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03-31-2010 01:55 AM  10 years ago
THX1138

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Indianapolis, IN

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Pic a tail rotor blade and throw tape at it till the vibe goes away. If the vibe gets worse, throw tape at the other blade...

Lou
Just give me five dudes and two flashbangs...
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03-31-2010 03:02 AM  10 years ago
Aaron29

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USA

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It can only make it better.
Not if you add weight to fix a static issue that just then becomes a dynamic issue.
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03-31-2010 03:12 AM  10 years ago
Aaron29

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USA

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Pic a tail rotor blade and throw tape at it till the vibe goes away. If the vibe gets worse, throw tape at the other blade...
I can see how this would work. What would you use to get it spinning?

I've seen the "compressed air and ice cube" technique for a fan and think it is superior to balancing a fan on the high point. Essentially, the fan is suspended by collets and rod on two frozen ice cubes for minimal friction. Tape is added in areas as you mention. Compressed air is used to spin the fan, and you touch the ice cube to feel the vibration. When the vibrations decrease, you're in the right area, keep adding it until the vibes stop decreasing, and then you're pretty much there. If you add tape and it increased, you've added to much.

Of course the fan has 360 degrees of available area to add tape and is somewhat complex since you can move radially and on the arc.

Balancing a head or tail ought to be even easier as it has only two sides. Except you'd need a method to get it spun up and a way to feel for vibes.

But again, I wouldn't bother, as I've never had a head related vibration issue despite just balancing blades.

That said, I don't think any method that could dynamically balance the head/hub would be counterproductive. Maybe just degraded to "waste of time" status.

The tail would be more critical. It is traveling at higher speeds, and is way out on a long rod we call the tailboom that can amplify vibration even more.
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03-31-2010 03:59 AM  10 years ago
c54fun

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Dallas Texas

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THX1138,
Pic a tail rotor blade and throw tape at it till the vibe goes away. If the vibe gets worse, throw tape at the other blade...
That does work great. I have done it with the paddles also. Unreal how it smooths out.
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03-31-2010 01:00 PM  10 years ago
S Bell

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Nova Scotia Canada

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If I could find the technical information on dynamic balancing I would apply this to our helicopters.
I've tried it with statically balanced blades and the situation can be improved slightly. Small changes in tracking not evident to the eye can alter balance readings either for the better or for the worse.In my opinion dynamic balance is not worth the effort and the largest readings within a competent build come from the engine power pulses (during a spectral sample). What we do now as modelers is adaquate and sensible for both the main and tail rotors. Matching blades accurately as sets works very well for both Main and Tail rotors. If you can see the vibration or have physical evidence then it should be addressed through maintenance.
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03-31-2010 05:17 PM  10 years ago
AirWolfRC

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42½ N, 83½ W

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A word on Dynamic balancing . . . it's not helpful on heli blades.

Dynamic balancing is balancing while in motion. What makes this better (generally speaking) than static balancing ?

Three dimensional objects like crank shafts and car wheels benefit from dynamic balancing while two dimensional objects like flat plates and rotor disks do not.

Three dimensional objects can have one end out of balance one way and the other end out of balance in a different direction. When such an object is static balanced (a process that can not detect this condition), and then spun up, there will be a wobble in it's motion. That wobble is what dynamic balancing detects and corrects. Rotor blades are essentially two dimensional and do not have this problem.
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03-31-2010 05:37 PM  10 years ago
Yug

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UK. Herts

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Dynamic balancing - is used to correct for small differences in blade profile between the 2 blades.
I fit blades and hub to a dremmel, spin it up and the vibes are easily felt by the hand. Some time and patience is required to determine which blade and where needs tiny changes in weight to accomplish zero vibration. I also made a small jig so I could also incorporate the pitch slider so that the vibes could be explored under different pitch settings. It's very hard to perfectly match profiles so I generally do this adding weight somewhere. Thin tape is used as the precursor to using tiny weany amounts of CA
Vegetable rights and Peace
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03-31-2010 05:44 PM  10 years ago
AirWolfRC

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Blade profile ? That's aerodynamic, not a mass issue. Aerodynamic forces and mass acceleration issues are two different animals.
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03-31-2010 08:02 PM  10 years ago
CX1

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Canada

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Just fly it

" Team Protoast "
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03-31-2010 11:27 PM  10 years ago
Yug

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UK. Herts

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Blade profile ? That's aerodynamic, not a mass issue. Aerodynamic forces and mass acceleration issues are two different animals
Absolutely, but I found that a small amount of weight placed just right goes along way to evening out the overall balance.
Vegetable rights and Peace
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04-01-2010 12:02 AM  10 years ago
xeon1000

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florida

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I finnaly figured it out the other day as I I was reading through schluter's rc helicopter manual. Dynamic balance was Important in the 70's and early 80's as almost all helis had a coning angle built in. I now realize a static balance is all you need for todays helis. I became a believer when I saw a heli with one blade and a counter balance fly.
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04-03-2010 04:45 AM  10 years ago
Aaron29

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USA

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Good golly I think xeon1000 is right.

Did a little homework. One centripetal force equation is...

Force = 4 x pi squared x mass x radius / Time for one rev

Which means for a given RPM, we can assume that 4, pi, and time for one revolution are held constant on each blade. Force is then linearly related to mass x radius (spanwise CG).

Just like moment is mass x radius (spanwise CG).

If you have static balance you should have balance at speed since the mass x radius will be equal for each side, creating the same centripetal force each side.

I'll be!!

(Kudos to Airwolf for pointing this out for 2 dimensional force equations. I only realized that when I reread this thread.)
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04-03-2010 04:49 AM  10 years ago
Aaron29

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USA

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Only gotcha is my fictitious head would still vibrate horribly due to lift distribution.

But this is more of a out of plane vibration consideration and for equal length blades should not exhibit itself in more realistic scenarios.

Does make you wonder how the single bladed heli doesn't vibrate?

The lifting blade would be constantly wobbling round the body with no opposing blade lift to balance it. You'd probably get a nice big vibration unless there is some dynamic balancing of the counterweight to account? I don't see how and it would depend on RPM, airspeed, etc.

That takes nothing away from the smarter guys here - there would be no in plane produced vibrations from a single blade/counterweight that is properly setup, or for a statically balanced system.

Thanks! I guess you can high point away and it isn't counterproductive after all. This marks the third time I've changed my mind on this. Hopefully a month from now I'm not convinced I need to spend an hour putting my head on the high point.
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04-03-2010 08:53 AM  10 years ago
Yug

rrMaster

UK. Herts

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Aaron29 - you sure about that ?

F=m*v*v/r where v=2*pi*r*rpm/60

So, F=0.0109*rpm^2*r*m
Vegetable rights and Peace
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04-03-2010 04:03 PM  10 years ago
Aaron29

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USA

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Yes, quite sure. Mainly because I didn't come up with it myself.

These are all the same equation, just solving for different values, be it radians per second, RPM, time for one rev, etc.

Your equation is closer to a this one...

http://www.ac.wwu.edu/~vawter/Physi...petalForce.html

F = M x r x w squared,

Where w is radians per second. Radians per second is just a fancy way of expressing rotational velocity and when multiplied by 2pi/60 becomes RPM. Again, the linear relationship exists between Force, mass, and radius.

Even with your proof (which simply is F=mxrxw squared solved for RPM instead of rads/sec if you look close enough), there was a linear relationship between force, mass, and radius. The only thing that was squared was RPM, which is the same on each blade (assuming no blade lead/lag which we don't even want to get into).

Anyway, even with this third equation, we see a linear relationship between force, radius, and mass. Anyway you slice it.

High point away if you want. If you stay away from the collets your aren't hurting anything. I'm more confident in high pointing the tail while on the shaft than ever and completely reverse my statement that it can be counterproductive so long as collets are not used.
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04-03-2010 04:50 PM  10 years ago
AirWolfRC

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42½ N, 83½ W

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What I have previously posted for centrifugal force,

https://rc.runryder.com/p2243030/
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04-05-2011 11:27 PM  9 years ago
jonnywoods

rrNovice

courtenay bc

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every single helicopter in the entire world gets dynamically balanced after any change or modification to the rotor head on all types of head designs before its deemed airworthy and signed back into service. tell any aircraft maintenance engineer that dynamic balancing isn't important and he or she will laugh at you. rc is more tolerant to vibes so it isn't as important as the real ones. most rc products are manufactured "good enough" to just bolt on a fly but your electronics and bearings would benefit from accurate balancing of your components. saying 2000 rpm is to slow to worry about is ridicules. every vibration in every frequency is the number one enemy to a helicopters health besides corrosion witch is not a problem with rc.
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04-05-2011 11:31 PM  9 years ago
jonnywoods

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courtenay bc

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Oh and the hiller hornet was a heli with rockets on the main rotor blads tips and it had a single tail rotor blade with a counter wight opposite and its worked terrible.
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04-06-2011 12:34 AM  9 years ago
ShuRugal

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Richmond, VA

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besides corrosion witch is not a problem with rc.
Tell that to OS .50 Hyper owners
AMA 700159
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04-06-2011 02:11 AM  9 years ago
helidoc2

rrApprentice

so.cal

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Johnnywoods...you are correct...After our 100Hr inspections we dynamic balance tail/Rotor...and Driveshaft..if within limits..not a problems..if not..we balnce to within..
As Far as a model is concerned, i like to balance fan, i like to check that stuff due to the high RPM.
I once asked a tech from Chadwick, how would you check a model Heli, just for the heck of it, he said the accelerometer, could only check lateral..to me if you build the heli right, not much concern.
On real helis, its everything!
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