I just finished putting my Twist 3D 60 together and I'm in the process of balancing it.
The book say's 5 7/8ths to 6 1/2" for CG. I've got my Great Planes balancer set at 6 1/4" .....Here ismy problem.....

The plane will drop the tail making me think it's tail heavy,BUT, If Ipush the plane level or just past level, the nose will drop like it's nose heavy? What's going on?

You're on the c.g. exactly.
Large airplanes tend to be finicky as to remaining level on balancers.

Pall is correct. The plane is top heavy - That is, the mass on top of the pivot point is greater than the mass below it, so anytime that mass goes forward or aft of the pivot, it will continue in that direction.

Awesome!! Thanks guys!
This is my first "bigger" plane so yeah, the others just sit level no problem.

Thanks again

Balance it upside down then.....but the real test comes in flight

As stated, balance a low winger upside down.

It is upside down but just won't sit still

Then turn it over. It will be "stiller" one way or the other. (Unless the CG is exactly on the centerline of the wing, halfway from top to bottom.)

You can actually balance a plane very accurately on a "wide board." I do it all the time, but there is a trick to it.

I make building cradles out of insulation board. It's about 3/4" wide. I've got a number of them around the shop. I pick one that's too big to cradle the fuselage, that hits the wing on each side before the fuselage touches the trough. Then I balance the thing. OK..... actually, I put a strip of masking tape along where I think the CG is going to be discovered.

With the thing balanced, I get a pencil and start shifting the plane foreward. At some point, it will just start to tip nose down. It's easy to hold it and mark the masking tape. Now start shifting the plane back the other direction. When it just starts to tip tail down, hold and mark. Halfway between the marks is exactly where the CG is.

Calm, no hassle, and no real effort way to find the CG. And it's marked on the plane... uh, marked on the tape on the plane.

And no special tool is required. There is a very slight bit of touch to it. You'll pick up the touch right off.

I found the CG on that Warhawk with that foam cradle in the attached picture. It needed the battery moved back so took the wing off, moved the battery, wing back on and found the new CG was where I wanted. When the plane went to the field, the fuselage rode in the trunk in the cradle like you see it.

Use a Vanessa rig, and don't look back:

http://home.mindspring.com/~the-plum...%20Machine.htm

http://www.rcuniverse.com/forum/m_70.../tm.htm#708060

As explained above, I believe that the problem you have is related to the natural instability of the situation where the CG is above the pivot point (skin of the wing in contact with the balancer).

As the schematic shows, the situation in which the CG hangs from a pivot (Vanessa), is self-stable.

Regards

I highly recommend the Vanessa rig described on another thread. This is very simple to build and use, works great. I very rarely use my GP center of gravity unit anymore.

bhady

if the plane will not sit steadyon the Cg machine then its best to use the Vanessa rig or calculate the CG using the sum of moment method.

I had this very problem with an extra 300 and was told if thats what its doing then its on the CG. WRONG!!! the flight test nearly ended in tears, it was tail heavy and friction in the cg machine ball joints was keeping it level.

When i calculated the CG it was 1/2 inch behind the recommended ideal. Thats enough to ruin a flight.

Vanessa rig or calculate. you won't be sorry

Appreciate the links!!
That looks like a must have

OK, I'm a little slow so bare with me....
Since I haven't used this system I don't quite get the whole thing.

You strap the plane in and get it level by twisting the dowel, then the plumb bob will be pointing at the CG which you mark with tape. From there, how do you get the plane on the CG mark and balance it?

There is another option for balancing your Twist 3D 60 using your Great Planes balancer:

Hang equal weights at the exact same distance from the recommended CG, aft and forward.

By doing that, you will make the actual CG relocate itself below the wing, and then, your balancer will work properly.

I like the Vanessa rig if I can figure it out... Thanks for the suggestions though!

I like Vanessa too.

The principle is simple: for two bodies hanging from the same point (or axle), the CG of each body will be located over the vertical (or plumb) line that extends from that point down.
This is true, regardless the position of each body around its own CG (or attitude) or the part of the body from which it hangs.

For this rig, one body is the airplane, the second body is the plumb.

Vanessa is the name of the daugther of the man who invented this balance rig: Jim Archer; unfortunately, he passed away some years ago.

Mark the position where the plumb bob points to. Assemble the plane and check the balance where you marked it. Adjust the balance as needed by moving the battery or adding/removing weight until the plane balances where you marked from the vanessa rig.

HI
BUILDING for 26 yearsfly more than most-just use you fingers most balance between 1/2 to 1 inch behind the wing spar do balance low wings upside downbe slightly nose down no fuel in the tank
if you do not know the CG-start at 33% of the wing cord
fly the plane hold the amount of up elevator you like on the left and right turns if the nose drops with your up elevator input-add tail weight ,PLANE SHOULD TURN THE CORNERS LEVEL with your up input
if balanced correctly the plane should fly inverted with a little down elevator -DONE

ENJOY BEST REGARDS TONY

Many modern models have foam wings without the traditional spar at the high point.
Finding the mean aerodynamic chord and locating the c.g. on that is more practical.
And swept wings.... the m.a.c. is way behind the wing at the root.

.
I expect the rig is older than Vannessa.
I just my Team Kadet on her.
I have the c.g. so the plane will -just- rock back on the mains, for a tail-dragging takeoff or extended landing.

How do you balance a plane with sweep back wings?

Put the supporting sling around the fuselage, adjust the lengths to a level fuselage.

CGCINC, as some folks have previously stated, it is easier to build and use a Vanessa rig than it is to explain it. The first thing you do is consult your plans and mark the manufacturer's suggested CG on the surface of your plane. Then you load your plane into the rig. Adjust your plumb bob so that it hangs about a quarter inch above your plane. Now, before you even look at where the plumb is pointing, level the horizontal stabilizer. This is done by rotating the axle that the plane is hanging from. I use a bull's eye level but a small line level will work also. After you level the tail of the plane, check to see where the plumb bob is pointing. If it is pointing exactly at your mark, you are good to go. If not, move something around in the plane. NOW, LEVEL THE TAIL AGAIN by rotating the axle. The tail has to be level every time you check the plumb bob. If the plumb bob is closer to your mark, move something the same direction that you moved it before. Now, level the tail again. Check your plumb bob again. Repeat this process until the CG is where you want it. Remember, the plumb bob point where the CG is, not where you want it. When the two come together (the plumb bob is pointing at the mark you put on the plane before you even hung it in the sling), you are balanced. That axle (dowel) has to be tight enough that it will not rotate on its own but not so tight that you can't rotate it when you want to change the attitude of the plane. And LEVEL THE TAIL each time before you check the plumb bob.

I wasnt getting "turning the stick" thing till I watched it super hi tech video.

[youtube]http://www.youtube.com/watch?v=h57T8OXH4qk[/youtube]