Question about blade COG

[darkchiild]

Why does the COG of the blade matter? As long as both blades are the same weight, you have the same amount of centrifugal force on either side of the head right? Regardless of whether one blade is heavy on the outside, and the other heavier on the inside?

Am I missing something?

[TheBum]

The weight of the blade tells you the mass that must be swung and the CG represents a point on blade where that mass is centered. You can use the analogy of two strings, each with identical fishing weights attached at the CG distances. Even though the weights are the same, if the distances are different, you will get a wobble when you spin them around the same axis because the centripetal forces exerted are proportional to distance as well as mass.

[darkchiild]

Diggity.. Good explanation. The bulb over my head is blinking.

Thanks!

[BarracudaHockey]

Its just like an airplane, if the spanwise cg is forward they are more stable, if its aft they are more aggressive (or cause flutter)

Same with lengthwise weight distibution, the farther out the weight the more stable.

[RHash]

The weight of the blade tells you the mass that must be swung and the CG represents a point on blade where that mass is centered. You can use the analogy of two strings, each with identical fishing weights attached at the CG distances. Even though the weights are the same, if the distances are different, you will get a wobble when you spin them around the same axis because the centripetal forces exerted are proportional to distance as well as mass.


Yep -- this sounds correct. From my (dim) recollection of physics, I think centrifugal force is directly proportional to mass x CG radius (called the moment??)

So . . you COULD also have two different masses, but if the heavier mass is shorter CG radius (so that the moments are equal), then spinning them should be balanced?.

Which brings me to a question I have regarding blade balancing -- the main way I've heard of doing this is to make each blade CG at equal point (radius), and then make each blade same weight. While this certainly would make their moments equal, this means that sometimes you would need to add weight to BOTH blades.

Would the blades still be balanced if I find the CG of each blade (probably have different radiuses from the mount point), find the mass of each blade (using a gram scale for this) and then add weight to only one blade (at that blade's CG point) to make the CG radius x mass equal? In essence, you are making the moments equal, but adding weight to only one blade to do this.

the disadvantage to this method is you would need a scale and ruler to measure the actual weight and CG radius of each blade, since you need to do calculations. The only advantage to this method I can think of is that you are minimizing weight addition to the blade system.

Not sure if this even matters or not compared to the accepted way. I may try this next time I balance my blades.

[fiveoboy01]

Bolt 'em together, put them on a seesaw, add tape to the tip of the light blade. Done.

[Pinecone]

That doesn't get teh CoGs balanced, just the overall blade weight.

1) Find each balde CoG, and compare. Add tape to the tip of the one where the CoG is closer to the hub, until CoGs match.

2) Then bolt together and check overall weigth balance and use tape ONLY ON THE CoG to balance.

One could identify the light blade first and when doing the CoG adjust the light blade only to try to get weights to match at the same time.

But good blades won't require much to balance either way so it isn't a big deal.

[spork]

That doesn't get teh CoGs balanced, just the overall blade weight.

Actually, his scheme gets the overall blade moments balanced - which is all that's necessary.

[sleddog7]

That doesn't get teh CoGs balanced, just the overall blade weight.

Terry's method is the proper and correct way to FULLY balance a pair of blades.
Anything less would produce a dynamic imbalance of the rotor disc.

All of Terry's steps are very necessary.

[spork]

Terry's method is the proper and correct way to FULLY balance a pair of blades.
Anything less would produce a dynamic imbalance of the rotor disc.

All of Terry's steps are very necessary.

Not true. Terry's method is fine. But it's not necessary to have your spanwise C.G. match on your blades. Have you ever seen the one-bladed choppers? They balance just fine. All that's necessary is a moment balance. A little weight far from the hub is the same as a lot of weight close to the hub.

[fogger]

On every set of blades I've played with the cg has been extremely close blade to blade, usually within a few mm even. (good quality CF blades) So I just mark the cg's, bolt onto the balancer and add tape to the light blade at the cg. Easy enough and I'm sure more than adequate. The last set of Radix I did needed about 2" square of thin clear packing tape to get as close to perfect as I care to try and get.

-Fog

[sleddog7]

Not true. Terry's method is fine. But it's not necessary to have your spanwise C.G. match on your blades. Have you ever seen the one-bladed choppers? They balance just fine. All that's necessary is a moment balance. A little weight far from the hub is the same as a lot of weight close to the hub.

A one-bladed heli has a counter-weight, a C/G match is n/a because there is no spinning, even span mass.

For two or multi-bladed heads a C/G match should be done... Personally I'll stick to the way the pros do it.

[spork]

A one-bladed heli has a counter-weight

Yes, and on a two bladed heli, each blade has a counter-weight - the other blade.

... a C/G match is n/a because there is no spinning, even span mass.

I don't understand your statement.

For two or multi-bladed heads a C/G match should be done...

You're welcome to do it. It will neither hurt nor help.

Personally I'll stick to the way the pros do it.

That's fine, but you might want to check and see how the pros do it. They don't all do a C.G. match.

For the sake of argument, maybe you can describe to me technically how a C.G. match helps.

[Pinecone]

Because it just IS. :)

Actually on my smaller blades I just weight match, and have no problems.

[spork]

Actually on my smaller blades I just weight match, and have no problems.

They don't even have to weigh the same. As long as they balance on a balancer (i.e. moment balance) the heli won't know any different.

[fiveoboy01]

WHY do some insist on doing it the difficult way? Time to waste?

[sleddog7]

For the sake of argument, maybe you can describe to me technically how a C.G. match helps.

For the sake of your interest and consideration, I will describe it to you... For the sake of an argument, Nah... I'm not into debates.

Maybe Finless could give you some information on it.

[spork]

For the sake of your interest and consideration, I will describe it to you...

Alrighty then - shoot.

I suspect Finless would agree that a moment balance is every bit as effective as a mass and C.G. balance. But I'm happy to hear what he (or anyone else) has to say. So far no one has suggested any evidence or reason to the contrary.

[spork]

Duplicate - sorry.

[sleddog7]

Spork,
On the first page here The Bum pretty much gives a layman's description of the importance of CG matching.

Your version and a few others here describe static balancing. The Bum is talking about dynamic balancing which is the really the nuts and bolts of a stable, spinning mass.... plain and simple.

[fiveoboy01]

That's not correct.

The CGs do not have to be at the same location on each blade for them to balance dynamically.

[spork]

Spork,
On the first page here The Bum pretty much gives a layman's description of the importance of CG matching.

Your version and a few others here describe static balancing. The Bum is talking about dynamic balancing which is the really the nuts and bolts of a stable, spinning mass.... plain and simple.

Given my master's degree in dynamics and controls I'm reasonably familiar with static and dynamic balancing. I thought you were going to offer some explanation as to why CG matching was important, or even useful. You said you would do so for my interest, and I am interested - and waiting.

For what it's worth, the Bum didn't say anything I disagree with. Nor did he say anything that supports your position. What he said is that centripetal force is a function of mass, speed, and radius. It's given by:

Fc = mass * Vel^2 / radius

All that's necessary is that this force be balanced. This means one blade can have more mass and a shorter moment arm than the other and be in perfect static and dynamic balance.

For the sake of your interest and consideration, I will describe it to you...

As I said before - shoot.

[TheBum]

Since I started this, I'll chime in.

It is true that the CGs do not have to be the same to have balanced rotation. However, it's also true that if the CGs are not equal, the masses of the two blades cannot be equal either. So, it goes back to the fact that balancing the blades for weight and discounting CG will not necessarily yield balanced rotation.

[spork]

So, it goes back to the fact that balancing the blades for weight and discounting CG will not necessarily yield balanced rotation.

Agreed. There's no need to balance them for weight though.

[Pinecone]

I only do it because of the small blade holes don't fit my balancer. :)

It works, so I am happy. Actually come to think of it, I CoG match, then weight match, I can do that without bolting the blades together or using a balancer.

Bottom line, whatevery works.