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COG question - 12/23/2012 9:46 PM   
sweatybetty


 

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Status: offline 		just finished a goldberg super chipmunk kit. do i balance the plane so the horizontal stab is level or so the leading and trailing edge centers are level, or should they both be the same?
thanks all!   sb


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RE: COG question - 12/23/2012 11:59 PM   
CGRetired



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Status: offline 		The Center of Gravity is set by the specs of the aircraft. The Chipmunk is a low wing plane. Mark the top of the wing to indicate the potential balance point, it should be a range of say, 3.5 to 4.25 inches in from the leading edge next to the fuselage.. or something like that. Then pick the plane up so that it's upside down and put your fingers at the marked points on the wings. It should balance. If it's nose heavy, excessively, add weight to the rear. If it's tail heavy, add weight to the nose. Ideally, it should not need weight.

If you mean laterally, well, once you get the correct Center of Gravity, then you can do a lateral balance check to make sure it does not tend to be either to heavy on the right or left.. If you built it correctly, the vertical stab should be perfectly perpendicular to the center line of the aircraft and the horizontal stab should be aligned with the wings which should be perfectly positioned for flight so that they are perpendicular to the fuselage.

I don't know if this is what you mean by your question, but that's what I think you wanted to know.

CGr.

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RE: COG question - 12/24/2012 12:32 AM   
Gray Beard


 

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Status: offline 		I think he is asking about where to look or place a level to see if the CG is correct. On your plans there is usually two lines running through the center of the crank shaft of the engine. One is on a slight angle and is the engines thrust, the other line should be straight. Sometimes this line is drawn from end to end of the fuse. If not you can use this line and draw it from end to end of the fuse yourself. This is the datum line that measurements are made from and this line should be level when setting the CG. You can tape a small bubble level along this line over the marked CG and when your CG is correct the bubble will be centered.
Hope this made some sense, it's easier to show you then explain it.

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RE: COG question - 12/24/2012 12:50 AM   
CGRetired



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Status: offline 		ok.. I see that now. Thanks for pointing that out, Gene.

Dick (CGr)

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RE: COG question - 12/24/2012 1:00 AM   
Gray Beard


 

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Status: offline 		That is a guess Dick, I'm not real sure if that is what is being asked or not but I think so?

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RE: COG question - 12/24/2012 1:05 AM   
guver


 

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Status: offline 		I've never built kits and am very crude to setting balance point , but somewhere along the way I had a similar question with my foam planes.

That is when setting on a balance point on the CG machine at what pitch is considered "level" ? Somewhere I rx an answer that the stab should be level. It may be simplistic , but that's kinda what I thought the OP was asking. As far as the LE and TE centers IDK what OP means.

Looking forward to the "real" question.

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RE: COG question - 12/24/2012 1:13 AM   
da Rock



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Status: online 		The manual for the kit says 3-3/4 to 4-5/8 for the CG.

Most airplanes are designed to cruise with the fuselage 'level' in order to produce the least drag. So balance yours so that the fuselage looks level to you from the side.

Anywhere in that range will give a safe flying model. If you're one who wants to be on the safe side, simply make it balance between 3-3/4 and 4-1/8. If it balances level anywhere in that range you'll have a safe flying model that will be less sensitive to your elevator input. Balance behind the 4-1/8 and it'll be somewhat sensitive. The mfg really won't specify a range that would allow the model to be a handful.

Do the balancing so the plane is level. You will then know where the CG actually is. And you know the exact range for the CG, so do the balancing accurately and you'll know for sure what to expect.

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RE: COG question - 12/24/2012 2:33 AM   
Hossfly



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quote:

ORIGINAL: sweatybetty

just finished a goldberg super chipmunk kit. do i balance the plane so the horizontal stab is level or so the leading and trailing edge centers are level, or should they both be the same?
thanks all!   sb


This is an everyday question and I provide a true everyday answer for CG within subsonic convergent airflow. So many of these kits, these mfgers. and others misdirect you when finding a SAFE CG that I wonder how anyone flies if one has to ask the question.
A Super Chipmunk has a tapered wing. Measure the center chord. Measure the tip chord just inside the wing tip. Do the same at the center of the wing. Now you have two numbers. 10" at the chord and 12" at the center. Average is 11". find the average chord place along the wing which in a straight taper will be midpoint. Measure the 25% point back from LE there and at the other side do same. Draw a line between the two 25% points. Where that crosses the center is where you want your model to balance, especially for the first flights. 25% of mean aerodynamic chord will provide safe place to fly and especially for first flights. Then adjust a bit as you get used to the machine and if you are going for 3D then ease it back to 35-40%. If you are racing pylon, you may want the stability of 20%. However the 25% of MAC will keep you safe and the model whole unless you inject other items not part of aerodynamics in sub-sonic airflow. Your call.
OTOH if you are doing a Free Flight with a pylon mounted wing, you may wish to start at some 70% of MAC. BTDT
The mfger. of my eindecker called for 35% CG. I use 25% and it is just beautiful. At 30% it wants to 3D. Not my style.

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RE: COG question - 12/24/2012 3:50 AM   
sweatybetty


 

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Status: offline 		thank you all. i have a little trouble wording things right sometimes.   im trying to get a starting point for the maiden flight. what i ended up doing is balancing the plane (by moving the battery pack fore or aft) so the horizontal stab is level when the balance point is centered on the main spar.

a pic of the plane hanging on the center of the main spar



the stab level (if you were to put a bubble level on it)



and my pilot





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RE: COG question - 12/24/2012 4:23 AM   
da Rock



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Status: online 		You're doing it right.

The fuselage is level.

Now measure how far the points of your balance are back from the LE of the wing at the root, right where those suckers are in the picture.

You want to measure at the root of the wing. That's right where it comes out of the fuselage. Put your ruler on the fuselage and measure straight forward to where the yellow line is in the picture. You want to find how far the CG is back from the LE.



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RE: COG question - 12/24/2012 4:34 AM   
da Rock



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Status: online 		That's a pretty decent CG finder you got there.

It looks like the balance point in that picture is probably right on the spar. If that's the case, measure from where the spar is right against the fuselage.

As long as your CG is within the range Goldberg specifies you're safe.

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RE: COG question - 12/24/2012 9:17 AM   
jaka


 

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Status: offline 		Hi!
All you do is eayball the fuselage is level. Nothing more fancy!
I use my index fingers checking the Cof G on allmy planes be it small 1m to 2,5m in span.
What's important is that you have the Cof G a little forward the first time youcheck it/ fly...Not as in your pictures showing,the fuselage sitting level (horizontal) level!
If you have it level it could mean that Cof G is to far rearward (which is't that good). Better safe than sorry!

< Message edited by jaka -- 12/24/2012 7:21 PM >

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RE: COG question - 12/24/2012 5:39 PM   
Gray Beard


 

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Status: offline 		I'm a bit more anal then Jaka about the CG but I do make sure my first flights have the CG a bit nose heavy. I have made the tail heavy mistake once, well twice but the other one I put it forward of where the manufacture showed it and it was still tail heavy. Way tail heavy but it was a 3-D plane.
Going by the stab is also a good idea as long as the stab is set with a zero degrees incidence. I do go to the forward most of the marked CG shown on the plans for the first flights. I bought some of those bubble levels at the Dollar store and took them apart so I have a lot of really small levels. I tape them level with the datum line directly center to the marked CG.
As long as the plane is a bit nose heavy there shouldn't be any problems or surprises.
CG is important but you do have some room to be off a bit.

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RE: COG question - 12/24/2012 6:15 PM   
goirish



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quote:

ORIGINAL: Hossfly


quote:

ORIGINAL: sweatybetty

just finished a goldberg super chipmunk kit. do i balance the plane so the horizontal stab is level or so the leading and trailing edge centers are level, or should they both be the same?
thanks all!   sb


This is an everyday question and I provide a true everyday answer for CG within subsonic convergent airflow. So many of these kits, these mfgers. and others misdirect you when finding a SAFE CG that I wonder how anyone flies if one has to ask the question.
A Super Chipmunk has a tapered wing. Measure the center chord. Measure the tip chord just inside the wing tip. Do the same at the center of the wing. Now you have two numbers. 10'' at the chord and 12'' at the center. Average is 11''. find the average chord place along the wing which in a straight taper will be midpoint. Measure the 25% point back from LE there and at the other side do same. Draw a line between the two 25% points. Where that crosses the center is where you want your model to balance, especially for the first flights. 25% of mean aerodynamic chord will provide safe place to fly and especially for first flights. Then adjust a bit as you get used to the machine and if you are going for 3D then ease it back to 35-40%. If you are racing pylon, you may want the stability of 20%. However the 25% of MAC will keep you safe and the model whole unless you inject other items not part of aerodynamics in sub-sonic airflow. Your call.
OTOH if you are doing a Free Flight with a pylon mounted wing, you may wish to start at some 70% of MAC. BTDT
The mfger. of my eindecker called for 35% CG. I use 25% and it is just beautiful. At 30% it wants to 3D. Not my style.

Nothing like a good looking girl with a nice transmitter. You have a keeper there.

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RE: COG question - 12/24/2012 10:26 PM   
Gray Beard


 

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Status: offline 		I'm a stinker about building a nice plane then installing some stupid looking doll or toy in the plane for a pilot but when I saw your pilot I busted up, a true classic!! Love it!!

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RE: COG question - 12/27/2012 9:14 AM   
wilfredes


 

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Status: offline 		Thanks for all the reports guys, nice to hear that



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