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Old Jul 12, 2011, 05:32 PM
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Canada, QC, Saint-Hubert
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Center of gravity

So, I've seen people saying this plane is tail heavy, this plane nose heavy... I've looked trough past posts and cannot find what I am actually looking for.

Is there an easy way to know where IS the cg of an UM plane ? I tried using brochette sticks .. tried holding the wings ( where do you put your fingers? ) tried pencils under the wings...

How do you guys do it ?

Thanks !
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Old Jul 12, 2011, 05:38 PM
"Add Lightness"..Colin Chapman
Keystone State, USA
Joined Jun 2007
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Which plane or planes are you referring to?

Most every plane that is a BNF or ARF, etc has a recommended CG in the manual. You could or can start there.

Usually they use the front edge of the wing at the root (where it attaches to the main fuselage) as a reference point. Then you measure back from that point to XXmm (29mm or whatever the manual says). I usually just put a very small dot at that measurement. It's a good idea to have an accurate ruler or some other type of measuring utensil.
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Old Jul 12, 2011, 06:48 PM
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That is the picture of my F4U. I drew a line at 28mm exactly as the manuals says. So now with stock battery in, placed at the most forward position, if I understand right, I should be able to place my finger on the line that is now under the battery and the plane *should* hold straight. NOT ! It falls tail side, even with a dime.
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Old Jul 12, 2011, 06:55 PM
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Old Jul 12, 2011, 07:07 PM
"Add Lightness"..Colin Chapman
Keystone State, USA
Joined Jun 2007
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Ok

You're on the right track.

Do the same measurements, but further out on the wings or just put a dot on the wing that correlates to your line on the batt tray. It would also be better, I believe, if you do this on the top side of the wing so you can balance it UPSIDE down.

Turn the plane upside down and put your fingers on the dots on the wings (usually easier with one hand, thumb and index finger) and hope it balances.

If not, experiment. I do not own the UM corsair but I believe I've read it is tail heavy so that is what you are experiencing. If so, add (temporary) weight to the nose and see if you can get it to balance.
Then you can decide what to do from there. Such as carve a bit of foam to move batt further forward or
just add the least amount of weight to the nose. Weight is bad of course.

Do you have the landing gear in place?
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Old Jul 12, 2011, 07:53 PM
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I do have the landing gear, I did put a dime on and it flies better, I just want to understand, regardless of the model, how to find and then make the adjustements to be sure the center of gravity is ok.

So you'd put the battery on and reverse the plane upside down and hope it stays straight, with one finger on each wing at the *sweet* spot ?
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Old Jul 12, 2011, 08:08 PM
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Is this what you mean ? This is the plane, hanging upside down along the 28mm cg line would make it tip to the tail, with added dime on the nose. It's more of a 26mm tough. But, I still don't understand what I can do other than put more weight to make it stay at 28mm. Is that alright ?
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Old Jul 12, 2011, 08:14 PM
"Add Lightness"..Colin Chapman
Keystone State, USA
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Originally Posted by Mespert View Post
So you'd put the battery on and reverse the plane upside down and hope it stays straight, with one finger on each wing at the *sweet* spot ?
Yes.

It may not be crucial to turn it upside down but on larger, low wing models, that is how it is done.

To make it easier, that is so it won't slip off your fingers, you could put very small pieces of double sided tape on your finger tips then put them on the "sweet spots". It is hard working with these smaller models and trying to get them to balance without them falling off your fingers, etc.

You could also try different batteries all the way forward. For instance, a heavier battery all the way forward will affect the CG. Adding "dead" weight to the small UM's is just not a good idea. However a perfectly flying plane can be quite satisfying. I am having to add weight to my Sbach to get the correct CG (it is nose heavy) but only about 0.5gm or so.

Just saw your pic. Is that balanced at the recommended CG of 28mm WITH a dime on its nose?
I see the dime, sorry.
Your technique is correct.
You can add weight or, if possible, move the battery further towards the nose. It may only take a couple mm's to get it right!
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Old Jul 12, 2011, 08:18 PM
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The thing I really don't understand is that wouldn't it be simpler to find the cg *without* the batery in, then knowing where the cg is and having drawn a line , but the battery and move it accordingly to the line you found ?
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Old Jul 12, 2011, 08:22 PM
"Add Lightness"..Colin Chapman
Keystone State, USA
Joined Jun 2007
1,139 Posts
No. Because you will be flying the plane, at all times WITH the battery.

So the battery is an incremental part of finding or having the correct CG.
Consider the battery to be an actual part of the plane. If you found the CG without taking the battery into account, then added the battery and its associated weight, it would completely off.

Does that make sense?
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Old Jul 12, 2011, 08:26 PM
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Alright, so, I have to take into account that the manufacturer is right when stating that the F4U's cg is 28mm and that the T-28's is between 30-32mm ? And adjust to that, end of the line ?
Then, maybe I don't understand what is the purpose of finding the cg at all :P
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Old Jul 12, 2011, 08:37 PM
"Add Lightness"..Colin Chapman
Keystone State, USA
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Originally Posted by Mespert View Post
Alright, so, I have to take into account that the manufacturer is right when stating that the F4U's cg is 28mm and that the T-28's is between 30-32mm ? And adjust to that, end of the line ?
Then, maybe I don't understand what is the purpose of finding the cg at all :P
Yes! Each airframe will be different. You have to trust their measurments.

The purpose of CG is to have an airplane that flies in a neutral manner.
Best way I can explain it is that when you give it throttle it will not want to immediately climb, and when you cut throttle it will not immediately dive.
In a plane such as this you want a slight (very slight) nose heavy tendency.
Having a tail heavy plane can be disastrous. Perhaps not so on these UM's but in larger planes for sure.

Here is where I learned a lot, and all do respect to TurboParker as he is a heavy contributor to these forums. Check out his blog and read about
Flight trimming aerobatic planes. It explains nose heavy, tail heavy, etc.
Also, head on over the the UM Corsair thread, they can probably help you more than I with this plane.
Do a search within the UM Corsair thread for "CG" and you will get tons of info. I just did it to try to help you here. I found that many have the same tail heavy problem.
Here is the link to TurboParkers blog:
http://www.rcgroups.com/forums/member.php?u=271944

You're on the right track. Decide how you want to handle achieving the CG of 28mm and get there.
You will see a difference in flight characteristics for sure!

Edit: Here is a link for the CG search in the UM Corsair thread.
http://www.rcgroups.com/forums/searc...62223&query=cg
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Old Jul 12, 2011, 08:40 PM
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Thanks alot, indeed Turboparker helped me alot My F4U flies great because I followed the posts in it's own section, but I wanted to know more about adjusting the cg on all planes. I'll go a read now !

Thanks again !
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Old Jul 12, 2011, 08:44 PM
"Add Lightness"..Colin Chapman
Keystone State, USA
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Originally Posted by Mespert View Post
Thanks alot, indeed Turboparker helped me alot My F4U flies great because I followed the posts in it's own section, but I wanted to know more about adjusting the cg on all planes. I'll go a read now !

Thanks again !
You're welcome. I'm certainly not the expert. There are other techniques, calculations, etc. to determine CG for an airframe. I leave all of that to the more experienced guys. Then I learn from them!
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Old Jul 12, 2011, 09:17 PM
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Then, maybe I don't understand what is the purpose of finding the cg at all :P
The CG should be ahead of the center of pressure.

If the CG is behind the center of pressure, the aircraft will be unstable.

The center of pressure is at approximately 25-30% of the Mean Aerodynamic Chord (MAC).

Mathematically, the word mean means average. The CG should be ahead of the 30% point of the chord which divides the wing into 2 sections of equal area.

You can Google MAC, Center of pressure and Cener of Gravity for more info.

http://www.airfieldmodels.com/inform...amic_chord.htm

http://en.wikipedia.org/wiki/Chord_(aircraft)
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