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HeliTorque :: View topic - Vortex Ring Discussion
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HeliTorque Forum Index » Flight Dynamics

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Vortex Ring Discussion
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Heliwhore
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PostPosted: Fri Feb 20, 2009 4:27 pm    Post subject: Vortex Ring Discussion Reply with quote

Have you ever tried to get a 44 past the incipient stage? Have to say I struggle to get a 44 into it full stop.

When you get a 22 into it, you can raise the lever to show that it just descends even faster, every time I've tried this with a 44 it slows down and comes out of VR. Anyone else experienced this?
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veeany
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PostPosted: Fri Feb 20, 2009 5:39 pm    Post subject: Reply with quote

HW

You just know I wont be able to resist, so off we go into Vortex Ring.

What kind of Rate of Descent does it take to get into VRS ? In an R22 or in an R44 ?

Why do you think it comes out again when you try in the R44 ?

What methods are available to get you out of VRS.

Answers on a postcard to , well here will do ?

I can feel another thread split coming up !

GS
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Heliwhore
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PostPosted: Fri Feb 20, 2009 7:00 pm    Post subject: Reply with quote

Whoah, whoah, lets get this thread split before this goes on....

But, this is an interesting one huh?

It's great that it is a standard teaching of less than 30kts, greater than 300 fpm decent and power applied ( there are a couple of slight variations on these figures ). It's great because it should theoretically keep all pilots so far away from VR that they shouldn't get into trouble.

However, I did actually do the maths on the R22 a couple of years ago (I'm sure theres a thread on this on that other forum) and if I remember rightly, the figures were more along the lines of about 700fpm descent to get anywhere close. I'm sure Veeany has the actual figures to hand somewhere.

As far as the R44 question, I've always figured it was more an effeect that the pitot is in such a bad place for that sort of descent airflow, that when we think we're doing 0 airspeed, we could be doing any speed and not know, including negative airspeed. I didn't put the 44 into the formula, so if someone has, what sort of figures are we looking at for that.
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PostPosted: Fri Feb 20, 2009 10:22 pm    Post subject: Reply with quote

Heliwhore wrote:
Whoah, whoah, lets get this thread split before this goes on....


As you wish Wink

Sarah
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PostPosted: Sat Feb 21, 2009 12:10 am    Post subject: Reply with quote

Here's an article from Approach on VRS that's pretty interesting. (Approach is the Safety magazine for US Naval Aviation)
http://public.me.com/ng.pogue and download VRS.pdf
(7MB pdf file)
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PostPosted: Sat Feb 21, 2009 12:40 am    Post subject: Reply with quote

If you can pull the lever up and reduce RoD then you weren't in vortex ring.

I remember reading somewhere that the required RoD was either 1/2 or 1/3 of the speed of downwash through the rotor. Perhaps someone can confirm.

The R44 is a bit anti-FI when it comes to demo-ing VR. Unlike the EC120 which you had to wrestle with it even in incipient stages.

HW, do you use DJ's method written in the Handbook? Works a treat in the R22, in the R44 you just have to be alot more positive.

HH
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PostPosted: Sat Feb 21, 2009 8:25 am    Post subject: Reply with quote

HH

Its about 75-80% of downwash velocity.

Which brings us neatly on to, which is closer to VRS an R22 at 800 fpm ROD or an A109 at 1200 fpm ROD. Both Zero Airspeed at MAUW ?

If they were both lighter would it make any difference ?

GS
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Heliwhore
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PostPosted: Sat Feb 21, 2009 11:05 am    Post subject: Reply with quote

Ah hahh, The R22 is closer than the 109.

Now if we lose one passenger out of each machine, then I figure that the R22 has now lost a margin of almost 100fpm, whereas the 109 has lost only about 25fpm ( ish ).

Its hard to believe that you are safer being heavy than you are being light. Counter-intuitive me thinks.

Quote:
HW, do you use DJ's method written in the Handbook? Works a treat in the R22, in the R44 you just have to be alot more positive


I sure do, even still, when people think they're in VR in the 44, I think they're fooling themselves. And you know who I'm talkin about......
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PostPosted: Sat Feb 21, 2009 1:13 pm    Post subject: Reply with quote

Yup!
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PostPosted: Sat Feb 21, 2009 7:19 pm    Post subject: Reply with quote

Next point: how does having, say, 4 blades rather than 2, with the same rotor diameter affect VR risk/performance?
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PostPosted: Sat Feb 21, 2009 7:33 pm    Post subject: Reply with quote

Shouldn't affect it at all.

All the proper theory books (not the Janet and John ones) point to VRS being related to Disc Loading not blade loading. The downwash velocity dependant upon Thrust and Disc Area,

To produce the same thrust more blades could reduce disk area which would put Disc Loading Up, but at same radius should make no difference at all.

GS
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PostPosted: Sat Feb 21, 2009 8:02 pm    Post subject: Reply with quote

Who is DJ and how do you calculate downwash velocity for a certain type of helicopter anyway? Obviously bigger helicopters generate more downwash but is there a formula and does it not vary depending on how much pitch is applied to the blades?

Not really something I have ever considered and always up for learning something new!

Cheers,

Sarah
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veeany
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PostPosted: Sat Feb 21, 2009 9:14 pm    Post subject: Reply with quote

Sarah

From momentum theory (complicated way of looking at how much mass of air the rotor shifts)

Thrust = 2 * rho * A * (vi squared)

If you rearrange it you end up with

Vi = Square Root Of ( Thrust / (2 * Rho * A))

Where

Vi= Induced Flow velocity
Thrust in the hover (or steady state climb or descent vertically) = Mass * G
Rho is Air Density
A is Disc Area

If you play with my blade diagrams stuff you can change the mass and look in the table below for its effect in Vi in the hover (note that at the moment it doesn't modify it based upon blade pitch).

I'll make a friendlier version just to show this stuff later for now its still on
http://www.aviationwebdevelopment.co.uk/samples/bladeforce.aspx

And yes collective pitch will change the magnitude of the thrust.

Its not the bigger helicopters its those with high disc loading that create more downwash.
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