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Where is my CG

Old 09-11-2007, 07:23 AM
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delosdw
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Default Where is my CG

I was given an old (1960's) cub of some sort and am rebuilding it. There are no plans to tell me where the CG should be. Can anyone tell me what a good starting point for the CG might be? Thanks for any help.
Old 09-11-2007, 07:35 AM
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RCVFR
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Default RE: Where is my CG

Balance at the spar.
Old 09-11-2007, 07:40 AM
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horace315
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Default RE: Where is my CG

if you could post a few pictures someone here can tell you who made it im sure and get a more detail cg range.Jim Dines is right though most airplanes start to balance at or just after the main spar.
Old 09-11-2007, 09:21 AM
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Alex7403
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Default RE: Where is my CG

CG point is at the main spar (beam).

Center of Gravity: The point which can hold the whole weight of the plane.
The plane can hang from that point in balance.
We want to make this point strong as possible, so we put two spars and something to hold the gap between them.
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Old 09-11-2007, 10:34 AM
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MinnFlyer
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Default RE: Where is my CG

I don't know where you got that definition, but not all planes put the spar on the CG.

But it's still a good place to start.

Balance it so it sits level or VERY slightly nose-down when held at the spar - Then fly it.

If it lands hot - That is, if you try to drop the tail for a 3-point landing but instead of settling in, it starts to climb and fly again, it may be nose-heavy.

Keep moving the CG back till it flies the way you want it to. But if it starts to get too sensitive, you either have too much elevator throw, or you're getting too close to being tail heavy
Old 09-11-2007, 10:50 AM
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frednjess
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Default RE: Where is my CG

Wow! What ever happened to 25% of the wing chord. A cub has a straight wing with a relatively simple wing tip, right? On either side of the fuselage measure the width of the wing. Let's say it's 12". A good place to start for your cg would be 25% of 12". That puts your starting point 3" behind the leading edge of the wing. Mark this point on the bottom of the wing on both sides of the fuselage. Fly it like this. If you want to plane to be more acrobatic, adjust the battery to move the cg back a bit. 33% from the leading edge is as far as you want to go, though. After this the plane will tend to become violently unstable and very "twitchy". On our sample 12" wing, 33% would be 4" behind the leading edge. Best of luck.
Old 09-11-2007, 01:16 PM
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Alex7403
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Default RE: Where is my CG

What MinnFlyer saying is working!

Center of Gravity:
The center of gravity of a collection of masses is the point where all the weight of the object can be considered to be concentrated.
Old 09-11-2007, 03:41 PM
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Default RE: Where is my CG


ORIGINAL: frednjess

Wow! What ever happened to 25% of the wing chord. A cub has a straight wing with a relatively simple wing tip, right? On either side of the fuselage measure the width of the wing. Let's say it's 12". A good place to start for your cg would be 25% of 12". That puts your starting point 3" behind the leading edge of the wing. Mark this point on the bottom of the wing on both sides of the fuselage. Fly it like this. If you want to plane to be more acrobatic, adjust the battery to move the cg back a bit. 33% from the leading edge is as far as you want to go, though. After this the plane will tend to become violently unstable and very "twitchy". On our sample 12" wing, 33% would be 4" behind the leading edge. Best of luck.

"What ever happened to 25% of the wing chord?" Good question frednjess. The most basic and easy information is seldom passed on in the so-simple form. Guess it's the information society.

25% of Mean Aerodynamic Chord (MAC) is the most reliable CG place one can possibly start with a conventional sub-sonic flying machine. On a straight wing 25% of chord IS 25% of the MAC. It works adequately for tapered wings in the model world.
Old 09-11-2007, 03:50 PM
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Default RE: Where is my CG

Does this work on all Planes? More specifically, I own a Hangar 9 Arrow, will it work adequately for that aircraft?

Even though it was flight ready 3 years ago, I want to check the CG before I take it up the first time to be sure since its been down a while, and is under repair for some minor stuff. (Read poor boy wants to make every inexpensive precaution short of not flying, because he cant afford a new plane.)
Old 09-12-2007, 03:25 AM
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delosdw
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Default RE: Where is my CG

Thanks All for the good info. I will post a pic sometime soon.
Old 09-12-2007, 08:46 AM
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frednjess
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Default RE: Where is my CG

Adui,
As long as the wing is straight and non-tapered, like a big rectangle, 25% MAC will work perfectly. If you've got some interesting geometry try a cg calculator site. You just have to enter a couple of measurements and it does the rest. Here's one...
http://sky.prohosting.com/air2/cg_calc.htm
Thisis a nice site for flying wing cg.
http://www.poseyflyers.com/caculator/caculator.htm
Have fun!
Old 11-05-2007, 04:57 PM
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Default RE: Where is my CG

I have a stinger cap 232 with a 80 inch WS where is the CG [ with a G - 62 on the fire wall its to tail heavy at 2.34 " at the root leading edge ] need help dose the engiine need to be move out ? [ hope not already cut cowl down ] if so don't know how far
thanks for any help bob

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