ORIGINAL: rgm762
ORIGINAL: flyX
1lb, that's a lot...that's on top of retracts ?
You might as will install a bigger engine becuase you're going to need the power to pull it.
That's a brick, brick.
there's my question, if you install a bigger/heavier engine, doesn't that change the cg? then how do you find cg when the formula doesn't ask for weight?
you guys are great and have helped a lot, and i'm sorry if i'm a pain, what i am doing is looking at the hanger 9 corsair, there's a thread on it under RTF/ARF with over 1900 posts. there's several posts about the cg being wrong, (among other things) but some say it's right and this is what's got me confused, the specs call for a 100 fs engine and they're installing 120's or 125's. so how are they finding the cg with the heavier nose weight than what ther manufacturer is calling for?
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