Wobble and vibration
This is a Post at RC Universe
By Soloboss
The first step for me when I had (have) these problems is to isolate the source. You have several places where the vibration can come from.
Flybar, upper rotor, lower rotor, outer shaft, inner shaft. If anything listed is out of whack, it will wobble the shaft assembly which makes the flybar think it's out of balance (even if it's fine!). SO, try this;
First remove all of the blades and the flybar. We need to verify the shafts.
When I get a bent shaft, I use this trick to isolate the two shafts so I can determine which shaft it is.
Move the throttle stick to the far right and then add a little throttle. That will run just the upper (inner) shaft. Move the stick to full left and add a little throttle to check the outer shaft. Got it? So, how did you do? Shafts straight? I have a straightening section below. If the shafts are good, your head should spin up to about half rpm and show no wobble at all. No need to run it at full RPM because with the blades on, the rotors will never spin that fast.
On the assumption that your shafts are good, drop the flybar into the hub on top. Fold the linkage over the top and stick it down with a little tape. You don't want that link to throw the balance off. Now spin up to about half throttle - just spin the inner shaft. OK? Now spin both of them. If you suddenly have a wobble that wasn't there with either single shaft, check to be sure your bearings are seated. I changed to the aluminum bearing cups and new bearings (it's a kit) and my heli got more steady. If your flybar caused the wobble, you know what to do. If not, remove the flybar and install the LOWER blades. Spin up the lower rotor. If you get wobble, you have out of balance blades or the pitch of one of the blades of goofy. Replace (DON'T Discard) the lower blades and try a again. We've seen blades that are balanced statically (with a scale or balance) but don't have the same pitch. If you get a shake out of the lower rotor, that will cause the entire inner / outer shaft assembly to wobble a bit. That throws the flybar off-center which makes the flybar hub spin off-center, which bends the flybar rods slightly and shakes the heli. But it looks fine when you look at it stopped.
OK, Let's assume that the lower rotor is spot-on. Remove the lower blades and install the upper blades. Do they move freely? Will they rotate when you pass your hand over them creating a little air movement? If not, fix it. If they move freely, spin up the upper rotor. How's the balance? No wobble?
So we have verified both shafts, the lower blade pair (separate blades don't count - you only verify pairs), the upper blade pair, and the flybar. Now connect the flybar to the hub. Check the ball on the upper blade where the flybar link will connect. Is it perfectly smooth? Do you use needle nose pliers to push the ball through the loop on the link? If so, look for a burr from the pliers. The ball has to be perfectly smooth. Snap the link onto the ball. Everything is absolutely free and almost moves by itself, right? Now spin the upper rotor - Hang on. Let's see those safety glasses. You think you use a lot of blades now, try flying blind! OK, spin the upper rotor only - and spin it fast enough so it tries to lift the heli. Checking blade tracking at low RPM is worthless. Bad tracking shows when the blades have lift. Is your tracking right on? If not it causes shake. Fix it.
Hmmm, what have I missed? If it still shakes, look at the lower blades and spin them up - just the lowers, and put some power to it. Look at the tracking of the lower blades. Of it's off, it's the blades and they got to be replaced. The lower rotor can suffer tracking issues, but there's not much you can do about it other than swap blades. Anyhow, if you keep swapping lower blades and one of the blades keeps showing in the reject pair, you got a dud. It happens. It's a buck. Deal with it. Or boil it in water. It may return to the correct form.
If you had a bent shaft, we need to straighten the bad shaft.
If it's the outer one, I take the inner shaft out, spin just the outer shaft and very lightly touch it with a felt tip to make a mark on the 'high' side. Then I pull it out, support the shaft on the ends and thump it on the felt tip mark, and try it again. With your heli lying on its side, you don't have to assemble the heli to run the outer shaft. Just stick it through the bearings and engage the drive gear and run it again with the heli on its side. TIP: It takes a pretty good thump to bend the outer shaft. It takes very little to bend the inner shaft. Keep working at it and you’ll get it. I’ve straightened mine several times.
The inner shaft is easy. You need a flat surface (table) so you can drop the flybar hub off the edge – then roll the shaft looking for the bend. Straighten it gently. It’s really soft rod. Or get a drill rod and make your own. Use a Dremmel and cut it to length and grind the little flats on it. The drill rod won’t bend and there’s no weight penalty. You want a 0.078 diameter rod. What’s that in fractions?? You, there in the back row asleep – yes YOU!! C’mon, divide 5 by 64. The calculator shows 0.078, thus .078 is a 5/64ths drill rod. And you thought you couldn't do this.
Lots of words. That's just me. Maybe the answer to your problem lies in one of these suggestions. And of course, if anyone disagrees with my 'article', post my error. We all learn.
GOOD LUCK!!
COPYRIGHT © CX2PARTS.COM 2005 thru 2009
