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  #1  
Old July 1st 03, 06:04 AM
Stephen Austin
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Which basically amounts to the same thing, doesn't it? You're still going to
have to have sufficient altitude to nose over to gain airspeed either way or
can you recover more easily once the VRS is halted and the rotor blades are
flying in relatively clean air again?


Hmm, no, not really. I'm just suggesting another way to skin a cat. It would
be possible to get into SWP in a helicopter and not having the luxury of forwad
flight...confined area for instance. Granted, if one were in that position, it
would be about as hairy as it gets, but it is possible. And in that case I'd
sure want to at least try the auto angle before I deliberately flew into
something. :^)

I'd doubt there are many who have been in that position, and I've certainly
never heard anyone telling that tale. As you noted it's going to take clean
air to get out of the situation. I don't really know what would happen if one
were forced to auto out of SWP...I have the sneaking suspcion that it would be
a hail Mary collective pull at the bottom. It seems to me that if one got into
established SWP in a confined area, did a vertical auto to get out of it, and
then pulled power to stop the descent again, SWP would be reestablished and it
would all be for naught. That's why I'd think it would be necessary to auto,
hold what you got, and then pull like heck at the bottom. But, mind you,
that's just thinking out loud. The important thing about SWP is to recognize
the conditions necessary to get into it and then avoid those conditions like
the plague. And it's always seemed to me the easiest way to avoid it is to
control your rate of descent.

One thing I was wondering, when I pushed cyclic to get out of the VRS, I
found
that my cyclic control authority was almost gone. The model was very
sluggish
in response.


Same in the Bell 47's we did the demos in.

Come to think of it, cutting power and
entering an autorotation as you suggested above, would have negated that
problem. It's just not something that would have occured to me in the heat
of
the moment.


And it's really easy for me to sit at this computer at my cluttered desk and
think about what I'd do if put in that situation. I'd like to think that I'd
react in a proper manner, but that "heat of the moment" you mentioned above can
sure make things trying. One of the old guys that taught me to fly used to say
"anyone can be taught to fly. It is not a difficult thing to accomplish. It's
knowing what to do when things go to worms that makes a true pilot." I tend to
agree with statement wholeheartedly.


Stephen Austin
Austin Ag Aviation
Charleston, Missouri
Ads
  #2  
Old July 1st 03, 01:57 PM
Rhodesst
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And it's really easy for me to sit at this computer at my cluttered desk and
think about what I'd do if put in that situation. I'd like to think that I'd
react in a proper manner, but that "heat of the moment" you mentioned above
can
sure make things trying. One of the old guys that taught me to fly used to
say
"anyone can be taught to fly. It is not a difficult thing to accomplish.
It's
knowing what to do when things go to worms that makes a true pilot." I tend
to
agree with statement wholeheartedly.


Stephen Austin


You'll get no arguments from me on that one. One thing about the "heat of the
moment" from my POV as an RC pilot. Regardless of what happens to the model, I
know I'm going home, alive and well! I'll have a bit of a pain in my wallet
after buying the parts to fix the model but that'll be the extent of my
injuries. I can only imagine what goes through your mind, knowing your life is
on the line. It certainly increases the "incentive" factor for you 1:1 scale
pilots.

Fly Safe,
Steve R.
  #3  
Old July 4th 03, 06:24 AM
Stephen Austin
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Stephen, think about it this way: For a helo to hover, it needs to
generate a thrust which is equivalent to the weight of the helicopter
(the rotor needs to move the same weight of air as the weight of the
helicopter). If gross weight is increased, the thrust must increase,
and since the area of the is not increased, the velocity of the air
must. What has occured is an increase in the disc loading, which can
also be accomplished by changing the size of the rotor. So the
velocity of the down wash is directly linked to weight & disc loading.
You can use a simple formula to calculate the velocity of your own
downwash.



Yes, but that downwash is not able to continue unabated. Immediately on
leaving the disc it is compressed against surrounding air which slows it
considerably.

Your assertion was that for a large helicopter to get into SWP it would take
"considerably higher" speed than a brisk walk down the stairs. Yet, in all my
training, I have never heard any other reference used. Please define
"considerably higher".

You mention a simple formula for calculating downwash velocity yet you didn't
post it. I'd be interested in knowing this too.


Stephen Austin
Austin Ag Aviation
Charleston, Missouri
  #4  
Old July 4th 03, 09:05 PM
Jim Eli
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"Michael McNulty" wrote in message ...
"Stephen Austin" wrote in message
...

snip


Yes, but that downwash is not able to continue unabated. Immediately on
leaving the disc it is compressed against surrounding air which slows it
considerably.


Wrong. In hover the downwash actually doubles in speed as gets leaves the
plane of the disc, due to wake contraction. This is a basic fact of
helicopter aerodynamics.


Your assertion was that for a large helicopter to get into SWP it would

take
"considerably higher" speed than a brisk walk down the stairs. Yet, in

all my
training, I have never heard any other reference used. Please define
"considerably higher".

You mention a simple formula for calculating downwash velocity yet you

didn't
post it. I'd be interested in knowing this too.


Here's a simplified example for an AS-350b2:
Main Rotor Radius: r = 17.5 (ft)
Gross Weight: gwt = 4961 (lbs)
Disc Loading: DL = gwt/pi*r^2 = 4961/3.1415*17.5*17.5 = 5.16 (lbs/sqft)
Air Density: rho = 0.002377 (standard day sea level)
Downwash Velocity: v = (DL*2/rho)^1/2 = (5.16*2/0.002377)^1/2 = 65.87 (fps)
v = v*3600/5280 = 44.9 (mph)
Which is considerably faster than a speed I can attain walking.
  #5  
Old July 5th 03, 02:37 PM
Stephen Austin
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Here's a simplified example for an AS-350b2:
Main Rotor Radius: r = 17.5 (ft)
Gross Weight: gwt = 4961 (lbs)
Disc Loading: DL = gwt/pi*r^2 = 4961/3.1415*17.5*17.5 = 5.16 (lbs/sqft)
Air Density: rho = 0.002377 (standard day sea level)
Downwash Velocity: v = (DL*2/rho)^1/2 = (5.16*2/0.002377)^1/2 = 65.87 (fps)
v = v*3600/5280 = 44.9 (mph)
Which is considerably faster than a speed I can attain walking.



So are you telling me that you believe that it will take a 44.9 mph descent
before an A-Star will get into SWP? I'm sorry, no offense, but I find that
hard to believe. I've got a little A-Star time, but I never entered SWP with
it. I do remember during my checkout though that the IP was adamant that
standard precautions against it (SWP) be maintained. Heck, if what you're
saying is true it would be pretty much impossible to get into SWP in any
aircraft.

The 3rd rule of thumb in the 1-2-3 SWP is a greater than 300 feet per minute
descent. I've always heard 4 miles per hour is a pretty average walking speed.

4 mph = 21,120 ft/hr / 60 = 352 feet per minute. Which is a speed that is not
considerably faster than I can attain walking.


Stephen Austin
Austin Ag Aviation
Charleston, Missouri
  #6  
Old July 7th 03, 01:05 AM
Greg Whyte
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I've read through this exchange with interest and I am, of course,
obliged to say that the Chapter on Vortex Ring State in my book, Fatal
Traps For Helicopter Pilots addresses this quite extensively and gives
some Accident Report excerpts in Case Studies (see
www.fataltraps.com).

But, in response to the very first message posted, I would have to say
that in any helicopter I would expect VRS to be demonstrated at
nothing less than 4000' ASL and a full briefing to be given before the
event - aside from recovery the most important thing to learn from the
demonstration is recognition of the early signs of onset - it should
be stressed that these can vary greatly depending on the make & model
of helicopter. I doubt that entry to VRS was an intentional one on the
part of the Instructor (who was, after all, the Pilot in Command)...
if it was intended, it was poor practise indeed.

I note with interest the mention of the fact that the rotor thrust
must equal the weight of the helicopter. I was fascinated with a
display with a model helicopter I once saw on a Science program on TV.
They weighed the model on a scale and then hovered it over the same
scale (which had a large plate placed on it and reset to zero). Sure
enough, the weight of the downwash onto the plate was exactly that of
the model. With enough decimal points you could actually see the
figure steadily decreasing with fuel burn-off. I'm not saying I ever
doubted the principle, but practical demonstrations (safe ones) are
always interesting and valuable.

As a newcomer to this site, thanks all for your input, makes for
interesting reading.

Greg Whyte
 




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