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HelicopterBlades and Night Flying › Why static and dynamic
12-16-2003 12:45 AM  14 years agoPost 1
helicopter34

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New Jersey, exit 82

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The other night I got to thinking. Why both. All you need for a balanced rotor is that the centripetal (or centrifugal I forget which) of one blade is equal to the other blade. Well if you look at a little mass element on blade dM, the centripetal force on it dF=dM(V)^2/x (for simplicity lets say the blade is sitting on the x-axis and the tip is at x=Length of blade and the grips are at x=0). We if you have an angular velocity w=rpm then the velocity of one of these dM is just V=x*w*2*Pi. Plug this into the original equation dF=dM(x*w*2*Pi)^2 /x or dF=dM(w*2*Pi)^2*x
and if you assume that both blade spin at the same rpm (reasonable) then it is constant for both blades so after we place all the constants out front and cancle them on both the left blade and right blade integral, the integral of the dF of the left blade has to equal the integral of the dF on the right blade
integral of(x*dM)left=integral of (x*dM)right so in other words as long as these integrals are the same, the blades will balance.

Well one simulation of testing this integral of use another physical situation the uses the same integral, the moment of a dM.
If you bolt both blades to a balance with the folcrum at the center of the block they are bolted to, then the blades will balance if the total moment of the right blade equals the total moment of the left blade. So the moment on a dM is the force applied to the dM times the distace (x) from the folcrum. The force is g*dM and x so the total moment is the g*integral (x*dM)
so if the left blade's integral equals the right blades integral then it will balance on this seasaw. But if the blades balance on this seasaw then they have satisfied the same integral that they needed to satisfy for the centripetal force integral and they should balance when spun.

So why isn't just balancing the blades on a balance like that (dynamic balancing i think its called) sufficient to ensure balanced blades in rotation. Why is static needed. Are there more factors I have incorporated.

Make it idiot-proof, and someone will make a better idiot

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12-16-2003 12:41 PM  14 years agoPost 2
Steve Campbell

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Baton Rouge, LA

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If the chordwise CGs are off, each blade will react differently to aerodynamic forces as the blade moves about the feathering axis.

I don't know the math, nor can I give you a "scientific" explanation; but I know, from practical experience, that if you have mismatched CGs, chordwise, spanwise, or both, your head will shake. To what degree depends upon how far off the CGs are, head speed, and the mass of the blade.

It is a similar phenomenon you encounter when a plank is tail or nose heavy; it doesn't fly worth a crap. A good rule of thumb on plank wings is to have the CG at about 30% of the chord, as measured from the leading edge of the wing. There is a like figure for rotor blades, although I'm not sure what it is.

Bottom line, if a blade/s CG is very far from this ideal chordwise location, you will have flutter and vibration problems. This is why some blades flutter on certain machines, and others don't. Combine it with a design flaw, such as a pitch moment on the trailing edge of the blade grip, and you're guaranteed headaches.

Proper blade balancing, like setting up a tail rotor with a non-HH gyro, is becoming a lost art. We have become spoiled , and when those fancy, expensive carbon-fiber blades that were rolled between the thighs of a Cuban virgin, etc., etc., are off, you must correct the out-of-balance condition or put up with the vibration.

I might be flayed for saying this (and flail away to your heart's content), but truth is truth; bolting on a set of blades right out of the package is like breaking in an engine in the helicopter. You might get away with it; and you might not...

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12-16-2003 01:31 PM  14 years agoPost 3
Doug

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Port Saint Luice Florida....

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The key thing to remember is that the blades do not remain "in plane" during flight if the CM is not the same the result is the shake generated by the "out of plane" moment.

First member of Member of Bearings Anonymous

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02-16-2004 02:08 AM  13 years agoPost 4
RaptorDude

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Madeira Island, Portugal

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Very well put, helicopter34!!! Nice question!
Finally I find someone who poses the exact some question I came across when I started thinking on this subject!

Below, I'll reproduce a post I made quite a while ago on RR (http://www.runryder.com/helicopter/t64133p1/).

I sincerely hope this will be of some help to you.
If you feel like discussing a little bit more on this subject, feel free to drop me a line by e-mail.

««
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…)

Louis
»»

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02-16-2004 02:47 AM  13 years agoPost 5
kc2huv

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Cornwall, NY

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Bottem line: is it safe to assume that if the blade mass is equal and the CG is at the same point on each, I should be ok?

Thanks,

EVO 50 Futaba 9CHP (50 MHz) Futaba GY-401/9253 O.S. Max 50 SXh

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02-16-2004 04:43 AM  13 years agoPost 6
Helipilot01

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ColliervilleTn

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Blade balance.

Awesome subject!!

I wrote an article on this subject that appeared in Model Helicopter Technique a couple issues ago. This article talked about blade design and how blade weight,legnth,cord wise and span wise CG's and bolt hole location effect a blades performance. In the next issue to be released ,Issue 39 I believe,I follow up with how to balance blades using the Koll Rotor Pro.

On the commercial jet engines that I balance,the difference between blades can only be 1/10 of 1 percent between all the blades. This is for STATIC balance. Even with this near perfect match of blades,the entire assembly is then run at rated power rpms and DYNAMICLY balanced. In every case,a lower imbalance ( vibration ) is acheived by dynamic balance.

At this time,all we have in the hobby is static balancers. Maybe in time,a dynamic balancing system will be made for our us in the hobby.

MIke Fortune
Fortune Model Products
Team JR
MHT Technical Editor.

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02-16-2004 12:48 PM  13 years agoPost 7
Steve Campbell

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Baton Rouge, LA

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kc2huv,

Yes; as long as you're matching BOTH CG's (spanwise and chordwise).

The easy way to do this is balance the blade at a 45 degree angle across a straight edge, draw a light pencil line across the blade in line with the straight edge, then do it the other way. You should now have a "X" drawn on the blade's upper surface. Where those two line cross is the center of gravity, and it is easy to measure the exact location.

All you need to do now is get that point to the same spot on the other blade. The heavy blade is the master blade; you add weight to the light blade as needed to move the CG of the desired point and match the weight of the heavy blade.

This is why, if a set of blades are off (different from one another) by more than a gram, I won't fool with them. That much weight is a LOT of tape. DC and Ray H used smashed lead fishing sinkers, CA'ed to the lower blade surface, for years with great success. I just cannot bring myself to do that...

.

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02-16-2004 09:41 PM  13 years agoPost 8
RaptorDude

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Madeira Island, Portugal

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The links in my previous post don’t seem to work anymore…
Sorry about that.
The digital-fusion.ca domain seems not to be available anymore.
The other two links are still working, but were ruined upon copying&pasting them.
Please refer to my original posting in http://www.runryder.com/helicopter/t64133p1/.

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02-16-2004 09:50 PM  13 years agoPost 9
RaptorDude

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Madeira Island, Portugal

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Nice piece of advice, Steve.

I agree width Steve: if the mass difference amounts as much as a gram… firstly, I wouldn’t even consider to attach them to the rotor grips… and I wouldn’t try to balance them with tape…. that’s IMHO, at least.
The original woodies that came with my R30V2 kit had a weight imbalance of 5 grams!!!! That’s way too much!
I was considering adding some lead weights and securing them with epoxy…. but I soon abandoned the idea… I was afraid of compromising the strength of the blades… plus… if the weights would happen to come off, they would be as dangerous as a stray bullet… not a nice thought…. I’m not saying it can’t be done safely… but, IMO, it’s not worth the risk…
So I decided to go along with the advice I got from Al Magaloff in these forums, and buy a “cheap” set of fibreglass blades…. actually, I happened to go for a carbon set… I bought a pair of SAB550…they were only marginally more expensive than the glass version and I figured I could accommodate them into my budget…
SAB blades are renowned for their quality and came highly recommended.
I have nothing but good things to say about these blades. Build quality is top notch. They have a polyurethane foam core and are perfectly finished in white gel coat (except for a narrow band near the root, which is covered in clear gel coat, revealing the carbon fiber weave inside….cool! ).
(I don’t think these are rolled between the thighs of a Cuban virgin, Steve…hehe… They’re made in Italy… so maybe they’re rolled between the thighs of an Italian virgin… but I guess those are so rare nowadays that, if that was the case, they would cost more than a box of “Monte Cristos”… hehe… sorry… just an “innocent” joke… I don’t mean any bad thing about Italian girls….)
Anyway, they balanced perfectly right out of the box. I weighted them on my 0.01 resolution scale (I happen to have one of those in my chem. lab) and they matched to the tenth of a gram. Dynamically, they tracked perfectly also. And my Rappy looks steady as a rock, even with high rpm…
(BTW, I came to know about their insides in the worst manner: I crashed… … I’ve already ordered two more pairs )
Bottomline is: I lost my faith on wooden blades… I’m sure there are lots of guys who use them with no problem… after all, they’re cheap…and they come already with most kits… but, IMHO, it’s almost impossible to achieve a good manufacturing consistency with a material like wood… plus, they have a nasty tendency to absorb water, which worsens even more the balance problem…. and, from what I read in some posts, the heatshrink cover that comes with them tends to deteriorate too often…
Sure they’re cheap… but if they cause your heli to vibrate, they will become expensive in the long run….
I’d like to read some other opinions on this subject but, as far as I’m concerned, (and this may prove controversial) wooden blades are simply not trustworthy and should be considered obsolete…
I’m not saying that’s an exclusive problem of woodies… I haven’t that much experience in this hobby… probably there are some bad quality composite blades out there… IMO, it’s always a good idea to check the blades for balance before trying to fly with them…

Have fun.

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02-16-2004 09:55 PM  13 years agoPost 10
RaptorDude

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Madeira Island, Portugal

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Yeah… I agree with Helipilot01…it IS an awesome subject. I sure would like to read your article but I’m not fortunate enough to be a subscriber of Model Helicopter Technique. I’m considering a subscription. Your job must be fascinating. I guess blade balancing must be a very critical aspect of jet engine building, at the working rpm of those machines….(I don’t know, but they must be in the order of tens of thousands rpm, right?). At that rotation, a tenth of a gram might generate centripetal forces of several kilograms, I guess… fortunately, compressor/turbine blades are not build to flap up&down like in helos… that must simplify matters quite a bit…since the CoG position becomes less important… I maybe too simplistic about this… so, please correct me if you can…
It would also be interesting if someone came up with some practical tool for dynamically balancing heli blades… I, myself, had a couple of ideas a while ago… one was purely mechanical… other involved piezo pressure sensors and an interface to a PC, for data collection and analysis… I even considered pursuing the construction of these devices… but I soon abandoned the idea… I think it wouldn’t justify the resources… today, I think it’s possible to achieve good enough results (statically & dynamically) without the use of expensive or sophisticated apparatus…

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02-16-2004 10:30 PM  13 years agoPost 11
RaptorDude

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Madeira Island, Portugal

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My 2 cents on blade balancing

I'll try to veer this discussion into a more pragmatical orientation, rather than a purely theoretical approach. As much as I would love to embark on a physics/technology discussion, I think it would be better to the general RunRyder user to keep it as simple and practical as possible.
Hope it's of some help.

I guess some of the most fundamental and frequent questions us heli-addicts usually pose are:

1 – How important/critical is statically balancing the blades?

I guess we all agree that this is the easiest an less controversial question to answer to. In fact, it’s a typical “newbie” question. Every experienced heli flyer knows that unbalanced rotor blades cause vibration… and a “vibrating heli” not only looks bad… IT IS BAD… vibration ruins the performance of piezo gyros and, worse, can cause material fatigue, over time…parts can break (both plastic and metal) links, bolts and nuts can get loose…electronics may fail… and all this can lead, ultimately, to a catastrophic failure, with all its potential nasty consequences… if we’re lucky, only the aircraft gets damaged… if not, someone can get very badly injured… so, the answer should be: EXTREMELY IMPORTANT.

2 – How much imbalance is acceptable/tolerable?

Now… this one is a bit more difficult to answer… Ideally, blades should be perfectly balanced so that no vibration whatsoever originates at the rotor assembly. Well, this would be in an utopical world. In the real world this never happens. But one should do whatever one cans to get as close as possible to this. I read somewhere the following rule of thumb: a weight difference of 1 gram at the Center of Gravity of a typical rotor blade set will amount to circa 1 Kg of centripetal force imbalance. This means a force of 1Kg pulling/pushing the heli sideways at each rotor revolution!!! Likewise, if the blades weigh the same but their respective CoG differs in 1 inch spanwise (along the main blade axis), you get 1Kg side force too. If you compare this with the typical weight of a heli (maybe between 3 or 4 Kg), that just gives you an idea of how important and critical it is.
I went on and did some simple calculations to validate this (it’s only basic physics: I’m not going to throw a formula at you face…but its roughly the mass times the radius times the angular speed) and I tell you that, for a typical setup (rotor diameter and rpm), this is roughly accurate.

3 – Which is most important: mass or CoG?

Well… BOTH!!! Actually, we cannot consider mass as a separate factor. You CANNOT use a scale to evaluate blade balance! Where the mass is is equally important (i.e. the Center of Mass/Gravity or the abstract point where you can imagine the mass is concentrated… pardon the simplistic definition).
Actually, if I were to be strictly and scientifically correct, I should be talking about the “Center of Gyration”, since a blade actually is rotating when it’s doing what’s supposed to do….and the mass close to the tip contributes more to the centripetal force than the part closer to the root… the Center of Gyration is, hence, a dynamic concept… they are 2 different points: they don’t coincide…
But, since I’m trying to keep things as simple as I can (honest! I’m not trying to post a whole compendium…) and we’re talking about static balancing and… they’re closely interconnected… I’ll just refer to the CoG….
Well, getting back to the central subject, the question should, maybe, be rephrased as: “Which is more critical?”
Well… since a difference in weight of 1g is roughly equivalent to a difference of 1” in the spanwise position of the CoG, I’m inclined to say that, IMO, the weight is more critical than the CoG (I know I’m exposing myself on this point…).
Plus, I think that the question of the spanwise (mark this word…I’ll refer to the chordwise later) position of the CoG is a bit over-rated… (I’m always open to some discussion…).
Let me explain why I say so:
The spanwise CoG only reveals its importance when the blades “cone-up” or “flap-up”. You can refer to my previous post where I explain why is it so…
And the more the blades cone-up, the greater the effect of CoG position dissymmetry.
Actually, what counts is that as the blades flap up/down, their centers of mass, gyration and lift get closer to the rotation axis…and that’s what causes the undesired effects.
Well, and how much coning happens in our typical model helicopter?
Well, really not that much… in a real scale helicopter, it might amount to 5 or 10 degrees (I don’t know for sure…)… but, in our small-scale aircraft I figure it’s much less… so, I’m assuming this effect is sufficiently negligible.
Basically, the coning angle depends on the following:
a) the weight (mass) of the blades -> the greater it is, the larger the centripetal force, thus, the smaller the angle;
b) the rotor speed -> the more, the smaller the coning (again the centripetal force will be greater);
c) the lift force (equals the heli weight on hover…) -> more means more coning (this is what causes the coning, anyway);
d) the position of the force centers (center of lift, center of mass, center of gyration…) on the blade -> the farther the center of lift is from the rotor center, in relation to the other centers, the greater the coning angle;
e) the mechanical rigidity (or the elastic modulus) of the ensemble: blades, grips and flapping mechanism -> it works basically like a spring: more stiff equals less coning;
(...hope I didn't forget something... )
Regarding this last item, we have to know the type of flapping mechanism of our heli. If it’s of the “flapping hinge” type (like in the Concept models), there will be potentially more coning. In this type, each blade has the liberty of flapping independently from the other. Each can rotate about it’s own flapping hinge, which is basically a pivot point located on the grip. That’s the system most real-scale helis have.
Now… in other models – like the Raptor I own – the mechanism is different: both blades are solidly connected to a rather rigid shaft (called the feathering spindle or shaft). This shaft can rotate about a pivot point in the center of the rotor hub. Movement is restricted/dampened by two rubbers…
Now, mechanically, this system does not provide any liberty for coning. If one blade goes up, the other comes down. Simple as that…. if materials were perfectly rigid, no coning whatsoever would happen… of course, this is not the case. The shaft flexes (but not much, I assume), the grips also flex (in spite the reinforcing ridge) and, ultimately, the blades themselves also flex a bit. Although I suspect not much also: I’m using carbon blades which are very rigid by nature (CF reinforced composites are - if not the most - one of the more rigid man-made materials).
Plus, all this rotates much faster that a real-scale heli, with immense centripetal force pulling the blades away…
I must tell you that, from experience, in real flight it’s difficult to detect any coning at all in my Rappy with CF blades, with bare eye observation.

So, all I’m saying amounts to this: if you can statically balance a pair of blades by placing a tape at the tip of one of them, then, by all means, place it there. (I can sense some eyebrows being risen )
First, by placing it at the tip, you’ll need roughly half the amount of tape if you were to put it at the Center of Mass.
Secondly, it´s faster and simpler. Forget about finding the CoG.
In fact, placing a bit of tape at the tip won’t disturb the position of the CoG in any significant amount. I’ll bet the displacement of the CoG will be in the order of magnitude of the error you’ll commit when finding it by the “razor edge method” (just speculating…).
By the way, this is what my Raptor manual recommends: tape the tips (manuals are not always right…but that doesn’t mean they should be underestimaded).

Now… what about the “chordwise” position of the CoG (that means the position in regard to the edges – leading and trailing).
Well, in this respect I’m inclined to agree with Steve.
If there’s enough difference between the two blades chordwise CoG position then, we might get some bad vibes…
Why? Because this will impact the lead/lag angle.
That is the angle that the blades get from rotating about the bolt that secures them to the grips… this liberty of movement is essential to the dynamic behaviour and controllability of the aircraft.
When the blades are rotating, they tend to align themselves by their CoG in such a way that, if you draw a line between the two, it should, pass exactly at the center of rotation. Now, ideally, this should be applicable to the centers of lift. That is, if you draw a line joining the two, it should pass through the center of rotation. If the CoG of the blades is symmetrical and so is the lift (we’re supposing they have exactly the same shape) then we can say that the lift is balanced and the heli will be stable.
But, if the chordwise CoG of one blade is closer to the leading edge than the other, then this blade will “lag” more than the other - i.e. will “get behind”. Vibration then comes into place. That’s because the line between the 2 CoGs still goes through the rotation axis but, the line that joins the centers of lift will no longer…. another way of putting this is that the combined center of lift of both blades no longer coincides with the axis of rotation…. in fact, it revolves around it with every rotation…and that’s what causes the vibration.
The fact that the center of lift is located closer to the tip than the CoG exaggerates this effect even more.
OK… so I say the chordwise CoG is more critical than the spanwise CoG. So, what can one do to remedy a pair of blades with unmatched chordwise CoGs?
Well… it’s not that easy… at least I don’t see how you can cure that easily with some masking tape… you have to judicially calculate and place weights in one or both blades to correct it….
So, if you own a pair of blades that suffers from a considerable dissymmetry of chordwise CoGs… I think you might consider returning them or discarding them…
Anyway…if that’s the case, it reveals a great lack of craftsmanship and quality control on the part of the manufacturer… IMHO, anyway…

4 – Do one needs any sophisticaded/expensive tools to achieve a good balance?

Well, of course, my opinion on this matter is…just that: my opinion. It’s debatable and I even might change it if confronted with some valid arguments.
Helipilot01 mentions the Koll Rotor Pro. He’s obviously very knowledgeable on the matter (I’m sure more than I). After all, he’s the pro
This tool is highly reputed and I don’t doubt that it’s very good. It has been recommended to me by an experienced pilot. And I’ve seen it being recommended in forums more than once. And I seriously considered ordering one when I was confronted with that imbalance in my stock woodies. (at that time, I even thought it was a normal situation to have such an imbalance).
But, what I wonder is: in this day and age, when there are increasingly good quality blades available (I’m excluding wooden ones) with reasonable price tags, do we need another expensive tool in this already expensive hobby? Well, unless you crash your heli every other flight, you might just spend that money on a good set of blades…
This is what I did and what I can recommend from my rather limited experience. I didn’t invent the method. I saw it on a web page…I can’t recall the URL right now…
I just joined both blades with a bolt and nut through their holes and supported them between two glasses, with the bolt hanging on the rims of the glasses…
If you do it very carefully, you can correct imbalances of much less than a 0.1g, I assure you!
The key is to find the correct angle between the two blades… it has to be the shallowest possible. The thing is that the Center of Gravity of both blades must be positioned just below (and I mean “just”… a mm or less) the rim of the glasses. You do that by carefully adjusting the angle between blades (in practice, you’re building sort of a scale with them). In that way, the slightest imbalance will reveal itself. If you manage to do it properly and balance a set of blades and if you add just 10mm (1/2 inch) of tape to the tip of one of them, you should see them tilting towards the side that has the extra tape! And if you have a precision balance and weigh that piece of tape, it’ll hardly be as much as 0.1g! That’s enough precision IMO.
You should do that with glasses (made of glass! not plastic!) and in a perfectly level working surface, of course. The reason to use glasses has to do with the fact that glass, being very rigid, provides a very low drag pivoting point.
I’ve seen some recommend to suspend the blades from a wire… but the glasses seemed to work fine enough….
Of course, if you want the chordwise CG of you blades with a decent precision, then I guess the Koll Rotor Pro will be a valuable tool… I guess it’ll be better than the “razor blade” method… but, then again, is this situation the rule when we consider reasonable quality blades?

I’m sure some of the things I said will prove controversial. I’m looking forward to an interesting and enlightening discussion…

Well… I seem to have far exceeded the two cents…
Sorry abt that

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02-16-2004 11:37 PM  13 years agoPost 12
jb_turner

rrElite Veteran

USA

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Whislt you all are discussing the mathmatics on blade balancing I just bolt on carbon blades and fly....

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02-16-2004 11:45 PM  13 years agoPost 13
RaptorDude

rrApprentice

Madeira Island, Portugal

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Hehehe... good point
I guess that's the ultimate purpose of it all...
But whilst one is grounded, it doesn't hurt to tickle one's brain...

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02-17-2004 02:21 AM  13 years agoPost 14
Helipilot01

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ColliervilleTn

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The Koll rotor pro has many merits but certainly one of the most important is that you can match cordwise CG. This is very often over looked as being important,but if your performing 3D areobatics,you might want to consider giving this another look. Here's why....In hover with the blade weight and span wise cg matched but the cord wise cg is not,the machine can tolerate and absorb some of this imbalance. Just t like the other great example described by Raptordude explaining what happens when the blades flap and the span wise cg moves closer to the center ( Mainshaft ) so it goes when the blades experience large changes in pitch during aerobatics. Pitch =lift and lift =drag and drag is just a nice word for resistance. When the blade is changing pitch,it is moving back and forth on the blade bolt. The larger the pitch change,the larger the blade movement. If your still not sure of this,pull the blade bolt out and you'll see it's polished where the blade bushing contacts the bolt. Now,combine this imbalance with the imbalance from unmatched spanwise cg and you can see now the blade is way out of it's optimum position. This will almost always occur at a very unwelcome time!!

For my machines,I balance my blades as close to perfect as I can get them. It's cheap insurance.

Great discussion...Keep it up!

Mike Fortune
Fortune Model Products
Team JR
MHT technical Editor.

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02-17-2004 12:23 PM  13 years agoPost 15
RaptorDude

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Madeira Island, Portugal

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Thanks alexander!

Thank you for the updated links!
I truly appreciate it! And so should everyone who’s interested.
They are really good resources. I felt really disappointed when I found out they no longer worked…
I again encourage everyone to check them out.

I try to keep my postings short as I can. Honest! But the subject is so vast and intricate that it becomes very difficult to summarize without compromising the understandability.
I really just barely scratched the surface of this matter…. believe me…
It’s not that the physics involved are very complicated. It’s just that the effects involved are rather numerous.
That’s why it’s very important to check out those links. The information therein is much more comprehensive (although, by no means complete). And it’s illustrated… so it becomes much more clear to visualize and understand.

I perfectly understand the spirit of jb_turner when he says: “I just bolt on carbon blades and fly”…
But as much as many of us would like this hobby to be “plug’n’play”, we all know it’s not really so…
I believe it’s important to know at least some basic principles. I think it helps us to fly better and, no less important, to keep our machines in shape. And this, in turn, can keep our hobby safe and, maybe, a bit more economic… (if we can apply the word )
Also, I believe that knowing the theory behind helicopter flight can actually enhance the pleasure we take from the hobby.

I’ve learned so much from the experience and knowledge from the guys here at RunRyder… I feel obligated to retribute… I’m not very experienced but I feel I should share the little I already know…

Have fun… but be safe…

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02-17-2004 03:54 PM  13 years agoPost 16
RaptorDude

rrApprentice

Madeira Island, Portugal

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About the Koll Rotor Pro and the chordwise CG

Thanks for your valuable contribution, Mike!

I sure share your view about the importance of checking the CoG, specially its chordwise position. And I’m pretty convinced that the Koll Rotor Pro is, probably, the best device available for the precise determination of this (Mike, I don’t know if you had the necessary resilience to read through my posting, by I do say that near the end).
Of course, if one owns such a nice tool it’s only natural to put it to use before installing a new pair of blades.
What I don’t know is exactly what can be done to remedy the situation if one encounters a discrepancy between the chordwise CG of a pair of blades.
Using tape, for instance, doesn’t seem (at least to me) very adequate or viable in this case.
Let’s say I own a Koll Rotor Pro and, by using it, I find a discrepancy between the chordwise CG of my newly acquired carbon set. Let’s say a couple of millimeters. Is this acceptable? What difference would be tolerable? If it reveals to be to much and to cause vibration (a couple of mm, probably won’t) shouldn’t I simply return them? I mean, if I can’t fix this in an easy and more or less elegant way I shouldn’t probably see it as acceptable.
I mean, if the difference can’t be fixed with tape, I can’t imagine how it can be done. IMHO, it’s unthinkable to drill a hole through the gel coat and fiber and to put a lead weight in the middle of the polyurethane foam!
So, Mike, can you please tell us what’s your opinion regarding these questions? What do you usually do in a circumstance like this? How do you fix a bad chordwise CG?

Regarding the lead/lag caused by air resistance: I know that the lead/lag angle variation on the blades depends, directly and proportionally, to the aerodynamic drag force as they swish through the air (there are other effects that contribute to this also, like the Coriolis Effect… but let’s leave that out for the moment). More pitch means more drag which, in turn, means more lag.
Knowing this, I chose, deliberately, to leave it out of my explanation for simplicity sake… because (correct me if I’m wrong), if the blades are aerodynamically equivalent and the pitch of both is the same, they will lag by practically the same amount, regardless of them having symmetrical CoG positions. Of course, if they have different chordwise CoGs, they will present different initial lead/lag angles. But the variation of the angle (as the aero-drag changes with collective pitch and the relative movement of the heli through the air) will be roughly the same for both of them.

Mike, you say that having the same chordwise CoG is specially important in regard to 3D flight (were the most extreme pitch angles are achieved). I quite agree but, if you let me, I will extend the scope of that statement and say that it is equally important to every style of flight.
Let me try to clarify my view regarding the importance of the chordwise position of the CG:
Let’s suppose we have a pair of blades that statically balance, perfectly. Unfortunately, their chordwise CG is different. Now, let’s assume that the heli is on the ground and that the pitch angle is 0 (meaning no lift at all, for symmetrical blades). We spool up the rotor to the hovering speed. If the blades are not over-tightened at the grips and can, more or less, freely rotate about the bolt, they will automatically align themselves in such a way that the centrifugal forces cancel each other perfectly, regardless of the difference in their CG positions. This means that they will have different lead/lag angles, of course. But there will be no resultant sideforce whatsoever and the heli will appear perfectly stable and steady.
Now… what happens when we apply collective and bring the heli into a hover (i.e. we apply upward lift force to the blades)?
The increase in pitch causes an increase in aero-drag force, so the blades will both lag a bit more… but, roughly, by the same amount.
Since the blades have the same shape, the Center of Lift (CL) lies in the same spot on each of them. If you imagine a straight line between the two CLs, the combined resultant CL lies in the middle of the distance between the two.
Well, since the lead/lag angle is still different in both blades (due to the different CGs) the combined CL will not coincide with the rotation axis. And, what’s worse, will revolve around this axis with each rotation. Another way of seeing this is that the lift force varies cyclically its position with regard to the heli’s CG. The net effect of this is that the heli will “wobble” (like a gyroscopic precession) at the rotor frequency. This maybe not visually too evident, but can even disturb the gyro in such a way that the tail may start wagging…
If we apply full collective, the effect will be much more noticeable (since the lift will be even greater). So, if our heli vibrates or wags its tail on full power ascents, it may be due (among other possible causes) to a mismatched chordwise CoG of our blades! So, you see, IMHO this matters not only when we do 3D stuff with steep cyclic inputs….

Another crucial matter with regard to the chordwise CoG is the position of the Center of Lift in relation with the that imaginary straight line between both CGs in a rotating pair of blades. Ideally, the CL should stay slightly behind (I mean closer to the trailing edge) this imaginary axis. At the most, it may be right over it but never ahead.
Why? Well if the CL is not over this axis, a torque will be generated when there’s some lift. This torque will try to “twist” the blade, reinforcing or dampening (depending if it’s ahead of behind the axis, respectively) the blade pitch.
If it’s ahead of this imaginary line, the lift force will try to further increase the pitch of the blade (especially if the blade is not that rigid). This causes kind of a snowball effect which can lead to stability and controllabity problems.
This problem worsens considerably if there’s a chordwise asymmetry between the CGs of the blade. That should be easy to understand, because one of the blades will gain slightly more pitch than the other creating – again – a dissymmetry of lift that will cause the blades to not track and the heli to vibrate. This can be slightly counteracted by changing link rod lengths… but this will not solve the problem completely.

So, there you have it… the chordwise position of the Center of Gyration of a blade is very important… and it’s more critical than its spanwise position, IMO... this much I know…. I just ignore exactly what can be done to solve, effectively, such a situation…

Well, much more remains to be said about this subject… but I guess I overdid it again! So I’ll stop for now…

Looking forward to some feed-back…

Louis

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