The CG Sling, Balancing your airplane

[MrClean]

The easiest way to check airplanes for their current CG that I have found is to hang the airplane in a sling. This method uses a sling made from a loop of some heavy 1/8" chord. My loop is made from a 14 foot piece of chord which makes a diameter of about 4.5 feet. You also need a plumb bob and I made mine from an extra piece of wooden dowel. For a suspension point you can use a nail in an exposed rafter or a plant hook in your ceiling anything that can withhold the weight of your plane. In my example I used a simple piece of chain that is hooked onto a ceiling fan. The Chord loop is wrapped around one part of the airplane, up through the chain suspension point and back down to another part of the plane. Here I have it wrapped around the nose of my F-22 foamy. It then goes through the chain and then wraps around the cutout before the elevators.



Also suspend the plumb bob from the chain between the chords temporarily tying the string so that you can raise and lower the plumb bob. I usually wrap the string around the chain a couple times. My wooden dowel doesn't weigh much so it will hold still. Set your airplane so that it hangs level in flying attitude. Now slide the plumb bob down so that it hangs over the highest part of your fuselage. When everything stops swinging around, the plumb bob will point to the current CG of your airplane.

How's it work? You've essentially made two plumb bobs and both the plumb bob and the airplane will be hanging from the same point and the Center of Gravity of both will be on a line from that point to the center of the Earth's mass. Hang a piece of weight off the tail and the plane will shift a little forward. This will make the plumb bob point a little farther aft, just like it should. The lateral balance of the plane works the same way. Put a piece of weight on the left side, and the plane shifts a little right. In the end, balance your plane by adding weight or moving components so that the plumb bob points to the desired CG of the plane, as in this picture...



And that's all there is to it.

WRC

 

[PaulT]

This sling will not work well, in my VERY humble opinion.
Why, you ask?
Please go to this link and open the pdf file.
Your concept is sound but you need to attach the sling differently for it to work properly.
You have too many points that are binding which renders this set up as being only a good way to store your plane out of the way.
But, then again, maybe I'm missing something?!?!?!

http://www.rcgroups.com/forums/attachment.php?attachmentid=4861746

I still prefer the very easy and dowel method.

 

[earthsciteach]

It took me a while to figure this out, but I do think it is sound. Since the plane hangs from a single point, it will naturally level out so that the plumb bob points to the cg. This cg is NOT necessarily the correct one. It just is the cg of the moment. If you know the desired location of the cg, you can mark it on the plane, then shuffle components around such that the plum bob points to that location. It doesn't matter at all if the plane is level, if the plumb bob points to your wanted cg location, you're good to go. (I think).

 

[MrClean]

I've seen the Vanessa stuff before. First time I saw it was about 10 years after I sent in my article. This is not to say that it does not predate the one I use. I am not the originator just an ex newsletter editor. That being said. Do it as I do it works 100 percent. Just simple physics. U can hang the plane by the tail wheel and the plumb bob, if not interfered with, will point to the center of the airplanes mass.

But fear not. I've demonstrated the rig. Measured weight and datum to figure moment arms. Added weight on paper and done that math. Added it on the plane hung it and by golly the plumb bob is pointing where the math said it should and the feller looked at it and said. "Nope can't work."
It's simple as pie.

PaulT, it does not matter how many points you have attached to the airplane as long as the two loops are suspended at the singular overhead point. In the "Tail Heavy" thread earthsciteach posted a picture of a Viking Ride and pointed out it is essentially what I'm talking about. Put more people in the back, the front tips up and a plumb bob from the fulcrum would point towards the rear.

Why use this instead of the dowel? When my plane slipped and rammed one of those lovely dowels through the upper and lower Silkspan of my just completed PAMPA stunter, I went for something different. I've more then once test flown a plane for a third party that turned out to be obviously tail heavy. Can't be, the guy said, put his fingers on the planned CG held it up and sure enough it was level, when he used HIS fingers. Seems you can lean into it and change the CG, which he was already at the rear side of and now was over a quarter of an inch off. But these are random instances that is why I use a sling and recommend them. Just another way to reliably locate and adjust the CG of your airplane.

Wow, just skimmed through that rig again. That's when heck of a lot of work to do what I've done with a piece of kite string, a 1/4 x 20 nut for a plumb bob and 14 feet of spare cordage. Other then that, it works the same.

 

[00redmd]

CG

PaulT you are correct the previous method of the sling is not correct. Move the plane in the sling and the bob will stay the same (hence a second CG on same plane) This will show where CG is but not where it should be.