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CG: how do you do this?

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Old 03-09-2006, 08:29 AM
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jigeye
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Default CG: how do you do this?

I need to get the proper CG on a trainer that I changed engines on. The new engine is heavier. I've seen CG machines that do this, however, I'm sure there is a technique that most modelers use without a machine to properly balance a trainer. Advice please on how to do this.
Thanks,
Mike
Memphis TN
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Old 03-09-2006, 08:50 AM
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Default RE: CG: how do you do this?

Mike,

That is easy. Just mark the spot on the wing on both sides of the fuselage right against the fuselage, make sure the tank is empty and everything else is installed on the plane and lift it with your fingertips and see if it sits level or very slightly nose down.

If it's a high wing plane then you mark the bottom of the wing and lift it, if it's a low wing plane then mark the top, turn it upside down and lift it.

I just responded to your 4* thread. If you are talking about this plane then mark the top of the wing, flip it over and lift it up. If you are unsure of where the CG should be on the 4*, look at the wing close to the fuselage and about 3 inches back from the leading you will feel/see and piece if wood that runs out to the wing tip, that is called the wing spar. The balance point on the 4* is right on the spar and for that matter most every trainer I have ever seen is balanced right on the spar
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Old 03-09-2006, 12:26 PM
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Default RE: CG: how do you do this?

That's what I call my "Digital CG Machine".

I use my left index "digit" and my right index "digit" placed on the recommended CG location and see what happens
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Old 03-09-2006, 01:01 PM
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JohnW
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Default RE: CG: how do you do this?

The finger, er I mean the Digit method of balance described by Pettit is good enough for most planes. Do what Bubbagates mentioned about making marks. A felt tip pen will wipe back off.

As for where to make the mark, use the location indicated in your manual, i.e. X number of inches back from the leading edge, or however they described the location.

If you added a heavier engine, you will most likely be nose heavy. This is show up when you balance. I.E., you will find the plane balances forward of the desired CG location you marked with the pen.

If that happens, to resolve you will need to either move equipment (mass) around, or add weight to achieve your desired balance point. Adding mass (lead strips, etc.) is the least desirable method to solve. So, see if you can move your battery to achieve your desired balance, or maybe you can shift the engine back some on the moter mount. Do what you can with moving equipment, and if it isn’t enough, then add some lead weights.

Nose heavy is a lot easier to fix than tail heavy (on traditional tailed planes.) This is all becasue of moment arms, i.e. you have a much longer lever from the balance point to the tail vs balance point to nose. So a little tail weight will fix a nose heavy problem, but a lot of nose weight is needed to fix a tail heavy problem.

Remember, the recommended CG isn’t an exact point and every plane will fly across a range of possible balance points, with the nose heavy side of the range increasing pitch stability and the tail heavy side decreasing pitch stability. If you are not exactly on the recommended balance point, that may still be fine, but we’d need to know more about the plane, specifically the model make and/or wing chord.

Cheers
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Old 03-09-2006, 02:04 PM
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Default RE: CG: how do you do this?

ORIGINAL: pettit

That's what I call my "Digital CG Machine".

I use my left index "digit" and my right index "digit" placed on the recommended CG location and see what happens
Consider this term "stolen"...I like it lots [sm=thumbup.gif]
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Old 03-09-2006, 03:13 PM
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Default RE: CG: how do you do this?

Good starting point:

Measure the wing (closest to the fuselage) from the leading edge to the trailing edge. Divide this number by 3. Then measure back this distance from the leading edge on both wings.Mark this spot on both sides of the fuselage. Stick your index fingers on these marks and lift the plane off the ground, noticing which way the plane pivots, nose down or tail down. Hopefully nose down. If tail heavy....add weight to the nose. Never fly a plane without first checking your CG. The results are entertaining to say the least.This would be a good point at which to work from. 1/3 of the way back from the leading edges of the wing, closest to the fuselage of course.
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Old 03-09-2006, 04:14 PM
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Default RE: CG: how do you do this?


ORIGINAL: bubbagates

ORIGINAL: pettit

That's what I call my "Digital CG Machine".

I use my left index "digit" and my right index "digit" placed on the recommended CG location and see what happens
Consider this term "stolen"...I like it lots [sm=thumbup.gif]
Ditto!

BTW, if this all seems confusing, here's a simple method - Put the old engine back in - mark the wing at the spot where it balances (Providing you liked how it flew before) Put the new engine back, and move the battery, or add lead until it balances in the same place.
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