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....?