Rotor Balancing - check THIS out!
Bill Ludwig
On my Nexus 30, I've been having wobble when spooling up and down (while
still on the ground) and occasional wobble in a hover. This is my second
set of Kyosho wood blades. We won't talk about the first set, although I
just bolted them on and flew - no problem with these...until, well, you
know.
For the second set, I thought I'd do it up right and get a Kyosho balancer
and get them right on. Well, that's where the wobble comes in. I had to
put two pieces of tracking tape on the end to get them to static balance.
Pulling the tape off this afternoon resulted in NO WOBBLE! How can this be?
Only thing I can think is that I didn't put the tracking tape on the CG of
the light blade...could this the problem???
Thanks for the help,
-- Bill in Tucson
DBone62556
>
> . . . . having wobble when spooling up and down and
>
Say Bill, I'm just guessing here but what kind of head speed are you running? I
can get any of my three Kyosho 30"s to wobble if I drop the head speed down
around 1400. Wind them up to 1600~1700 and all is smooth.
Just a thought . . . .
Dave Bone
Bill Ludwig
This is definitely a rotor imbalance problem, Dave. Per my original
post...ALL wobble went away when I took off the tracking tape that I used to
'balance' the blades.
-- Bill
Beavis
In article <19980122073...@ladder02.news.aol.com>, DBone62556
<dbone...@aol.com> writes
>>
>> . . . . having wobble when spooling up and down and
>>occasional wobble in a hover.
>>
>
>Say Bill, I'm just guessing here but what kind of head speed are you running? I
>can get any of my three Kyosho 30"s to wobble if I drop the head speed down
>
>Just a thought . . . .
And a particularly good thought Dave. All the Kyosho heli's have a
"Bootie Shake" if the head speed is low (below 1400). It doesn't seem to
worry them though, they just keep on flying no matter what:-)
Head speed up Bill, just like Dave says.
Beav
Please remove XXX from reply address.
I had to change my address due to huge amounts of spam!!!
Beavis@nachos dot demon dot co dot uk is what it should read.
Nick M.V.Salmon
Bill Ludwig <lud...@azstarnet.com> wrote;
: On my Nexus 30, I've been having wobble when spooling up and down (while
: still on the ground) and occasional wobble in a hover. This is my second
: set of Kyosho wood blades. We won't talk about the first set, although I
: just bolted them on and flew - no problem with these...until, well, you
: know.
G'day Bill
Spooling up wobble is usually caused by the blades CG
being out of alignment which is fixed by 'centrifugal force'
once up to speed...
Is this a relatively slow 'wobble' you're talking about
rather than vibration shaking things around ? Could be
your head speed was a little low too if it's more of a
'wobble'...
Shouldn't happen spooling down though...
: For the second set, I thought I'd do it up right and get a Kyosho balancer
: and get them right on. Well, that's where the wobble comes in. I had to
: put two pieces of tracking tape on the end to get them to static balance.
: Pulling the tape off this afternoon resulted in NO WOBBLE! How can this be?
: Only thing I can think is that I didn't put the tracking tape on the CG of
: the light blade...could this the problem???
Dynamics matter with helis to put it mildly and you moved the
blades CG with your tape as you realised...
BUT; The out-of-balance from the same weight should have
affected it even more than that small CG shift you caused...
If the buggers don't wobble or vibrate, just fly 'em 'til you bust
'em Mate... (If it aint bust, don't fix it !!!)
I'd check that Kyosho balance accuracy by balancing your *next
set* in both directions in case there's some inherent innaccuracy...
Ciao...
Nick...
--
Nick M.V.Salmon. Southwest UK.
+++++++++++++++++++++++++++++++
R&D on MH Design's Extant Helicopters
&
Competition Flying.
Web Pages: http://www.ftech.net/~nick
+++++++++++++++++++++++++++++++
Greg Bezjak
My Nexus 46 gets a pretty good wobble going as I spool up if I don't have
the rotor grips tightened down enough. If the grips are too loose, the
blades twist a bit when the head first starts to rotate. When I get the
head up to speed, the blades staighten out and all is well.
This really doesn't explain your wobble in hover though.....
Greg
DBone62556 <dbone...@aol.com> wrote in article
<19980122073...@ladder02.news.aol.com>...
> >
> > . . . . having wobble when spooling up and down and
> >occasional wobble in a hover.
> >
>
> Say Bill, I'm just guessing here but what kind of head speed are you
running? I
> can get any of my three Kyosho 30"s to wobble if I drop the head speed
down
> around 1400. Wind them up to 1600~1700 and all is smooth.
>
> Just a thought . . . .
>
>
> Dave Bone
>
Len Gaultois
I have had the same problem with a Z when I first started out. What I
found was the blades were well balanced but the Head was not! The
paddles were not the same distance from the head and not properly
balanced either. What I had to do was put some old wheel collars on the
flybar and then make sure the paddles were EXACTLY the same distance
from the center of the head. Then move the wheel collars until the
paddles were balanced (this requires the head be removed and balanced on
a HI-POINT or something similar). Finally put the main blades back on
and balance them on the head (still on the ballancer) then put the whole
assembly back on the chopper and do the final tracking. ** I realize
this may be like teaching some of you guys how to suck eggs --But this
guy needs the basics!!
John Bahr
On 22 Jan 1998 23:06:05 GMT, "Nick M.V.Salmon" <nm...@dial.pipex.com>
wrote:
>Bill Ludwig <lud...@azstarnet.com> wrote;
>: On my Nexus 30, I've been having wobble when spooling up and down (while
>: still on the ground) and occasional wobble in a hover. This is my second
>: set of Kyosho wood blades. We won't talk about the first set, although I
>: just bolted them on and flew - no problem with these...until, well, you
>: know.
>
>G'day Bill
>
>Spooling up wobble is usually caused by the blades CG
>being out of alignment which is fixed by 'centrifugal force'
>once up to speed...
How close do you need to be on the blade CG? Are we talking within
1/8" (on 515 or 530mm blades) or closer than that.
I have a Nexus also and have seen the same problems Bill is talking
about. I have not tried removing the balancing tape though. I know
it's gotta be the CG 'cause I static balance the blades within .01
grams of each other after checking the CG. (static balance tape going
over the CG of each blade)
I guess I need to get an RPM meter to check out my head speed.
Oooo... More gadgets!
John Bahr
(2-mile high flyer)
>
>Is this a relatively slow 'wobble' you're talking about
>rather than vibration shaking things around ? Could be
>your head speed was a little low too if it's more of a
>'wobble'...
>
>Shouldn't happen spooling down though...
>
>
>: For the second set, I thought I'd do it up right and get a Kyosho balancer
>: and get them right on. Well, that's where the wobble comes in. I had to
>: put two pieces of tracking tape on the end to get them to static balance.
>: Pulling the tape off this afternoon resulted in NO WOBBLE! How can this be?
>: Only thing I can think is that I didn't put the tracking tape on the CG of
>: the light blade...could this the problem???
>
>Dynamics matter with helis to put it mildly and you moved the
>blades CG with your tape as you realised...
>
>BUT; The out-of-balance from the same weight should have
>affected it even more than that small CG shift you caused...
>
>If the buggers don't wobble or vibrate, just fly 'em 'til you bust
>'em Mate... (If it aint bust, don't fix it !!!)
>
>I'd check that Kyosho balance accuracy by balancing your *next
>set* in both directions in case there's some inherent innaccuracy...
>
>Ciao...
>
>Nick...
John Bahr
jcb...@bahr123engineering.com
(Remove 123 for email replies)
Nick M.V.Salmon
: John Bahr <jcb...@bahr123engineering.com> wrote;
:
: "Nick M.V.Salmon" <nm...@dial.pipex.com> wrote:
:
: >: Bill Ludwig <lud...@azstarnet.com> wrote;
: >: On my Nexus 30, I've been having wobble when spooling up and down (while
: >: still on the ground) and occasional wobble in a hover. This is my second
: >: set of Kyosho wood blades. We won't talk about the first set, although I
: >: just bolted them on and flew - no problem with these...until, well, you
: >: know.
: > Nick;
: >Spooling up wobble is usually caused by the blades CG
: >being out of alignment which is fixed by 'centrifugal force'
: >once up to speed...
: How close do you need to be on the blade CG? Are we talking within
: 1/8" (on 515 or 530mm blades) or closer than that.
: John;
: I have a Nexus also and have seen the same problems Bill is talking
: about. I have not tried removing the balancing tape though. I know
: it's gotta be the CG 'cause I static balance the blades within .01
: grams of each other after checking the CG. (static balance tape going
: over the CG of each blade)
Slight misunderstanding there I think John...
Where I said "the blades CG being out of alignment" I mean the whole blade
was out of the usual line it will be in once the blades come up to speed...
ie. both the main blades and the head centre aren't in a straight line until
'centrifugal' force aligns them... If you have your blades too stiff in their
holders, it may cause 'wobbles' until quite high RPM... Hold the heli on it's
side and a gentle shake should cause the main blades to 'droop' an equal
amount... (I also use 'whipping' the head to ensure blade holder tension is
*exactly* the same...)
Blade CGs affect vibration when the blades are already up to speed, in flight...
Off hand, I'm darned if my 'forgettory' will play ball and let me explain the why
of it but I can assure you it's true... Ray's Heli Manual doubtless contains an
explanation...
: I guess I need to get an RPM meter to check out my head speed.
: Oooo... More gadgets!
Not entirely necessary but a good instrument to share within a club... Once
you've used it a few times you can tell what head speed you're running from
the engine note, just 're-calibrate' your hearing now and again... ;-)
: > Nick;
: > Is this a relatively slow 'wobble' you're talking about
: > rather than vibration shaking things around ? Could be
: > your head speed was a little low too if it's more of a
: > 'wobble'...
: >
: > Shouldn't happen spooling down though...
ie. Vibration may be CG or blade mass/'weight', 'wobble' may be just
speeding up misalignment or running too low a head speed in the hover...
Space Baron / Enforcer typically exhibits quite pronounced 'nodding', a
slow rotational vibration of the tail boom, at low head RPM... Resonance
can sometimes be a culprit too but it's usually a symptom of either
something very badly out of balance or much to low a head speed, ergo
needing too much pitch for lift off... You can't get a 'misalignment wobble'
spooling down unless your blade holder tensions are right-up-a-creek...
Nick M.V.Salmon
: : John Bahr <jcb...@bahr123engineering.com> wrote;
: :
: : How close do you need to be on the blade CG? Are we talking within
: : 1/8" (on 515 or 530mm blades) or closer than that.
Ooops, missed that bit...
I used to work to the nearest millimetre with built up woodies but
that's probably closer than necessary...
I don't bother with CG anymore for the NHPs I use exclusively
now but I do check the balance between blades and usually just
add a tiny sliver of tracking tape extra to one blade tip is all...
Maybe caused by me Cyanoacrylating the foam unevenly between
blades... Just stops them shedding foam if you clip some weeds...
I'm gonna have to look up the 'why' of blade CG matching unless
Steve Rhodes can elucidate..? Seem to recall summat to do with
coning but that may be way out of left field...
TaRa...
John Bahr
On Tue, 27 Jan 1998 12:40:54 +0000, Beavis
<Bea...@XXXnachos.demon.co.uk> wrote:
>
>...Bill...
>...Nick...
>...John...
>...Nick...
Oh...
I see where I was out-of-wack. Thanks Nick.
>Beavis:
>
>If I'm reading this thread correctly (and I answered the original
>question about the wobbles) I think there's DEFINATELY a little
>confusion creeping in.
>
>The heli in question (If I remember correctly) was a Concept that
>"wobbled" in flight. I can "see" this as I sit here, but we call it a
>"shimmy" and on the ocassions that I've come across it (No sexual
>innuendo there then!) it's not been cured by balancing ANYTHING, because
>nothing was out of balance in the first place. Just increasing the head
>speed was enough, and strangely it didn't seem to be because the blades
>weren't "pulled out" enough to properly align them, with either each
>other, or the head.
>
>How do I know?? Because after the shimmy stopped by increasing rotor
>speed, a very slow DECREASE in rotor speed brought the shimmy back.
>
>The "problem" was looked at by a few of us at the field, and the general
>consensus was that the blades were reaching their "end stops" in the
>flapping plane, and a change of teeter rubbers wouldn't hurt. So it
>turned out, and when I stripped the errant choppers head down, I found
>the original BLACK rubbers (Again, no innuendo there either:-)
>
>These were changed for new RED ones, and the problem took a holiday. I
>should imagine that when the red rubbers get shagged out, the shimmy
>will re-appear.
>
>SO....If all else fails, use new rubbers. You KNOW it makes sense:-)
>
>Beav
Beavis... That sounds like a good explanations, but the problem is the
helicopter was a Nexus Std (for Bill) and a Nexus Dlx (for me). I'm
not sure about the Std rotor head, but the Dlx doesn't have rubbers.
It's, as someone at my local shop called it, a full flopper. The
rotor grips move up and down about 20 degrees without dampening.
I do need to work on my rotor grip tension (I'll use those techniques
Nick), but I see wobble on a slight decrease in RPM also. I'll do
more testing and also make sure my head speed is up in case it's just
the Kyosho shimmies.
It hasn't caused me any trouble except once I noticed fuel bubbling
and the engine racing for short periods of time. That got my
attention since I've only done a few autos on the Sim so far. Before
then, I must have had an SEP field around it.
By the way... If I'm talking like a stupid git every once in a while
it only 'cause this is my first heli. I've wanted to fly choppers
ever since I had a ride when I was 8.
John Bahr
(2-mile flyer)
Beavis
In article <01bd2ad2$bd3afba0$LocalHost@oemcomputer>, "Nick M.V.Salmon"
<nm...@dial.pipex.com> writes
>: John Bahr <jcb...@bahr123engineering.com> wrote;
>:
>: "Nick M.V.Salmon" <nm...@dial.pipex.com> wrote:
>:
>: >: Bill Ludwig <lud...@azstarnet.com> wrote;
>: >: On my Nexus 30, I've been having wobble when spooling up and down (while
>: >: still on the ground) and occasional wobble in a hover. This is my second
>: >: set of Kyosho wood blades. We won't talk about the first set, although I
>: >: just bolted them on and flew - no problem with these...until, well, you
>: >: know.
>: >
>: > Nick;
>: >Spooling up wobble is usually caused by the blades CG
>: >being out of alignment which is fixed by 'centrifugal force'
>: >once up to speed...
>: 1/8" (on 515 or 530mm blades) or closer than that.
If I'm reading this thread correctly (and I answered the original
question about the wobbles) I think there's DEFINATELY a little
confusion creeping in.
The heli in question (If I remember correctly) was a Concept that
"wobbled" in flight. I can "see" this as I sit here, but we call it a
"shimmy" and on the ocassions that I've come across it (No sexual
innuendo there then!) it's not been cured by balancing ANYTHING, because
nothing was out of balance in the first place. Just increasing the head
speed was enough, and strangely it didn't seem to be because the blades
weren't "pulled out" enough to properly align them, with either each
other, or the head.
How do I know?? Because after the shimmy stopped by increasing rotor
speed, a very slow DECREASE in rotor speed brought the shimmy back.
The "problem" was looked at by a few of us at the field, and the general
consensus was that the blades were reaching their "end stops" in the
flapping plane, and a change of teeter rubbers wouldn't hurt. So it
turned out, and when I stripped the errant choppers head down, I found
the original BLACK rubbers (Again, no innuendo there either:-)
These were changed for new RED ones, and the problem took a holiday. I
should imagine that when the red rubbers get shagged out, the shimmy
will re-appear.
SO....If all else fails, use new rubbers. You KNOW it makes sense:-)
Beav
Please remove XXX from reply address.
Nick M.V.Salmon
John Bahr <jcb...@bahr123engineering.com> wrote
:
: I do need to work on my rotor grip tension (I'll use those techniques
: Nick), but I see wobble on a slight decrease in RPM also. I'll do
: more testing and also make sure my head speed is up in case it's just
: the Kyosho shimmies.
G'day John
See if it goes away too when you've got main blade holder tension
accurately matched...
: It hasn't caused me any trouble except once I noticed fuel bubbling
: and the engine racing for short periods of time.
Doesn't sound too good... Sounds like something's slipping and best
traced *before* the forced auto..! IMO, 'fuel foaming' comes from HF
engine related vibration, out of balance clutch, fan Etc...
: That got my attention
Good..!
: since I've only done a few autos on the Sim so far.
They're easy enough for real but best *planned* for your first few...
: Before then, I must have had an SEP field around it.
Que..?
: By the way... If I'm talking like a stupid git every once in a while
: it only 'cause this is my first heli. I've wanted to fly choppers
: ever since I had a ride when I was 8.
No worries here Mate, we all start somewhere..!
Let us know how it goes...
Ciao...
Michael Young
Nick M.V.Salmon wrote in message <01bd2adf$ca416980$2ffc82c1@oemcomputer>...
>Maybe caused by me Cyanoacrylating the foam unevenly between
>blades... Just stops them shedding foam if you clip some weeds...
I use the NHP tracking tips. Adds something extra to the balance equation.
>I'm gonna have to look up the 'why' of blade CG matching unless
>Steve Rhodes can elucidate..? Seem to recall summat to do with
>coning but that may be way out of left field...
Coning has more to do with lead/lag, and thus chordwise CG. Coning and
flapping aside, vibration is much more sensitive to spanwise CG. This is
easily seen when balancing a rotor head with blades of equal weight, but
different CG's. The longer lever arm on the blade with the outboard CG
overpowers the inboard CG, even though both blades weigh the same. The
system's natural center of rotation is the CG of the rotating mass. When the
blade CG's differ, the rotating mass is eccentric with respect to the mast
centerline. In other words, you could theoretically kink your main mast to
compensate. How closely do you hold the run-out on your mast? The rotor head
CG should be this close for spanwise CG matching to be meaningful.
Conversely, a mast need be no straighter than you're willing to hold rotor
head balance. OTOH, the other rotating parts -- shaft, metal head, seesaw,
upper swash, mixer, links, flybar, main gear, etc. -- contribute to the CG
location. Thus, the actual radius of gyration is somewhat smaller than the
blade CG difference. So, while it's not the end-all and be-all, spanwise CG
deserves all the attention you can spare it. Less than 1mm difference across
blades is close enough for me, and I'm upset when I can measure the runout
on a new shaft.
I think you had it right with your earlier reply. A wobble that stabilizes
on the first run up is probably nothing more than the blades seeking their
spot.
Michael.
Nick M.V.Salmon
Colin...
Sorry to disturb your v10.0 programming but can you help me here
please..?
Reason for matching blade CG and mass rather than static moments ?
TIA...
------
Can you explain it for me Stephen..?
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Michael Young <mi...@mcs.com> wrote in rec.models.rc.helicopter;
:
: Nick M.V.Salmon wrote;
Snip only...!
: >Maybe caused by me Cyanoacrylating the foam unevenly between
: >blades... Just stops them shedding foam if you clip some weeds...
:
: I use the NHP tracking tips. Adds something extra to the balance
: equation.
Yo Mike...
Yeah, they're neat and they reduce drag from the squared off NHP sport II
tips too, I must remember to check balance the blades before fitting and
use a dab of extra glue on the light blade, if there is one... Need to fix
them pretty good although the only time I've had one come off was in a
crash...
BTW, I don't *weigh* main blades, just use a ballraced balancer that is
the equivalent of a bolt and two glasses... CG's I checked, (don't bother
now I'm using NHPs) by putting a screwdriver handled ball driver shank
on the bench at 90Deg. to the edge, lay a (covered..!) blade on it parallel
to the edge and roll the driver along until the blade balances, press down
just enough to mark the covering and highlight with felt tip pen... Transfer
the position to the blade's leading edge with a set square, hang the two
blades from a nail in the wall and check CGs are the same, chordwise
whilst you're at it too but it isn't really critical, a millimetre difference in
'angle of dangle' at the tip is no HuHu... If spanwise CG is not the same I
put some small nails (Brads) onto the inboard end of the blade (Woodies
only !!!) with the outboard CG until they match and then press them into the
*root's* thick trailing Balsa edge, angled so they can't be dragged out by a
lack of centripetal 'grip', checking the CG again afterward... Balancing is
then completed by adding blade covering on the light blade's CG... NB: If
it's the heavy blade that has the far CG then add extra wide tracking tape
to the tip of the light blade to avoid too much balancing tape on the CG...
BTW; If chordwise CG is way out then send 'em back for a better pair !!!
: >I'm gonna have to look up the 'why' of blade CG matching unless
: >Steve Rhodes can elucidate..? Seem to recall summat to do with
: >coning but that may be way out of left field...
:
: Coning has more to do with lead/lag, and thus chordwise CG.
*Dynamic* lead/lag or 'hunting' (Stephen Bell has started using
'hunting' on the h-list and I like this as it saves confusion with static
lead...) is caused by the apparent horizontal shift of *spanwise* CG with
flapping and the resultant coriolis forces; flap up -> CG moves inward ->
blade speeds up -> blade/CG moves ahead and vice versa... If you think
about it you can see that this will lead to the CG's being a *long* way out
of alignment across the head with excessive coning angle (One blade
leading, one blade lagging...) which you will have at too slow head RPM
thus the 'wobbles' which are accentuated by resonance with some part of
the heli at various RPM... Ok, hunting lead/lag angles would be unequal
with different blade CGs but it *isn't* equal and opposite anyway !!!
Static lead is certainly set by chordwise CG and chordwise bolthole
position... I have never seen blades with static lag inbuilt, I doubt our
servos could handle it with the dynamic reduction in lead due to blade
profile drag and the extra drag when any pitch is applied...
: Coning and flapping aside, vibration is much more sensitive to
: spanwise CG.
I do understand what you are saying but I still can't remember exactly
why it matters... Please read on...
: This is easily seen when balancing a rotor head with blades of equal
: weight, but different CG's. The longer lever arm on the blade with the
: outboard CG overpowers the inboard CG, even though both blades
: weigh the same.
Assuming you actually compared blade *weight* rather than *moments*
with a blade balancer... Blade mass can be different but they will still
balance with CGs at different positions on a 'seesaw' balancer...
: The system's natural center of rotation is the CG of the rotating mass.
Yes...
: When the blade CG's differ, the rotating mass is eccentric with respect
: to the mast centerline.
If each blade mass is the same... Yes... But what if static moments are
the same and not actual mass..? I'm quite sure it does matter but I still
cannot remember why.......... ^&$^*%"£**^&(&*) Forgettory..! Both Dave
Day and Wil Snitjer gloss over it, can't find Ray's and lent Dieter
Schluter's book out a short while ago... Wish there was a *modern* heli
aerodynamics text book but I somehow doubt there would be sufficient
market...
: In other words, you could theoretically kink your main mast to
: compensate.
<G>... F()&(((^&*%((*)k that !!!!!!!
: How closely do you hold the run-out on your mast?
As near to zero as possible, nil 'wobble' when rolled free on plate glass...
: The rotor head CG should be this close for spanwise CG matching to be
: meaningful.
Yes, full rotorhead CG, sure, and blade CG affects this too BUT;
Why ?... Your explanation works Ok assuming the blade mass to be
equal and the spanwise CG incorrect but it still doesn't explain why
you get vibration with statically balanced moments...
: Conversely, a mast need be no straighter than you're willing to hold
: rotor head balance.
Nope... Blade CG only affects blade moment and proportionally less
the whole head, mainshaft straightness directly affects the whole head's
mass..! The two error figures don't need to match within the same order...
: OTOH, the other rotating parts -- shaft, metal head, seesaw, upper swash,
: mixer, links, flybar, main gear, etc. -- contribute to the CG location.
Yes... But, the *moments* imparted are small in comparison to blade
mass; I have tried balancing a head alone on a Dubro Hi-Point and I could
add up to 2gms of lead to one blade holder before the dang thing moved so
I don't bother anymore !!! Now I've got a metal Robart hi-point I may try
again but I'm danged if I want to bend the wheel pins with all that weight...
I also want to use the same balancer for fans & other stuff revolving at
17,000 RPM and my plastic Dubro hi-point was wrecked by the weight of a
head cantilevered on a spare mainshaft...
: Thus, the actual radius of gyration is somewhat smaller than the blade CG
: difference. So, while it's not the end-all and be-all, spanwise CG deserves
: all the attention you can spare it. Less than 1mm difference across blades
: is close enough for me, and I'm upset when I can measure the runout on a
: new shaft.
Reasonable analogy I suppose but not directly comparable, as you say
above... A mainshaft with 0.1mm runout is for sure BENT !!!
: I think you had it right with your earlier reply. A wobble that stabilizes
: on the first run up is probably nothing more than the blades seeking
: their spot.
Agreed... Only thing is they were getting a similar 'wobble' whilst
spooling down too and the blades should be reasonably aligned by
then !!! Maybe it was just mismatched blade holder tension, that'll
produce odd wobbles throughout the flight regime...
Nick M.V.Salmon
G'day Folks
Herewith the reasons for balancing spanwise CG from Stephen Bell,
MHW columnist...
Perhaps I'll be able to remember it now..!
Ciao...
Nick...
----------
From: Stephen Bell <hel...@ns.sympatico.ca> [I added the anti-spam XXX]
To: nm...@dial.pipex.com
Subject: Re: Main Rotor balance... Everything you ever (didn't) want to know... ;-)
Date: 29 January 1998 14:32
Nick,
Sorry that I was not clear enough on the static verses dynamic balance
thing. While the differences are small they are none the less present when
the spanwise c of g is not the same but the assembly is still statically in
balance. In a flat rotor disk condition things will be fine. When the rotor
is disturbed by means of coning and or cyclic deflection with blades
without identical c of g but never the less statically balanced higher
vibration will be encountered. Here is how it works in one situation. The
numbers are only for example and the true effect is smaller for the average
blade builder!
100grams X 1cm =100cm grams moment.
10 X 10cm= 100cm gram moment.
This is statically balanced and dynamically so.... at a flat disk.
With equal coning in the hover the blade c of g moves closer to the
rotational center. Eg .25cm or 2.5mm change:
100grams X .75cm=75 cm/gram moment
10grams X 9.75cm= 97.5cm/gram moment.
To further complicate matters the c of g is moving unevenly in a vertical
manner too so balance has been thrown off in another (very minor and an
acceptable fact of life to all helicopters) plane. Keep in mind that all
conditions cannot be covered by matching c of g's but we are never the less
closer to perfection. Me I don't worry about it too much with models due to
lower coning angles but try to build my blades close. Make any sense now?
Mr Koll had a hard time understanding this when he wrote his articles in
past RM issues.
See ya...
Stephen
Michael Young
>BTW, I don't *weigh* main blades, just use a ballraced balancer that is
>the equivalent of a bolt and two glasses... CG's I checked, (don't bother
>now I'm using NHPs) by putting a screwdriver handled ball driver shank
I don't weigh mine, either. I match CG's, and balance the rotor head. The
mass has to be equal to get the matched CG's to balance. I'm still using the
Robart hi-point with the back standard flipped to hold the shaft down.
Michael.
Nick M.V.Salmon
Michael Young <mi...@mcs.com> wrote;
:
: I don't weigh mine, either. I match CG's, and balance the rotor head. The
: mass has to be equal to get the matched CG's to balance. I'm still using the
: Robart hi-point with the back standard flipped to hold the shaft down.
:
: Michael.
Yo Michael
True, same CG, same mass when balanced on a 'seesaw' or Hi-Point
assuming distance from centre on the head is exactly equal of course...
Don't think much of the Dubro HiPoint though, the wheel pins ate into
their plastic bearings on mine with the weight of a 60 head + blades
cantilevered on it...
Just got a Robart like yours and it looks better able to take the weight so
I'll give it another try... I got two heads that are suspect at the moment
even though I've balanced everything else in sight seperately...
Ciao... :-)
Dale Smith
Here is the way to balance blades that my Japanees instructor showed me and
it only takes 10 minutes.
1. Glue the rotor on grips in not already done. If you just bought the
blades and the grips are attached check them to ensure the grips are glued
on and not just screwed in place. THIS IS A VERY INPORTANT SAFETY ITEM!!!
2. Paint one of the tips a bright color such as international orange or
fire truck red. Painting takes the place of tracking tape and adds very
little weight. You only need to paint a 1/4 inch of blade to see blade
tracking since you will be looking at the tip anyways. Testor's plastic
model paint will work fine and it comes in small sizes and every color in
the rainbow. What I've wanted to try is to paint one tip blue and the other
yellow. If the blades are tracking you should see green in theory.
3. Plum the blades. Hang the blades, with both faceing the same
direction, from the hole the rotor grip attaches to any round objest that
will fit in the hole and allow the blades to move. Hang a plum line to see
how balanced the blades are. Both blades should hang together and if
everything works the leading edge should be within a few degrees of parallel
to the plum line. The closer the better but the importent thing is the
blades are parallel together. I haven't had a problem here so I don't know
how to fix it.
4. Find each blades CG. Roll the blades on anything round. I use my
X-acto handle. Roll the blades sea-saw wise and mark the position on the
blade where they balance. Use a felt tip pen so you don't dent the wood
that a ball point will. If the CG's are not in the same spot don't worry
about it for now. If you want to get fancy rotate the blade 45 degrees and
roll them again to find the CG center.
5. Find each blade's weight. For this you will need a .1 gram scale or
something simular. With a felt tip pen write on the blade the weight too
help you keep track of which bladle weighs what.
6. Matching blade weight and CG. Now you have to decide where to add
weight if needed. You'll be working with the light blade here. Check to
where the CG is on the light blade as compared to the heavy blade.
a. If you're lucky the CG is towards the tip. If the tip is heavy you
will be adding weight to the rotor grip end. The best way to add weight at
the grip is to find a small wood screw that is same weight needed to balance
the blade. A heavier screw is better since you can file it untill it is the
weight you need. Drill a small pilot hole in the blade and install the
screw.
b. If the CG is towards the grip them you will have to add weight to
the tip. Tracking tape will work but you can also use clear packing tape.
Weigh out a piece of tape equal to the wieght neede to correct. If the tape
is not big enough to wrap around the blade cut the tape into two equal
pieces. (One piece goes on top and second goes on the bottem just like the
wheels on you car. Split the weight and add to each side.) Use super
sticky tape since it will need to hold at +250 mph speed. If you can wrap
the tape around the blade keep the edges down wind so it woun't peel off.
Now if everything worked out the blades should match. Rerun step 4 and 5 to
check. If you added weight with a screw glue it in place with epoxy. I
used this procedure on three sets of blades with no problems.
If you still notice wobble start checking the head balance, stabilizer
balance and anything else on the helo that is spinning. RCM had an article
a few months back on how to check the head.
Good Luck
Dale Smith
US Navy Yokosuka Japan