CG question
#1
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CG question
Okay it has been a long day so the answer is probably simple but i cant grasp it at the moment. In the pictures below if the section in front of and behind the wing are supported and the plane is level then how is it going to move to show that it is nose or tail heavy? To me if you had lets say a single hook on the cg on the plane and a single hook in the ceiling with only one string connected then i could see the nose or tail dipping but the way it is in the pictures it doesnt seem that the nose or tail can drop.
#2
The setup you have is similar to a Vanessa Rig. But, it is missing the important part, the dowel control section. Go to Tips and Techniques Forum and search for Vanessa Rig. There is a long thread about the Vanessa Rig.
As your glider is setup, if the glider is level than the plumb bob is pointing to the CG. If this is within your CG range you are good to go. Your setup does not allow for adjustments.
In my picture I am checking CG with some equipment installed just to see where I am at in the CG range. The plane is level. So let us say I add some weight over the battery. Now the plane will be nose heavy and the nose drops down. By rotating the dowel, I lower the tail, and reference the level I placed on the tail, until the plane is level again. This will cause the plumb bob to move forward and point to the new CG location. A simple process. Making a Vanessa Rig and using it give you a better understanding how it works.
As your glider is setup, if the glider is level than the plumb bob is pointing to the CG. If this is within your CG range you are good to go. Your setup does not allow for adjustments.
In my picture I am checking CG with some equipment installed just to see where I am at in the CG range. The plane is level. So let us say I add some weight over the battery. Now the plane will be nose heavy and the nose drops down. By rotating the dowel, I lower the tail, and reference the level I placed on the tail, until the plane is level again. This will cause the plumb bob to move forward and point to the new CG location. A simple process. Making a Vanessa Rig and using it give you a better understanding how it works.
#4
The key to using this rig, IMO, is having an easy way to return the plane to level. This is where the dowel comes in. An example: I built an Ugly Stick and after the build I decided to change engines. I went from a heavy engine to a lite weight engine. The plane was tail heavy. I moved the servos but still needed more weight up front. Using the rig, I slowly added weight to the engine mount checking CG time. I did this several times until I determined I needed 3 ounces to balance. Easy to do using the dowel in the rig, more difficult repositioning the aircraft on the rope. Not an expert at this but I like this Vanessa Rig.
Last edited by ETpilot; 02-10-2017 at 11:10 AM.
#5
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In my original post the pictures were not of my plane it was from another post i was just curious. I have decided to go with the electronic scale method to find the correct balance thanks for all the help.
#6
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Just curious p3arljam, what exactly is the airplane that you are trying to CG? Also when you say "the electronic scale method" are you talking about the Zicoy electronic thee point system and computer (a wonderful device but quite expensive) or are you talking about using a standard electronic scale measuring each of the three points and manually computing the moment/arms to arrive at your CG?
Because yes the Vanessa rig is effective and works well for some while others have problems with it simply because it is too cumbersome to set up. I work out of a wheelchair and it is a real pain in the butt. You need to know there are other methods that can be just as effective some even costing nothing. It could be as simple as making two tiny bubbles of epoxy under the wing at the target CG and lifting with two fingers on bubbles. This can even work well with many large and heavy airplane working with a team mate.
So what is the airplane that is the subject of this thread?
John
Because yes the Vanessa rig is effective and works well for some while others have problems with it simply because it is too cumbersome to set up. I work out of a wheelchair and it is a real pain in the butt. You need to know there are other methods that can be just as effective some even costing nothing. It could be as simple as making two tiny bubbles of epoxy under the wing at the target CG and lifting with two fingers on bubbles. This can even work well with many large and heavy airplane working with a team mate.
So what is the airplane that is the subject of this thread?
John
#7
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Just curious p3arljam, what exactly is the airplane that you are trying to CG? Also when you say "the electronic scale method" are you talking about the Zicoy electronic thee point system and computer (a wonderful device but quite expensive) or are you talking about using a standard electronic scale measuring each of the three points and manually computing the moment/arms to arrive at your CG?
Because yes the Vanessa rig is effective and works well for some while others have problems with it simply because it is too cumbersome to set up. I work out of a wheelchair and it is a real pain in the butt. You need to know there are other methods that can be just as effective some even costing nothing. It could be as simple as making two tiny bubbles of epoxy under the wing at the target CG and lifting with two fingers on bubbles. This can even work well with many large and heavy airplane working with a team mate.
So what is the airplane that is the subject of this thread?
John
Because yes the Vanessa rig is effective and works well for some while others have problems with it simply because it is too cumbersome to set up. I work out of a wheelchair and it is a real pain in the butt. You need to know there are other methods that can be just as effective some even costing nothing. It could be as simple as making two tiny bubbles of epoxy under the wing at the target CG and lifting with two fingers on bubbles. This can even work well with many large and heavy airplane working with a team mate.
So what is the airplane that is the subject of this thread?
John
#8
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Cool, I had one of the old Great Planes (I think) ARF stuka some years ago and rather enjoyed it. I think I later used it for trading stock
One thing about that airplane is since the vertical center of mass is so much lower on that airplane with so much of the wing structure, landing gear, pants etc. much lower than a more conventional low wing airplane is it will be difficult to balance if the balance pins or rods are located close to the side of the fuselage. If they are spread outward with the airplane inverted to roughly the location of the landing gear in now inverted crouch of the wing the airplane will be much more stable on the stand and easier to obtain the CG.
Hey using the full scale methods is great too if ya like though.
John
One thing about that airplane is since the vertical center of mass is so much lower on that airplane with so much of the wing structure, landing gear, pants etc. much lower than a more conventional low wing airplane is it will be difficult to balance if the balance pins or rods are located close to the side of the fuselage. If they are spread outward with the airplane inverted to roughly the location of the landing gear in now inverted crouch of the wing the airplane will be much more stable on the stand and easier to obtain the CG.
Hey using the full scale methods is great too if ya like though.
John
#9
I think if you had a single line from the ceiling hook to the sling . Then put each sling side around the wing on each side of the fuse so the plane could be leveled you would be good with the sling method.
#11
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Because he device holds the plane while you move things around or add weight to achieve the correct CG, the plump bob tells you when you hit the mark, you don't have to continually add weight pick up to test,add weight pick up to test,add weight pick up to test,, repeat 20-30 times.
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Well i used the scales method under each wheel and it was the best thing i could have done. Just level the plane do a little math and you are done. No more picking up the plane and turning it over. Thanks for all the help guys. https://www.youtube.com/watch?v=9EgC6rX24AU
Last edited by p3arljam; 02-17-2017 at 01:15 PM.
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My plane has a top hatch its not the one in the picture. If you use the scale method i just open the hatch and move the RX battery and reweigh. If you are using the vanessa rig the same thing applies. If you have a plane where there is no top hatch i guess you would have to remove the wing or just add weight.
#15
There's a great deal of strange contraptions and unnecessary CG huffing and puffing, sweating and angst. Finger it until its about right, then fly and adjust as required. As long as you're in the zone, you'll be fine. Go grab a beer and put your feet up.
#16
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My plane has a top hatch its not the one in the picture. If you use the scale method i just open the hatch and move the RX battery and reweigh. If you are using the vanessa rig the same thing applies. If you have a plane where there is no top hatch i guess you would have to remove the wing or just add weight.