My 2 cents on blade balancingI'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