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Autorotation ? R22 for the Experts



 
 
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  #1  
Old February 27th 04, 06:06 PM
Eric D
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Default Autorotation ? R22 for the Experts

Here is a good question for the more experienced helicopter pilot or
mechanic. A friend of mine that flies a late model Robison R22 Beta
II asked this question of a number of individuals and no one seem to
have an explanation. This included people at the manufacturer. I
believe I know the answer, however, I would like to put the question
out and see what others think before tainting their thoughts with my
ideas.

So here is the question: With one person flying the R22 (170lb),
simulating an autorotation by rolling off throttle, the collective
goes full down and pretty-much stays there to maintain proper rotor
rpm. With two people, same maneuver, some collective has to be pulled
to keep the main rotor from over-speeding. The amount of collective
will obviously vary dependent on a number of factors, but for this
example let's say about 1 inch off the stop. One more time with the
same maneuver, with two on board, except let the governor control
engine speed, and not rolling off throttle. In order to maintain
rotor speed the collective has to be on the stop. The question is
why? Please post your thoughts.

Thanks,

Eric D
  #2  
Old February 28th 04, 02:18 AM
Bob
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In the first instance, you are not "simulating" an autorotation; with no
engine OR no engine power applied to the rotor (throttle rolled off) you are
"performing" an autorotation. With the engine throttle left in a flight
position, the governor is GOVERNING rotor R.P.M. due to it's linkage to the
collective and the engine which is what under any circumstances what it
ACTUALLY GOVERNS, and ONLY THEREFORE can govern your rotor R.P.M.
SO...collective bottomed...engine "running", governor GOVERNS rotor R.P.M.
NO engine running (or no throttle applied) YOU govern rotor R.P.M. The
other variables you alluded to will include gross weight per fuel on board,
etc.
Bob


  #3  
Old February 28th 04, 04:54 PM
herbie
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"Bob" wrote in message ...
In the first instance, you are not "simulating" an autorotation; with no
engine OR no engine power applied to the rotor (throttle rolled off) you are
"performing" an autorotation. With the engine throttle left in a flight
position, the governor is GOVERNING rotor R.P.M. due to it's linkage to the
collective and the engine which is what under any circumstances what it
ACTUALLY GOVERNS, and ONLY THEREFORE can govern your rotor R.P.M.
SO...collective bottomed...engine "running", governor GOVERNS rotor R.P.M.
NO engine running (or no throttle applied) YOU govern rotor R.P.M. The
other variables you alluded to will include gross weight per fuel on board,
etc.
Bob


One pob = collective fully down to maintain rotor speed.
Two pob = collective not fully down to prevent overspeed.

If helicopter is heavier will not the coning angle be greater and
therefore the rotor speed higher because of less effective rotor
diameter. Hence the need to pull some collective at heavier weight to
slow blades and increase rotor disk diameter?

Or maybe I'm talking b****x! It has been known!!
Herbie
  #4  
Old February 28th 04, 06:03 PM
Bob
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Herbie...
YES. BUT...that wasn't the question. The question was given
weight-throttle on vs. given weight-throttle off.

Bob


  #5  
Old February 28th 04, 06:27 PM
Bob
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And come to think of it...actually NO. Collective down, there's no cone to
speak of or your rotor speed would be drooping.


  #6  
Old February 29th 04, 03:28 PM
Steve R.
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"Bob" wrote in message
news
Herbie...
YES. BUT...that wasn't the question. The question was given
weight-throttle on vs. given weight-throttle off.

Bob



I'm not sure I'm getting this either. The original poster was confused
because, engine out of the equation, they had to "raise" the collective to
prevent an overspeed. With the engine / governor still in the equation,
they "didn't" have to raise the collective to prevent an overspeed.

Now, if the collective is bottomed out and the MR blades are at an
autorotative angle, wouldn't the rotor system try to outrun the engine /
drive system? Or, is the engine acting as a brake in the situation, kind of
like a car coasting downhill, using engine compressionas a brake?

Just wondering?
Fly Safe,
Steve R.


  #7  
Old February 29th 04, 03:32 PM
Steve R.
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"Bob" wrote in message
...
And come to think of it...actually NO. Collective down, there's no cone

to
speak of or your rotor speed would be drooping.



No cone? I'm not sure I agree with that. Even with the collective all the
way down, there's still positive lift being generated by the rotor system.
Maybe the coneing angle isn't what it would be in steady state, powered
flight but I'm not sure it's accurate to say that there's "no" coning with
the collective full down.

Fly Safe,
Steve R.


  #8  
Old February 29th 04, 04:14 PM
Bob
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Steve...
Kindly read my post carefully. I said NO CONE TO SPEAK OF. As in...it
would not to compare to the cone present when trying to yank the helicopter
off the ground at Max Gross Weight. The lift generated with the collective
down is what keeps your rate of decent an "autorotative" rate, rather at
terminal velocity

As to your other post...when the collective is down with no engine power (on
a real helicopter) the pitch links are adjusted to keep the auto-R.P.M.
within an acceptable RANGE. That is a standard maintenance function, but it
dosn't mean the pilot can go to sleep during an auto. Collective down, if
rpm is too high- it's not adjusted properly, but the pilot can jockey the
collective. If its too low it's DEFINATELY not adjusted properly and rots
o' ruck.

In my first post in this string ALL I was trying (apparently not too
clearly) to explain is that if the engine is running (at all) the governor
knows it. I am a heavy helicopter maintenance type and can't even spell
Robinson, BUT even the most "rudimentary" if you will, helicopters have to
operate with some derivation of dynamic principles and proven hardware
concept application. The bottom line is in an auto, to keep the rotor in
the green right off. Too High is always better than Too Low because the
pilot can fix that.

Good luck,
Bob


  #9  
Old February 29th 04, 05:19 PM
John
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You will have almost as much blade cone in a STEADY auto rotation as
in hover. The cone angle is the tangent of the vertical force vector
verses the centrifugal l force vector generated by the rotation of the
blades. The only force not supported by the blades in auto rotation
is the force generated upward on the body of the helicopter by the
vertical wind component. Due to this you will still have 90% or more
of the cone angle when in hover. You can reduce the cone angle by
speeding up the RPM of the blades since the centrifugal force
generated depends on the square of the RPM. Most people keep the
blade RPM at the top of the green in auto's anyway.
John

On Sun, 29 Feb 2004 15:14:24 GMT, "Bob"
wrote:

Steve...
Kindly read my post carefully. I said NO CONE TO SPEAK OF. As in...it
would not to compare to the cone present when trying to yank the helicopter
off the ground at Max Gross Weight. The lift generated with the collective
down is what keeps your rate of decent an "autorotative" rate, rather at
terminal velocity

As to your other post...when the collective is down with no engine power (on
a real helicopter) the pitch links are adjusted to keep the auto-R.P.M.
within an acceptable RANGE. That is a standard maintenance function, but it
dosn't mean the pilot can go to sleep during an auto. Collective down, if
rpm is too high- it's not adjusted properly, but the pilot can jockey the
collective. If its too low it's DEFINATELY not adjusted properly and rots
o' ruck.

In my first post in this string ALL I was trying (apparently not too
clearly) to explain is that if the engine is running (at all) the governor
knows it. I am a heavy helicopter maintenance type and can't even spell
Robinson, BUT even the most "rudimentary" if you will, helicopters have to
operate with some derivation of dynamic principles and proven hardware
concept application. The bottom line is in an auto, to keep the rotor in
the green right off. Too High is always better than Too Low because the
pilot can fix that.

Good luck,
Bob


  #10  
Old February 29th 04, 06:59 PM
Bob
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Default

The string starts with a guy asking a question of a pilot or a mechanic. I,
being a longtime mechanic, try to answer his questionand stay ON TOPIC. The
guy, and his original question, disappears!

No offense to John, but we end up with a physicist.

Start a new string...pullleeeeez!!!!
Bob


 




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