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changing your CG?

Old 01-31-2007, 01:44 PM
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pywackit
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Default changing your CG?

Guys, I read a comment where someone changed his CG to make his plane handle different. I didn't even know that was possible, what do you have to do...move the wing forward or backwards. I can see where this can come in handy on certain planes.
Old 01-31-2007, 01:52 PM
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TLH101
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Default RE: changing your CG?

You can change the CG by moving weight forward or rearward. Or you can simply add weight to the front or rear of a model. As an aircraft gets more tail heavy, it becomes more sensitive to the elevator inputs. To a point this can be good, but go too far, and it can be disastrous. As it get nose heavy, it will track better and smoother, be less "twitchy", and you can loose elevator response at slower speeds, making landing difficult. An aircraft's CG is a delicate balance. Remember always though, a nose heavy model fly like crap, a tail heavy model flys once.
Old 01-31-2007, 03:13 PM
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pywackit
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Default RE: changing your CG?

TLH101, Changing the CG seems easy enough but how do you determine just where the CG is on a plane if you don't know?
Old 01-31-2007, 03:24 PM
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Default RE: changing your CG?

You did balance you model to start with, right? Same way you did it originally. here is a good way: http://home.mindspring.com/~the-plum...%20Machine.htm
Old 01-31-2007, 04:32 PM
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RCKen
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Default RE: changing your CG?

As you start moving the CG around on the plane you'll need to remember the following saying:

Overly nose heavy planes fly poorly, overly fly only once!!

Be careful as you move the weight around to shift the CG. Unless you know what to expect with a tail heavy plane you are more than likely going to crash it, and if it's too tail heavy it's assured that you will crash it. Your best bet is to move the CG backwards in 1/4" increments, and then fly it after each adjustment to see how the plane will fly.

Hope this helps

Ken
Old 01-31-2007, 05:11 PM
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pywackit
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Default RE: changing your CG?

Guys, the CG on my plane is right where the instructions told me to put it and there's no way I messin around with that. If it works right, don't fix it is what I believe. After reading some of the other posts here some of these more experienced builders talk about stuff I'm not sure about...so I ask. I've got an Alpha 40 trainer that's been doing just fine this year and hopefully for a lot more.
Old 02-01-2007, 08:55 AM
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Default RE: changing your CG?

If you like your plane the way it is, then just leave it alone. But subtle CG changes can also have good (or bad) effects.

For example, a plane that is slightly nose heavy will be a bit more stable, but less aerobatic. So maybe your plane flies well, but you can't get it to spin or snap.

Now, if you'r flying a trainer, you may not be ready for spins or snap rolls, so you think, "That's ok, I'll leave it alone"

But that same plane, while being more stable, may need a faster landing speed, or you may find that you CAN'T slow down as much as you would like to before touching down. Adding a little tail weight may be just what you need to make it settle in nicely.
Old 02-17-2007, 03:25 PM
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pt19 flyer
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Default RE: changing your CG?

hi
a rough rule of thumb is 25% back from the leading edge of the wing, sometimes on the wing spar location, done with the tank empty. a good method is to use #8 or #9 buckshot. this enables fine adjustment by gluing with epoxy. if nose heavy the further back toward the tail will require less weight to be added. best to stay with 1/2 inch forward of rearward of point on plans.

good luck and happy flying

pt19 flyer
Old 02-18-2007, 05:54 AM
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Default RE: changing your CG?

I have a GP ultimate bipe where the CG is aft of the limit in the manual. I did this the increase its aerobatic capability. I moved the CG to the aft limit and then snuck it back a little at a time (by moving the battery) until I was at what I considered to be my limit. Here in pitch it is unstable. This means that it is almost impossible to trim the model to fly level. It either wants to climb or decend. It sounds bad but its not to bad to fly. When the CG is back here it does the most insane negative snaps of any model I have seen.
Old 02-18-2007, 06:44 AM
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Ed Smith
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Default RE: changing your CG?

A lot of people talk about minor adjustments in balance point position and it's effects. Yet most airplanes today have a balance point that changes during the flight, every flight.

One fluid ounce of glow fuel weighs approximately one ounce. If a twelve ounce fuel tank is positioned in the nose of the airplane then during the flight THREE QUARTERS OF A POUND WEIGHT is removed from the airplane. Now surely, nobody will try and tell me that the trim of the airplane will not change. If as suggested in a previous post that 1/4" change in balance popint will make a difference what will taking THREE QUARTERS OF A POUND out of the nose do?

Put the tank on the balance point. Only then can the airplane be trimmed accurately for the flight duration.

Ed S
Old 02-18-2007, 12:31 PM
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Capt. Crackup
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Default RE: changing your CG?

The CG that is in the instructions or plans is for the most part, a starting point. If you balance it here, the plane should fly fairly stable enough to guarrentee a good test flight. The CG should only be adjusted after a few flights unless of course the plane feels unstable and is overly sensitive at landing speeds, and maybe doesn't drop the nose when reducing power. In this case you would consider moving the CG forward. If you move it back the plane begins to "come alive". In addition to becoming more sensitive to control inputs, you will find that it will most times land slower and use less elevator. So expect as you shift the CG aft, you will need less elevator throw. Moving the CG either direction can fine tune your airplane to your flying style. It can make it more stable on final (fwd) or perhaps hover or roll better(aft). But if you move the CG move it a small amount. Maybe 1/8-1/4" at a time then evaluate it over a couple of flights. Most importantly remember that nose heavy planes can be hard to fly slowly, but tail heavy planes are hard to rebuild.
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