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Center of Gravity contraption

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Old 09-01-2010, 03:30 AM
  #1  
JollyPopper
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Default Center of Gravity contraption

I've read here on RCU a couple of times about a machine that you can hoist your airplane in and a plumb bob would point to the center of gravity. I could not visualize how the thing would work so I built one. And I still can't figure out what it is telling me. If I hoist a plane in the loops of this machine and then manually level the horizontal stabilizer using a carpenter's level, the plumb bob actually does point diredtly to the center of gravity. However, if I rotate the dowel around which the loops are wrapped, raising or lowering the nose of the plane, the pointer points at another spot. This seems to be telling me that the CG changes with the attitude of the plane. It moves back a little bit as the nose rises and vice versa. That doesn't seem right. The CG should remain constant regardless of the attitude of the plane, right? What did I do wrong? This thing looks like a really neat contraption if I can figure out how it works.
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Old 09-01-2010, 04:08 AM
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mintie
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Default RE: Center of Gravity contraption

You are correct in that the cg does move when the attitude of the a/c is nose high or nose low. The c/g does NOT remain constant regardless of attitude.
When a Full size A/C is having the weight and balance done to determine the cg ,the a/c has to be in the flying position as it would be when trimmed and flying straight and level. [this is in the manufactures spec]
If you change that positon then the c/g also changes. This is what you are seeing on your machine. Beleive what it says and balance your plane with that ..I have one and it is very accurate in determining the c/g ... cheers.
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Old 09-01-2010, 05:32 AM
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Default RE: Center of Gravity contraption

The CG does in fact remain constant in space. What you have discovered is that the CG does not lie on a the flat plane of teh wing but is infact somewhere inside the fuselage.

If you draw th plumlines straight through the fuselage in the nose up and nose down attitude and level planes, the poin where those line intersect is the CG.

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Old 09-01-2010, 05:34 AM
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Default RE: Center of Gravity contraption

JollyPopper,

Would you please post the instructions for this "contraption", I need a better method than the two pencils stuck in board that I have been using.

Gary
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Old 09-01-2010, 05:43 AM
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Default RE: Center of Gravity contraption


ORIGINAL: Roo Man

I need a better method than the two pencils stuck in board that I have been using.
No, you don't
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Old 09-01-2010, 05:47 AM
  #6  
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Default RE: Center of Gravity contraption

Minnflyer,

I have trouble getting the wing to stay on the pencils especially on larger planes. I am trying to re-balance my 60 size U-Can-Do and the problem here is that there is no sheeting under the balance point. I tried using a wing formerbut have same problem with the wing moving on the pencils. What am I dping wrong?

Gary
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Old 09-01-2010, 05:49 AM
  #7  
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Default RE: Center of Gravity contraption

The Great Planes CG machine is essentially two pencils stuck in a board zooted up.

It works well.

No CG machine gives you the actual cg position. All it provides is the distance from the leading edge the plane balances. CG is inside the fuselage..

a CG machine is a 2 dimesional device, CG is 3dimensional point in space
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Old 09-01-2010, 05:54 AM
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Default RE: Center of Gravity contraption

you need to make up a wing shaped cradle with a little detent in it for the pencil head to rest in, sort of like a ball and socket
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Old 09-01-2010, 06:16 AM
  #9  
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Default RE: Center of Gravity contraption

Timble,

I'll try that, thanks.

Gary
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Old 09-01-2010, 06:36 AM
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Default RE: Center of Gravity contraption

You can also tape two pieces of stronger sheet (of any kind) to the surface where the two pencil erasers will contact. Basically you "cover" that area with hard covering. Or move your pencils together so they're not out where the wing's open bays are. Very few wings are open bay right next to the fuselage. But if they are, the sheeting will work. Last time I did the sheeting trick, the sheeting was thin clear plastic cut from the packaging of stuff just purchased. Blister packs of merchandise sold from racks has that plastic.
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Old 09-01-2010, 06:48 AM
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Default RE: Center of Gravity contraption

Just use your fingers.

People tend to get way too critical when it comes to balancing a plane. While it is true that balancing is vitally important, the exact spot is much less so.

You must have noticed that a manufacturer will not give you a CG "point", but rather a "range". Exactly where inside that range the plane "should" balance is completely up to YOU.

If you balance the plane exactly on the recommended CG it SHOULD fly, but will it fly the way YOU want it to? Maybe, maybe not. A good example of this is my dad and I. He prefers a plane that leans slightly toward the nose-heavy side, while I prefer the same plane to be slightly on the tail-heavy side. So there is no "Perfect" point.

Here's how I balance a plane:

With the plane in a ready-to-fly configuration (minus fuel) pick it up at the recommended CG and see if it sits level. If it does not, shift or add weight until it sits slightly nose down when picked up at the CG point. Now of course comes the usual question: How much is "Slightly" nose down?

Simple answer, only enough so that you can say it is NOT nose-up. In other words, you might look at a plane and say, "Is that exactly level?" Do you need to break out a level and check it? No. Just add or shift weight until you can look at it and say, "It is definitely not sitting level." At this point, you KNOW that it is NOT tail-heavy

The reason for this is because, as we all know, a plane that is nose heavy will still fly. It may not fly as well as you want it to, but it will fly. Whereas a tail-heavy plane can be, or quickly become uncontrollable.

So, now you know that your plane is flyable. Now, fly it.

How does it fly? Is it perfect? Great! leave it alone.

But maybe you notice that it's a little sluggish on the elevator response, or it won't slow down enough on landing. If this is that case, you probably want to move the CG back a little. So, do you break out the balancer? No! Just shift the battery back a few inches, or stick a 1/4oz of lead on the tail and fly it again.

Keep repeating the process until the plane flies the way YOU want it to. At this point, do you need to check the CG? No, why would you need to find out where the balance point is? The plane is balanced and flies well, so WHERE it balances is a moot point.

The only time you may care to check the balance is if you should decide to change an engine, or add a smoke system, or do anything else that will drastically alter the CG. In that case, once again, lift it with your fingers and mark the point where it balances. Now make whatever changes you want to make and re-balance it to the same place that it balanced before.

So you can see that there is really nothing critical about balancing. It's not like you only get one shot to get it right. As long as you start in a slightly nose-heavy condition, you can always do the fine tuning later.
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Old 09-01-2010, 07:16 AM
  #12  
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Default RE: Center of Gravity contraption

Fingers.Under the fattest part of the wing... by coincidence you'll normally find the spar there.Sports aircraft are not that CG sensitive... balance slightly nose down at that location using your fingers.Fly and adjust from there.
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Old 09-01-2010, 07:20 AM
  #13  
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Default RE: Center of Gravity contraption

Here's what you need:

http://home.mindspring.com/~the-plum...%20Machine.htm

You didn't do anything wrong. You adjust the plane so that the horizontal stabilizer is level. Then the plumb bob gives you the exact CG.

Two nice things about it: You can leave the plane hanging there all day while you move batteries etc. around inside. And you can balance a low-wing plane right side up. Try doing those things while holding a 20-pound plane up with your finger tips.
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Old 09-01-2010, 07:52 AM
  #14  
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Default RE: Center of Gravity contraption

thats a lovely device. A good modification is too mount the loop dowel in the of its holder with a bolt so that it can mimic roll. This wil help you with the lateral balance of the airplane at the same time. i fyo clever you will make make a cradle to hold the loop dowel and its holder so that teh loop dowel holder can rotate in roll and use a pin to lock it in place while you balance the model along its long axis. Thenremove the pin to balance the plane laterally.

nice one
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Old 09-01-2010, 08:11 AM
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Default RE: Center of Gravity contraption


ORIGINAL: Top_Gunn

Try doing those things while holding a 20-pound plane up with your finger tips.
Ok, I'll give you that one

Yea, for a huge plane lifting with your fingers isn't easy - That's when I say, "Oh sweetheart, can you come down here for a minute?"
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Old 09-01-2010, 08:55 AM
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Default RE: Center of Gravity contraption

I use a version of the 'contraption' which is essentially the same. EXCEPT the plane is supported not from the wing dowels but rather from a string attached to the engine crankshaft and the other attached as close to the tail as possible. This is the way it was described in the original article from around 1979. I don't know if the attachment points make any difference but all planes do not have wing dowels and logic tells me that the closer one supports the plane toward the ends of the plane the more accurate the reading. Lateral balance is also very simple with the fore/aft support setup. I've found that the weight necessary to balance the aircraft at the center of the CG range is normally pretty close whether I use the contraption or the pencil through the wings routine. However certain aircraft seem so sensitive to the pencil technique and are much easier to balance on the contraption. The GP Seawind (glow model) sticks in my mind as I balanced one for a friend on the contraption.

Given that, using the contraption, I have never once had a new plane take off and exhibit tail heavy behavior. I use the characteristics from my first few flites to adjust the CG to my preferences but the adjustments have always been minor. Maybe I do spend too much time on CG balancing but after months of building an aircraft why not give it the best chance to fly well? This also includes final checking of all incidences, prop balancing and range checking.
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Old 09-01-2010, 09:56 AM
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Default RE: Center of Gravity contraption

Fingers do work great. I have the Vanessa rig and a home built balancer, but what I use almost 99% of the time? Yes, my fingers. I can get the plane just about spot-on to suit me now. Just takes some practice and you get the hang of it. Big planes just need a buddy and you are good-to-go

We had a plane one time that was around 30% size and it was the wildest thing I have seen. It had a super lightweight canopy that was just about over the balance point. You could remove the canopy and the plane would quickly tilt as if you had removed the battery pack. We were all astounded that this light weight canopy could change the balance so drastically! Never saw another one do that.
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Old 09-01-2010, 11:42 AM
  #18  
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Default RE: Center of Gravity contraption


ORIGINAL: MinnFlyer


ORIGINAL: Top_Gunn

Try doing those things while holding a 20-pound plane up with your finger tips.
Ok, I'll give you that one

Yea, for a huge plane lifting with your fingers isn't easy - That's when I say, ''Oh sweetheart, can you come down here for a minute?''
I just did a 15-1/2 lb. model that way (fingertips) and it flew 'hands off" with no trim required on the maiden. I also balance laterally, and the jig is no help there.

The C.G. from the plans/manual is just a ballpark estimate, anyway. You need to fly the individual model and adjust accordingly. Can also depend on your flying tastes. I am usually well behind the recommended range before I am satisfied.
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Old 09-01-2010, 01:09 PM
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Default RE: Center of Gravity contraption

JollyPopper,

For the short answer, see the attached diagram.

Here is the 1000 words’ answer:

ORIGINAL: JollyPopper

If I hoist a plane in the loops of this machine and then manually level the horizontal stabilizer using a carpenter's level, the plumb bob actually does point diredtly to the center of gravity. However, if I rotate the dowel around which the loops are wrapped, raising or lowering the nose of the plane, the pointer points at another spot. This seems to be telling me that the CG changes with the attitude of the plane.
The CG DOES remain constant regardless of the attitude of the plane.

The CG is just a concept:
Each cubic inch of the airplane feels a force pulling it directly to the center of Earth. We call that force the weight of that cubic inch of plane.
Since it is hard to compute the many weights of each cubic inch of any airplane, we try to find that unique point in the entire body of the airplane for which all the individual weights on the right side equal all the individual weights on the left side of that point.
We call that unique point CG, and consider that such a point is feeling a force equivalent to the addition of all the individual weights of the entire body of the airplane (on the right and left sides).
(That is only true for a bi-dimensional analysis (pitch balance), in actuality; CG is the balance point for all the individual weights surrounding that point spatially).

According to that, the only way to relocate that theoretical point (CG) is by altering the balance of forces (individual weights) between right and left.
If we add a piece of lead to the tail, there will be more forces pulling down towards the tail than towards the nose.
Our old CG is not such “center” anymore; hence, we have to find the new CG, which will be a little closer towards the tail.

The weight of each cubic inch and its distribution in any airplane has nothing to do with the attitude or position of that airplane respect to the ground.

ORIGINAL: JollyPopper

I've read here on RCU a couple of times about a machine that you can hoist your airplane in and a plumb bob would point to the center of gravity. I could not visualize how the thing would work so I built one. And I still can't figure out what it is telling me.
In summary, the device is nothing more than two independent plumbs, pendulums, or hanging bodies pointing vertically down.
Naturally, the CG of each body will place itself at the lowest possible point.
If both bodies are suspended from a common point, then both CG’s will be located over the same vertical line that starts at the hanging point (although not necessarily at the same spatial point).
Once any swinging stops, the plumb points in the direction of that vertical line, and the airplane CG is located somewhere along than line.
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Old 09-01-2010, 02:15 PM
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Default RE: Center of Gravity contraption

Hi!
As I have said many times before...Don't complicate things!
Use your fingertips!!! It's that easy!
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Old 09-01-2010, 02:45 PM
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Default RE: Center of Gravity contraption

Once any swinging stops, the plumb points in the direction of that vertical line, and the airplane CG is located somewhere along than line.
True. But you don't know where along that line the center of mass is. So, to determine the balance point, you want the plane level (i.e. horizontal stab level). It isn't that the balance point changes when the plane is tilted, it's that you can't tell for sure where the balance point is with the plane tilted.
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Old 09-01-2010, 03:00 PM
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Default RE: Center of Gravity contraption


ORIGINAL: mintie

You are correct in that the cg does move when the attitude of the a/c is nose high or nose low. The c/g does NOT remain constant regardless of attitude.
When a Full size A/C is having the weight and balance done to determine the cg ,the a/c has to be in the flying position as it would be when trimmed and flying straight and level. [this is in the manufactures spec]
If you change that positon then the c/g also changes. This is what you are seeing on your machine. Beleive what it says and balance your plane with that ..I have one and it is very accurate in determining the c/g ... cheers.
Very WRONG. The CG is a fixed point regardless of the position of the model. The only way to move it is to redistribute the mass withing the model (including structure) or adding additional mass somewhere within or on the model.
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Old 09-01-2010, 04:37 PM
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Default RE: Center of Gravity contraption

How do these guys flying 100 pound models balance their machines?
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Old 09-01-2010, 05:49 PM
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Default RE: Center of Gravity contraption

Hi

Have a look at this

http://www.youtube.com/watch?v=h57T8...layer_embedded

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Old 09-01-2010, 06:13 PM
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Default RE: Center of Gravity contraption

What the guy didn't notice in the video link above, was that there was a bug crawling around on the rudder. That bug completely threw off the whole CG procedure. EVERYONE KNOWS YOU CAN'T BALANCE A PLANE WITH A BUG CRAWLING AROUND ON IT!!

Here's the formula I use to figure the center of gravity. It's really simple!

CG = cubic inch ² x m² / (π x r³) - EM¹ x -1
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