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ashan412
5th Jun 2017, 07:21
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

hueyracer
5th Jun 2017, 11:51
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.

Hughes500
5th Jun 2017, 19:04
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.

NickLappos
5th Jun 2017, 21:29
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
http://www.copters.com/aero/pictures/Fig_2-82.gif

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.

puntosaurus
5th Jun 2017, 23:25
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.

megan
6th Jun 2017, 00:42
but the number of accidents due to "falling through" while trying to enter an OGE hover is very real and happens very oftenMy 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.

NickLappos
6th Jun 2017, 02:28
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.

http://i118.photobucket.com/albums/o95/ichris7/PRvsAirspeed22_zps8f6a68b5.jpg

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.

6th Jun 2017, 05:41
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.

LRP
6th Jun 2017, 06:23
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.

Same again
6th Jun 2017, 07:55
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.

6th Jun 2017, 08:32
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.

Same again
6th Jun 2017, 09:55
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.

6th Jun 2017, 12:48
Then you could use 50 kts and 200'/min and be even safer :rolleyes:

SASless
6th Jun 2017, 13:43
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.

malabo
6th Jun 2017, 14:56
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.

Same again
6th Jun 2017, 15:05
Then you could use 50 kts and 200'/min and be even safer

No thanks. 400' and 40 kts works just fine for me.

albatross
6th Jun 2017, 15:52
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.

NickLappos
6th Jun 2017, 17:07
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.

albatross
6th Jun 2017, 17:22
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

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

albatross
6th Jun 2017, 21:00
Graph labelled Vortex Ring State, does not show VRS, re-inspect
Sorry I do not understand your comment.

Vertical Freedom
6th Jun 2017, 21:03
High ROD doesn't matter a zack provided Your above ETL :eek: & why not a decelerating approach....why fix a speed to it? ie. start at 120knots & keep slowing down till You stop, arrived over the H :ooh:

aa777888
6th Jun 2017, 21:16
This low timer says: best thread in a while, learning/confirming a lot :)

6th Jun 2017, 21:21
the graph is labelled 'Flow states in descending forward flight' at the bottom but the very top line says 'Vortex Ring State'.

Fully re-inspected:ok:

Reely340
6th Jun 2017, 21:40
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.
I fail to grasp that. What is wrong with the following description:
a) we hover OGE at 80% power
b) setting power to 77% will start descent, with an accelerating(!) rate of descent,
e.g. VSI needle will slowly wander(!) towards max descent
(like free falling with the rotor downwash compensating almost all of earths gravity,
thus lift force < gravity force, hence we will accelerate downwards, I get that part)
c) we restore power to 80%. Momentary sink rate is preserved,
won't increase and won't decrease, e.g. VSI needle negative, but steady.
d) we raise power to 83%. Sink rate will reduce, VSI needle still negative, but starting to climb.
e) when we reach sink rate 0 we could reduce power to 80% and HOGE, or leave it at 83% and start to climb.

You were insinuating c) to be wrong. why is that?:confused:

NickLappos
7th Jun 2017, 01:15
thanks Reely340 for so clearly stating the question!


What I am stating is that in c below, the application of original power of 80% will not steady the descent, because in a slight descent of say 250 fpm, the power needed is more than 80%, it might be 85%. So if you restore the power while descending at 250 fpm to 80%, the aircraft continues to have a power deficit and will increase its descent rate. To arrest a descent of 250fpm , you must pull 85% torque. (all these numbers are hypothetical, but illustrate the issue.


If I could draw a power required curve for you, it would look like the line for -500 foot rate of descent crossed over the level flight one, so a steady descent of 500 fpm takes much more power than steady OGE hover.

Reely340
7th Jun 2017, 08:17
So if you restore the power while descending at 250 fpm to 80%, the aircraft continues to have a power deficit and will increase its descent rate. To arrest a descent of 250fpm , you must pull 85% torque. (all these numbers are hypothetical, but illustrate the issue.Interesting, :ok: very important to memorize, but why is that so?

If reapplying the (ficticious) original HOGE power of 80% after inducing a 250fpm descent does NOT arrest descent rate, e.g. sink rate keeps increasing, this would mean that a descending rotor produces less lift than a hovering rotor (pitch being identical).
I can imagine that as pitch were identical, AoA is higher with the rotor descending.
But this condition should produce MORE lift than in HOGE, right?

I'd say this: if (blade) pitch during -250fpm is kept identical to HOGE pitch, AoA will be higher due to descent. Higher AoA means more drag. More drag at same power setting results in rrpm decay. Rrpm decay is compensated by closed loop rrpm control via increase in torque. Now the increased drag from increased AoA is nicely compensated, pitch and rrpm still at HOGE setting, e.g. same lift as in HOGE, but TQ has climbed to a higher level to reestablish equilibrium. How does that sound? :8

If above is true, merely wanting to control/arrest a desired descent rate will produce requirement for more torque. I think I got that, not?

I can see now, that a piston powered helo (the ones I'm flying) can effectively run out of power and settle_with_power from trying to achieve too high a sink rate on approach.

But wouldn't that mean, that people descending with insufficient turbine TQ margin would rather overtorque their engine,
(fuel control trying to maintain rrpm), till the gearbox fails, than "falling through" from settling-with-(lackof)-power?
I always picture turbine helos as excessively overpowered with the gearbox as the limiting factor, in non hot-and-high conditions.

Reely340
7th Jun 2017, 10:02
Nice post I made but it still feels wrong, looking at the bigger picture:

Going 50 knots I know, that
- climbing at 400 fpm needs a lot of power
- climbing at 200 fpm needs less
- straight cruise needs even less power
- descending at -300 fpm definitely needs less power then straight cruise
- autorotating at -600 fpm needs no power at all.

Why would that change if hovering?
Steadly lifting something upwards, needs more energy
("stored" as altitude energy, converted into kinetic energy upon free fall and heat upon impact ;) ).
Steadily descending vertically should "produce" energy, or need less than hover.

*sigh* I definitely need an aerodynamic explanation.

7th Jun 2017, 12:35
In a vertical descent the rotor tips are experiencing a flow from beneath, increasing the tip vortex and creating additional drag - so to re-establish the hover from a descent you must first overcome this extra drag with power.

At the root end, any increase in RoD will increase AoA markedly, very probably past the stall so the effectiveness of that part of the blade is also reduced so more drag there as well to be overcome with power.

Reely340
7th Jun 2017, 13:49
tip vortices, induced drag. Slick explanation, makes sense, thx a lot!

albatross
7th Jun 2017, 18:15
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.

One thing I did not mention was that the above scenario used to happen after you had been in a stable hover at 2-6000 agl in light or no wind conditions for 3-4 minutes ..so you had been hovering +_ 12 inches creating a downflow of rotor down wash unaffected by wind thereby creating a column of down flowing air. That might be a clue as to what caused the sudden downwards departure into VRS. I understand that this case was not something you normally do in ops but it was a very interesting scenario.
Perhaps I was creating my own local downdraft?

fadecdegraded
7th Jun 2017, 20:13
Have experienced the same as you Albatross while sitting on the end of a longline for extended periods holding a load in position with no change in power.
Normally did if by default anyway but used to move the machine around to stop VRS/settling from developing as we never had the luxury of height for the recovery. ( normally moving around s bit anyway)
I wonder if a prolonged stationary hover gave the induced flow a chance to accelerate to a higher level than normally would happen and then a little decent would start due to reduced A of A, a small pitch pull to hold the height and then VRS.
Essentially what Nick said, settling first then VRS.

paco
8th Jun 2017, 11:01
According to the books I have here, the downwash velocity about 2 disc lengths below the machine is twice the induced velocity, so maybe getting anywhere near that may be a problem.

Phil

albatross
8th Jun 2017, 17:34
Makes sense FADECDEGRADED.
Thanks PACO for your imput.
It was a long time ago but it sure was interesting.

It became predictable and was never a problem but the first time it happened it sure got my attention!