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CG - easy way to check and adjust

Old 08-17-2010, 09:27 PM
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Dash7ATP
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Default CG - easy way to check and adjust

Checking and adjusting the CG.

I had a CGProblem on the maiden flight of my Top Gun Aircraft F-15 yesterday. It was pretty nose heavy. Ihad checked the balance point and I'm pretty careful about that. Inearly lost a model dueto a rearward CGa few years ago caused by a modification to the airframe. So, after posting here for comments, and getting an answer from the designer, I went back and rechecked. It was very close but slightly forward of his suggestion.

A few years ago (maybe ten?) I was the JPO District IVRep, and Ihad a web page called Joe's Jets. On that site I had shown this technique to locate your CG and easily make adjustments to bring itto the desired location. Icame up with this when trying to devise a way to balance my new Yellow Aircraft F-4. The long fuselage and short sweptback wing made setting it on sticks, or holding it on your finger tips just wasn't feasable. For some reason, something clicked in my mind, and I realized that any object that is hanging from a single point will have it's center of mass (or CG) directly beneath that point. Idevised a simple sling from which the model could be suspended, and made it easy to adjust to keep the fuselage level. Now a small plumb bob hanging from the center of the sling would be directly over the current Cg.

If you look at the photos below, you can see the model suspended in the sling. This is just a heavy cord with a loop in each end. This passed over the nose (or prop spinner, or shaft for non-jets), and around a convient location in the rear. This sling is suspended by a piece of heavy cord whcih has a loop in the end. Iran the sling through a piece of Silicon fuel tubing. This has enough friction to prevent it from sliding but still allows for the sling to be adjusted to keep the model level. My plumb bob is a bullet Ifound with a piece of light fishing line CA'ed to it. A bent pin allows this to be inserted into the vertical line at a point just keeping the bob off of the model. It's important to keep this plumb bob line directly inline with the main suspension line, and not off to the side.


The plane will come to rest with the CGdirectly beneath the bob. If it is not level, just slide the sling in the tubing until it is. If you add a weight to one end or the other, the model will move to the new at rest position. It may be neccessary to readjust the sling until you are very close to your desired CG. The really nice part about this technique is how safe , and convienent it is. You can put a hook through a ceiling joist right over your bench, or a clear spot in the floor. I have my sling go through a small pully and lead the end to a nearby vise. You only need to lift it high enough for the wheels to clear, and you can begin to move your batterys, servos, etc, to find the correct location for the propper CG. If that's not enough and you need to add some weight, tape a small ziplock bag to the nose or tail and fill it with shot until your model is level and the bob is over the propper spot. Then you know how much you need to add to bring it into balance. You don't have to fool around with hanging the model upside down over some jury rigged supports in fear that it may slide off or punch a hole in your wing.

If you want to try this very quickly, just make up a sling for a yard stick, and add a weight to one end. Support the sling with a simple slipknot and hang it somewhere. Make the bob out of a fishing sinker and piece of thread, and see how easy it is to see where it balances. Change the location to see the CGpoint move, and relevel the yard stick.
Very simple.

This system works on any and all models. Ijust finished a 24 pound P-47 and did the CGcheck the same way. Again, just clear the floor, and level the model, and you know immediately what you have to do to "get it right".

Alook at the photos should make this all perfectly clear In one photo, Iadded the cockpit to the side of the F-15, and if you look closely, you can see that the bob moved forward about 3/8 inches. Actually, the model moved aft under the bob to it's new position slightly more nose heavy.

Give this a try. Ithink you'll like it.

Dash
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Old 08-17-2010, 09:33 PM
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Gordon Mc
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Default RE: CG - easy way to check and adjust

I remember the first time I saw this technique, I got a chuckle when the poster said he called his rig something like "Veronica". When asked why, he said it was in honour of his first ex-wife because it reminded him so much of her ... quite elegant, yet incredibly simple
Old 08-17-2010, 10:14 PM
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Default RE: CG - easy way to check and adjust

I don'tknow if the first wife story is true, but I have used the Vanessa for many years.Iit is slightly different in design but works the same way. I have a picture on RCU balancing my 53 pound Mustang, no problem. It is still the most accurate way in my opinion.
there are build instructions at the same site I got it from on here maybe 10 years ago.
http://home.mindspring.com/~the-plum...%20Machine.htm
Dash's design looks good too and a little simpler. I think the original may have a slightly tighter levelling adjustment, but I wouldn't see that as a problem except on heavier planes.
My setup is on a rope and pulley in my garage. I set the plane under it, hook up the sling and haul away.

good to see your setup Dash.
Paul
Old 08-17-2010, 11:10 PM
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Dash7ATP
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Default RE: CG - easy way to check and adjust


ORIGINAL: dragoonpvw

I don'tknow if the first wife story is true, but I have used the Vanessa for many years.Iit is slightly different in design but works the same way. I have a picture on RCU balancing my 53 pound Mustang, no problem. It is still the most accurate way in my opinion.
there are build instructions at the same site I got it from on here maybe 10 years ago.
http://home.mindspring.com/~the-plum...%20Machine.htm
Dash's design looks good too and a little simpler. I think the original may have a slightly tighter levelling adjustment, but I wouldn't see that as a problem except on heavier planes.
My setup is on a rope and pulley in my garage. I set the plane under it, hook up the sling and haul away.

good to see your setup Dash.
Paul
Paul,

Ihad not seen or heard of the Vanessa, but essentially, it does the very same thing my system does. It's somewhat more complicated, however. All you need with mine is a few pieces of rope and a way to hang it. Exact same principal.
Thanks for sharing that.

Joe
Old 08-18-2010, 08:00 AM
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Default RE: CG - easy way to check and adjust

This system works on any and all models.
Even delta wind config?.
Old 08-18-2010, 08:47 AM
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Default RE: CG - easy way to check and adjust


ORIGINAL: basimpsn

This system works on any and all models.
Even delta wind config?.
YES. Even delta wing models. It will show you where your CGis during the procedure. You have to know where it's supposed to be and adjust your onboard components or add weight to make the model balance at that point.

This is a very simple to use method of balancing a model. Give it a try.

Joe
Old 08-18-2010, 09:54 AM
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Default RE: CG - easy way to check and adjust

Thx
Old 08-18-2010, 10:00 AM
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Default RE: CG - easy way to check and adjust


ORIGINAL: Gordon Mc

I remember the first time I saw this technique, I got a chuckle when the poster said he called his rig something like ''Veronica''. When asked why, he said it was in honour of his first ex-wife because it reminded him so much of her ... quite elegant, yet incredibly simple
LOL
Old 06-20-2014, 04:28 AM
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How did it fly with c/g in pics?? Looks way to far forward.
Old 06-20-2014, 07:49 AM
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The F-15 Cg was where the plans called for. It was slightly nose heavy. The molded cockpit with pilot was pretty heavy, On the first flight, it took a long takeoff roll to unstick. After taking out the cockpit (it was just stuck in place in the canopy) it tookoff and flew as good as any other jet I have.
Dash
Old 06-20-2014, 09:50 AM
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My method is similar but more simplified, I drill two 3mm holes in the wing root at the required cg position its usually a glass fuz so no problem. I then have two lengths of 3mm piano wire each bent in an L shape and each with a loop at the top end to which is tied a piece of strong nylon cord and the two are fixed together at slightly more than the width of the fuselage. I simply push the L piece through the hole in the root either side and then I centre the cord that is tied to the top of the wires and attach it to a strong hook I have on another piece of cord which passes through a pulley fixed to a beam and there is also a tie off. With the wings pushed on and the plane positioned on a table I simply haul it off the deck slightly and will immediately notice which way it is balanced, I just do this making adjustments along the way until I have my required balance point. I am able to do this with quite large models without help. On one model I had to glue a piece of scrap ply inside the root as the glass was very thin but normally it's ok.
Old 06-20-2014, 02:49 PM
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Been using the sling rig for a couple of years. I have it on a pulley system rigged to ceiling. Just hoist the model clear of floor and set CG. Works perfect every time

shepp
Old 06-20-2014, 04:14 PM
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How do you transfer the plumb bob CG location to the plan (required) point, which many times is measured on the wing root somehow?
Old 06-20-2014, 05:13 PM
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Matt,

Normally the plans will give you a reference point from which to measure the suggested CG location. It can be from the Leading edge of the wing, or from a bulkhead, In any case, I usually try to locate that point on the top of the fuselage. I normally put a small piece of masking tape on the fuselage and put the mark on the tape. Actually, all you need is just a dot.

As for the previous technique mentioned, using a wire through the fuse at the desired location, you give up one advantage of the sling method, which is the lateral balance. A heavy wing will show up and you can also add some weight to a tip if it appears necessary.,

Dash
Old 06-20-2014, 06:29 PM
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I use a dowel running span wise and then wrap my sling around it about three turns. Them I twist the dowel to level the model. I hang two plumb bobs from the dowel that are now on either side of the fuselage will try to take a pic tomorrow. Very simple system that works on any model regardless of weight
Old 06-20-2014, 09:15 PM
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For lateral balance using my wire method I use a small pulley block at the end of the vertical cord and feed the cord from the wires through, it is a separate function after the cg balance as I work alone usually I also weigh the wings and get them matching weight wise which is generally enough.
Old 06-21-2014, 05:29 AM
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kool.. thanks for info .. if you use the "% 25 percent" method ,,does it come close your location?? from pics. doesn look like it would
Old 06-21-2014, 07:12 AM
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Dash7ATP
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Originally Posted by RAPPTOR View Post
kool.. thanks for info .. if you use the "% 25 percent" method ,,does it come close your location?? from pics. doesn look like it would
If you are asking about my F-15, you must remember that it has a highly swept wing, and for purposes of CG calculations, the area inside of the fuselage must also be considered. To do this you have to extend the leading edge of the wings to the centerline of the aircraft, That's a lot of area in the case of the F-15. The area of the intakes has an effect also, as would the leading edge extensions on many jets. Since I did not design this model (The F-15 - I wish) I did not make the initial CG calculations. I have to assume that this was done on the initial flight testing of the model and feedback from early builders of the model. As I said above, it was pretty close in my case, and removing the cockpit interior and pilot, which were somewhat heavy, it flew just fine. It's always better to be a little nose heavy than tail heavy.
Dash
Old 06-21-2014, 10:02 AM
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Dash how is your F15 powered?
Old 06-21-2014, 11:04 AM
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Dash7ATP
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Originally Posted by Bob_B View Post
Dash how is your F15 powered?
It has an O.S. .91 VRDF in a DynaMax fan. It goes pretty good!
Old 06-21-2014, 11:54 AM
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Sweet I had one of the original TG F15 with the pusher fan. Flew great!

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