Corona Main Rotor Balancing - RC Groups
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Feb 08, 2004, 03:34 PM
Registered User

Corona Main Rotor Balancing

ARGH! I feel like such a dope. Have I finally gotten to the age where the brain doesn't function right?

I'm at the point in construction when I'm now balancing the subrotor and mainrotor. It ain't goin' quite too well...can I please get a pointer or two. It's probably a no-brainer for Gary Morris, Matt and others, but it's got me a slight be stupped.

I set up the crutch/main rotor head as per the manual and balanced the subrotor. One blace seemed a touch heavy so a bit of tape was applied to that blade and it appeared balanced. I can spin it and it will stop dead with no back rotation in either direction. I spun it mulitple times in both directions and noted where it stopped. It would stop each time with no indication of wanting to move toward a heavy blade.

When I attach the main blades and spin it, it will always stop and find it's way to the taped subrotor blade down.

I dissassembled and when back to checking the balance of the subrotor and again it seemed(?) balanced.

Any tips on what I'm doing wrong or missing here? On first blush, it seems as thought the subrotor isn't actually balanced. I'm thinking of going back to square one and removing the tape from the subrotor and recheck it.

Don't get me wrong, the Corona is full to build, but it's nice that I shave my head to I'm not tempted to start pulling my hair LOL.

Thanks to the group.

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Feb 08, 2004, 04:23 PM
Your attitude is your alt
Gary Morris's Avatar

Balancing...a delicate act! ;)

Ok try this my friend, take off all the tape you have put on the subrotor and start from scratch. Spin the subrotor, mounted on the main shaft with the screw thru the shaft and nut in place. Spin it being sure the two links for cyclic aren't in the way. It should come to rest horizontally if it's balanced, if one blade is low add weight to the light one until the subrotor remains horizontal. Now put the blades on per the instructions, one is placed on upside down in relation to the other. Spin the rotor and again if the main blades now stop horizontally then it's balanced. If on the other hand it does not, i.e., one blade is lower add tape to the underside of the light blade until you can spin it and it will come to rest in a horizontal position. You need to be patient and take your time since this is important to keep the vibration to a minimum.
Hope this helps!

Gary Morris
Feb 08, 2004, 04:48 PM
Registered User

Back to the beginning

Thanks, Gary.

Yes that's what I'm now going to do. I even went so far as to remove the main shaft, sand it with a touch of fine grit sandpaper, and rebuilt the spacer/bevel gear stack. This is to try to eliminate any unwanted friction. I also found that the upper and lower bearing blocks can be mounted with an every so slight skew to them causing the main shaft not to rotate as freely as it could.

One question: When the subrotor and/or the main rotors are spun to determine balance, the blades will rarely stop in the horizontal position. Wouldn't it also be true that the blades are going to stop where they choose, and any evidence of a heavy blade would cause the assembly to slightly back rotate toward the heavier side?

Feb 08, 2004, 04:55 PM
Your attitude is your alt
Gary Morris's Avatar

Blade stop

If the bearing aren't binding or the shaft isn't bent it should stop horizontally everytime you spin it. Since you want both blades to weigh the same if both are the same then they will balance much like a teeder tooder. If each blade has the same weight then it's force will be equal and the blades will come to rest horizontally. If you think that your bearing are binding you can easily tell this by slowly rolling the main shaft with your fingers. If it feels rough or it binds anywhere as you turn it 360 degrees then you need to find out why.
Hope this helps! Also see my latest post about vibration for some more tips to look for.

Happy Hovering

Gary Morris
Feb 08, 2004, 05:07 PM
Registered User
Thanks again, Gary

Yeah, you're right about the horizontal positioning, now that I think about it.

I just read your post on your vibration problem. GREAT READING!!

I'll readjust the 1 1/4" dimension and "indicator wire" check the main shaft to see if there is any bend in it.

Thanks again for the help. This group is priceless for help.

Feb 08, 2004, 07:12 PM
Registered User
Karl -

I just went through this aggravation myself about 30 minutes ago. I personally found that loosening the Upper and Lower Bearning Blocks some will make the shaft spin so much looser. This allowed me to finally dial in the blades.

One other thing I read, when balancing your main blades, make sure you angle both of them up very slightly as per the manual. Once you get this, it shoud work for you as it did me.

I just powered this thing up again and it has a small vibration at the beginning, but then goes away just before hover speed.

Hope this helps.
Feb 08, 2004, 07:36 PM
Registered User
Here's a progress(?) report.

Dissassembled main shaft assembly. Sanded shaft and reassembled. VERY smooth. Using a wire feeler gauge, checked to see if shaft was bent. It was not. Reattached rotorhead. Balanced subrotor. When spun, it stops each and every time in a horizontal position. This is good! Attached main blades, one right side up, the other up side down. Balanced blades. When spun the blades stop in a horizontal position each time.

So what's the problem, you ask?

When the main blades stop in their horizontal position, the subrotor blade with the tape is alway down! When I rotate the blades 180 degrees, so the taped sub blade is up, it will rotate to the down side, with the main blades horizontal.

This can't be right!!!!

How can the subrotor be "balanced", the assembly with the main blades attached be "balanced" and yet is exhibits a "heavy side"??????????

I just cannot figure out what's not balanced?

Getting frustrated (but not giving up)

Feb 08, 2004, 08:05 PM
ab amicis auxilio parvulo
Aerospacer's Avatar

If your subrotor was truely balanced before you attached your main rotor blades, it should stay in ANY POSITION you place it, not just horizontal.

I believe the problem is that the arms perpendicular to the subrotor are out of balance. This is the feathering plate with a cutout in one side for a link to pass through. I found this needed a screw in the feather plate's end hole opposite the link to get this subrotor balanced.

Then with this balanced, I used a prop balancer with a block that I made to attach the blade grips/main rotor baldes to. I was careful to place all balance tape at the CG of the main rotor blade, not at the tip. This way the main rotor blades are moment balanced for the different diameter that the feathering plate holds them than the shorter block I used on my prop balancer.
Feb 08, 2004, 09:13 PM
A-4 nut!!
skyhawk's Avatar
I ran into the same problem you did. One problem comes when the lead and lag axis of each blade is slightly out. The easiest way to balance this is to balance your subrotor and put it on. Next you MUST balance each main rotor. No matter what heli I fly or what blades I use, I have found no two blades the same.

Take each blade by itself and find the c of g. Measure it from one end to where it balances. Next, take the second blade and find it's c of g (Doing this on your fingure is good enough). Next weigh each main blade and cut a piece of tape - put it on the light blade to make them weigh the same. Finally move that piece of tape to a position on the blade that makes it have a c of g equal to the other blade. Install blades and go fly.

This sounds complicated but it is really easy. Once you do this stick the blades on and hover it - if it doesn't shake it's fine. I have always done this and always come out very smooth.
Feb 09, 2004, 12:10 AM
Registered User
OK, heres your problema. Although the post before this is a great method to balance blades. When attaching the main blades for balancing your rotor you'll notice that when attached one up one down you are basically making these blades CG equall. I belive the position of the head on the jesus bolt makes a little difference, but also try placing the taped rotor at the top position while you 2 main blades are up 5-10 degrees from vertical. You will probably see that it stays there and your subrotor taped blade does not fall to the bottom.
Feb 09, 2004, 06:14 PM
Registered User
To all who replied -- thank you very much!! I studied all your suggestions, planned another attack at it and, lo and behold, I got the darn thing to balance. Double/trippled checked it and the results were delightfully the same.

Now it's on to further construction and waiting for the next gremlin to rear it's ugly head. LOL

The ultimate satisfaction will be when this thing gets off the ground.

Thanks again, guys, and when I run into another glitch I'll be sure to raise my hand again.