RunRyder - 3+ blade balance ?

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Scale Model RC Helicopters > 3+ blade balance ?

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

How do you achieve a blanced set of blades with 3 or more blades ?

07-15-2008 04:54 PM

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eggmcmuffinplz
Key Veteran
Location: Los Angeles, CA

Use one blade as the master blade, and balance all the others from that.

07-15-2008 05:00 PM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

Sure, but what balance method ?
I want to see how many variations there are.

I have a simple method that is sure to prove controversial, but works.
I'm talking moments and want to see how many are using that method.

07-15-2008 05:08 PM

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Copter Doctor
Elite Veteran
Location: daleville/ft.rucker, al

i use a regular see saw type balancer, bolt the two blades on and keep the heaviest blade til i got them all checked. now that heaviest blade is what i balance all the rest to. some may need more tape to balance to the heaviest blade some may not. this has worked for me for years with the odd amount of blades, ie; 3,5, 7 etc.
for the even blades, i just balance the two opposing blades and go.

drive a rotary, fly a rotorcraft

07-15-2008 07:00 PM

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AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

I had in mind a spring loaded rig that held one blade horizontal by the blade root (cantilever) and the spring would be adjusted to level the blade with the heaviest "master" blade. Adjust all other blades to match that one with tape on the tip.

That way you could do it like the big boys. Adjust the replacement blade to a known master so field balance is not needed, just tracking.

It's interesting to note that (with a 2 bladed head) a 0.1gm weight offset on the tip of a 710 blade is equivalent to the main shaft being out by 0.22mm or 0.0086"

07-16-2008 06:46 PM

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sparkyman
Senior Heliman
Location: uk

...I use to be happy just matching blade weights so they're equal, but this just gets the static balance correct.

When the disc is spinning, dynamic balance comes into play and this is not only determined by blade weight, but also blade spanwise CoG AND chordwise CoG. - I'm aware that the Koll Balancer can determine both spanwise & chordwise CoG.

I'll be trying a simple method soon to see how it works out - experimenting to try & find a better balance than just pure static (equal blade weights). I intend to weigh the blades first to find the lightest and cut a piece of tape to get the light blade upto the same weight as the heavy blade. Next, balance both blades on a knife edge beam to find the spanwise CoG (tricky) and mark each blade spanwise CoG accordingly. Finally, place the cut tape, on the light blade, on the marked CoG.

In theory, both blades will now have the same weight & spanwise CoG.......(I may be wrong - the experts out there may tear my theory to shreds !!

)

Of course, you can do this with multi blade sets, always using the heaviest as weight reference.

Chordwise CoG remains unchecked....the Koll would be the next step forward for the ultimate blade balance, but I admit I don't know how the Koll works.

07-22-2008 12:20 PM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

Simply matching blade weights is NOT an answer.

You need to match moments.

To do that, do what Copter Doctor outlines above.

You can try the Koll balancer but that's a lot of work for no additional gain.

07-22-2008 02:56 PM

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sparkyman
Senior Heliman
Location: uk

Correct - Thats why I find the spanwise CoG of the blades, so the weight/moments can be matched....

...at which point, along the span of the blade, is Copter Docter adding his tape....and how much tape is he adding. ?

I read that he's adding tape to get the blades horizontal, but he could add a lot of tape at the root, or a small amount of tape at the tip and still get the same result ? The moment arm is the same for each blade, but the CoG's are different.

he could add enough tape to the light blade, so that it weighs the same as the heavier blade and the blades balance perfectly horizontal on his see-saw, but what about the CoG of each blade - how does he determine that the CoG is the same for each blade - the blades need identical CoG to have good dynamic balance, as this is where the wieght of each blade is centered. If the blades weigh the same and have the same CoG then you will have both perfect static & dynamic balance?

I see Copter Doctors method gives him good static balance, but how is he finding the spanwise CoG and where is he putting his tape ?

07-22-2008 04:30 PM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

I, and I believe Copter Doctor is putting the tape at the tip only.

I keep telling people that CG does NOT have to be the same.
I keep telling people that weight does NOT have to be the same.
Only the moment needs to be the same.
Moment is weight times distance.
That's what you get when you see-saw two blades against each other.

It really is that simple.

07-22-2008 04:51 PM

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sparkyman
Senior Heliman
Location: uk

I have to disagree.....why did Koll invent his balancer to measure both Spanwise & Chordwise Blade CoG? There is a need to know these points in order to effectively balance the blades dynamically.

The CoG is important as it is the point where the weight is centered in the rotating mass, and is therefore the point where resulting forces, due to rotation of the mass, effectively act.

If the CoG of 2 (or more) blades differ, then the forces created by the rotating masses differ - the blade with the CoG furthest out, spanwise, will have greater forces acting upon (for the same mass) because the effective mass is rotating at greater velocity than the effective mass of the other blade(s). This imbalance of forces between the blades will manifest as a vibration.

I see a 3 Blade rotor (for example) with a 1" blade weighing 1oz with a 12oz weight on the tip, a 4" blade weighing 1oz with a 3oz weight on the tip & a 12" blade weighing 1oz with a 1oz weight on the tip. All three blades have the same moment arms (13oz/in) but the CoG's are extremely dissimilar - will this 3 blade rotor rotate smoothly at 1000rpm or will it shake itself to bits ?

here's another couple of links I found last night;

http://www.helifever.com/articles/1/1/Blade-Balance

http://www.dream-models.com/eco/Blade_balancing.html

07-23-2008 10:16 AM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

Quote
why did Koll invent his balancer to measure both Spanwise & Chordwise Blade CoG? There is a need to know these points in order to effectively balance the blades dynamically.

He saw a market oportunity (people didn't understand balancing) and exploited it.

Quote
All three blades have the same moment arms (13oz/in) but the CoG's are extremely dissimilar - will this 3 blade rotor rotate smoothly at 1000rpm or will it shake itself to bits ?

YES . . . . it will run smoothly.

This is high school physics - - why are you having such a problem with it ?

07-23-2008 03:18 PM

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sparkyman
Senior Heliman
Location: uk

..just having a problem with your statement that CoG doesn't have to be the same

The picture shows that the guy has put a mass, no doubt equal to the mass of the opposing blade, on an arm of the correct length to ensure that the CoG of the mass on the arm is the same as the CoG of the blade.....

Maybe your definition of Moment Arm is my definition of CoG....I'll see what I can find - we could be talking about the same thing ?

07-23-2008 04:13 PM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

Quote
The picture shows that the guy has put a mass, no doubt equal to the mass of the opposing blade, on an arm of the correct length to ensure that the CoG of the mass on the arm is the same as the CoG of the blade.....

No, the masses are NOT equal and neither are the CG distances.
- - - but the moments are equal.

Why do I feel like getting out the message on blade balance is more of a crusade than a simple statement of physics ?

07-23-2008 04:29 PM

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Scale Model RC Helicopters > 3+ blade balance ?

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