Theoretical blade balance question

[kenkadish]

When I balance blades, I use one of those see-saw type balancers, and have a question.

Suppose I hook up the blades, and one side winds up being slightly higher/lower than the other, yet not so much as to cause either blade to touch the work table.

Seems to me that in a perfect world, the heavier side would continue to rotate to a vertical position, and not wind up only slightly lower than the lighter side.

I'm dealing with friction here, right?

Just a reality check

Ken

[mkovalcson]

If you move them, you should be able to put them at level and they should stay put.

However it is better to weigh the blades to see how far off they are with an inexpensive $10-$12 scale accurate to 0.01 gram.

Then see where the balance point CG of each blade is.
If the CG's match then mark the CG and put tape symetrically on that spot on the lighter blade until they are the same weight.

If the CG's are off add weight to the lighter blade until the CG's match and then continue to add weight to the lighter blade at the CG until they are the same weight.

[nameci]

see-saw type balancer is using exact same principle as balance scale or lever. There is always the friction that comes into play with not well made see-saw type balancer but what you see(one side lower than the other) is not from friction.

It will rotate until there is an equilibrium of torque. One side is lower than the other means that side lower has higher mass or center of gravity of one blade is further out from the root than the other side. Torque is calculated by force(force acting perpendicular to distance) times distance from the fulcrum(point where it turns). Even though seesaw type balancer does not look very scientific and is very simple, it does little more than just comparing of the mass/weight of two blades on each side as center of gravity location of the blades also contributes for balancing on this kind of balancer.

For example, two blades measuring 100g may not balance perfectly on the see-saw type balancer if their center of gravity is different.

Digital scale will give you the weight/mass of the each of the blade but will not give you the balancing based on the center of gravity. As Mark stated above, you need to go additional steps for CG and also balancing.

[danfornow1]

There is another factor that comes in to play here just to further confuse a fairly simple process. The cg of the blade is highly over rated in balancing these blades. (its not even factored in on full size helicopters). As correctly stated above, 2 blades could weigh the same amount and not balance on a see-saw balancer because of cg. With that being said, if you balance them on a see-saw balancer they will balance dynamically because of the moment of the arm.
Picture an oil drilling rig, or the hammer head carnival ride with the heavy weight on a short arm counter-acting the long arm on the other side. In each case, these are dynamically balanced.
I'm not trying to start a big argeument here and not trying to tell anyone to stop locating the cg of their blades because there are also some other rather complex dynamics that come into play here that makes the cg benificial, just saying its not as important as people think and as long as you are in the ball park then things will be fine when you do your static and dynamic balancing.

[dcope17]

The reason your blade see saw doesn't keep rotating until one blade is straight up and one down is because the pivot point is above the center line of the blades.

[BillWJr]

Moment is a force which results from an object's weight acting at a distance. If the blades are the same weight but the cg is not the same the blade balancer will perceive this force as unequal weight.

[Solmanbandit]

I have gone through dozens of blades over the years with doing nothing more than balance with a see saw. I still haven't found any reason to do all that other work and my heli's fliy great.

[EEngineer]

Originally Posted by mkovalcson

If you move them, you should be able to put them at level and they should stay put.

However it is better to weigh the blades to see how far off they are with an inexpensive $10-$12 scale accurate to 0.01 gram.

Then see where the balance point CG of each blade is.
If the CG's match then mark the CG and put tape symetrically on that spot on the lighter blade until they are the same weight.

If the CG's are off add weight to the lighter blade until the CG's match and then continue to add weight to the lighter blade at the CG until they are the same weight.

Please let me know where I can purchase a $10-12...or even a $50 scale that can measure 200g with a precision of 10 milligrams.

FWIW

[Mad Russian]

Originally Posted by EEngineer

Please let me know where I can purchase a $10-12...or even a $50 scale that can measure 200g with a precision of 10 milligrams.

FWIW

http://www.midwayusa.com/product/175...grain-capacity

I use one of these to reload ammo. It measures to a 1/10 of a grain which is a much finer measure than grams. 1 gram = 15.432 grains.
Also measures in grams...

[distructor]

Originally Posted by dcope17

The reason your blade see saw doesn't keep rotating until one blade is straight up and one down is because the pivot point is above the center line of the blades.

Right answer.

Sent from my SAMSUNG-SGH-I317 using Tapatalk 2

[mkovalcson]

Originally Posted by EEngineer

Please let me know where I can purchase a $10-12...or even a $50 scale that can measure 200g with a precision of 10 milligrams.

FWIW

I've got one of these for $14.29

http://www.amazon.com/American-Weigh.../dp/B003STEIYY

[flyinfool]

Balancing is not as hard as it used to be. Most modern blades are pretty close. Back in the old days they were not real close. But I do still check every set of blades that I acquire. Mains and tails.

As mentioned several times above, The reason that your balancer only tips a little is because the pivot point is well above the centerline of the blades. I you look close at the balancer you will see that as it tips to one side due to an out of balance condition, the center of the tray that the blades are resting on moves toward the light side relative to the pivot point. It will only move until everything is once again in balance.
This is done on purpose in the balancers design so that it will always tip to show an unbalanced condition. Unlike a balacer that would be used for a planks prop, or a fan and clutch assy. In this case since the pivot is exactly thru the center of the mass, once the item is balanced it will stay anywhere in the 360° rotation.

I have seen blades with tape on both blades to try to get them balanced using the weight and CG method. When I see tape on both blades I know that some one did something wrong. I prefer the see-saw type balancer since the end result is the same, balanced blades, and it is a lot faster and easier and you will (should) never have tape on both blades.

[nameci]

Originally Posted by dcope17

The reason your blade see saw doesn't keep rotating until one blade is straight up and one down is because the pivot point is above the center line of the blades.

Originally Posted by distructor

Right answer.

Sent from my SAMSUNG-SGH-I317 using Tapatalk 2

I guess I was not the only one who did not pay attention in highschool/college Physics class.

Here is a simple explanation if I was not clear enough in my post above.(those of who actually paid attention in Physics class, please chime in if I'm not stating something correctly).

When beam is in balance(not moving) on a pivot point, either parallel to the ground or in an angle, it means there is a equal amount of torque/moment applied on both clockwise and counter clock wise at the pivot point.

To make it simple, start out with the beam in parallel to ground with weight on both side.
In order to calculate the torque/moment, Torque = Force * Distance to fulcrum(pivot point) Where Force is Mass * Acceleration(in this case is the gravity). If left side and right side of the pivot point has opposing torque that is equal, it is canceled out and beam will be balanced.

Now introduce the angle. If torque on one side is greater than the other side(due to difference in mass), beam will rotate until there is equilibrium(with beam not parallel to ground/floor). In order to calculate the torque with the beam in an angle, you need to use the force acting perpendicular to the beam. Torque = Force * Distance to fulcrum * Sin (A) Where Angle "A" is an angle of force measured against the beam.

Assuming Mass located is same distance from the pivot point for both side of the beam(or same COG in our application), beam will rotate if mass on one side is slightly greater until there is equilibrium in torque. Keep in mind that Torque(counter clock wise and clock wise) will vary depends on how much the beam has been rotated(Torque = Force * Distance to Fulcrum * Sin (A))

Additionally, if Mass is same but COG is different = distance will be different from the pivot point. Again, beam will rotate until Torque introduced by left side of the beam is equal to right side of the beam.

[Solmanbandit]

I personally believe that people over think blade balancing. Back in the old days, I can see that because there were mores issues with balance being off. Nowadays with any decent brand, chances of being off to where you will notice a problem is going to be minor. Using just the see saw, I haven't had vibrations issues or anything else go wrong because I didn't do a full regiment. I also recall talking to Jason Krause once and he said that he doesn't balance his blades at all.

[cbdane]

Originally Posted by nameci

I guess I was not the only one who did not pay attention in highschool/college Physics class.

Here is a simple explanation if I was not clear enough in my post above.(those of who actually paid attention in Physics class, please chime in if I'm not stating something correctly).

I paid attention all the way through my PhD. The other two posters are correct. The blades will continue to rotate until the center of mass for the assembly is located directly below the point of rotation. When they are properly balanced, the assembly will hold its place at any angular orientation, not just horizontal.

[smsodhi]

Could one of you very clever people explain the below to me. I posted it over on the minicopter forum recently. It still has me stumped. Thank you.

Originally Posted by smsodhi

I spent the afternoon shopping with the family and after coming home did a bit of work, here is what I found.


First I weighed all the tail components. I have marked one side with a black spot to differentiate between the sides.


Tail grips and bearings...*.13.40g....13.40g
Rail blades 106..................5.79g......5.79g


So the weights of both sides of the assembly are identical, the blade weights are identical and the COG are also identical.


I made this up with razor blades to balance the tail assembly on.






As you can see in the following pic the side with the black mark on is "heavier", despite identical weights.





Now if I turn one of the the blade grips by 180 deg, so it is in the position it would be for flight this happens.





When I put the blades on this is what happens





And when I rotate one of the blades 180 deg to put it into flight position this happens







My question is why when the components weigh the same on both sides is there a balance discrepancy, and why is it worse when the blades are in flight position?


In which position should I be balancing?
Seems a bit screwy to me. Is there a logical engineering explanation for this?

Should I put tape on the lighter side, on the blade or grip, but then this would affect the weight of that side. I thought that both sides have to weigh the same? They do now but after putting tape they will not.
I am finding it frustrating but very interesting. Learning as I go along.

[cbdane]

Perfect example. I'm really not too clever but I'll try. In this case, the axle hole in the tail hub is probably very slightly off center. Not just along the length of the axle, but also in the narrow dimension at 90 degrees to the length. The reason that the effect is more pronounced with the grip arms on opposite sides of the assembly is that this is the theoretically balanced configuration. A perfectly balanced hub should hold its position in any rotational position. The fact that it has a preferred stopping orientation close to horizontal (but not quite) means that the center of mass is not exactly on the axle hole but slightly below it, at that small angle. This indicates that most of the decentering, based on the angle, is in the narrow dimension of the hub; not along the length. Note that this conclusion assumes that all other components are as you state perfectly match in mass.

When you rotate the grips arms both down, then the center of mass shifts even lower. You'd expect even a well balanced tail assembly to always seek exactly horizontal in this case (rather than stopping at any angle). It's not quite horizontal, though, because of the axle hole being very slightly off center. Notice that the angles with and without blades are the same for each configuration, indicating that the blades are not contributing to the overall imbalance.

The problem with correcting it, though, is the right answer is not to weight the blades. You'd ideally like to place weight near the top of the hub until the whole tail assembly (with grip arms opposite one another) will stop in any rotational orientation; i.e. not seek a preferred stopping point, even if this stopping point is horizontal. Adding weight to the hub may not be practical, however. I would order another center hub from the supplier and see if you see an improvement.

Just my thoughts.

[smsodhi]

Wow, thank you for a very comprehensive and informative answer.
I have read it a few times and will a few times more but it's making sense.
I have some new hubs to experiment with. I'll give it a go and see.
BTW the reason I looked into this was due to vibrations in the tail getting worse at a certain RPM

My grips have Chinese weights, I could add or remove weight from these, to balance the tail assembly. Would this be a good idea?

[PcChip]

Originally Posted by danfornow1

The cg of the blade is highly over rated in balancing these blades. (its not even factored in on full size helicopters)

Full-size helicopters don't change pitch radically like RC helicopters do.

We can go from -13 degrees to +13 degrees in half a second, and if the CG is off from one blade to the next, then they will respond to the pitch change differently and cause vibrations during the pitch change.

Sure, it will smooth back out once the pitch is constant, but why deal with it to begin with ?

[PcChip]

smsodhi - in your pictures, one side is very slightly heavier than the other side. The difference in weight is so small your scale can't accurately measure it (but your razor-blade balance can show it)

When you rotate them around, the center of gravity changes, so the whole assembly shifts

You want it to balance out perfectly level when it's in "exact flight configuration" - just like it will be when it's spinning

That said, the balance does not look far enough off to warrant "messing with it"

(Also, FYI - looks like it's the metal tail pieces (not your Rail tail blades) that need the balancing)