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cg dilemma

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Old 05-14-2008, 07:22 AM
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tazzzz
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Default cg dilemma

I purchased a Carl Goldberg Tiger 2. It was built from a kit. I checked on line for the CG, from CG themselves. It states the cg should be from 3 1/2 to 4 3/8" from leading edge of wing at the fuselage ( im assuming that mean with the plane upside down seeing how it is a low wing symmetrical wing. Well I went back 3 3/4". I got it to balance fine,,but when I turn it over (right side up) it seems to go tail heavy,,but if I push down a little on the nose itll go nose heavy. This happens only on the upright position. Shouldnt it balance the same either way, or should I stick with the up side down balance and go with it?

Thanks Guys-TAZZZZ[sm=spinnyeyes.gif]
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Old 05-14-2008, 07:29 AM
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Ed Smith
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Default RE: cg dilemma

Balance it the right way up with the supports at the extreme win tips, at the correct position chordwise of course. If balanced it will not tip backwards or forwards.

Ed S
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Old 05-14-2008, 07:47 AM
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Default RE: cg dilemma

G'day Mate,
It's a low wing plane, so balance it upside down, because if you try to balance it upright it will be unstable, as you have noticed.
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Old 05-14-2008, 08:07 AM
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JohnBuckner
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Default RE: cg dilemma

First yes the airplane should balance the same either way. The problem arises and what you experiancing when you turned it back up is there is not enought stability to be able to balance it. In other words the vertical center of gravity needs to be sufficiently lower than the balance supports to be able to balance the airplane. Trying to balance many low wings upright is next to impossible to do accurately, Just like trying to balance a pencil on its point.

If the low wing has considerable dihedral then it may be possible to balance upright if the supports are fairly wide apart. However most commercial balancing devices won,t spread that far.

If the airplane is perfectly level upside down at the point you are looking for then that is the balance point.

John
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Old 05-14-2008, 08:22 AM
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Default RE: cg dilemma

Balance it heavy side down (that's inverted for a low wing, upright for a high wing).
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Old 05-14-2008, 08:29 AM
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Default RE: cg dilemma


ORIGINAL: tazzzz

I purchased a Carl Goldberg Tiger 2. It was built from a kit. I checked on line for the CG, from CG themselves. It states the cg should be from 3 1/2 to 4 3/8" from leading edge of wing at the fuselage ( im assuming that mean with the plane upside down seeing how it is a low wing symmetrical wing. Well I went back 3 3/4". I got it to balance fine,,but when I turn it over (right side up) it seems to go tail heavy,,but if I push down a little on the nose itll go nose heavy. This happens only on the upright position. Shouldnt it balance the same either way, or should I stick with the up side down balance and go with it?

Thanks Guys-TAZZZZ[sm=spinnyeyes.gif]
Question? Is this a taildragger???????????? If so I have the solution for you. I have a Excel spread sheet set up to show you the correct balance at the CG point. All you will need is a small postal scale and a tape measure. This spread sheet will work on any size aircraft from our smallest models to full scale.
If you would like a copy just PM me with your e-mail address and I will send it along to you.
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Old 05-14-2008, 08:52 AM
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Default RE: cg dilemma

I noticed the same thing when I bought my first low wing. Theoretically the plane should balance the same upright and inverted, but as another has pointed out, the plane is unstable and unpredictable right side up.

Right side up the plane's center of mass is just too high (normally you would say CG, but that'd be a little misleading in this case) to be repeatable. You'd have to balance the plane on two pins and add lead a gram at a time. A bit too tedious(SP?) for my taste.

I've balanced my low wing inverted since I've bought it. Err on the nose heavy side to make sure. You can always remove lead after you fly it.

I have a PVC rig for checking CG. I measure from the tip of the spinner to the table and that gives me a definite(SP?) measurement to adjust from once I fly the plane.
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Old 05-14-2008, 09:28 AM
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tazzzz
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Default RE: cg dilemma

Yea,,,Sounds interesting,, Its not a taildragger,,,although some day it might be. I bought this plane at a smokin' deal I just couldnt pass up


My e- mail is-eaglelite@aol.comThanks...TAZZZZ
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Old 05-15-2008, 06:59 PM
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Default RE: cg dilemma

Balance it the right way up with the supports at the extreme win tips, at the correct position chordwise of course. If balanced it will not tip backwards or forwards.
I stand by what I said. balance the airplane in the attitude it flies.

Ed S
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Old 05-15-2008, 07:48 PM
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Default RE: cg dilemma

You know why the Pyramids sit with the wide side on the desert floor? CG problems.

The CG of the Pyramids don't change when they're pointy side down or pointy side up. But they were easier to keep level when they have most of their weight below the CG. Same with our models.

Balance however it's easiest to keep the model from tipping. You've just seen that it's almost impossible one way. Don't worry that you won't be able to find the CG because you will. It's not more accurate either way, just nearly impossible one way.
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Old 05-16-2008, 10:56 AM
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Default RE: cg dilemma

balance a low wing plane upside down...high wing plane right side up...balance your fuel tank empty because the tank is forward of the CG point.....remember xx amount of inches from the leading edge means draw a line up from the leading edge and measure from there...don't measure along the wing from the leading edge,,,,
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Old 05-16-2008, 11:15 AM
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Default RE: cg dilemma

Here's how I balance my planes. Low wing, high wing, doesn't matter, always right side up. I feel for the spar with my fingertips, if I am within a half an inch or so of the spar (usually to the tail side of the spar), I go fly it. I then go inverted in a 45 degree upline and feel how much pressure I have to put on the elvator to keep it at 45 degrees. I adjust accordingly. I will also fine tune my CG by doing flat spins. The flatter the spin, the closer I am to it's sweet spot.
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Old 05-16-2008, 01:15 PM
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Default RE: cg dilemma

Be sure to start with your plane level; if it is sitting tail down or up this will influence the balance.
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Old 05-16-2008, 02:21 PM
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Default RE: cg dilemma

tazzzz


I have not had a Tiger 60 for quite a while but I remember that it will do as you say. If you push the tail down, it will stay that way even if it is balanced properly. Move the main gear back a little.

Paul
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