This is where you are getting yourself confused. The RECOMMENDED Center of Gravity (CG) position is the point where the SHOULD balance out. This point does not matter what weight is there. For the plane to fly properly that CG should not move if you add more weight. So in your question, if you add a bigger engine that adds weight to the nose of the airplane you have to ADD weight to the tail in order to keep the CG at the recommended point.

Let's say that we have a plane that calls for the CG to be 3" back from the leading edge of the wing. So you build the plane and balance it so that the CG is right where it's supposed to be. So now you add a bigger engine, and the CG of the plane will shift forward because there is more weight on the nose. So for sake of argument let's say the CG now has moved to where it's 2" behind the leading edge of the plane. But the PROPER place for the CG in order for the plane to perform like it's supposed to is with the CG at 3", therefore we now have a nose heavy plane. In order for the plane to fly properly you need to move that CG back to where it's supposed to be. So you are going to have to add weight to the tail plane in order to bring the CG back to where it's supposed to be.

All of this adding of weight also has another effect. You are adding weight which increased the wing loading on the wing. A plane with a high wing loading needs to fly faster in order to stay airborne, which means you landings are going to get faster. Plus, high wing loading leads to other bad habits like violent snaps when a plane stalls, among others.

So to address your original questions. No, you should not recalculate the CG position of a plane. The better manufacturers, in this case Hangar 9 which is a good manufacturer, will put a lot of research into their planes. They will recommend a CG range for you to achieve when you build your plane. So when you have your plane assembled you need to balance it, and if the CG is not in the recommended position you will need to shift components inside the plane or add weight in order to bring the CG into the proper location on the plane. For an initial flight of a plane you ABSOLUTELY SHOULD have the CG exactly where the instructions call for it to be. Once you have the plane trimmed out and you are comfortable with how it flies you can start experimenting with moving the CG. But to recalculate the CG BEFORE you fly it is just asking for disaster. If you do that I would recommend bring hefty bags to the field so you have something to take the plane home in.

There is an old saying that comes in to play here.... "When all else fails, follow the instructions." They are your friend in getting a plane in the air.

Ken

thank you, that makes sense, and now see where i got confused, sometimes i'm not the brightest bulb on the christmas tree and really thankful for everybodies help

Adding a heavier engine moves the CG, but it doesn't change where it ought to be.

The location of the CG in an airplane is almost solely a function of 4 things. The wing area, the wing's chord, the area of the horizontal tail and how far aft it is from the wing. There are a number of other modifiers for those 4 but they aren't important. If you know those 4 things, you will know where the CG can be. You then place the heavy bits where they and the body of the airplane will cause the CG to go. It has to go where those 4 things want it to go.

The CG placement is decided by the aerodynamics of the wing and tail. We then have to arrange the weighty things so the airplane with all it's parts balances at or near where the CG is supposed to be.

The center of gravity is shown on the plans or described in the manual - i.e. 3-1/2" from the wing's leading edge at 12" from fuselage's centerline on either side.

Doesn't matter if you've added a Chevy 350 engine or taped a duck to the nose, the center of gravity will still be there once you've compensated with sufficient tail weight.

thanks guys, just want to make sure here, last thing i want to do is crash on take off of the maiden flight, build the plane, balance where manufacture recomends, fly it, trim it out, i'm asuming here, but when trimming out if you have to use a lot of up elevator trim, possible sign that your nose heavy, do flight tests to see if cg is correct. this sound close?

Yes, flight test for CG. CG and wind incedence interact, so it is hard sometimes to figure out which is best to adjust. Study a trim chart for details. http://www.wtp.net/DBEST/trimchrt.html

I was just saying, from the original post. If 1 lb of weight needed to added to nose to balance the CG correctly.
You might as will add wieght that will produce power instead of dead wieght.

The over all weight effects the model's power to weight ratio.

I just fine it odd that you would need to add 1lb of weight to an arf or a model , especailly when there's retracts.
The retacts components are mounted forward of the CG (tail dragger). If anything, one would think it might had came out nose heavy.

CGRetired had said that he used right around a pound of lead in the nose of his Tiger 120 to get it balanced, but flying it, you couldn't tell about the added weight, it handled the weight well.

i understand what you were saying now. if you had to add that much weight, why not add it in engine weight? makes perfect sense plus a bigger looks alot better than some ugly chunks of lead, just beef up the firewall

thanks John, the chart will be a big help

CG confusion? How can anybody be confused about the Coast Guard?

Gee, where should I start??? Ok, first of all..... It's the Coast Guard!!!! Nuff said???






I do hope you know that I am kidding. Your service to the country is appreciated!!!!!

Ken

Umm... after 26 1/2 years, I sorta gotta agree with ya!!!

Howz it goin, Ken?

Just got back from England. Was supposed to be there for over two weeks, but testing was quicker than anticipated.. so homeward bound.

CGr

CG,
I did 10 in the Army and had my career cut short by a back injury. I would have retired in 2006 if I hadn't been hurt.


Been doing ok. Just been busy. So, in other words....... normal!!!

Ken

I had a raven profile .40 size. The guy that built it got smart and decided to dope it and painted it.
I gave it to my instructor..he then gave it to someone else.lmao

I was thinking if I slap a .60 with a tune pipe on it..it might pull it.lol
That thing weight more than my .60 4*[X(]

He freaken mounted the wheels just a little too far back for my taste.
It was like 1 1/2-2" behind the LE.lol

Hi Ken.

Yep. I understand.

I was in England for the past week, work related. Was supposed to be there until next Friday (9/26) but we finished early so homeward bound.

Nice flight, US Airways Heathrow to Philadelphia, Airbus 333-300. Nice plane, lots of room to stretch out.. about half full.

So, back home now, but headed to Salzburg, Austria on the 1st for a week vacation. Gonna visit Munich on one day.. take in a little Octoberfest... should be fun.

CGr

For new flyers, the simple CG setup is to fly it and see if you're losing up elevator on the landings. If the plane drops its nose and you can't control it even with full up elevator, you're too nose heavy. Start on the nose heavy side of the range and work back slowly from there.

Many times a kit will have instructions that say something like: "Balance the model at 5-7/8" behind the leading edge of the wing." A range may not be specified. You'd not believe how many calls we get when a range IS specified: "Where in the range do I balance it?"...."The range says 5-7/8" plus or minus 1/2, mine is on the edge, is that OK?"?......and on and on.

Also, if the model is at or below the manufacturer's suggested maximum weight once it's balanced, it's OK. There are just some designs that cannot fly without large amounts of weight in the nose or tail. That may not be due to "poor" design, but due to the configuration of the model. Many WWI and WWII models have very short noses, which means that there's little chance to use the engine and RC equipment to balance the long tail. Adding weight is the only solution.

With sport models, variance in the density of the wood can mean that the amount of weight needed will vary from model to model. Again, as long as the model's within its weight guidelines, it will fly OK. However, the lighter the model is, the better.

Hey CGHave a good time on vacationtell them you know me but don't tell them where I am.

However, if you HAVE put a duck in the nose, you're not going to be adding tail weight. Unless it's to the duck's tail.

And if you do THAT, then don't tape or glue it to the feathers. They will not hold over an ounce each before pulling out. Plus the duck will be pissed off.

Choose a goose instead. They're heavier to begin with and if you have to attach the weight to those feathers, they hold better.

Hi Irish.

Thanks. This is probably my fifth time to Salzburg. Julie's brother George lives and works there (teaches Classical Piano at the Mozarteum) so we go often... we stay at Casa George ..

We were thinking of going to Venice (four hour drive from Salzburg) and still may. We will see.

Meanwhile.. what's for breakfast?

I tried to post a reply to Bingo Field about the Goldberg Tiger 120 and the CG problems I had but for some reason, I could not get the posting to save.. so here it is (just to keep in the subject matter). The only thing I didn't like about the Tiger 120 WAS the needed weight for the CG. But, as I said, it flew just fine. I would have liked to see how it would have flown if it did not need all that extra weight. But, it is not a pattern plane, nor is it supposed to be that aerobatic.. but it does fine as a easy to fly, relaxing aircraft.

CGr

Dick,
I've been to Venice so many times I was bored sick of it. When I was stationed in Italy I was just about 20 minutes north of Venice. I think I've been there 8 or 9 times. Had several visits from family when I was over there, and they all wanted to go see Venice!!!

Ken