Big blade weight imbalance

Hi! Hope someone will be kind enough to help me with this.
Just finished assembling my Raptor 30 V2 kit.
Prior to mounting the blades, I decided to check their static balance (like the Manual says...).
I was quite surprised to verify that they presented a relatively big difference in weight: nothing less than 5 grams!!! (I'm lucky to own a precision balance)
Thunder Tiger says (in the manual) that they strive to make them balanced, out of the factory. I wonder if 5 grams is considered acceptable.......
By my calculations, a 5g imbalance in the blades would account for lateral forces in excess of 5Kg at typical rpm (>=1500).
I'm naturaly very reluctant about trying to fly the heli in such circumstances!
The remedy the manual gives is to add tape at the tip of the lighter blade... but I guess that would take quite a length of tape!
Obviously, impractical for this weight difference.
I'm considering the possibility of embedding some lead weights (approx. 2.7g) at the tip of the blade, securing them with epoxy.
This, ofcourse, would offset the center of gravity. I'm not sure about the importance of maintaining the c.o.g. of the blade...
(by the way, the c.o.g of both blades are coincident... almost perfectly)
From the point of view of balancing centrifugal forces alone, this doesn't present a problem.
If I add the weight at the c.o.g, it would take almost twice the weight required... plus, I fear to disturb the structural strength of the blade.
The c.o.g question makes me wonder... because in every website I checked info abt blade balancing, they always say that maintaining the c.o.g is of capital importance.
They don't explain why, though... btw, they all prescribe the tape remedy...so I assume it's NOT NORMAL to have such a weight difference..
Other than that...I'm considering returning the blades...
So, please, has anyone experienced such a problem? And can anyone suggest a solution?

Thanks...

Louis

[Daz(DJLFlapper)]

Hi,

I have had this problem and actualy butchered some blades to get them sorted. The tape if used needs to be applied over the centre of gravity point. The problem I had when I did this was that when finaly I got the weights level. I had a depth of about 1-2mm of tape on the blade that was out.
I actualy used this and while it performed ok, after the flight the tape had unserprisingly started to move toward the end of the blade from the top. send them back and get them changed. There should be no comebacks if they really are that far out.

I did cut mine in the balsa area after and used car body filler at the centre of gravity point to gain extra weight. This did actualy work and When the blades were recovered they flew very well. Of course they needed re balancing and where only just out so it was actualy quite a good outcome. but I would only suggest doing this if you are confident with a file because you don't want to take out more balsa than you need to get the balance sorted and file the filler down not sand. this will leave the filler flush and when recovered you wont know you done anything.

If this seems too much like hard work or you can get them changed with less expense then do that.

Just to confirm the C of G is very important in the resonse of the helicopter to the inputs you give if the C of G is off it can make the heli unstable. one blade will have a wider CofG ark and try to pull the heli over causing a very bad wobble. So any weight you need to add must be at the CofG, When I finished mine the New CofG was only 1mm different. I had no problems in flight.

hope this helps,
Regards,
Daz

I begin by thanking everyone who helped me sort out this issue, very specially Daz(DJLFlapper)! Thanks Darren!

I hope the following lines help to clarify the subject of how the position of the rotor blades center-of-gravity impacts the stability of a helicopter, from the vibrational point-of-view... and not from the control perspective (that'll be for later on...;-)), since there is so little on the web - strangely - about this subject.
This is, by no means, substitute for good literature on the matter. I don't pretend to be an authority on it, either... for goodness sake, I'm but a beginner on this great hobby. This is just my humble attempt at sharing the little that I learned on my quest for answers of how things work, in the hope that it'll be of some use to somebody. I also, probably, incurred in some incorrections, imprecisions and over-simplifications... For that, I apologize, in advance. My aim is solely to explain things as simple as I can?. (enough excuses...)

When I posted my original message on this forum, I was puzzled by the fact that every page I came across about blade balancing methods pointed out the importance of achieving perfectly symmetrical CGs in both blades. None of them, unfortunately, mentioned WHY this is so important.
My reasoning was: the centrifugal force depends, linearly, on the mass and the distance from the CG to the axis of rotation. Ok, let?s assume this rather unrealistic example: One blade weighs 1Kg and its CG is 10cm from the rotor axis; the other weighs 100g (10x less) and its CG is 1 meter from the axis (10x more). Notwithstanding this huge difference in weight and CG, they would, at first glance, balance? since the mutual opposing centrifugal forces would be the same (better said, symmetrical), thus canceling each other. The heli wouldn?t ?shake?! Why isn?t that so in the real world?

Well? I already knew about the necessity for the blades to flap up and down to counteract the dissymmetry of lift in forward flight (another interesting subject, altogether). I just failed to take this into account while trying to find an answer to the CG problem.
My brain was trapped, somehow, on thinking about the rotating blades as a disk... in fact, if that were the case (i.e. a rigid rotor head - no flapping blades) my initial assumption would be correct: it wouldn't matter if the CGs were on different positions (actually, what matters is the center-of-gyration...and not the CG itself... but, since they are linearly inter-related, it doesn't matter for the current purpose). At zero pitch, a heli with flapping blades (the type we actually fly), in fact, wouldn't also shake.
But, as soon some collective is applied, the blades will ?cone up? and this is where things start to get a bit rough......
What happens is that the coning angle of the blades with different CGs wouldn't be the same. The blade whose CG was closer to the root would rise more than the other. It doesn't take knowledge of advanced physics to understand that... it's enough if you understand how a lever works: the center-of-lift of both blades is the same, since they have the same shape; the lift force would rise both blades until an equilibrium is reached with the centrifugal force pulling away horizontally at the CG. It?s like lifting a wheelbarrow. The closer the weight gets to your hands, the harder it is to lift, and vice-versa.
O course, once a blade tilts up, the CG also gets closer to the rotation axis, decreasing the centrifugal force. Consequently, the blade rises even further!
The first obvious undesired effect of this is that the blades wouldn't track! They may be generating the same amount of lift but it's now impossible to tell this by simply checking, visually, if they follow the same path!
Secondly, both CGs would now be in complete imbalance, vertically and horizontally: the blade whose CG is closer to the root would have it's CG in a higher position and, horizontally, it would be closer to the rotor axel! So, I wouldn't like to be a passenger on such a heli! I would get a bad case of ?the shakes?....
Thirdly (yes...it gets worse), a more unexpected thing happens: Enter the Coriolis Effect! Yep, the same effect that makes the water twirl when going down the drain on a bathtub... and makes cyclones and tornadoes, etc....
Well, cutting it short, Coriolis Effect will cause the blades to lead/lag in different amounts.... more shakes!

And I rest my case? for now. I would encourage anyone who got up to this point (not many people, I figure? ;-)) to further complement this rather imperfect and simplistic explanation by taking a look at the links below. These pages I dug up on the web are where I -- finally -- got my inquiry mind reasonably satisfied:
http://www.digital-fusion.ca/sbr/baldedesign.htm - Excellent! The best!
The other technical section on the same site are also worth taking a look at: http://www.digital-fusion.ca/sbr/TechMan1.htm
Another excellent site (which, in fact, I came across before my blade problem) is this one:
http://www.bartolo.worldonline.co.u...0-%20910&11.htm
It explains the details about what causes lead/lag... the center-of-gyration vs. center-of-gravity....etc...etc... also worthwhile reading.
It pays also to read the other sections: http://www.bartolo.worldonline.co.u...%20Mill-1&2.htm

Summing up: Who said helicopters were simple??!! The sheer amount of effects and counter-effects is just staggering! That's why it amazes me that these things really can fly!!! (and I bet I've just scratched the surface)
Before I joined the hobby, I thought helicopters were a "done deal". Easy: rotory wings to provide the uplift and tail rotor to counteract the torque... easy. HOW I WAS WRONG!
With what I know today, I'm absolutely amazed at the fact that it's possible to fly the damn thing in a controlled manner!

Have Fun! (?and learn a bit in the process?)

a little extra info

i found that the best balancer is the miniature aircraft
colrotor pro.
you can check the span wise and the cord wise balance points of the blades,this way once you've found the amount of weight neede to corect,you have the exact position where to locate the extra weight.it's a long process but if perfect blades is what you want this it the way to go

steff

Thanks for sharing the tip, Steff.
I'm very willing to give a look at the device you recommended.
Could you tell where on the net one could get more info on it and, eventually, order it online?

Thanks again,

Louis

you can see what it looks like at rick's rc heli www.ronlund.com
go to online store,,,,,,tools,,,,,balancing
not much info here but if you go to miniatureaircraft
they would surly have lots of imput
will check myself to see,,actualy caled koll rotor pro

steff

I'm checkin'it right now...
Thanks again

For anyone interested. Here's the picture of the Koll Balancer Tool:
http://www.ronlund.com/images/ma/ma0514.jpg
A copy of its manual can be found at Al Coelho's website:
http://www.rchelibase.com/tools/koll.pdf
(link posted by Al somewhere else on this board)