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Old Aug 06, 2003, 03:06 AM
Rhodesst
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

>
>Without the paddles, the bar would not have any incentive to "maintain it's
>position 90 degrees
>from the rotor mast".


Hi Steve S,

I disagree. The flybar will try to maintain a position, 90 degrees to the
rotor mast. The reason for this is simple centrifugal force. There will be a
time lag between the time that the system is disturbed and the time the flybar
reorients itself, but it "will" attempt to reorient itself.

Fly Safe,
Steve R.
Old Aug 06, 2003, 03:06 AM
Steve Simpson
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

> I disagree. The flybar will try to maintain a position, 90 degrees to the
> rotor mast. The reason for this is simple centrifugal force. There will be a
> time lag between the time that the system is disturbed and the time the flybar
> reorients itself, but it "will" attempt to reorient itself.
>


While that does seem logocal, it is incorrect.

Gyroscopic force would hold the spinning bar in a single plane unless acted upon by an outside
force. Absent paddles, there is no outside force . . until the mast bumps the bar.

Consider that centrifugal force exerts a force 90 degrees to it's OWN rotation. In this case the
relevant revolution is about the middle of the flybar, not the center of the mast. The mast tilt
has negligible effect because it is hinged at the bar and therefore cannot exert influence.


Old Aug 06, 2003, 03:06 AM
Lars Blaabjerg
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

Hi,

Thanks for your answers, I'm trying to learn something new here. I'm still
unclear about a few things, see the comments below

"Steve Simpson" <simpson34@coxpanties.net> skrev i en meddelelse
news:MlQXa.36193$Bp2.12512@fed1read07...
> > I don't understand how inconsistent head speed can affect pitching up
> > tendencies, can anybody elaborate on this?

>
> Head speed has a significant effect on handling characteristics. If a heli

has a tendency to
> pitch up, then varying head speed may accentuate or reduce that tendency

making the handling
> inconsistent if the head speed is changing during flight.


Shouldn't it then have an equal tendency to pitch down? I'm not sure I
understand why the head speed is the determining factor when a heli is
pitching up.

>
> > Anyway, you say that the inertia of the flybar is enough to help keep

the
> > head speed constant? One should think that the weight of the rotorblades
> > themselves carried a LOT more inertia than the flybar . . .

>
> They do . . . I think you may have misinterpreted the comment.


Then why are flybar-less heads more prone to irregular rotor speeds than
heads with a flybar?

-------snip------

Thanks

Lars


Old Aug 07, 2003, 03:02 AM
Rhodesst
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

>
>Consider that centrifugal force exerts a force 90 degrees to it's OWN
>rotation. In this case the
>relevant revolution is about the middle of the flybar, not the center of the
>mast.


Well, exactly "where" is the middle of the flybar, if it's not in the middle of
the rotor mast?? The rotor mast "is", in this case, the center of rotation for
the flybar and it will try to orient 90 degrees to that point.

The mast tilt
>has negligible effect because it is hinged at the bar and therefore cannot
>exert influence.


I would agree with this IF the flybar hinge point was movable in "all"
directions, like the center of the swashplate. Only thing is, it's "not!" The
flybar can teeter in one plane only, relative to the rotor mast. Now, if the
rotor mast is changing it's axis at the exact instant that it's in line with
the teeter axis of the flybar, then for that brief instant, the flybar will
maintain it's plane of rotation and everything will be happy. Only problem is,
90 degrees later, the flybar is being forced off of it's plane of rotation
because it can't teeter in all directions, relative to the rotor mast. It
won't like that and "will" try to reorient back to a position 90 degrees to the
rotor mast.

FWIW, I'm getting this from a video I own, bought it years ago along with
several others when I first got into RC helicopters. I was trying to learn as
much as I could about how they fly. Anyway, it's part of the "Aviation A.V.
Library" and I got it from a company called:

Aircraft Components, Inc.
Benton Harbor, Michigan 49022
616-925-8863.

The publisher is:

Ferde Grofe Films
3100 Airport Ave.
Santa Monica, CA. 90406
213-397-7524

The specific tape is called "Attack Copter" and includes six parts titled:

1. Evolutions of Attack Helicopters - Concepts of Deployment. It has footage
of the Huey, Cobra, and Cheyenne.

2. Tactical Formation Flying

and the parts I was most interested in at the time!

3. Helicopter Aerodynamics - Rotor blade actions

4. Helicopter Aerodynamics - The Stabilizer Bar (which is the basis for most
of what I'm trying to talk about here)

5. Helicopter Aerodynamics - Dissymmetry of Lift

6. Helicopter Aerodynamics - Rotor Blade Angles.

These are basically old army training tapes and they are pretty dry watching a
lot of the time. They do have good info on some basics which didn't hurt me to
learn about. I have no idea if these tapes are still available. The copies I
have are over 20 years old now.

Fly Safe,
Steve R.
Old Aug 07, 2003, 03:02 AM
Steve Simpson
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters


> >Consider that centrifugal force exerts a force 90 degrees to it's OWN
> >rotation. In this case the
> >relevant revolution is about the middle of the flybar, not the center of the
> >mast.

>
> Well, exactly "where" is the middle of the flybar, if it's not in the middle of
> the rotor mast?? The rotor mast "is", in this case, the center of rotation for
> the flybar and it will try to orient 90 degrees to that point.
>


The centers may be coincident, but the cetrifugal force acting on the flybar is not effected by
the mast.

> The mast tilt
> >has negligible effect because it is hinged at the bar and therefore cannot
> >exert influence.

>
> I would agree with this IF the flybar hinge point was movable in "all"
> directions, like the center of the swashplate. Only thing is, it's "not!" The
> flybar can teeter in one plane only, relative to the rotor mast.


Quite a brain twister, huh?

You had me convinced there for a second . . . . . :-)

The flybar IS hinged in all directions . . think about it. It is a gimbaled mount that can
teeter and rotate (paddle pitch).

However, even if the model's bar was not rotatable, absent paddles, the bar's rotation about
it's own centerline would suffice as the 'hinge' perpendicular to the teeter 'hinge'.

In many cases, it is correct to think of a spinning bar or rotor as a solid 'disk', and in this
case, a disk hinged in only one axis would indeed behave as you describe. However, a bar needs
no second hinge becuase there is nothing (like a disk or paddles) connected to it so it can
simply rotate about it's own centerline.

> FWIW, I'm getting this from a video I own, bought it years ago along with
> several others when I first got into RC helicopters.


Sounds like my kind of video tape!

> 4. Helicopter Aerodynamics - The Stabilizer Bar (which is the basis for most
> of what I'm trying to talk about here)


I think you'll find that the stabilizer has mechanical dampers which force it to follow the mast
inclination.

Never the less, I am very curious about what the tape says about our interesting debate.

I have a finely honed skill called "failing to recognize the obvious" which may be at play here
.. . . . . . :-)






Old Aug 07, 2003, 03:02 AM
Steve Simpson
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters


> > Head speed has a significant effect on handling characteristics. If a heli

> has a tendency to
> > pitch up, then varying head speed may accentuate or reduce that tendency

> making the handling
> > inconsistent if the head speed is changing during flight.

>
> Shouldn't it then have an equal tendency to pitch down? I'm not sure I
> understand why the head speed is the determining factor when a heli is
> pitching up.


Methinks you are finding information that no one is actually providing. I don't see where anyone
said that head speed was a 'determining factor' in a heli pitching up.

> > They do . . . I think you may have misinterpreted the comment.

>
> Then why are flybar-less heads more prone to irregular rotor speeds than
> heads with a flybar?


Once again, I don't see where anyone has made such a claim.

Debate works better if you read carefully and don't fill in the blanks with your own
assumptions.


Old Aug 07, 2003, 03:02 AM
Beav
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters


"Lars Blaabjerg" <lab@*nospam*ready.dk> wrote in message
news:mW1Ya.46357$Kb2.1983006@news010.worldonline.d k...
> Hi,
>
> Thanks for your answers, I'm trying to learn something new here. I'm still
> unclear about a few things, see the comments below
>
> "Steve Simpson" <simpson34@coxpanties.net> skrev i en meddelelse
> news:MlQXa.36193$Bp2.12512@fed1read07...
> > > I don't understand how inconsistent head speed can affect pitching up
> > > tendencies, can anybody elaborate on this?

> >
> > Head speed has a significant effect on handling characteristics. If a

heli
> has a tendency to
> > pitch up, then varying head speed may accentuate or reduce that tendency

> making the handling
> > inconsistent if the head speed is changing during flight.

>
> Shouldn't it then have an equal tendency to pitch down? I'm not sure I
> understand why the head speed is the determining factor when a heli is
> pitching up.


It's not the speed changes that cause the pitching tendencies, it's just
that the changes make the pitching more obvious with a flybar-less rotor
head. The REASON for pitch instability is usually more to do with the
relationship between the positions of the centre of lift and the centre of
gravity of the blades. A blade with a rearward C/G will pitch up a lot more
readily than one with a forward C/G, which is one reason why we see large
lumps of lead in the leading edge of rotor blades these days. (Model AND
full size)

>
> >
> > > Anyway, you say that the inertia of the flybar is enough to help keep

> the
> > > head speed constant? One should think that the weight of the

rotorblades
> > > themselves carried a LOT more inertia than the flybar . . .

> >
> > They do . . . I think you may have misinterpreted the comment.

>
> Then why are flybar-less heads more prone to irregular rotor speeds than
> heads with a flybar?


They're not, in fact it's the opposite IF the blades are the correct weight.
Inertia must play it's part and as we all know, inertia has the effect of
trying to keep moving things moving and stationary things stationary, so the
heavier the blades, the less prone to speed changes they are. They slow down
more slowly and they speed up more slowly which is good for flybar-less
flying, but a governor and a powerful engine help too.

The problems all come when there IS a speed change of the rotors and a
simple examination of the rotor system as a whole will demonstrate why they
pitch up as speed increases. The advancing blade picks up lift and the
retreating blade loses lift, even given the fact that pitch differences
exist on both blades, so if the blades speed up, the advancing blade will
provide even MORE lift, but the result of the force on the advancing blade
isn't seen until 90 degrees later (which places the effect perfectly for
raising the nose, so up goes the nose of the machine and you've encountered
pitching up from nothing more than an increase in rotor speed.



--
Beav


Please note my E-mail address is "beavis dot original at ntlworld dot com"
(with the obvious changes)

Beavisland now lives at
www.beavisoriginal.co.uk


Old Aug 08, 2003, 03:01 AM
Lars Blaabjerg
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

Sorry Beav, I think I probably misinterpreted your first post. All sorted
now :-)


"Beav" <beavis.original@ntloxoworld.com> skrev i en meddelelse
news:aPcYa.1808459$mA4.253348@news.easynews.com...
>
> "Lars Blaabjerg" <lab@*nospam*ready.dk> wrote in message
> news:mW1Ya.46357$Kb2.1983006@news010.worldonline.d k...
> > Hi,
> >
> > Thanks for your answers, I'm trying to learn something new here. I'm

still
> > unclear about a few things, see the comments below
> >
> > "Steve Simpson" <simpson34@coxpanties.net> skrev i en meddelelse
> > news:MlQXa.36193$Bp2.12512@fed1read07...
> > > > I don't understand how inconsistent head speed can affect pitching

up
> > > > tendencies, can anybody elaborate on this?
> > >
> > > Head speed has a significant effect on handling characteristics. If a

> heli
> > has a tendency to
> > > pitch up, then varying head speed may accentuate or reduce that

tendency
> > making the handling
> > > inconsistent if the head speed is changing during flight.

> >
> > Shouldn't it then have an equal tendency to pitch down? I'm not sure I
> > understand why the head speed is the determining factor when a heli is
> > pitching up.

>
> It's not the speed changes that cause the pitching tendencies, it's just
> that the changes make the pitching more obvious with a flybar-less rotor
> head. The REASON for pitch instability is usually more to do with the
> relationship between the positions of the centre of lift and the centre of
> gravity of the blades. A blade with a rearward C/G will pitch up a lot

more
> readily than one with a forward C/G, which is one reason why we see large
> lumps of lead in the leading edge of rotor blades these days. (Model AND
> full size)
>
> >
> > >
> > > > Anyway, you say that the inertia of the flybar is enough to help

keep
> > the
> > > > head speed constant? One should think that the weight of the

> rotorblades
> > > > themselves carried a LOT more inertia than the flybar . . .
> > >
> > > They do . . . I think you may have misinterpreted the comment.

> >
> > Then why are flybar-less heads more prone to irregular rotor speeds than
> > heads with a flybar?

>
> They're not, in fact it's the opposite IF the blades are the correct

weight.
> Inertia must play it's part and as we all know, inertia has the effect of
> trying to keep moving things moving and stationary things stationary, so

the
> heavier the blades, the less prone to speed changes they are. They slow

down
> more slowly and they speed up more slowly which is good for flybar-less
> flying, but a governor and a powerful engine help too.
>
> The problems all come when there IS a speed change of the rotors and a
> simple examination of the rotor system as a whole will demonstrate why

they
> pitch up as speed increases. The advancing blade picks up lift and the
> retreating blade loses lift, even given the fact that pitch differences
> exist on both blades, so if the blades speed up, the advancing blade will
> provide even MORE lift, but the result of the force on the advancing blade
> isn't seen until 90 degrees later (which places the effect perfectly for
> raising the nose, so up goes the nose of the machine and you've

encountered
> pitching up from nothing more than an increase in rotor speed.



Old Aug 08, 2003, 03:01 AM
Nick M V Salmon
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

"Steve Simpson" <simpson34@coxpanties.net> wrote
> > I disagree. The flybar will try to maintain a position, 90 degrees to

the
> > rotor mast. The reason for this is simple centrifugal force. There

will be a
> > time lag between the time that the system is disturbed and the time the

flybar
> > reorients itself, but it "will" attempt to reorient itself.
> >

>
> While that does seem logocal, it is incorrect.
>
> Gyroscopic force would hold the spinning bar in a single plane unless

acted upon by an outside
> force. Absent paddles, there is no outside force . . until the mast bumps

the bar.
>
> Consider that centrifugal force exerts a force 90 degrees to it's OWN

rotation. In this case the
> relevant revolution is about the middle of the flybar, not the center of

the mast. The mast tilt
> has negligible effect because it is hinged at the bar and therefore cannot

exert influence.

Steve S., you're wrong mate - think about it again..........

If a flybar is at anything less than 90 Deg. to the mainshaft then the
centres of gravity of the flybar halves will be closer to the axis of
rotation, ie.the mainshaft - since there's nothing holding these Cgs closer
in, ie. no centripetal force, they will try to move outward to their maximum
distance away from the mainshaft, which is with the flybar is at 90 Deg. to
the mainshaft.

Admittedley, the apparent 'centrifugal force' will be quite low with the
flybar close to 90 Deg but it will be 'not zero'.

Try it with just a flybar carrier and flybar on a mainshaft in a power drill
or just twizzle it in your hands - the flybar will align itself at 90 Deg.
to the mainshaft...

Ciao...

Nick...
--
Nick M V Salmon Master Mariner MN(Retd.)

My four initials@btinternet.com


Old Aug 08, 2003, 03:01 AM
Steve Simpson
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

> Beavis wrote:
> -------------
> Perfectly feasible even on a Piccolo, but it's a bit easier to maintain a
> constant head speed when the blades have enormous amounts of inertia, and
> it's variations in head speed that makes flybar-less flying more problematic
> than flybar flying.
>
> The Jet Ranger didn't appear to have any serious pitching up tendencies (a
> claasic symptom of poor rotor speed control) and as all turbine models are
> governed, that aspect is a non issue.
> -------------
>
> As I read it, he states that a governor cures pitching up tendencies. So
> please don't make me out as an idiot. It is insulting.
>


I don't recall commenting on that particular mis-conception.

In any event, you have my apologies. I can see now where a novice reading Bevis' description
could reach the conclusion you came to.

However, you have made another mis-interpretation if you believe I intended to 'make you out as
an idiot'. I simply advised that you were making incorrect assumptions and should go back and
read more carefully since the answers you sought were already there.

The be clear about it, if I thought you were an idiot I would not be having a serious discussion
with you at all. I am not one who suffers fools.

Don't be offended by my blunt style. I have only a small amount of time to spend on this group
so I don't waste it with pleasantries.














Old Aug 08, 2003, 03:01 AM
Courseyauto
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

Dont argue with an Idiot,they bring you down to their level and then beat you
with experience....................................
Old Aug 08, 2003, 03:01 AM
Courseyauto
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters

>
> The Jet Ranger didn't appear to have any serious pitching up tendencies (a
> claasic symptom of poor rotor speed control) and as all turbine models are
> governed, that aspect is a non issue.
> -------------
>



Am i missing something here. Turbines might be goverened for max RPM, but i
don't think they are goverened like a nitro engine, they dont have the throttle
response like a nitro engine.
Old Aug 08, 2003, 03:01 AM
Steve Simpson
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters


> > Consider that centrifugal force exerts a force 90 degrees to it's OWN

> rotation. In this case the
> > relevant revolution is about the middle of the flybar, not the center of

> the mast. The mast tilt
> > has negligible effect because it is hinged at the bar and therefore cannot

> exert influence.
>
> Steve S., you're wrong mate - think about it again..........
>
> If a flybar is at anything less than 90 Deg. to the mainshaft then the
> centres of gravity of the flybar halves will be closer to the axis of
> rotation, ie.the mainshaft - since there's nothing holding these Cgs closer
> in, ie. no centripetal force, they will try to move outward to their maximum
> distance away from the mainshaft, which is with the flybar is at 90 Deg. to
> the mainshaft.
>
> Admittedley, the apparent 'centrifugal force' will be quite low with the
> flybar close to 90 Deg but it will be 'not zero'.
>
> Try it with just a flybar carrier and flybar on a mainshaft in a power drill
> or just twizzle it in your hands - the flybar will align itself at 90 Deg.
> to the mainshaft...
>


Good to hear from you, Nick!

It is quite the treat for me to debate a point of physics with two people who's opinions I
respect. Quite the challenge to take on both you and Steve at the same time . . . ;-)

I understand what you are saying, but I still contend that I am correct. The ends of the flybar
will (try to) move away from the mast as you stated, but that is only coincidence because the
center of mass of the bar and the centerline of the mast happen to also be coincident. However,
the mass of the mast and the mass of the flybar are separate and hinged. i.e. the spinning
flybar does not 'see' the inclination of the mast . . . other than through bearing friction
which should be negligible.

Your experiment is interesting, but since we are talking about gyroscopic force, you would agree
that the mast would have to be spinning fast enough to generate such a force at the bar. A fast
drill might suffice, but even if I were so inclined, my natural fear of being impaled by metal
rods acts as a rev-limiter while 'twizzling' such objects by hand . . .:-)


As long as we are observing physical experiments, how would your theory account for the old
circus act of spinning plates on the top of a stick. Obviously the plate remains horizontal no
matter the inclination of the stick. In what way would that be different from the relationship
of the flybar and mast?

As I told Steve, I do sometimes fail to recognize the obvious, but in the hypothetical scenario
we are discussing (hinged bar with no links or paddles), I see nothing that would provide the
force to disrupt the bars gyroscopically fixed position.









Old Aug 08, 2003, 03:01 AM
Beav
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters


"Lars Blaabjerg" <lab@*nospam*ready.dk> wrote in message
news:u9oYa.46717$Kb2.2009540@news010.worldonline.d k...
>
> "Steve Simpson" <simpson34@coxpanties.net> skrev i en meddelelse
> news:JaaYa.37372$Bp2.28902@fed1read07...
> >
> > > > Head speed has a significant effect on handling characteristics. If

a
> heli
> > > has a tendency to
> > > > pitch up, then varying head speed may accentuate or reduce that

> tendency
> > > making the handling
> > > > inconsistent if the head speed is changing during flight.
> > >
> > > Shouldn't it then have an equal tendency to pitch down? I'm not sure I
> > > understand why the head speed is the determining factor when a heli is
> > > pitching up.

> >
> > Methinks you are finding information that no one is actually providing.

I
> don't see where anyone
> > said that head speed was a 'determining factor' in a heli pitching up.
> >
> > > > They do . . . I think you may have misinterpreted the comment.
> > >
> > > Then why are flybar-less heads more prone to irregular rotor speeds

than
> > > heads with a flybar?

> >
> > Once again, I don't see where anyone has made such a claim.
> >
> > Debate works better if you read carefully and don't fill in the blanks

> with your own
> > assumptions.
> >

>
> I may have misinterpreted something due to my incomplete knowledge of the
> subject, which is why I posed the question in the first place. It is easy

to
> read between the lines when possessing an inexhaustive knowledge on a
> subject. I DID read it carefully, so don't knock me for being rash. I

don't
> consider my apparent miscomprehension of the following strange:
>
> Beavis wrote:
> -------------
> Perfectly feasible even on a Piccolo, but it's a bit easier to maintain a
> constant head speed when the blades have enormous amounts of inertia, and
> it's variations in head speed that makes flybar-less flying more

problematic
> than flybar flying.
>
> The Jet Ranger didn't appear to have any serious pitching up tendencies (a
> claasic symptom of poor rotor speed control) and as all turbine models are
> governed, that aspect is a non issue.
> -------------
>
> As I read it, he states that a governor cures pitching up tendencies.


Maybe I wasn't as clear as I could've been Lars. A governor won't actually
cure anything, as all it does is control rotor speed. Now rotor speed
variations will show up any tendencies for a heli to be pitch sensitive, but
the actual CAUSE of the pitch sensitivity is usually poorly designed blades
where the C/G is too far back along the chord, and it's the poor design AND
rotor speed variations which both need to be addressed. The gov manages to
address one, but not the other, but one is better than none.


--
Beav


Please note my E-mail address is "beavis dot original at ntlworld dot com"
(with the obvious changes)

Beavisland now lives at
www.beavisoriginal.co.uk




Old Aug 08, 2003, 03:01 AM
Beav
Guest
n/a Posts
Re: Large Scale turbine Jet Ranger at 3D masters


"Lars Blaabjerg" <lab@*nospam*ready.dk> wrote in message
news:OcoYa.46720$Kb2.2009613@news010.worldonline.d k...
> Sorry Beav, I think I probably misinterpreted your first post. All sorted
> now :-)


That's fine


--
Beav


Please note my E-mail address is "beavis dot original at ntlworld dot com"
(with the obvious changes)

Beavisland now lives at
www.beavisoriginal.co.uk


 


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