“AH - YES”! The fine art of balancing of blades. ( don’t forget to balance the pre-built blades also )
The main objective and end result is, that you want to end up with a set of blades that have the same center of gravity or cg, both length wise and cord wise and weigh the same. This way if your rotor head is balance and you install these blades you will have a shake free helicopter, or at least shake free from the main shaft out to the tips of the blades.
Build your blades to the manufactures recommendations. Don’t skimp! Don’t use damaged blades. You will end up getting hurt, or hurting some one else, not to mention it will cost you $$$$$.
The equipment I use is a High - Point Balancer, a long main shaft from Yale Hobbies, and a triangle shaped draftsman’s ruler. If you have a gram scale great. If you use other items, the end result will be the same. Its just how you get to the end will be different. Follow your manufactures recommendations.
First we have a balance rotor head right? ( with out the blades ) No, ok. Put the rotor head on the long main shaft. Lay it horizontally on the high - point balancer. The purpose of the long shaft is to counter- balance the weight of the head. If one of the paddles is heavy you will need to move either of the fly bar weights to balance the head. If you do not have fly bar weights then you can either add some or use wheel collars on the fly bar as some. You could also add some tape to the light paddle if its possible on your setup. Rotate the assembly 180 degrees and recheck. If everything is straight you should get it to balance. If you can not get it to balance on both sides, it could be due to the fly bar is bent and both paddles hang in the same direction and so its heavier towards one of the blade grips. You will have to fix this and then go on.
I use the triangle ruler to check the cg’s of the blades. You can use other things as simple as a round ball point pen, anything to let the blade free balance. I lay one of the blades on the ruler, bottom side up, and perpendicular to the ruler. Move the blade around to balance the blade. This is to check the length wise cg’s. Use a pencil or some type of erasable item to mark the cg. Then rotate the blade 45 degrees in both directions and mark these balance points. Once you are done with the first blade, do the same for the second one. Now compare these points and hopefully will be with in 1/16 of an inch. If so great. If not then we will have to fix that. Mark on the blades which way the cg’s will have to be moved so they will be within 1/16th of an inch to each other.
OK now we have a balanced head. Lets attach each blade to the rotor head. Lets do them one at a time so we can align them at the same time. Tighten the mounting bolts just snug enough so you can move the blade easily, but tight enough so it will stay. This is what I do to align them. They should lay straight out from the center of the bolt mounting hole, through the cg to the tip. The way I check this is to remove any push rods or links from the blade holders. The blade should have no lead / lag. If the blade has "lead" it will rotate forward towards the leading edge, and drop in that direction. If it has lag it will rotate towards the trailing edge and drop in that direction. If its laying straight out it will not rotate in any direction, this is what you want. Tighten the mounting bolt. Do this to the other blade, being careful not to move the other one. Another way is to attach both blades like above ( with all your links attached ) and run up the helicopter just below a hover so the blades will assume there correct lead / lag position. Then you let it spool down on its own and stop on its own. With out moving it tighten up the bolts so the blades will not move when on the high point. Remove the head again.
Now we have the blades attached to the head, and they have a good lead / lag. Put the long main shaft in the rotor head and lay it horizontally on the high - point balancer. Ok which blade falls? This of course will be the heavy blade. Now the hard part. If the cg’s were the same, just add some blade covering to the light blade ON the cg marks that you made earlier of the light blade. This will make it heavier, balance and the cg’s will still be the same. If the cg’s are off, this is where it gets hard. The light blade will need weight. Now if the light blade needs the cg moved towards the tip to make it like the other blade, then add the weight to the tip of the blade to make it balance and to move the cg towards the other cg. If it needs the cg moved towards the root, then add the weight to the root to make it balance and to move the cg towards the other cg. Rotate the assembly 180 degrees and recheck. Most likely the lead / lag was not perfect and it will now hang different. Lets say the first side was right on, and this side is about 2 inches off, measured at the tips. I balance the blades by splitting the difference. This will compensate for the lead / lag being off. If its more than this, recheck the lead / lag you set earlier, and redo the balance. When your happy hopefully the cg’s will now be with in the 1/16 of an inch. If so good, if not it gets tricky from here. So you need the cg’s moved but now they weight the same, If you add move tape to move the cg of one blade, it will become heavier. So add this tape to move the cg and then you will have to add more tape on the other blades previously marked cg line to make them weigh the same. ( so the cg's will not change ) Rebalance the assembly again.
Replace the rotor head. Attach the push rods. Give it a pre flight check and when your sure, hover the helicopter. I like to run the blades at a medium tight condition. You can either run them fully tight or loose. If they are to loose, you are more likely to have a boom strike if you hit hard. Also when spooling up the helicopter the blades will lag and there is a chance you will hit the holders or have a boom strike. This is why I like to run them in a medium condition, but that’s up to you. If you like them tight then the best thing to do is leave them after you have high - pointed them.
Now your hovering lets check for a shake. If its in the head you will probably see a low speed shake of some sorts. Its more likely to show in the skids or canopy. If you have one, land and let the blades stop. Take a small piece of tape and add it to one of the blades on the cg mark, don’t forget to clean the blade first. Run up the helicopter again and see if the shake gets better. If it does, land and add another piece of tape and see what happens. If it gets even better add another. If it gets worse you were Ok at the point you were. If the first piece of tape made it worse try it on the other blade and see what happens. If it gets better, add another. If it gets worse, you were Ok where you were. I use a piece of tape about 1/2 inch wide x 2 inches long.
There you have it. Go FLY!,FLY!
Different kinds of shakes are hard to understand and there are some that will look like one kind and end up being another.
The first type of shake is a high speed or high frequency type of shake. This will show up at either the tips of your fins or you might have a fuel tank fuel of fuel that is churning or foaming. This is mostly caused by the engine, either the mixture is off, your clutch shaft is out of tolerance or some other fast spinning part on the engines drive train.
The next type of shake will be from the head being out of balance. This will be a lower frequency type of shake. It is most noticeable in the skids or the canopy will shake. This type can be some times show up at the end of the tail boom. If its here it will usually have a up / down and side ways motion. Its really easy to get this mixed up with the tail rotor being out of balance. Another place a head shake can come from is if the 2 flybar paddles are not aligned in the same flying plane as each other. They should also be aligned to the swash plate.
The next type of shake will be the tail rotor out of balance. This is low frequency type of shake but will be most noticeable at the end of the tail boom and the tail rotor. You will see the end of the tail boom bouncing up and down with out any side ways motion, like the head shake.
This is not usually a shake but can be mixed up as one. If the helicopter oscillates in a what looks like the skids are going in a circle, this is some times cause by the dampening in the head. It will usually be that the dampening is to stiff. You can try either a different type of blades, raise the head speed, or change to a dampener that is softer.