I'm setting up a Rearwin Speedster ARF - 100" WS, 15#. The instructions are calling for the CG to be:
The wing has a constant chord of 16". Using their formula puts it at 3-1/4" to 4". I don't remember for sure, but it seems to me that the standard formula calls for 33% mean chord as a starting CG, which would put it 5-1/4". If that's right and i can't remember for sure , that's a very big difference.

Can anyone tell me what the formula is?

Much appreciated!!!

The standard for all aircraft in the sport catagory as well as the recomended location for inital test flights in the aerobatic catagory is 27.5% of the mean aerodynamic chord +/- 2%. 33% is considered by most to be in the 3-D range and I don't think your speedster will like it there. At 20-25% it will track like its on rails but you may run out of elevator authority at lower speeds just when you need it the most. I would contact the manufacture and verify that number it might be a typo.

Here are a couple of things for you to consider with regard to the manufacturer's CG recommendation.

First, the manufacturer has to consider the safety and flyability of his kit. Since the plane is a high wing plane, he has to figure that Joe Doofus who has just bounced a couple of landings on his trainer may consider himself ready for a scale plane and build the kit. If I'm putting out the kit, I'm going to recommend a real safe, nose heavy CG for a starting point so I don't get accused on the web of having an unstable plane. If the builder is a good flier, he's going to think like me and figure the manufacturer is just being safe and start with a CG somewhere around 30%. From that point, I can adjust either way depending of flight characteristics.

Next, scale planes normally have small stabs. Anything horizontal tail smaller that 20% of the wing area is, in my opinion, too small for an RC model. I prefer mine closer to 25% of the wing area. The larger horizontal tail surface gives you much more lattitude in CG location. Warbirds, in particular, tend to have small tails. If you are flying scale, you may have to live with it and balance nose heavy to compensate. But, many kit designers of scale planes do enlarge the tails for better model flyability.

The large tail is off your subject, but here's one more item in that area. I once had an 80 inch original acro plane that I took out a Quadra and replaced it with a Super Tigre 3000, effectively losing at least 2 pounds in the nose. The CG went back to around 50% of the chord. The plane was definitely pitch unstable, but it was flyable, goofy, but flyable. You could blip in either up or down and release the stick and the plane would complete a loop on its own. For landing you had to hold down elevator to "prop" the tail up. It did have a large tail, around 24%. This made the pitch divergence less abrupt and more controllable. It was uncomfortable because you couldn't relax or look away for an instant or it was up/down somewhere else. After 6 or 8 flights, I lengthened the nose to get the CG back into a normal range.

Third, some airfoils are more pitch stable than others. Many people designing a giant scale model will use the scale airfoil, but for the most part, 40-60 sized models use dependable, proven airfoils that fly well on models. What's the use of producing a lit with a scale airfoil it it tip stalls easily and most people can't fly it successfully. Why not go with a good semi-symmetrical airfoil so everyone can fly it. Take the new Hangar 9 60 size Mustang. Everyone here on RC Universe is raving about how it flies. I'd bet a lot of money it doesn't have a scale laminar flow airfoil.

I have digressed here, but check the tail area. If it's greater than 20% of the wing area, try a 30% CG. If it's less, make it a little more nose heavy.

Didn't I just say that?

Thanks guys!!!

I think I'll start at 4-1/2" and go from there. That sure helps. I thought I was going to have to add some extra nose weight after putting a homelite on the nose. Couldn't believe it would be tail heavy.

Now go fly!
George

Aeronautical engineering aside, I have always balanced my planes (at least 50 planes over 30 years) 1/3rd back from the leading edge at the root with an empty fuel tank unless the plans indicated otherwise such as on a biplane where it's normally near the leading edge of the lower wing.

I always had good results without calculating 25%, 27.5%, 30% etc. Unless you are using the datum line, arms, weights, and moments to mathematically arrive at the CG, verses the "two fingers under the wing" method that modelers use, I doubt that you will come out at exactly 27.5% verses 30 or 33%. Your fingers are not that accurate but fortunately, they dont have to be. I doubt you could tell the difference between a plane balanced at 30% and 33%. Some of you are worrying too much.

The only plane I've ever had a CG related problem with was a new Tiger Moth that I knowingly flew tail heavy many years ago because I was at the field when I remembered to check the CG, did not have anything to add as nose weight, and I wanted to fly! Everything was fine until I stalled it turning final and could not recover from the ensuing spin. STUPID!!!

Yak

The method of using 1/3 of the chord at the root is not a very good way to balance. The shape of a wing has more than just a little to do with balance point. If the leading edge is swept hard {F-20 tiger shark} as apposed to a straight leading edge {EDGE 540} the 1/3 rule will not work. I have been flying pattern since the 70's and TOC for the last 5-6 years and I can say without any doubt the plane will fly very differant at 30% than 33% MAC. Also the smaller the plane the more CG affects its performance. If you are serious about building and flying, a incidence meter and a balance stand are as important as an razor knife IMHO.