Steve76,
I switched providers, but will erect that set of pages in the next few days.
NIck

Thank you all. I'll also have a look at Nick's site when it comes up in a couple of days.

For those interested while waiting for Nicks page to be restored, this link will take you to info on the vortex associated with the osprey accident a couple of years ago.
It has been posted here before.

That Vexing Vortex Ring



PS: Steve, got the calendars two days ago. Thanks mate!:ok:

As for a high rate of descent required for Vortex Ring State, I would point to an accident involving a Canadian Sea King in1992 at an airshow in Schenectady NY in which the Helo started in the hover at about 100 ft on a hot day. The video is around. Someone on these threads will have a link.

High rate of Descent
 

Whirlybird,
I'm not sure that a 'high rate of descent' is required to enter Vortex Ring. The figure of 400fpm is used widely, and that seems to be a relatively slow rate of descent, ie 2.5 mins per 1000'.

Hilico
Although you'll experience vortex ring in training, and read about it in the books, you're also very welcome to post questions here.
We've had some very good threads which started with a simple student or PPL question.
Someone gives the 'book' answer, somebody else says the 'book' is wrong and .............. etc

pohm1
I agree. It may be more likely to occur at a high rate of descent, but I stand by my 'medium' rate of descent posted above. Although the books often say 500' per minute, I believe it's been known to happen as low as 300 fpm. It will be interesting to see what Nick's page says.

less than 30 Kts air speed.
Greater than 300 FPM decent.
power applied.

Take one of the above away = No Vortex ring (Or settling with power if your in the USof A)

Another way NOT to get into it by the sounds of your post, is to engage another instructor !

Safe flying.....

Question: Is it possible to for the tail rotor to enter vortex ?
Humm:confused:

Here is the link to a discussion with data on VRS

http://nicklappos.0catch.com

For Happy landing! the Tail Rotor certainly experiences VRS, usually at about 75% of its downwash speed. Try to go 25 knots rightward sideways in a Boelkow 105 and you will see the pedals dancing while you try to hold heading. In the S-76 it is about 25 knots as well.

This is brilliant. Can I now throw something else in.

Do you mean 'power applied' or 'pitch applied'? What would happen if you're in auto (say an EOL), and having levelled the skids, keep 10 knots across the disk and lift to cushion the landing?

I don't suppose VRS would have much time to develop, and it isn't the kind of thing I'd want to try at altitude (I've done the exercise in a -300 with bringing the rpm to the bottom of the green, and the gauge became more and more interesting the further down it went).

Thanks again Nick.
;)

Hilico :

"Power" applied. In autorotation, the airflow is mostly UP through the disk. In powered flight, the airflow is mostly downwards - yes, I know it's a simplification, but hey, it's early.

No power = no chance of physically forcing air into an already disturbed airflow.

No pitch at bottom of auto = strong chance of a sore back !

Excellent stuff on Nick's pages, never seen that detail before and it was a timely reminder why I never became a test pilot :O

Hilico,

The best "pilot's perspective" I was given about how to visualise vortex ring was this:

Imagine a doughnut of air (annular / ring vortex) encompassing the outer part of the rotor disk, with the rotor mast in the hole in the middle of the doughnut. The air travels downwards through the disk, then out and upwards over the outer edge of the disk then back in over the tips and recirculating down again.

From this it might be easier to see why forward airspeed prevents the vortex ring building around the disk.


I had it demonstrated very briefly to me once in military training and some years later, very spectacularly at night by the relatively inexperienced pilot in the RHS. I was a QHI on type but acting as co-pilot on the flight. At the end of a long night he was attempting to achieve a hover at around 8,000 ft (military observation job). At very low IAS he allowed the ROD to build up, which I pointed out, but he took no recovery action and he eventually lost the plot. The aircraft suddenly began random pitching and rolling and the VSI went hard to the bottom stop so I yelled "airspeed" a couple of times more before I intervened and pushed the cyclic well forward for him. We flew out a few seconds later, having lost well over 3,500ft.

VRin autos
 

To anyone interested:
Now it was explained to me once, that you may experience VRS at the bottom of an Autorotation, while flaring, particularily in a slight downwind situation. Now I've been told that this is not possible, yet I have read about it in the aerodynamics column in R&W. perhaps Nick would share his views on this?

Thanx
Winnie:O

The concept of VRS is intriguing, all the more so because of the miseries of the V-22 these days. The web site I put up attempts to explain the basics, I will try to add to it as we see new questions (such as this!)

Autorotation is called the windmill brake state, since the flow (going upward through the disk) is driving the rotor, and the rotor serves as a windmill. There is no such thing as VRS while in this state. When the pitch is pulled at the bottom of the auto, the rotor goes from Windmill Brake State to normal powered state, but the descent rate of the helo in this brief transient region is nowhere near that needed for VRS, nor do I know of VRS in auto touchdowns as a problem when we do the procedures during development test work.

Hello Nick,

I have held an anenometer close to a hovering Puma and have recorded speeds of circa 60kts of wind and it certainly felt like it.

The calculated downwash speed using your formula comes out at only 47.76ft/sec (circa 28kts). The S92 is very similar. These figures seem very low.

I know the downwash will be deflected by ground effect etc which will distort an anenometer reading but this seems a huge difference. Have I made a calculation error or is there another factor i am not taking into account ?

By the way, your website must have taken lots of hits as your host has closed down access to it due to it exceeding its download limit. Lots of interest in vortex ring which is very healthy !!

Thanks Nick....

interesting concept. Would it therefore lead to a stall ? and if it did, what could be the net effect given the worst case senario?

Happy !

Coalface,
The formula is one that assumes the rotor produces a uniform stream of moving air, and the velocity is a constant, based on the weight of the machine (thrust produced).
In reality, there is a distribution of little streams within the wake that have various velocities, and these can be considerable greater. I would not be surprised to find strands of 60 knots but would be surprised if they were very large. On the other hand, when you are on the ground and the stream has bent 90 degrees along the ground, especially if a wind has helped concentrate the flow, then there might be plenty of concentration.

In measurements of the S-92, we found knots mostly in the 40's, which is close to the theoretical.

Remember that if the downwash is faster, this would make VRS even less likely, which is what sparked this whole thread.

Happy landing! The VRS is totally unrelated to rotor stall, which occurs at maximum lift for the blade, usually near maximum speed. I can popst some of that stuff if you'd like.

Nick

Thanks Nick that would be great if you could,

Realy useful stuff. I hope the thread initiator is still reading it and taken on your comments.

You can't teach experience !

Happy !

If you don't get in on the first try, keep checking back on my website each day. As there is no free lunch, that free site shuts down at 20 megs of data each day, probably about 30 hits.
does anyone out there have any web space they are not using? If your ISP provides web space, I could post on it, if you'd let me. My cable service does not provide web space.

Nick

inflow question
 

Hi Nick,I've got a Question regarding inflow during autorotation.

Thanks

Chiplight