That used to be the RAF taught technique for the Puma HC1, especially when night flying to a field using a basic NATO “T”. Flaring too hard on that type of helicopter could result in the engines backing right off and because they had no collective anticipators, fail to respond in time to prevent a big Nr loss and possibly a heavy landing (as some discovered). The problem became more acute when the draggy old metal blades were replaced with more “slippery” composite ones.

Semantics alert!!!!

The vibration is not transverse flow - it is the boundary of ETL (not incipient, not fully developed just ETL) and is due to the vortices clearly shown in the diagram. It happens too quickly to separate in minute detail - the vibration happens and almost immediately the nose pitches up (flapback) and the aircraft climbs (ETL).

On the way down the approach you can ride the burble (vibration), as Robbie describes, and are remaining just above the boundary of what you could call non-ETL. You can feel if you lose that ETL as the burble disappears and your power required to maintain the angle of descent increases - you have lost ETL.

see above, to get to that situation, you have to reduce speed - if you reduce speed too much you lose ETL.

You can't ignore the aerodynamic effects of transitioning from the hover when you are considering the approach to the hover - what goes up must come down. The figure from Prouty is very relevant.

I'm not an aerodynamicist either but I do know what a helicopter does and doesn't do.

Hmm,...

Maybe next time I'll just say, "I like to just sit on top of ETL and crawl in". :-)

Had to turn on few more processor cores to comprehend those last few posts. I think I will dig out the Prouty book and give it a second read.

The figure out of Prouty is not relevant to a descent. It describes the ground vortex that rolls up in front of a helicopter travelling in ground effect.

After thirty four years and 16,500 hrs. on helicopters I am so glad that air is invisible.

Have you heard of brown mist? :E

Only after a Screaming Jesus vindaloo or a Sichuan Hotpot.

Still in denial aa777888?? Do you think the rotor vortices disappear when you are further from the ground??

I was carrying a load of pax around Sydney on a sparkling winter's day, blue skies forever, vis around 60nm, when a booming American voice comes over the intercom:
"You know what's wrong with this country?"
"Ummm. I guess you are going to tell me anyway..."
"You can't see the air! I live in LA, and I've been in Mexico City, and in both places you can grab a handful of air and throw it at somebody! But this place... I don't even know what the hell is keeping us up here!"

Seems to me that the air gets more complicated every time I read PPRuNe.

This technique when power limited or VIPs onboard or if it's a tricky to judge approach such as a confined area or a raised pad.

Variable speed: you establish your approach angle and decrease your speed gradually in proportion with your distance to the pad, all the way working the collective to stay on the glide slope. Finish with very gentle leveling off. [This allows you to climb on the back of power curve super progressively]

This technique, or similar most other times: (my advice is to do it not pointing at the pad, leave a 90deg turn-on for the latter stages, that way your judgement of the decel is not critical, you just decel, and turn the last 90deg when you have the crafts speed at a manageable pace)

Constant speed: establish your approach angle and keep your speed 60 knots or above as long as practicable (collective hardly requires any work because all things remain equal) finish with a solid flare and leveling off. [The later part requires you to climb on the back of power curve very quickly]

passenger comfort, risk profile, landing situation, weather, type of aircraft. what factors come into play?[/QUOTE]

When you have hours under the belt, you can blend the techniques seamlessly, varying approach style according to the situation.

I always liked the v low level, v max, downwind approach, followed by a pull up to a wingover around 250ft, spiral descent down to the low hover or 0/0 on the spot. Always surprised the marshaller that one! :E

Absolutely, positively not. But that figure does not show rotor vortices. Not only is entirely clear from the drawings in the figure, the damn figure is labeled "Effect of Ground Vortex on Inflow Patterns" (emphasis mine).

I'm sure you can find a figure in Prouty that shows rotor vortices, but that certainly isn't the one!

For someone who demands strict accuracy and precision from others, crab, or at least appears to, I would expect better.

https://cimg7.ibsrv.net/gimg/pprune....29e3ee9ae1.jpg
Here's an interesting picture from a paper on Ground Vortex. ;-)

So you think the ground has a vortex????

The ground vortex in the diagrams is caused by the rotor and the point I am still trying to make is that you have to get through those vortices to get to ETL - it is just more noticeable nearer the ground because the rotor wake doesn't contract as it would in a free air hover. And then as you reduce speed, those same vortices impinge on the rotor. In both cases they cause vibration. Remember what I said about a boundary for ETL.

As I said before - just go out and try my slow and steady transition to forward flight and tell me if it rolls BEFORE the vibration or not. Perhaps you will then acknowledge that transverse flow just produces roll, not vibration.

And before you say we are supposed to be talking about an approach - the same rotor vortices are what the blades are bumping into to cause the vibration when you make an approach just above ETL.

Yes. Ref: https://etd.ohiolink.edu/!etd.send_f...ion=attachment

This describes the effect in the Prouty figure in much more detail.

That is where I got the picture above from. At least as interesting is this:

The flow around a helicopter is very complex; it becomes much more complex when it comes close to the ground. The presence of the ground changes the aerodynamic characteristics of the rotor and the flow environment becomes much more complex compared with that of flight out-of-ground effect (OGE) and hence the behavior of the rotor wake in the vicinity of the ground is challenging to predict. Under in-ground-effect(IGE) conditions, the wake collides with the ground and causes a significant perturbation to the flow near the blade. Significant interactions between the main rotor wake and the ground have been associated with the formation and passage of the ground vortex in forward flight. The presence of a ground vortex affects the handling qualities of the helicopter.

I'm not sure what you don't get - the ground doesn't produce the vortex, it comes from the rotor wash reacting with the ground. The rotor vortices exist without the presence of the ground as well.

The 'rotor wake' is a clue to the origin of the vortices.

F-E-D, #46..shouldn`t it be `44`yrs....?

aa777888 - another thing for you to try next time you go flying.

Make a normal constant angle, speed reducing approach until you get to the burble/vibration and then hold the speed steady.

Once happy in that condition, note the power required to maintain your descent angle.

Now reduce speed slowly until the vibration goes away and stabilise again at that new speed.

Note the power required to maintain your descent angle.

I expect two things to be obvious, 1. you need more power to maintain the angle as you have lost ETL and 2. you are not having to make roll corrections with cyclic during the journey through the burble/vibration because it isn't transverse flow effect.

Transverse flow (inflow roll) is noticeable on a transition from the hover because of the difference in inflow angles between front and rear of the disc - I hope we can all agree with that - but is barely noticeable on the approach, yet the vibration occurs clearly in both transitions - discuss:ok:

Now that's a sensible post. Obviously, like any helicopter pilot landing a helicopter, I've done this innumerable times, but I can't say I've ever taken note of the actual MP, other than to keep it out of the red and that it obviously increases the slower you go until ground effect. But I can take special note of the MP at a speed above the vibration and then when "riding" the vibration down.

Definitely agreed, no discussion required on that part of it.

great:ok:so what is your explanation of the vibration on the approach?

I don't have one. The books I have say transverse flow. Right or wrong, that's what they say. And when it's time to take my next written knowledge exam, I will parrot the FAA answer. Meanwhile you say something else. I don't actually care about the cause, but I do care about whether or not I have ETL when I feel the vibration and that will be the experiment I perform when I next get the chance.

Excellent - and don't forget the transition into forward flight - nice and slow or it all happens at once - it will want to roll to the right (transverse flow) before you get to the vibration (boundary) and then ETL and flapback will pitch the nose up and start a climb.:ok:

Crab/aa77788,

Ref the vibration you are feeling on approach (forget T/O, that you are about in this last couple of posts, are you talking about the Nper rev vibe at about the 45->25KIAS regime on decel/finals? Especially noticeable in the Aw139???

Exactly! When push comes to shove, if you wanna pass the test, you have to go with what the book says!

Personally, I've often wondered if both cause a vibration,...but then these two phenomenon happen so close together do I really need to take out that fine toothed comb and say, "Ok, this vibration is ETLLLLLL and now its Transverse Flow!"?

Anyway, clearly the guys who write these books have their own arguments, and since we're pilots, not engineers, I guess we're just at their mercy?

No, I'm talking about an approach in just about any helicopter but particularly in this case light singles and around the 20 Kt IAS mark.

What crab said.

Wait a second...holy crap, I agreed with crab! :}

A more simple way of expressing this subject is:

"If it changes from what you want, fix it!"

From 60kt and 300', when your aiming point is about 3 finger-widths below the horizon, set the hover attitude and keep it,
Reduce power to make the aim point stay steady in the window - AND KEEP IT THERE,
And the apparent walking pace over your toes will be apparently steady, as the altitude and speed both decrease - You kept it there,
And if the nose tries to kick up as speed changes slowly downwards and power moves upwards, you fix it,
And if the aim point moves (not through attitude, because you haven't let that change) then fix it with power.
And you will get to the hover, with little if any attitude change, but a few inputs from power, pedals, and fiddling with cyclic.

All because "you fixed it."

The quality and finesse of the approach will exactly depend on that: the method above and the rate at witch "you fixed it" until it becomes imperceptible corrections to the observer

there is naturally a lot of practice correcting pedal setting with power setting it is anticipated in motor control for a great part, so the first sign of good pilot is that the tail remain dead steady throughout.
it gets harder anticipating longitudinal attitude change with power setting during the approach, because the correlation lags behind and is oscillatory in nature.

in level flight an exercise consists in holding the longitudinal cyclic firmly at a fixed spot and reduce the collective (while holding the longitudinal cyclic firm)
the attitude will nose down progressively --> the speed will pick up --> flapback will increase --> speed will slow down

that long period oscillation is one I sometime get stuck with at a moderate level at least for one small cycle, and requires full attention to get ahead of.

Stick fixed instability. It will start as soon as the aircraft moves forward from the hover, and increase until you crash 3 oscillations later.

If it moves, fix it.

Hi, AC I don't get the "walking pace thing of my toes".
I understand that you want the exact location where the line of sight to the landing spot pierces through the bubble stay fixed.
This requires a) a constant angle approach AND b) unchanged attitude, as failing to achive any of the two will cause that point on the bubble to move.
But while satisfying both a) and b), the air speed can be either constant or progressively decreasing (I fvor the latter).

So where's that "walking speed over my toes" thing come in here?

I once had a FI demonstrate an approach where he setup ship at app. 60 kn @300ft and apparently didn't move any of the controls while we went down the line of sight like a cable car, constantly loosing speed.
He simply waited until we fell through ETL, pulled a tad pitch and we hovered at 3 ft. I was very impressed
Maybe he did the walking toes thing, I don't know. ;)

Let me try to answer for AC and he can confirm if I get it right. as you get closer to the ground the apparent speed (the one your eyes feel) will increase
in other words, far from the target, at 300 ft it does not look like 60 knots but close to the ground it feels mighty fast.

therefore if you set yourself to look rather down and keep the perceived speed constant your actual speed will precisely decay with respect to your altitude above ground.
it works well when the ground has regular features like grass, not so good good luck to make it work on snow or water.

in that case I look at the airspeed, say its 60 knots choose a feature at mid point say a strange tree, then aim for 30 knots at the tree, then repeat the process of halving the distance until the pad.

Yes Agile - it is apparent groundspeed. When you fly in an airliner at 30,000 ft the ground doesn't seem to be moving past you very fast, even though you are doing in excess of 500Kts. Come down to 3 ft and the world is hurtling past at a scary rate.

You can either look down past your toes, if your aircraft has a nice bubble or chin window, or scan out to the side to keep the apparent rate of movement across the ground the same as you get lower by gradually reducing the IAS.

aa777888 - :ok:

OMG!!!
I agree with Crab too!
What is the world coming to....

My way of thinking...:E

thanks guys, I think I've understood.
Next time i'll pay attention to the perceived speed of ground vanishing behind my toes during approach.
(the S-300C has excellent bubble canopy for that exercise)

I struggle with much of the 'black magic' that is helicopter aerodynamics, and so attempt to look for the simple answers (which is not to ignore that a lot is occurring behind the scenes). Since the vibration associated with ETL can occur when on approach to an elevated helipad (i.e. when there is not yet any hard surface for the rotor wash to interact with) the vibration cannot simply be associated with the so-called 'ground vortex'. I have observed rotor wash in a still-air hover (e.g. over a body of water) and have seen visualisations that show the classic donut vortices evenly distributed around the rotor tip path. I have also observed the effect of rotor vortices in flight (both actually and virtually) where they resemble conventional fixed-wing tip vortices. At some point as a helicopter accelerates, the vortices that circulated around the entire tip path are forced to separate and migrate toward the extremities (the 'wing tips'). Noting that this separation and movement of vortices is quite violent, and that it is also likely to impact on the airflow over the tail rotor, it seems reasonable to me to assume that vibration will be marked. I'm happy to be corrected, but this is the simple explanation I give to my students.

The "rigid rotor" MBB Bo105 has a pronounced vibration as you transition "back" through ETL. The speed range is quite wide, and the vibration goes away once you're fully in a hover. Sometimes, if you can't get directly into the wind for some reason, you sit there, anxiously wondering if the vibration is the "normal" 105 roughness or the onset of VRS. Fun times!

Many 105 pilots will ride the approach down, smack-dab in the middle of the vibration range, making for a teeth-rattling, rivet-loosening, gyro-killing approach. Savvy 105 pilots will decelerate to just above the onset of the vibration and hold it there. As you come to your termination point, a tiny bit of aft cyclic is all you need to drop the rotor completely back through ETL with (hopefully) no appreciable power change. (I say "hopefully" because those dang ol' C20B's ran so hot in the summer time and you don't want to be yanking in a bunch of collective at the bottom.) Using this technique, the duration of the teeth-rattling/rivet-loosening is truncated. The key is very precise attitude control on the way down. The only other multi-blade rotor system I've flown is on the Sikorsky H19, which has less vibration coming back through ETL than the 105, but the technique works equally well. To a pilot trained in single-engine piston helicopters, who was taught to stay well out of the shaded-area on approach, this technique seems painfully, dangerously slow. But it is nice and calm and predictable, and requires no big pitch attitude or power-change at the bottom. Added bonus: no yaw change either.