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Home✈️Aircraft🚁HelicopterHelicopter Main Discussion › 3 blade heads why the new thing ?
07-05-2014 09:52 PM  6 years ago
BobOD

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Well, you're using the term a bit wrong is all. The balance method you're describing is not a dynamic balance, it's a static balance. However, it does equate to a dynamic balance as the blade disk has no significant height.
Probably wasn't worth the insult though. Professors being professors and all.
Team POP Secret
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07-05-2014 10:50 PM  6 years ago
AirWolfRC

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In this much simpler heli blade example... first you add whatever weight is needed to get each blade to balance at the same point on each blade, then when you balance one blade against the other, you add the weight to the CG point of the lighter blade. Once the CGs are identical...by adding small weights at the CG point a dynamic balance has been achieved.
Some people just like to do things the hard way . . .

Why not try my way, and report back,

Balance one blade against the other and
add weight to the tip of the light blade to balance . . . done.
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07-05-2014 11:40 PM  6 years ago
wjvail

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Balance one blade against the other and
add weight to the tip of the light blade to balance . . . done.
Why add weight to the tip? Why not the root, or center?

Bill
"Well, nothing bad can happen now."
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07-05-2014 11:57 PM  6 years ago
Heli_Splatter

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While it is done statically... because of the simplified situation of this case, it has the same effect as a dynamic spin balancing machine. Other more complex objects, such as spinners are not quite as easy to balance. They need to be done in two axis.

The whole idea is to get the item to balance without any item caused vibrations when spinning/or in motion - thus I would call it a dynamic balance... how you get there is of no accord.

The subject is vast and the RPM can have an effect on the dynamic balance. At certain rpms the item is fine, other resonant rpms, the item will tear itself apart.

OK.. maybe we have agreed on definitions now, but the balancing weight is never put on the tip it is always added to the CG point of the blade. That is what the argument is all about.

My way works, I know that.
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07-06-2014 12:04 AM  6 years ago
Four Stroker

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Shade tree physics always amuses me. Wiki probably has more than you ever wanted to know. Look up principal axes of inertia. Hint: It is the rotor shaft in our case.
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07-06-2014 12:19 AM  6 years ago
AirWolfRC

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OK.. maybe we have agreed on definitions now, but the balancing weight is never put on the tip it is always added to the CG point of the blade. That is what the argument is all about.
My way works, I know that.
I know your way works . . . but it's the hard way.

My way works too . . . and it's the easy way.
. . . and put the weight on the tip . . . ONLY.

Tell me . . . how would you apply your process on a one blade heli ?

. . . or is that where your theory falls apart . . .
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07-06-2014 12:46 AM  6 years ago
Heli_Splatter

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why would it fall apart... exact same weight at the exact same distance will counterbalance a single blade. Moment of inertia.

I did not sleep through the many physics classes that I took. The mechanical engineering helped a great deal too!

You may think your way works, but in fact it is causing vibrations. Maybe you could try a vibration device such as the Robird FBL unit to prove or disprove the theory.

Either way, the newer CF blades are coming from the factory in such a great shape with matching pairs that this blade balancing becomes less and less of an issue. Who flies wood blades anymore?
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07-06-2014 12:50 AM  6 years ago
AirWolfRC

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I did not sleep through the many physics classes that I took. The mechanical engineering helped a great deal too!
I think you did . . .

I'm content to let you keep doing it the hard way.

. . . including letting you put balance weights out 6" on a 4" counter weight for a one blades heli . . .

There was also a day when most people thought you would fall off the edge of the earth if you sailed over the horizon
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07-06-2014 01:17 AM  6 years ago
wjvail

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My way works too . . . and it's the easy way.
. . . and put the weight on the tip . . . ONLY.
Why only the tip? If instead of 2 grams at 700mm, won't the teeter balance still work if I put 4 grams at 350mm or 20 grams at 70mm?... or 200 grams at 7mm? Maybe 2KG at .7mm? Taken to an extreme, why not 20KG (10ish pounds) at .07mm. Using your teeter system and CG x distance physics, these should all spin at 2500 rpm with out vibration.

Isn't mass moment of inertia a radius squared function? Going back to my college statics/dynamics classes, moment about an axis is linear (radius x force) but moment of inertia is a rᶺ2 function.

Why "ONLY" the tip?

Bill
"Well, nothing bad can happen now."
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07-06-2014 01:33 AM  6 years ago
AirWolfRC

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Yes, all those combinations will work.

Why do I say only at the tip ?

. . . because that's the least amount of weight to get the job done.
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07-06-2014 02:42 AM  6 years ago
wjvail

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Wait. Isn't centripetal force F = m(vᶺ2/r). That is the force exerted by a mass rotating about a center equal to it's linear velocity squared divided by the radius from the center of rotation.

So by your teeter method a 700mm blade with a center of mass of 100 grams at 500mm from the hub should be equivalent to a 700mm blade with a center of mass at 250 mm and 200 grams. 100x500=250x200. That is static balance.

Sooo... 2000 rpm and 500mm equates to a linear velocity of 105m/s and that suggest a centripetal force proportional to 2,205. (105ᶺ2/500)*100

The same 2000 rpm at 250mm equates to a linear velocity of 52m/s and that suggest a centripetal force proportional to 2,163. (52ᶺ2/250)*200

In this case you have two blades that will balance on a teeter balancer statically but exert decidedly different forces on the main mast in dynamic rotation.

Mass moment of inertia is a calculation that really only applies if the rotor head is accelerating. That acceleration could be an increase or decrease in RPM but is could also imply a change in the plane of rotation - but in either case, moment of inertia will NOT be the same for two blades with different CGs but the same moment. This difference will cause unwanted effects.

Bill
"Well, nothing bad can happen now."
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07-06-2014 03:16 AM  6 years ago
BobOD

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Your math is not quite right. 100 grams out 500mm will have exactly the same centrifugal force as 200 grams out 250mm.Team POP Secret
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07-06-2014 01:38 PM  6 years ago
wjvail

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Your math is not quite right. 100 grams out 500mm will have exactly the same centrifugal force as 200 grams out 250mm.
You're right. I stand corrected. I rounded off the velocities leading to the apparent different forces.

While I calculated manually, you can use this to calculate centripetal force:
http://easycalculation.com/physics/...petal-force.php

This may help convert rpm (rotational velocity) to linear velocity (Ï–r).
http://www.endmemo.com/physics/rpmlinear.php

Bill
"Well, nothing bad can happen now."
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07-06-2014 01:50 PM  6 years ago
MattJen

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adding weight at the tip is wrong unless you want to move the C of G, period.

You find the c of g, match the c of g, add weight on the c of g.
Airwolfs way does work, but only at a fixed certain RPM... on a nitro/gasser/turbine the governors are always playing catch up.. electrics you may get away with it.

When the rpm changes the blades will come out of balance and out of track because the c of g is different and you have altered the polar moment of inertia. ie when spooling up, and spooling down, or when you pump in pitch on a downwind run, and when you land on hard surface your machine will start resonate as the blades spool up/Down at the low speed they will be out of balance.

It is not the hard way, it is easy, yes it takes time, but so what,
I spend time balancing my blades,the proper way..hence I get no vibration or any bits gradually cracking and falling off my machines. And when filming in 4K any slight vibration ruins the picture, so I had to have my machine balanced perfect.. adding weight at the tip DOES NOT WORK unless you are using or running a constant head speed, but the head speed will always change when you spool up and spool down or get into vortex ring, that is when the practice of adding weight to tip becomes flawed..
All The Best
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07-06-2014 02:03 PM  6 years ago
wjvail

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Below is a nice online calculator for moment of inertia. 200grams at 250mm is not the same as 100 grams at 500mm.

http://www.endmemo.com/physics/momentinertia.php

Bill
"Well, nothing bad can happen now."
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07-06-2014 02:46 PM  6 years ago
Heli_Splatter

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When there is a right way and a wrong way to do things, I choose to learn and do it the right way.

This is crystal.
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07-06-2014 04:09 PM  6 years ago
BobOD

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Yes, the moment of inertia will be different. So if we accelerate or decelerate the rotational speed, the blades would react differently. Honestly though, we are not changing that all that much and we're talking about a piece of tape here.
A non-uniform mass distribution is happy to spin about its COG. You'd have to show us why it would not before I'd declare either way being wrong.
Team POP Secret
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07-06-2014 04:59 PM  6 years ago
AirWolfRC

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Wait. Isn't centripetal force F = m(v^2/r). That is the force exerted by a mass rotating about a center equal to it's linear velocity squared divided by the radius from the center of rotation.
I prefer . . . F=mrv² where V is angular velocity in radians/sec (like rpm)
From 8 years ago - https://rc.runryder.com/t287507p1/?p=2243030#RR

But that's over complicating things.
Getting the moments (r x m) equal is all that's needed.
If you cancel out the velocity (because all blades are moving at the same rate) that's exactly what you have.
adding weight at the tip is wrong unless you want to move the C of G, period.
Where the CG is doesn't matter . . . just the moments.
Why the obsession with individual CG's ? ?
When the rpm changes the blades will come out of balance and out of track because the c of g is different and you have altered the polar moment of inertia. ie when spooling up, and spooling down, or when you pump in pitch on a downwind run, and when you land on hard surface your machine will start resonate as the blades spool up/Down at the low speed they will be out of balance.
The math (above reference) does not support that conclusion.
When there is a right way and a wrong way to do things, I choose to learn and do it the right way.
Both ways are right . . . just one way is a lot easier.
Yes, the moment of inertia will be different.
No, it will still be the same
. . . for all blades that balanced statically
. . . regardless of rpm.

Moment of Inertia does not change with rpm. The kinetic energy in that mass or centrifugal force is proportional to the speed . . . but still the same for all blades balanced statically.
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07-06-2014 06:03 PM  6 years ago
wjvail

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So if we accelerate or decelerate the rotational speed, the blades would react differently.
Isn't it more than that though? Isn't it more inclusive than just accelerating or decelerating the rotational velocity (i.e. change RPM or, head speed).

Isn't doing a flip or roll for instance "accelerating" the disk/mass/rotating body into another plane of rotation? I would expect two blades with different moments of inertia to track differently during maneuvering.

Let me go back to my original statement. If you are simply opening a bag of quality, modern, carbon, blades, sure, if you balance or how you balance is really picking nits. You probably will never be able to tell the difference. If however you are trying to produce 3 rotor blades using two sets of two different packages of blades, you might want to pay more attention to HOW you balance.

For the record.-
Two blades that weigh exactly the same but have different CG's, are not balanced.
Two blades that sit perfectly on a teeter balance (same moment) but have different weights, are not balanced.

Blades that have the same weight, located in the same place, are balanced.

Finally, I encourage everyone to use whatever works for them. I will continue to use my Koll Rotor Pro and scales. I personally find my routine fast, easy, and effective. But, I see where there are other ways that work for other individuals.

Wow did this thread take off on a tangent.

Cheers,

Bill
"Well, nothing bad can happen now."
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07-06-2014 06:06 PM  6 years ago
AirWolfRC

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Two blades that sit perfectly on a teeter balance (same moment) but have different weights, are not balanced.
That is completely wrong.

When you balance blades . . . you are in fact balancing the moments.
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