I've secured an older EXTRA 230 (built from Kit) removed all components, checked frame integrity, addressed any covering issues and reinstalled new engine, fuel system, battery, servos and receiver...

My question the former owner didn't have a user manual so I do not know how to determine CG. Is there a formula?

I was going to use the point on the wing where the rib is thickest as the CG point. Am I way off?

Would you be way off? Probably not.

Since the CG location is really a function of how big the horizontal tail is, how far back it is, how big the wing is, and what the wing chord is, why not use an online application that takes those numbers and spits out where the CG should be? We place the CG to control the pitch stability. The tail and those other things are what controls that stability.

http://www.geistware.com/rcmodeling/cg_super_calc.htm is an online application that does the figuring for you. You just need a yardstick and time to make 9 measurements.

Thanks for the link...

Here are my results: Would you confirm my interpretation?

Mean Aerodynamic Chord (MAC) = 12.34

MAC distance from root chord (d) = 15.3

Wing Aerodynamic Center aft of root LE = 3.59

Ideal Center of Gravity %MAC = NaN%

Actual Center of Gravity %MAC = no value

Ideal CG aft of root LE = NaN

Neutral Point %MAC (NP) = 44.6%

Neutral Point aft of Wing's root LE (NP) = 6

Distance between Ideal CG and NP (E) = NaN


Would I set the CG at 44.6% of 12.34 inches?

Go back and rerun it without the "%" sign in the desired static margin.  That should fill in the NaN nonsense with numbers you can use.  The final location for your balance point will be the number it gives you for the distance back from the LE at the root.  that's where it's easiest to measure.

The 44.6% point is the Neutral Point.  It's the furthest back you can put the balance point and still fly without the model trying to flip around and fly backwards.  Contrary to what it says on the notes about setting the value for the static margin 3D models are often set up with SM's between +5 and -2'ish to achieve the ability they have to flip around.  But it increases the work load on the pilot to keep the model under control. 

You may eventually end up at that point but for now your 10% value for stability margin is a good beginning.  But depending on how you like to fly it won't be your final point.  I would suggest running the numbers again for an SM of 3 as a guide for the rearward limit so you know what sort of tolerance you have for later and can work your way back in small steps until you're totally happy with how the model flies and it's not too much workload to keep it under total control.

"LE at the root"

LE is leading edge

What does root mean?

I appreciate your time giving me a hand with this.

The tool is useful to get to a CGthat is flyable. But easier yet, You most likely have an old ACE Extra 230. Set the CGon the blade joiner and go fly. Let the airplane tell you where to adjust from there. If the airplane is built strait a neutral CG will mean little or no elevator trim will be required. Thats test one. Test two is to pull a 45 degree upline, roll inverted and see what she does hands off. My personal preference is for it to self level inverted in about 5-6 seconds.

The wing or tail root is where the wing or stabilizer joins the fuselage.

A useful trick for determining the accuracy of CG-locating softward is to plug in the dimensions of a successful canard (tail first) airplane, just to see the whether the algorithm gets totally confused, or takes it in stride. Da Rock's suggested program, copied below, failed miserably, unless I screwed up in some way, so I must conclude that it contains significant errors, not only for canards, but for conventional configurations. Maybe someone else could throw a canard at it, and see how it handles it. A good program should be able to work out the distribution of lift of a canard, as well as a normal layout, sufficiently accurately to locate the CG. If not, it is probably in error in calculating the effect of downwash on tail lift.

http://www.geistware.com/rcmodeling/cg_super_calc.htm

For the really nit picking guys the "projected root" is often used .
The c/l of the fuselage is this point
figuring total wing areas it then gives you 1/2 total span and the area differences can be quite large.
You also want to project the Leading edge and the trailing edge to this point.
For most setups -as noted - the point where the wing panel and fuselage connect.

Rock,

I'm flying a World Models Intruder with its adjustable incidence stabilizer. Set the initial incidence with an incidence meter... then needed a good bit of elevator to fly straight and level. Started tweaking the stab incidence by cranking in more negative (Stab LE lowered) incidence. Finally got the airplane to fly straight and level without up or down elevator (actually, there is a slight amount of down trim that requires a little up pressure on the stick, that I like to fly with). Now the airplane needs a lot of down elevator to fly straight and level inverted. Balance point is where it's supposed to be, just a tiny bit forward of the main gear wheels. I'm out of ideas.

ChiefK

ChiefK, sounds like your CG is a bit too far forward. You might try a bit of tail ballast, which will also remove the need for a lot of the negative stab incidence, and also dictate the use of lower elevator travel, to avoid the increase in pitch sensitivity that accompanies CG that is further aft. I like to set the CG so that the airplane requires virtually no "down" elevator while flying inverted, but this can sometimes make the airplane a bit hard to land smoothly in a gusty wind.

Thanks for your suggestion. I was told early on with this airplane that the cg should be a little further aft than the instructions specified, so I made the cg as aft as possible. This airplane is now nearly balanced on the main gear. If I remove the propeller and spinner, the nose wheel will lift and the plane will drop the aft end to the ground. I too would like having to use as little down elevator as possible to maintain straight and level inverted flight. Just don't know how to get there with this model.

ChiefK

ChiefK, there's flying and there's sitting on the ground.  You're on the right track for getting the flight trim closer to neutral by continuing to move the CG back until you only requre a small amount of down pressure to fly level inverted.  The first step would be to adjust that stabilizer back to mid point as what you used it for is not why it is set up this way.  If the main gear legs are mounted in grooved blocks as was very popular back then start by ensuring that any bend is to the rear.  If needed bend the gear about a half inch back or even get some music wire and bend your own legs up that has the right angle back to achieve a stable stance.

There's some good pattern model flyers that were flying a lot back in the day when the Intruder was hot stuff on the contest scene.  Hopefully a few of them will chime in about this issue.

My Intruder is not the original Kirkland design from the 70's, it's the World Models ARF version currently being produced. It has fixed tricycle gear. I will move the cg back and see how that affects the trim - next weekend. Darn work keeps me busy during the week.

ChiefK