition but those figures haven't changed from the days of underpowered helos with very low disc loadings and very low downwash speeds.

Modern helos (even the R22) are much more powerful and have higher disc loadings and downwash speeds so your RoD must be much higher to get even close to VRS.
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Old 6th Jun 2017, 09:55
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40kts and 400' is far too conservative a set of figures - see Nick's graph.
Which is precisely why it works and why I am still alive after 40+ years and 11,000 hours of flying.
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Old 6th Jun 2017, 12:48
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Then you could use 50 kts and 200'/min and be even safer
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Old 6th Jun 2017, 13:43
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Another rule of thumb......prior to take off...note hover power while into wind....depart and hold that power setting.....determine the airspeed that provides a 500 FPM ROC.

Prior to landing....adjust power to achieve the same airspeed as determined at takeoff and note the power required.....then pull power to achieve the 500 FPM ROC and note the poorer required.

Ensure the pre-landingCheck is done in calm air.....and not in up-rising air!

This gets you to thinking about power management at both ends of the trip and is a measure of power available at both ends of the trip.
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Old 6th Jun 2017, 14:56
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Thanks Nick, excellent explanation, I'm going to crib your notes for our instructors.

Standard oral question on the annual Canadian PPC ride (required per the guide): explain the difference between settling with power and VRS. I've noticed among new pilots it seems to be the new bogeyman, to the extent they are now being stupid on confined areas for fear of VRS.

To the OP's specific question on the 412, I've never encountered LTE or VRS in several thousand hours working bush with it, even tried unsuccessfully to induce it for demonstration when teaching type ratings. Must be there theoretically, but not something you're going to find flying good airmanship profiles. If you need a number, the 40/400 is plenty conservative.
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Old 6th Jun 2017, 15:05
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Then you could use 50 kts and 200'/min and be even safer
No thanks. 400' and 40 kts works just fine for me.
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Old 6th Jun 2017, 15:52
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Used to do a thing called "laser survey"
Hovering at 3-6 thousand feet maintaining position +_ 12 inches over a 10 amp gyro stabilized laser pointed veritically and impacting on a 24 inch screen on the bottom of the helicopter. The laser dot could be seen on a 4 inch tv screen in the cockpit...just keep the dot in the middle of the screen...that part took a lot of practice! LOL Some guys never could do it but with 4-6 hours of training most could ...some could never stop climbing, some could, some could also descend while keeping the dot centered and one eye/hand genius I knew would happily do hover turns as he climbed, hovered, descended and smoked a cigarette.
Sometimes ( a lot actualy ) you would be sitting there in a hover and the vsi would flicker downwards.
Add a little power ..vsi would flicker to 1-200 fpm and then WHAM you would be in fully developed Vortex Ring State...lots of fun but we could never figure out how it developed so quickly. Had lots of time to explore it as we plummeted downwards with the VSI pegged and the cyclic like a wet noodle..( once you lost the laser you had to go back to groundlevel hover over the laser to get the point back in the screen then climb vertically back up. The surveyors used to laugh when the helicopter rapidly disappeared verticaly downwards out of the field of view of their transits...the told me that they detected no downward movement of the helicopter before the abrupt downward departure and they had the helicopter in the crosshairs of a 50x theodolite.
Basically we were using the helicopter as a very tall stadia rod.
So anyone have an idea as to why the helicopter would suddenly enter Vortex Ring State from a stable hover? We were very light..usually only the pilot aboard and perhaps 3/4 fuel - AS350D in my case but we also did it using 500D, Gazelle and Alouette.

Last edited by albatross; 6th Jun 2017 at 16:55.
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Old 6th Jun 2017, 17:07
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albatross,
I think the explanation is that it entered Power Settling and then degraded to VRS as the power-induced descent increased. The terrible thing about rotors is that as you go from a stable OGE hover downward, the power needed to hold a steady slight rate of descent is HIGHER than the power needed for steady hover. In effect, a piece of VRS is introduced to the rotor and the pilot must raise the collective to add some torque to hold the slight descent. If you don't raise the collective, the slight power deficit of the descent makes the descent rate increase, and down you go.

AND our training has said that because VRS is the dreaded beast, never raise the collective in a vertical descent. Well, that is bunk. If you have the engine power, raise the collective and stop that moderate descent. Up to descents of maybe 700 to 2000 feet per minute (depending on the disk loading and downwash velocity of your helo) you have NO danger of VRS at all.

In short, up to horrendous vertical rates of descent, the issue is PERFORMANCE and ENGINE POWER, not VRS.
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Old 6th Jun 2017, 17:22
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We were usually into wind .. we hoped .. but at 3-6 thousand .. who knew except for cyclic position and airframe attitude LOL.
You would feel a nibble and the VSI would indicate 1-200 FPM down, just a flicker on guage really, so you would add a bit of power to stop that ..needle might stabilize then another flick and WHAM away you went...the strange thing was the surveyors never detected any descent before Gawd sucked all the air out from under you ..it became quite predictable.. for added fun pull 100% as you departed downwards..much giggling and laughing. We did a lot of this work and I spent a lot of time exploring VRS on the way down exploring various recovery methods ..the easiest was a small 45 degree movement of the cyclic at any power setting..straight ahead was slower. LOL
Took a fellow pilot on one of these flights and had time to say "We are about to enter Vortex Ring ..hold my beer and watch this!" Down we went..I held it there and recovered a couple of thousand feet later..he was amazed. Oh check yer PMs

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Old 6th Jun 2017, 18:55
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Graph labelled Vortex Ring State, does not show VRS, re-inspect
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