Question about main blade balance...

[Gerard]

(I know all you experts fly with carbon fibre blades that are
perfectly balanced out of the box but some of us have still
got wooden blades...)

I've got a slight imbalance in my blades when I check
them by dangling them from the blade holder bolt hole, but
when I test holding them out like this:

Woodenblade--^--edalbnedooW

(if you get what I mean) they are in balance. This presumably
means that the lengthwise COG of the heavier blade is closer
than the COG of the lighter one.

I could add trim tape to the lighter blade but that would mean
I'd have to add more tape to each blade (presumably to the
inside of the light blade and the outside of the heavy blade) to
maintain the lengthwise balance as described above.

But do I need to bother? Even though the blades have slightly
different weight won't the turning moments be the same? (and
similarly the centrifugal force "felt" by the rotor head, since
F = angular velocity x mass x radius, and angular velocity
is the same for each blade* and since they are in balance
the mass x radius product must be the same too.

* Yes perhaps not in forward flight, since as I understand
it the blades move slightly in the holders. When I get up
to forward flight I'll worry about that!

Note that when I tested the lengthwise balance I supported
the blades at the same distance apart as they would be in
the blade holders.

Gerard

BTW: changed over the Futaba GY240 for a JR G490T.
That has solved the problem with the (painful to watch)
extreme rotation of the tail servo during the gyro warmup.
Don't know yet how it's going to perform - time will tell.

[Olaf Greck]

On Sat, 15 Jan 2005 10:13:41 GMT, "Gerard" <lfismdlfk4e@hotmail.com>
wrote:

>(I know all you experts fly with carbon fibre blades that are
>perfectly balanced out of the box but some of us have still
>got wooden blades...)

ACK, when I moved up from unbalanced blades to balanced ones without
knowing it I tried to re-calibrate my balancer....
("this can't be! There must be an imbalance in brand new blades.. Why
is this s%^$! thingy showing balance?".)

>

>But do I need to bother? Even though the blades have slightly
>different weight won't the turning moments be the same? (and
>similarly the centrifugal force "felt" by the rotor head, since
>F = angular velocity x mass x radius, and angular velocity
>is the same for each blade* and since they are in balance
>the mass x radius product must be the same too.

No. The formula is assuming a point like weight, so this point (CG)
is distance x from the center for one blade and creates force X.
The other blades CG point is at a different distance (radius) so at
the same number of revolutions the force is different
[I jumped here from angular vel. to revolutions because I still have
problems to understand angular velocity.]

Depending on how much unbalance you have:
Easiest to check is if both blades have the same level. Move
one blade to the front and measure the distance from blade tip to
ground, do the same for the other blade and try to adjust the pitch
till both are similar (same).

If that does not help then the full balancing story at
http://www.dream-models.com/eco/Blade_balancing.html
can keep you busy for days ;-)

best regards

Olaf

[Gerard]

Thanks Olaf,

I looked at dream-models.com but it did not significantly
add to other material I had on blade balancing. In particular
it didn't specify what to do if the blades are of slightly different
mass (< 1%) but are balanced when tested held out lengthways.

I fired the heli up today and found that there was no vibration.

The tracking was slightly out initially but I increased the pitch
of the low blade and they now seem to be tracking well.

Incidentally this was my first solo! I only flew it for a few
seconds, with a training undercarriage on, but at least it's
a start...

Gerard



"Olaf Greck" <olaf_greck@virgilio.it> wrote in message news:5frhu0t3h5d0kmrd5lgbmau1ebprbu79o1@4ax.com...
> On Sat, 15 Jan 2005 10:13:41 GMT, "Gerard" <lfismdlfk4e@hotmail.com>
> wrote:
>
>>(I know all you experts fly with carbon fibre blades that are
>>perfectly balanced out of the box but some of us have still
>>got wooden blades...)
>
> ACK, when I moved up from unbalanced blades to balanced ones without
> knowing it I tried to re-calibrate my balancer....
> ("this can't be! There must be an imbalance in brand new blades.. Why
> is this s%^$! thingy showing balance?".)
>
>>
>
>>But do I need to bother? Even though the blades have slightly
>>different weight won't the turning moments be the same? (and
>>similarly the centrifugal force "felt" by the rotor head, since
>>F = angular velocity x mass x radius, and angular velocity
>>is the same for each blade* and since they are in balance
>>the mass x radius product must be the same too.
>
> No. The formula is assuming a point like weight, so this point (CG)
> is distance x from the center for one blade and creates force X.
> The other blades CG point is at a different distance (radius) so at
> the same number of revolutions the force is different
> [I jumped here from angular vel. to revolutions because I still have
> problems to understand angular velocity.]
>
> Depending on how much unbalance you have:
> Easiest to check is if both blades have the same level. Move
> one blade to the front and measure the distance from blade tip to
> ground, do the same for the other blade and try to adjust the pitch
> till both are similar (same).
>
> If that does not help then the full balancing story at
> http://www.dream-models.com/eco/Blade_balancing.html
> can keep you busy for days ;-)
>
> best regards
>
> Olaf
>

[Olaf Greck]

On Sun, 16 Jan 2005 06:22:55 GMT, "Gerard" <lfismdlfk4e@hotmail.com>
wrote:

>Incidentally this was my first solo! I only flew it for a few
>seconds, with a training undercarriage on, but at least it's
>a start...

That's a good feeling, isn't it?

>In particular
>it didn't specify what to do if the blades are of slightly different
>mass (< 1%) but are balanced when tested held out lengthways.

On my Eolo the CG is out by ca. 5 millimeters and I don't notice
vibrations. IIRC the pre-balanced blades have a CG difference by 1-2
mm. [But those are two different blade pairs (length, width, profile,
everything) and are for different models.]

I guess it is a matter of experience because nowhere I have seen
something written down. One can probably calculate the resulting force
and define a threshold, but then I like to have that good feeling and
lack time for maths. ;-)


best regards

Olaf

[Boo]

Gerard wrote:

> Thanks Olaf,
>
> I looked at dream-models.com but it did not significantly
> add to other material I had on blade balancing. In particular
> it didn't specify what to do if the blades are of slightly different
> mass (< 1%) but are balanced when tested held out lengthways.

Add tape at the CG of the lighter blade.


--
Boo

[Gerard]

Thanks, Boo.

Nice try but wrong :-).

Adding tape at the COG of the lighter blade won't change its COG,
but adding tape anywhere along the lighter blade will increase
its turning moment over that of the (currently) heavier blade.

If I'm to balance the blades by mass _and_ COG, I'll have
to add mass at the inner end of the light blade (to minimise
the additional turning moment of the added mass) and then
add more mass equally to both blades, at the inner end
of what was the lighter one and the outer end of the heavier.

My question remains, do I need to bother? I strongly suspect
yes when I get up to forward flight (because the rotational
speed of the blades in their holders is not constant throughout
the rotor cycle) but probably not the level I'm at, which is just
learning to hover.

An analogy might be a big guy & a little kid balanced on a seesaw,
with the guy obviously close to the pivot and the kid at the end.
Now if the seesaw is rotated around its yaw axis, centered on
the pivot, it'll remain balanced, even though the arms each
have different masses.

I think.

Gerard

PS: the most important thing seems to have been overlooked:
my heli flew!!!!! Whoo hoo!!


G


"Boo" <Boo@spam_me_no_spam.net> wrote in message news:41ea380a$0$4375$fa0fcedb@news.zen.co.uk...
> Gerard wrote:
>
>> Thanks Olaf,
>>
>> I looked at dream-models.com but it did not significantly
>> add to other material I had on blade balancing. In particular
>> it didn't specify what to do if the blades are of slightly different
>> mass (< 1%) but are balanced when tested held out lengthways.
>
> Add tape at the CG of the lighter blade.
>
>
> --
> Boo

[Boo]

Gerard wrote:
> Thanks, Boo.
>
> Nice try but wrong :-).
>
> Adding tape at the COG of the lighter blade won't change its COG,
> but adding tape anywhere along the lighter blade will increase
> its turning moment over that of the (currently) heavier blade.

But the question I answered was :

>> it didn't specify what to do if the blades are of slightly different
>> mass (< 1%) but are balanced when tested held out lengthways.

Which is the case where you don't want to change the COG ?

--
Boo

[Gerard]

Sorry, Boo, I'd respond privately but somehow I suspect it
wouldn't get to you through your spam protection!

Since the blades are balanced when held out (with about
14 cm between the bolt-holes), the only place I can
put anything without unbalancing things is at the COG
of the system - ie at the pivot point itself

This obviously doesn't help to make the light blade heavier.

As well as a math analysis I've also conducted two experiments
which suggest to me that I don't need to worry too much
1) an arrangement of (would you believe..) lego, spun around
a central axis with a long light arm and a short heavy arm* and
2) the helicopter itself.

No vibration was evident in either case.

Cheers,

Gerard

* the lego arrangement obviously was statically balanced at
the pivot point.

"Boo" <Boo@spam_me_no_spam.net> wrote in message news:41eaf490$0$15404$db0fefd9@news.zen.co.uk...
> Gerard wrote:
>> Thanks, Boo.
>>
>> Nice try but wrong :-).
>>
>> Adding tape at the COG of the lighter blade won't change its COG,
>> but adding tape anywhere along the lighter blade will increase
>> its turning moment over that of the (currently) heavier blade.
>
> But the question I answered was :
>
> >> it didn't specify what to do if the blades are of slightly different
> >> mass (< 1%) but are balanced when tested held out lengthways.
>
> Which is the case where you don't want to change the COG ?
>
> --
> Boo

[Boo]

> Sorry, Boo, I'd respond privately but somehow I suspect it
> wouldn't get to you through your spam protection!

Possibly true... :-)
>
> Since the blades are balanced when held out (with about
> 14 cm between the bolt-holes), the only place I can
> put anything without unbalancing things is at the COG
> of the system - ie at the pivot point itself

Ah, ISWYM now. I thought you meant that the CG of the two blades was
the same distance from the hub.each balanced at the same point. I
didn't realise you meant they already were in balance with each other.
AYS CG positions are of secondary importance cf major balance and it may
not be worth bothering with.

--
Boo

[Gerard]

Thanks again.

This is the conclusion I am coming to, on the basis of my
simple experiments and my understanding of the math.

I guess I was really just looking for confirmation from an
experienced helicopterer (er?), since the books generally
make such a fuss of both mass balancing & COG bal-
ancing the blades.

I re-emphasise, I'm not proposing to do forward
flight without mass balancing the blades because
the assumption of constant angular velocity doesn't
apply when the blades are flapping etc during
fwd flight.

Gerard


"Boo" <Boo@spam_me_no_spam.net> wrote in message news:41eb9e32$0$15405$db0fefd9@news.zen.co.uk...
>> Sorry, Boo, I'd respond privately but somehow I suspect it
>> wouldn't get to you through your spam protection!
>
> Possibly true... :-)
>>
>> Since the blades are balanced when held out (with about
>> 14 cm between the bolt-holes), the only place I can
>> put anything without unbalancing things is at the COG
>> of the system - ie at the pivot point itself
>
> Ah, ISWYM now. I thought you meant that the CG of the two blades was the same distance from the hub.each balanced at the same
> point. I didn't realise you meant they already were in balance with each other. AYS CG positions are of secondary importance cf
> major balance and it may not be worth bothering with.
>
> --
> Boo

[Beav]

"Gerard" <lfismdlfk4e@hotmail.com> wrote in message
news:Cr3Hd.122969$K7.119960@news-server.bigpond.net.au...
> Thanks again.
>
> This is the conclusion I am coming to, on the basis of my
> simple experiments and my understanding of the math.

I'm late in on this (been away) but most of the talk about balancing blades
is wasted effort. If you set up your blades to resemble a see-saw (or
teeter-tottoer) using the rotor head as the centre and the flybar as the
pivot point (a couple of drinking glasses under the flybar works more than
well enough) that's ALL you need.

If one blade falls, add a wrap of insulating tape to the high blade close to
the tip and check again. When they blades hang level, you're done. You've
also (as a by-product) now got tracking tape on one blade and the other
doesn't need any.

No maths needed and no expensive balancing/weighing machines, and your heli
will be as smooth as any other heli using any other method.


--
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

[Boo]

> I'm late in on this (been away) but most of the talk about balancing blades
> is wasted effort. If you set up your blades to resemble a see-saw (or
> teeter-tottoer) using the rotor head as the centre and the flybar as the
> pivot point (a couple of drinking glasses under the flybar works more than
> well enough) that's ALL you need.
>
> If one blade falls, add a wrap of insulating tape to the high blade close to
> the tip and check again. When they blades hang level, you're done. You've
> also (as a by-product) now got tracking tape on one blade and the other
> doesn't need any.
>
> No maths needed and no expensive balancing/weighing machines, and your heli
> will be as smooth as any other heli using any other method.

But I just spent £20 on a digital scale that weighs accurate to .1 of a
gramme. Kindly retract that comment immediately :-)

--
Boo

[Alan]

"Boo" <Boo@spam_me_no_spam.net> wrote in message
news:41ed816a$0$13387$fa0fcedb@news.zen.co.uk...
>> I'm late in on this (been away) but most of the talk about balancing
>> blades is wasted effort. If you set up your blades to resemble a see-saw
>> (or teeter-tottoer) using the rotor head as the centre and the flybar as
>> the pivot point (a couple of drinking glasses under the flybar works more
>> than well enough) that's ALL you need.
>>
>> If one blade falls, add a wrap of insulating tape to the high blade close
>> to the tip and check again. When they blades hang level, you're done.
>> You've also (as a by-product) now got tracking tape on one blade and the
>> other doesn't need any.
>>
>> No maths needed and no expensive balancing/weighing machines, and your
>> heli will be as smooth as any other heli using any other method.
>
> But I just spent £20 on a digital scale that weighs accurate to .1 of a
> gramme. Kindly retract that comment immediately :-)

That's the way I was taught to do it by Beav and never had a problem! No
expensive purchases here!


--
Alan
alandotrait@btopenworlddot.com
Remove the dots to reply
http://heliweb.users.btopenworld.com/

[Gerard]

Beautiful! I like it! Thanks!

I take it that means my concerns about forward flight are baseless?

Gerard

"Beav" <beavis.original@ntloxoworld.com> wrote in message news:10uq2opaino4af6@news.supernews.com...
>
> "Gerard" <lfismdlfk4e@hotmail.com> wrote in message news:Cr3Hd.122969$K7.119960@news-server.bigpond.net.au...
>> Thanks again.
>>
>> This is the conclusion I am coming to, on the basis of my
>> simple experiments and my understanding of the math.
>
> I'm late in on this (been away) but most of the talk about balancing blades is wasted effort. If you set up your blades to
> resemble a see-saw (or teeter-tottoer) using the rotor head as the centre and the flybar as the pivot point (a couple of drinking
> glasses under the flybar works more than well enough) that's ALL you need.
>
> If one blade falls, add a wrap of insulating tape to the high blade close to the tip and check again. When they blades hang level,
> you're done. You've also (as a by-product) now got tracking tape on one blade and the other doesn't need any.
>
> No maths needed and no expensive balancing/weighing machines, and your heli will be as smooth as any other heli using any other
> method.
>
>
> --
> 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
>

[Beav]

"Boo" <Boo@spam_me_no_spam.net> wrote in message
news:41ed816a$0$13387$fa0fcedb@news.zen.co.uk...
>> I'm late in on this (been away) but most of the talk about balancing
>> blades is wasted effort. If you set up your blades to resemble a see-saw
>> (or teeter-tottoer) using the rotor head as the centre and the flybar as
>> the pivot point (a couple of drinking glasses under the flybar works more
>> than well enough) that's ALL you need.
>>
>> If one blade falls, add a wrap of insulating tape to the high blade close
>> to the tip and check again. When they blades hang level, you're done.
>> You've also (as a by-product) now got tracking tape on one blade and the
>> other doesn't need any.
>>
>> No maths needed and no expensive balancing/weighing machines, and your
>> heli will be as smooth as any other heli using any other method.
>
> But I just spent £20 on a digital scale that weighs accurate to .1 of a
> gramme. Kindly retract that comment immediately :-)

Slightly withdrawn :-)) Anyway, over the years, I've bought just about every
balancer this was/is to buy and I'm currently using a home made "mini rotor
head" which is a simple ali bar with a steel rod through it's centre and a
hole for the blade bolts at each end. This saves me from removing a rotor
head (or finding one if I'm just balancing a new set of blades) and it saves
me fiddling about with linkages before and after the balancing session.

The rod can sit on a Hi-Point balancer, or a pair of drinking glasses and I
can't tell any difference when the heli's are flying.

Cheap as cheapie cheap cheap:) (to rob a phrase)


--
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

[Boo]

> Even cheapo wooden blades are so close in balance these days that the quick
> and easy method I described works perfectly well.

Well I agree with this - on my new scales I weighed a pair of woodies
for a Hawk and they matched to a tenth of a gramme without any efforts
on my part whatsoever. I think they must sort them into matched pairs
at the factory or something cos I've had the same experience with blades
for my old Eco8 as well.

--
Boo

[Beav]

"Boo" <Boo@spam_me_no_spam.net> wrote in message
news:41ea380a$0$4375$fa0fcedb@news.zen.co.uk...
> Gerard wrote:
>
>> Thanks Olaf,
>>
>> I looked at dream-models.com but it did not significantly
>> add to other material I had on blade balancing. In particular
>> it didn't specify what to do if the blades are of slightly different
>> mass (< 1%) but are balanced when tested held out lengthways.
>
> Add tape at the CG of the lighter blade.

Why at the CG?


--
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

[Beav]

"Boo" <Boo@spam_me_no_spam.net> wrote in message
news:41ee64eb$0$15412$db0fefd9@news.zen.co.uk...
>> Even cheapo wooden blades are so close in balance these days that the
>> quick and easy method I described works perfectly well.
>
> Well I agree with this - on my new scales I weighed a pair of woodies for
> a Hawk and they matched to a tenth of a gramme without any efforts on my
> part whatsoever. I think they must sort them into matched pairs at the
> factory or something cos I've had the same experience with blades for my
> old Eco8 as well.

Well you can just about guarantee that the blades aren't "balanced" at the
factory, but they're weighed and placed in bins relating to their weight
(i.e. bin 1 for 195g, bin 2 for 196g etc)

That's usually plenty good enough, but it never hurst to do the see-saw
thing to ensure the blades are smooth running when you get out flying.


--
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

[Boo]

>>>
>>>I looked at dream-models.com but it did not significantly
>>>add to other material I had on blade balancing. In particular
>>>it didn't specify what to do if the blades are of slightly different
>>>mass (< 1%) but are balanced when tested held out lengthways.
>>
>>Add tape at the CG of the lighter blade.
>
> Why at the CG?
>
Well I think we've been here before, but I was under the
misapprehension that the OPs blades had the same CG but weighed
different amounts so adding weight at the cg of the lighter blade would
cure the major imbalance...

....but that was the wrong problem ;-)

--
Boo