Sig somethin' extra CG issue - RC Groups
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Oct 05, 2008, 12:36 PM
Gary in Fla
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Sig somethin' extra CG issue

Hi Gang,
I finally got around to getting my SSE together. I am using the GP CG stand and when I set up to the kit recomended CG (3.5") from the LE. When I set up and try to balance the model it may stay balanced for a few seconds then tip either to the tail or nose. It is not steady at all, even when I try the CG in a more aft position (3.75"). I have it balanced great lateraly. Here is my set up: OS 55AX with pitts pipe / TNT aluminum LG. Any ideas on what to do here are appreciated. The kit is an ARF.
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Oct 05, 2008, 12:42 PM
Registered User
ndb8fxe's Avatar
Are you balancing it up-side-down? If not try that and it may work out better for you.
Oct 05, 2008, 01:15 PM
God is good
Viper Pilot's Avatar
Many planes act this way. The balance may e dead-on, but will drop both forward and backward. This actually indicates that your balance point is good.

Like ndb said, make sure you're balancing with the canopy down.

When correctly balanced and the nose or tail drops, try to get it in a position that it stays stable. If you can accomplish that, it is on the spot.

The real trial is once you get it airborne.
Oct 05, 2008, 07:49 PM
Registered User
That "pendulum effect" is exactly why trainers are almost invariably high-wing cabin models ! There is a built-in stability in a high-wing plane that will make that plane tend to return to level flight if everything is properly balanced. But put the wings down low, and the weight of everything located above the wing tends to make the plane either tip forward or backward unless you are making corrections for it, almost full time.

Really accomplished pilots can do this without even thinking about it, but "newbies" find they have a handful keeping the plane straight and level, at times.

The dihedral will offset that "upside down pyramid" sensation to a great degree, ( think of a Corsair ) but if you are trying to balance that plane with your CG stand, you probably have too much of the airplane's mass above the centerline !

Good luck! hope this helps. Jim
Oct 05, 2008, 08:16 PM
God is good
Viper Pilot's Avatar
Uh, Jim . . . this airplane is a mid-winged, no dihedral model.

Nearly everything you said does not apply to this plane.

Gary, try again with the plane inverted on the CG Machine. If it still "see-saws" like you said, try to put enough pressure to get it steady. If you can, it's real close. Go fly it.
Oct 05, 2008, 11:23 PM
Gary in Fla
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CG issue

Ok... I'll give it a shot. It balances on the recomended marks for about a few seconds and then tips nose down or tail down. Moving the CG back or forth makes no difference, I adjust the battery pack and it does the same thing. I will let you all know how it goes.

Oct 06, 2008, 12:09 AM
Registered User
Whoops, Viper, probably a "senior moment". I was thinking about high wing trainers, vs. low wing craft, and not a midwing setup such as the Extra !

However, I will now ask some questions that maybe you can answer.

Where are the servos, and other somewhat heavy objects such as the battery pack, located in that plane ? How much weight is above the center line through the fuselage, vs. below that center line? Why exactly IS it easier to find the CG on an Extra by suspending it upside down, rather than rightside up if the wing is located on the center line ?

Maybe the plane tips because it is being suspended by half the thickness of the airfoil away from the true centerline of the fuselage. Would that not still be acting somewhat like the "upside down pyramid" that I was referring to in my previous post?

BTW, we had a "fly-in" at our local airport today, and I had a chance to see a "real" Extra 300, up close and personal. Such a beautiful plane !

Teach us all a thing or two. I'm always ready to learn something new !
Oct 06, 2008, 08:41 AM
God is good
Viper Pilot's Avatar
On my Somethin' Extra, I placed the battery pack directly behind the fuel tank, and used foam and a piece of spruce to hold it tight against the tank.

The servo's and receiver was placed as per instructions. It balanced perfectly.

Be sure to keep a little throttle for landing, especially if the wind is over 10 mph. The SE bleeds off speed pretty fast with that thick airfoil.
Oct 06, 2008, 10:25 AM
Registered User
Devil's advocate here, and I'm sorry for asking for a clarification again, but HOW did you determine that the proper balance had been achieved in your plane, if the wing cord is so thick that you cannot pivot the plane on the fuse/wing centerline ? I realize that the final solution can only be achieved by actually flying and trimming the plane in flight, but if a person wants to have a reasonable chance that the plane will not do something drastic and unexpected, is Gary's answer in post #6 satisfactory ?

Having some dihedral in a midwing plane tends to help, because you can always balance it by supporting it at the tips, which will probably be slightly above the fuse centerline, but it IS harder to do on a flat wing, such as the Extra has.

Thanks for teaching us ! Jim
Oct 06, 2008, 10:28 AM
God is good
Viper Pilot's Avatar
The Somethin' Extra IS NOT an Extra! Look it up before any further comments. Maybe it'll clarify things for you.

It's NOT a flat wing, but a fully symmetrical, thick airfoil with a constant chord.
Oct 06, 2008, 10:34 AM
Registered User
I realize all that Viper. I used the wrong word when I said "flat wing" . I meant mid wing with NO DIHEDRAL ! My question still stands. When you balance a midwing plane with a thick airfoil, how do you avoid the "upside down pyramid" effect ?

Sorry, I do not intend to ruffle your feathers, but I do believe we would all stand to learn something from your expert advice. This will be my last post on this subject.

Oct 06, 2008, 10:40 AM
S.A.D. member
ivanc's Avatar
Originally Posted by Viper Pilot
The Somethin' Extra IS NOT an Extra! Look it up before any further comments. Maybe it'll clarify things for you.

It's NOT a flat wing, but a fully symmetrical, thick airfoil with a constant chord.
Everything is correct... except the chord - the Sig SE has a tapered wing (at the TE).

With the thick wing in the middle of the fuse it does not matter much how you balance it - whether upright or upside-down. If it stays for a sec or so at the recommended CG before falling either to the nose or the tail it is balanced fine (VP already stated it in post #3).

The SE is forgiving if the CG is slightly off... for the experienced pilot. If you (the OP) are not, then have someone experienced maiden it for you - he'll tell you if the CG needs to be moved forward or back.

Oct 06, 2008, 10:53 AM
God is good
Viper Pilot's Avatar
Originally Posted by jim_ag3y
. . . When you balance a midwing plane with a thick airfoil, how do you avoid the "upside down pyramid" effect ? . . .
You can't avoid it. That is why I explained that this "effect" is normal. If by pushing the high end of the plane to balanced, and it tends to stay that way for a bit, it's balanced.
Oct 06, 2008, 08:31 PM
Suspended Account

Balancing midwing CG

Hi all,

Putting things in different wording.

The seesaw noticed is because the CG is in between the wing surfaces used for support.

Whether the airplane is right side up or upside down the point (line) of support will always be below the CG if the CG is between the lower and upper surface of the wings.

Using the string loop method solves this behavior and you do not have to spend money on a balancing machine.

Oct 07, 2008, 10:43 AM
Registered User
Thanks Zor, for a great point. Would you be so kind as to further explain the string loop method ( which I have heard of ) in more detail?

Cheers, Jim

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