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HomeAircraftHelicopterMiniature Aircraft Whiplash & Fury 55 › Dynamic and Static Balance
07-16-2002 11:51 PM  16 years agoPost 1
RussNY

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Stonybrook, NY

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I just got a K&S blade Balancer from Rick's to try to smooth out some minor vibrations on my Extreme.

The instructions say to 1st adjust the dynamic balance and then adjust the Static balance.
-The dynamic adjusts the CG to be the same length wise in the middle of the blade. (add tape to the tip of the blade with the CG closest to the root)
-The static adjusts the blades to each other. (add tape to the light
blade near the center of the blade at the CG.)

NOW HERE ARE MY QUESTIONs:

1. Why can't I just adjust the static balance by adding tape to the light blade tip? After all wouldn't a very light blade balance with a very heavy blade as long the heavy blade was short and the light blade was long. In other words a 1 lb weight would balance with a 2 lb wieght if the distance from the rotation point was twice as far.

2. Is there any thing else going on here than avoiding vibration?

thx
Russ

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07-17-2002 04:14 AM  16 years agoPost 2
steve9534

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yakima, wa.

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balance
Russ;
I'm over 25 years away from my college physics classes, and have long since forgot the equations. What I do remember is that the forces are dependent on the distance of the CG of the blade from the center of rotation, and vary with the speed of rotation. Two blades with different span wise CG's can be made to balance at rest or at any RPM; but unfortunately, will be out of balance at every other RPM. It's important that the blades not only balance at rest, but also have equal CG's, or you'll have a vibration problem. Hope this helps. Steve.

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07-17-2002 05:01 AM  16 years agoPost 3
ncostes

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Va, US

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Russ,
You're balancing the moments, which is ok, but then you must also insure that the 2 blades have the same mass.

Last place I read about this was in Paul Tradelius' "The Basics of Radio Control Helicopters".


You need 2 of the following three (from ch 12 of his book)...

1. The weight of the 2 blades is the same.
2. Their CGs are the same.
3. They balance level.

Pick any 2 and you are guaranteed the 3rd.

Good luck!
Nick

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07-17-2002 01:03 PM  16 years agoPost 4
mark fadely

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indianapolis, indiana

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hi,

it has long been a myth that spanwise balance has anything to do with a vibration in a machine. if this was the case then how do you explain single bladed propellers? the only thing a spanwise balance difference will do is cause one blade to dampen slightly different than the other. if you want to test this theory put some flybar weights on your heli and get it flying smooth. then move one flybar weight all the way in on the flybar and the other all the way out. surprising the heli will still run smooth and where is the spanwise balance?

mark fadely

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07-17-2002 01:30 PM  16 years agoPost 5
heliman41

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Valparaiso IN. USA

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Or you could just buy a good set of matched FRP or CF mains and be done W horsing around.

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07-17-2002 01:43 PM  16 years agoPost 6
RussNY

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Stonybrook, NY

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Well.. Last night I balanced both static and dynamic but I am still a bit confused about why 2 blades need the same weight. I agree with Nick about two items implying the other. But I am still confused about why one blade cant be very heavy and short and the other be very light and long... leading towards single bladed props that mark mentioned with a big counter weight much like a engine crank shaft.

I disagree (or atleast I don't understand the point) with Steve about the dependance on RPM and balance.

This is not really a fury thread so I will let it be and balance both ways to be sure in the future.

Regards
Russ

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07-17-2002 02:00 PM  16 years agoPost 7
Steve Campbell

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Baton Rouge, LA

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To further muddy the water, chord-wise CG is extremely important on 60 blades. This parameter is genrally ignored on smaller ships, and folks get away with it because of the relatively light mass. But it can cause major grief on large, heavy blades.

Curtis' book has the best quick'n'dirty method for blade balancing. I learned this technique from him years ago when he demonstrated it at a fun fly. It is not quite as precise as using a Koll device, but it is close enough; and determines span-wise and chord-wise CG simultaneously. Along with matching the weights, it makes for a smooth set of blades.

Steve

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07-17-2002 02:12 PM  16 years agoPost 8
vetrider

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Daleville, AL (Ft.Rucker)

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Steve, does the Curtis method you mentioned consist of the blade being balanced across a strait edge at an angle (45deg) to the edge?
Kinda like the old schluter method.

Nolan

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07-17-2002 05:44 PM  16 years agoPost 9
ncostes

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Va, US

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Hey just go for it - if you want to have a short blade that's 2x as heavy as the other, try it and let us know how it works out for you

Make sure to have the video rolling please when you first take off

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07-17-2002 09:49 PM  16 years agoPost 10
Steve Campbell

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Baton Rouge, LA

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Nolan,

Yup. But there's more to it than that.

Steve

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07-17-2002 10:25 PM  16 years agoPost 11
vetrider

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Daleville, AL (Ft.Rucker)

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I'm sure there is. I have one of the OLD blade balancers from schluter and it has some of that in the instructions. That thing is OLD like the guy who gave it to me. Hal Beauchesne (bow-shane)
It uses the big screw weight on a pivet pendilum.

Nolan

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07-18-2002 02:17 AM  16 years agoPost 12
steve9534

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yakima, wa.

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Balance
Mark;
You can balance a single bladed propellor, but only at one specific RPM. At other speeds there will be vibration. This because the centrifugal forces are dependent on the RPM and the distance of the CG's to the center of rotation. The forces don't increase in a linear fashion as you move the CG farther out, so if you have a set of blades with different CG's, it may be possible to balance them for one specific RPM, but they will be unbalanced at any other speed. The other problem that different CG's causes is that it affects the flapping of the blades so they won't flap or bend up and down equally in response to the flight loads. This difference in flapping is probably of small consequence in our models with their relatively short, stiff blades, but does cause some dynamic imbalance during maneuvers.
As to your reference to the flybar weights, moving the flybar weights doesn't change the balance of the blades, but does change the balance of the head. If you move one of the weights in and the other out, it will cause a vibration in a heli that was previously in balance. Whether you're able to see it or not, I can't say, but it's easy enough for me to see on my machines. Steve.

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07-18-2002 03:51 AM  16 years agoPost 13
Phil Cole

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Menlo Park CA

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If you do the sums you will find that for any constant RPM, all that's needed is to balance the moments.

Radial acceleration = r * w^2

r = radius to the blade CG.
w = angular velocity (RPM *2 *pi/60).

Radial force = mass * acceleration = m*r*w^2.

We want two blades to have equal radial force for balance,
so m1 * r1* w^2 = m2 * r2 * w^2.

Both blades rotate at the same RPM, so we get
m1 * r1 = m2 * r2.

m * r is the moment of the blade.

If you want the blades to balance while they are changing RPM rapidly then you need the CGs matched. Accelerate them slowly enough and the imbalance does not cause a problem.

If you buy reasonably matched blades (as most are these days) it's good enough to bolt them together and balance the pair on your fingers. In the hundred of so blades of various types I've used, I can't remember any that shook the head or whatever after simply putting some tape on the tip of the light blade to balance them against each other.

The following shows that the theory does work in practice. There are vibration problems of a secondary nature, if only because the lift of one blade will cause the centre of lift to rotate around the overall CG of the machine. If if was necessary to match CGs then the pilot would have been thrown from his seat the moment the rotor spooled up.

http://www.hubschraubermuseum.de/Hu...kow_bo_103.html

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07-18-2002 05:05 AM  16 years agoPost 14
steve9534

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yakima, wa.

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Balance
Phil;
Thanks for the equations. I remembered there was a square function in there somewhere, but couldn't remember if it was with the velocity or the mass. I would agree with most of what you've said except the assumption that the velocities of the blades are equal. Obviously they are, but the problem is that the velocity used for purposes of calculating the forces involved is the velocity at the CG of the blades. If this is not the same distance from the center of rotation, ie; the blades have different CG's, the calculated velocity will not be the same even though the actual velocity is. Since the centrifugal forces are a function of the velocity squared, the forces produced by the rotating blades will not be equal at all RPM's if the CG's are at different points spanwise on the blades, even though it may be possible to balance them at rest or some fixed RPM. Thanks. Steve.

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07-18-2002 05:56 PM  16 years agoPost 15
Phil Cole

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Menlo Park CA

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Steve,

While the linear velocities will be different if the CGs don't have the same radius, angular velocities will be the same if we ignore lead-lag.

The radial acceleration only depends on the angular velocity and radius. The linear speed drops out of the equation.

The radial acceleration can also be found from
a = (v^2)/r.

v is the linear speed in this case.

However, v = w * r
or v = (2*pi*RPM/60) * r.

substituting for v, we get

a = ( ((2*pi*RPM/60) * r)^2)/r
= (2*pi*RPM/60)^2 * r

If you do a google search for "uniform circular motion" you will turn up college style lecture notes which go through the full derivation.

http://oldsci.eiu.edu/physics/DDavi...woDMtn/UCM.html

is one that has some nice diagrams and goes through the angular velocity stuff.

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07-19-2002 12:55 AM  16 years agoPost 16
steve9534

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yakima, wa.

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Balance
. Phil;
Thanks for the patient explanation. I guess I'll have to eat some crow here, in that it looks like you're right in theory. In practice I've not been able to get the machine to smooth out w/o getting the CG's the same. I've read that having different CG's will affect the dynamic balance as the blades flap - maybe that's why I get the vibration. At any rate, I appreciate the help. Thanks. Steve.

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