Autorotation ? R22 for the Experts
Eric D
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
Bob
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
herbie
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
Bob
YES. BUT...that wasn't the question. The question was given
weight-throttle on vs. given weight-throttle off.
Bob
Bob
speak of or your rotor speed would be drooping.
Steve R.
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.
Steve R.
> 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.
Bob
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
John
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" <southb...@hotmail.com>
wrote:
Bob
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
Michael McNulty
The rotor coning depends on the rotor speed and the total rotor thrust
(weight and
load factor). It does not depend on the engine power or the rate of
descent. A max gross weight auto gives the essentially the same coning as a
max gross weight take off, or a max gross weight landing approach, or a max
gross weight hover, etc....
"Bob" <southb...@hotmail.com> wrote in message
news:ZLp0c.27102$Im5.2...@twister.nyc.rr.com...
Bob
OR only the last comment/response? Too bad we can't see where we started or
we would see how afar we wound up! The guy was asking about collective
position vs. engine RPM, but who could possibly imagine just what the
ORIGINAL question was now?
Michael McNulty
If you are so concerned with staying on topic, then why did you make your
original (incorrect) statement about coning? It has nothing to do with
autorotation except as it relates to rotor speed.
"Bob" <southb...@hotmail.com> wrote in message
news:Deu0c.27576$Im5.2...@twister.nyc.rr.com...
Steve R.
Bob,
My response was prompted by a statement you made that seemed to strongly
imply that a rotor in autorotation had either "no" coning or at least,
significantly less coning than a rotor in powered flight. That is something
I disagree with.
If I misunderstood you, then I apologize. OTOH, I've been known to "not"
state my point of view as precisely as I would have liked on the first
attempt and I've seen others do it too.
FWIW,
Fly Safe,
Steve R.
Steve R.
"Steve R." <srho...@houston.rr.nospam.com> wrote in message
news:m%v0c.19754$OH4....@fe2.texas.rr.com...
>
> "Bob" <southb...@hotmail.com> wrote in message
> news:Deu0c.27576$Im5.2...@twister.nyc.rr.com...
> > AND ONCE AGAIN...I DID NOT SAY HE IS INCORRECT. Does anyone read a
> STRING?
> > OR only the last comment/response? Too bad we can't see where we
started
> or
> > we would see how afar we wound up! The guy was asking about collective
> > position vs. engine RPM, but who could possibly imagine just what the
> > ORIGINAL question was now?
> >
> >
>
Ok Bob,
I did a little search and here's the conversation in full! You'll note that
in message #6, you said (and subsequently reminded me of) that there was "no
cone to speak of" which is essentially saying, "there's no cone!" That part
I disagree with.
You also never really answered "my" question, posed in message #5 regarding
why the pilot could go full down on the collective and have an overspeed
problem with the engine idled but not with the engine and governor still in
the equation. I'll repeat the question. Is the engine acting as a brake on
the rotor rpm, not unlike a car running down hill, in gear, with the drivers
foot off the gas?
I'm just curious about this. I'm not trying to pick any fights. You're the
one that brought up the coning angle part the first time in this
conversation and some of us have questioned that. I still don't feel like
there's been an answer to that part of the question, just some defensive
responses.
FWIW,
Fly Safe,
Steve R.
Start Quote:
------------------------------
Message 1 in thread
From: Eric D (erictig...@hotmail.com)
Subject: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-27 09:06:57 PST
Thanks,
Eric DPost a follow-up to this message
Message 2 in thread
From: Bob (southb...@hotmail.com)
Subject: Re: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-27 17:18:01 PST
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.
BobPost a follow-up to this message
Message 3 in thread
From: herbie (herb...@aol.com)
Subject: Re: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-28 07:54:02 PST
"Bob" <southb...@hotmail.com> wrote in message
HerbiePost a follow-up to this message
Message 4 in thread
From: Bob (southb...@hotmail.com)
Subject: Re: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-28 09:10:04 PST
Herbie...
YES. BUT...that wasn't the question. The question was given
weight-throttle on vs. given weight-throttle off.
BobPost a follow-up to this message
Message 5 in thread
From: Steve R. (srho...@houston.rr.nospam.com)
Subject: Re: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-29 06:28:27 PST
"Bob" <southb...@hotmail.com> wrote in message
Just wondering?
Fly Safe,
Steve R.Post a follow-up to this message
Message 6 in thread
From: Bob (southb...@hotmail.com)
Subject: Re: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-28 09:34:42 PST
And come to think of it...actually NO. Collective down, there's no cone to
speak of or your rotor speed would be drooping.Post a follow-up to this
message
Message 7 in thread
From: Steve R. (srho...@houston.rr.nospam.com)
Subject: Re: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-29 06:32:05 PST
"Bob" <southb...@hotmail.com> wrote in message
Fly Safe,
Steve R.Post a follow-up to this message
Message 8 in thread
From: Bob (southb...@hotmail.com)
Subject: Re: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-29 07:21:10 PST
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,
BobPost a follow-up to this message
Message 9 in thread
From: John (John@no_where.com)
Subject: Re: Autorotation ? R22 for the Experts
View this article only
Newsgroups: rec.aviation.rotorcraft
Date: 2004-02-29 08:20:29 PST
>Post a follow-up to this message
Bob
I've been looking out of helicopter windows at blade tip targets tracking,
balancing, and setting autorotation RPM since I was done teething. SO you,
your x,+,-,%, = signs, textbooks, drawing boards, engineering degrees, and
any other credentials you might present can go elsewhere and tell someone
about autorotations, heavy gross flights, hovers and blade tip paths. Don't
sell it to me because seein' is believin', and I've seen plenty.
Goodbye string...it's been a gas!
Eric D
posting.
Bob, Your response was super, and I understood what you wrote. I
believe what you described is right on the money for larger aircraft
that you mentioned you worked on.
The governor on the Robison R22 Beta II does not have linkage to the
collective. The scenario I described in my original post is due to
the sprag clutch. During low overrun speeds there is some drag
through the sprag. As the overrun speed increase this drag decreases.
To clarify, when speaking of overrun speed, it is the velocity across
the sprag clutch, not rotor speed.
I have limited helicopter maintenance experience on anything larger
then a Robinson R44. I received my A&P in the early seventies and
also have IA. I have worked on many types of aircraft over the years.
My passion for Robinsons came about four years ago, so I'm still
learning. I also have a fixed wing pilot's license and I have started
working on learning how to fly an R22. I'm finding it a blast and I
am really enjoying it.
Thanks again for the great responses,
Eric D
Bob
Freewheeling Unit or whatever it's called in the aircraft you are
referencing DISENGAGES the ROTOR from the engine drive as in autorotation it
disallows the engine to slow the rotor speed down AT ALL repeat AT ALL!
Bob
since I replied to your first question. Out of courtesy to the group, I'll
still consider whatever aircraft you were addressing a, uhhhhhh, helicopter
and leave the rest of this mess in your hands. If there was any help on my
part, you're most welcome.
Happy flying or whatever that contraption does :)
Steve R.
Got it! That's what I thought but maybe I missed something. Thanks!
Fly Safe,
Steve R.
Bob
Would you consider autorotation a LOW LIFT condition??? I kinda thought you
would...so please refer to the following and read about the CONING HINGE. I
and I'm sure the group will await your response.
http://www.copters.com/mech/mr_semi.html
Michael McNulty
No. A steady autorotation is not a "low lift condition"; it is a low shaft
power condition. In flight the lift is equal to the aircraft's weight times
it's load factor, g's. Low lift means on the ground, or in pushovers, or at
unusually low weights. Autorotation gives zero or negative steady blade
lag, but the coning does not decrease. In fact, if you measure it closely
enough you will see that the coning actually increases slightly in
autorotations because the blade lift distribution shifts outboard during
autos.
"Bob" <southb...@hotmail.com> wrote in message
news:PUz0c.7457$1e3.2...@twister.nyc.rr.com...
Jerry H.
news:a66b50a8.04022...@posting.google.com:
This gets a little complicated, but I'll give it a try. You'll probably
need to refer to some pictures, so try page 3-10 of the FAA's Rotorcraft
Flying Handbook, or pages 139 and 140 of Principles of Helicopter Flight,
by W.J. Wagtendonk.
First off, one of the previous replies mentioned that the ship is set up to
autorotate correctly by mechanics. In the R22, this is done by entering
autorotation at 50 kts and known ship weight with the collective full down,
and then adjusting the pitch links to set the full down collective blade
angle to obtain the correct rotor RPM from a chart in the maintenance
manual. For example, at max gross weight at sea level density altitude, the
RPM should stabilize at about 108%. At 2000 feet density altitude, it
should be about 110%. As you observed, at a lighter weight, the rotor RPM
will be less. This is the reason for the minimum pilot weight limitation in
the R22.
During autorotation, there are three regions of the rotor disk. In the
first, close to the hub, the amount of air flowing up through the disk
(remember, during autorotation, air flows up, and in powered flight, it
flows down) is large compared to the rotational velocity of the blade. The
result is a high angle of attack, and a stalled airfoil. As far as
generating lift and govering rotor RPM, not much is going on here.
As you move out towards the tip, the rotational veloctity increases and
thus the angle of attack goes down and the airfoil is now flying. The
upward airflow has the effect of changing the angle of the relative wind
across the blade, tipping the vector forward (here's where the pictures
help). Since lift is produced perpindicular to relative wind, this tips
the lift vector forward. Drag is relatively low, as rotational velocity in
the center of the blade is not too big, so the total resultant force vector
is still tipped forward. This forward vector is what drives the rotor
system during autorotation. That's why this part of the blade is called the
driving region.
As you continue to move outward along the blade, the rotational velocity
increases. This has two effects. First, since upflow is constant, and the
relative wind is the vector sum of the upflow and the airflow due to blade
rotation, the relative wind is more flat to the blade (aligned with the
chord). Lift is thus tipped forward less. Second, drag increases. Both of
these effects tip the total force vector aft (towards the trailing edge of
the blade). Once you've moved out far enough, the force will be pointed
backwards, and you will have entered the driven region. The total force
generated in this region tends to slow down the RPM.
Now, the size of each region is governed by several factors. Higher blade
RPM means more rotational velocity and thus a larger driven region. That's
why the rotor RPM tops out in the case of the 170# solo pilot. A higher
collective setting increases drag across the blade, also shifting the
driving and driven regions, and resulting in a lower RPM. That's why you
are able to keep the rotor RPM in the green at a higher weight.
Now for the tricky question. Why is rotor RPM higher for a given
collective setting with more weight on board. More weight means you come
down faster, and thus you have more upflow through the disc. Remember, the
upflow causes the relative wind to be tipped forward, which tips the lift
vector forward all across the blade. This has the effect of increasing the
size of the driving region, which causes the rotor RPM to increase. Drag
will then increase until a new steady RPM is reached.
By the way, when you enter a turn during autorotation, you effectively
increase your weight by pulling more than one "G". As you know, the RPM
tends to increase here also.
As for your last question, if you let the governer control the RPM, you are
probably not autorotating. Ask your friend if the needles are split. The
governer WILL maintain engine RPM at 104% (top of the engine tach green),
so in order to be autorotating, the rotor RPM would have to be higher than
104%. Since you said that the collective had to be full down to maintain
RPM, I'm guessing that it was not. I'll bet the descent rate was less than
in an autorotation, indicating that the engine was helping out.
Hope some of this makes sense. If there are any errors, I'm sure someone
will point them out.
Jerry