techrtr

I just finished balancing a new set of woodies for a new heli and the cg of each of the blades was really different. Used about 12, 1/2" pieces of heavy tape to get the cg right, and then another 6 pieces or so of tape to balance the blades. I'm kind of afraid to use them now and am wondering if I should just go and buy another set of blades. Have never flown this heli before so I might just use the wooden blades during set up and hover testing.

Thoughts?

Optimus110103

My Feedback: (5)

I would send them back, if they were that far off. I myself wouldn't fly them, my opinion is you got a bad set of blades. to balance my woodies took 3 pieces for cg and 4 for balance, granted that was a complete wrap around the blade.

Skarn

You really don't need to balance the blades for CG at all. Just balance using a standard method and put the tape on the tips.

But if they are REALLY out of balance I would send them back as optimus said. Other than that, unless you make them weigh a ton, as long as they balance, they should be fine.

Skarn

Optimus110103

My Feedback: (5)

balancing blade weight and cg is called dynamic balancing, and it really helps for a stable flying machine. take for example, if you take a baseball bat and sing it by the proper end, and then swing the same bat by the big end, it feels a lot lighter because of the cg, if it's closer to you it takes less torque to swing due to a basic leverage. anyone who says that rotor blade cg is not important must be misinformed.

evan-RCU

You should balance them both static and dynamic. The amount of tape you used, 6", is not that much for woodies, I've seen much worse.

Optimus110103

My Feedback: (5)

listen to him he knows better than i do fo sure

techrtr

Right-o, I'll use them for break-in and hover testing, and if they feel okay, I won't worry too much about them. I was intending to get some cf blades once I was comfortable flying the new heli.

Thanks for the help.

Skarn

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.

I got this from an interesting thread at helifreak. Check it out here: http://www.helifreak.com/showthread....&highlight=cog

After reading that thread and also Finless Bob admitting COG balancing is not necessary I tend to agree.....plus I've heard of many of the "pros" that don't COG balance on their heli's.

So I DON'T think they are misinformed!

Skarn

Optimus110103

My Feedback: (5)

apparently you can't read, I just read the thread, which btw I have read before, and I don't see a single person saying what you say, as a matter of fact, most every pilot on that thread is talking about the importance of proper dynamic balancing, and how to do it. get your techno jargen straight and start understanding what people are saying before you start giving people advice that is going to cause them to have an improperly setup machine. I'm done with this completely imature argument, and you need to do some reasearch.

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

evan-RCU

Yes very few pro's balance blades at all anymore but it's because they fly better blades, most always carbon fiber ones. I have found very few good carbon blades that need any balancing in the last 10 years.

This and the fact that the small electrics usually don't care if they are a little out of balance has caused many people to not check them at all. Is it right? NO... balance them both static and dynamic, you'll have a better flying helo.

dooleyje

Tell Them Evan!!! I balance everything, paddles, tail blades. Wow what a difference it makes.
I am also not afraid to use a little sand paper here and there. For plastic covered blades do not use sand paper on the covering.

Jim

Ecklund1

If you want to cut down on the amount of balancing tape you use, first see which blade is heavier.
Mark the CG on the heavy blade, transfer the mark to the lighter blade and balance IT with tape.
Then, balance the two blades and you'll find you have to use less balancing tape overall.
I found this out by doing it the other way and my blades looked like they belonged in the ER.

Oh, and these guys are right, balance anything that moves. (Our cat didn't like it, but I tried to balance it, too...)

Jeff[sm=thumbs_up.gif]

Skarn

LOL! Easy there fella! No need to get testy.

Did you NOT read where Spork gave the reason why it isn't necessary? And that he has a Masters in dynamics and controls? I'll be the first to admit if I'm wrong, but instead of flaming someone, why dont you post FACTS as to why it's so necessary? BTW, the text I quoted was from that thread word for word...so maybe YOU didn't read it correctly? LOL!

This post has me intrigued once again and once again I WILL apologize and admit if I'm wrong, I'm just trying to get the facts straight!

BTW, here is a thread I started with a poll on the subject. http://www.helifreak.com/showthread.php?t=59419

So far, most of the respondants DO COG balance, but as of right now there are 19 yes 8 no's......

Feel free to read it and add your input. Apparently there are others that feel it's not necessary eh? I can say that I stopped doing a COG balance and honestly cannot tell the difference....smoooth.

Skarn

Skarn

Sorry to offend anyone. Didn't mean to hijack thread.

Keep em flying!

Skarn

dooleyje

The reason to C/G the blades is for tracking purposes. I read hundreds of threads where folks bash a helicopter or blades because the blades will not track. They buy set after set of blades and say they have balanced the blades but I can tell you now that they probably did not. At least C/G wise and correctly. As in an airplane the wings have a c/g to allow the airplane track from nose heavy to tail heavy depending on the pilots flying style. Helicopter blades are wings and need to be on a constant plane with each other. Hence: Blade tracking. There is nothing more sweet than a well balanced set of blades span wise, C/G, dynamic, cord wise and just because we are wise. A set of blades may track at hover but may not be when the angle of attack of each blade increases or decreases. In an airplane if your c/g has the airplane tail heavy the controls will be super touchy. A slightly nose heavy airplane will have tamer response. Heli blades are the same and you have to find the balance of the response. We also want our blades to be a little nose heavy span wise and a little heavy at the tip for length wise with both blades the same. Blade flex will also have an effect.

Now to keep it real. Myself and EVAN have about XX years between us in this hobby with airplanes and helcopters. He's just better looking than me and a lot younger, a lot faster and can leap higher than me. We have a few tricks and experiences that we will share to anyone who will have an open mind and wants to learn.

I don't care if you paid $200 for the best chrome go faster blades on the market, Check them out. You may see where that blade flutter is coming from when you bang the pitch and cyclic.

Now I would want to check the credentials of your engineer. I have met a few and some are, just to be kind, some are real Goobers. Ha Ha

You are correct though. You do not have to check/correct your blades unless you want to do so. It just depends if you want a great flying machine or one that is constantly shaking itself to death. All you need is a pencil and some tape to check your blades.


Jim

Skarn

Thanks for the info!

Check out the thread I started on helifreak (listed in my above post). Many interesting replies....many from people that have been flying for years as well. Apparently, you can get a good dynamic balance with different cog's on blades. Also, about half the responses were people that do not cog balance them. Again, very interesting.

For me, the bottom line is due to such a disparity in agreement, and considering how simple it is to do.....I've decided why not just go ahead and do it anyway whether it is truly needed or not? We KNOW it doesn't hurt...at least everyone agreed on that one!

Thanks again,
Skarn

Sikorzky

Location of CG (COG as some say) in a pair of helicopter blades is important for proper system balance, performance, and vibration reduction. If you want to know why then read on.
It is true that centrifugal force should match, but this is not by any means the governing force in the system. Because the blades are hinged, their final positions (in the plane of the disc as seen when viewing from the top) are determined by a balanced set of forces. To make it simple you have the following forces:
1. The force supplied by the torque of the rotor shaft.
2. The force of drag (there is much involved here, but at a constant rpm and pitch this force can be reduced to a magnitude acting through a point, but this is dependant upon the orientation of the blade).
3. The force of the blades mass due to rotation about the rotor shaft axis (centripetal acceleration of the blade CG).
As the blades move, these forces will balance to determine a final position of the blade (when viewed from the top). Let’s assume the spatial geometry of each blade is identical, and so is the force applied to each blade by the rotor shaft. What is different between the blades? Let’s say their location of CG is different, which also implies an overall difference in their mass distribution. Differences in mass distribution also imply a different mass moment of inertia for each blade (think of the spinning ice skater pulling in their arms to spin faster). As the blades make a revolution around the common axis of the rotor shaft, we must remember that each blade also makes a revolution around it own CG. If each blade has a different mass moment of inertia, then the forces across the system will tend to be unbalanced to compensate. This imbalance will be achieved by changing the orientation of the blade, thus modifying its drag force point, also modifying its centrifugal force (only slightly), also modifying the line of action relative to the blade of the force supplied by the rotor shaft. Changing the orientation of the blade will change the orientation of its airfoil to the relative wind, causing more force imbalance in the system, such as one blade producing more lift than the other. This is a brief analysis of the forces acting in the plane of the disc, but far more is going on since the forces cause the blades to cone up....this is too much to consider here.

In short, balancing of the CG along the length of the blade is important, and easy to do. So is a balancing the location between leading and trailing edges of the airfoil, but this is a bit harder to do. Also important is the overall mass distribution of the blade. Blades should be balanced as well as possible to avoid ill effects such as reduced flight performance, and reduced lifespan of your rotor system, and reduced lifespan of the overall aircraft.

Blades should weigh the same and have their CG's matched. Use a good scale and determine which blade is lighter and by how much. Use the same scale and cut a single piece of tape that weighs the same as the weight difference. Put the blades in a moment balance and apply the piece of tape to the lighter blade in a position that balances the moments. Now you have matched the weights exactly and matched the CG along the length [edit:as demonstrated by phunk in the next post, this method will only work for very small cg and weight differences]. You could use other techniques to match the CG between the trailing and leading edge of the blade as well. What it boils down to is "what is overkill here, these are just models". I say, do as much as you are capable of doing, do as much as you are satisfied doing, and at the very least do something to balance your blades, because the heli will appreciate it. Hey, people fly with paint sticks, so these models are tolerant, but over time, these cyclic forces add up and break things.

This brief 2D analysis did not at all speak to the affects of cyclic pitch changes, but this is also intimately related to CG location relative to the pitch change axis. Maybe some other time.

phunk

The problem with that method is that if the blades are very close in weight but the cg is significantly off, that small piece of tape to match the weights will not be enough to move the CG, even if placed all the way at the tip.

I have a pair of blades here that are about 0.05g different, but the cg is off by 8 or 9mm. A 0.05g piece of tape, all the way at the tip, is not even close to enough to match the cg. It took about .8g of tape to move the cg to match. To do it with 0.05g would require placing the tape 16 times farther out than the tip of the blade.

Sikorzky

I agree with you phunk. There are situations where this method will work and situations where it will not work within the length of the blade. My statement although correct in math really can't work in many situations. Maybe I should revise what I said. Thanks for pointing it out.

Oliveirax

In that case, you'd put a 0.80g piece of tape on the tip of the lighter blade, and a 0.75g piece of tape on the CG of the heavier blade. Presto!