large scale aircraft COG balancing

[richard armstrong]

hi all

is the a stand or a way of balencing a large aircraft for CoG

i lookng for ideas to build a stand for the CoG or something that can help

all ideas welcome,

ric

[David Jones]

Hi Richard, we balanced the DC3 at the weekend, we used single strand electric cable tied in a loop around the wing joiner tube. (The C of G on the Dak is about a 1/3 of the way back from the front of the tube (see plan)). The other ends of the wires were then looped over a bar, the model was lifted and lead added until it balanced. We added about 2lbs of lead right at the front of the battery box and we think we've got it pretty much right, perhaps slightly nose heavy.
Another way we thought of doing it was to build in hard points on the C of G and screw in a couple of "eyes" to take the wire. Once balanced the eyes would be removed and the holes filled ready for painting. Either way, both of these systems require at least two of you do do it. I'm sure there are other methods and I'll follow this one with interest. Jonesey.

[John Rickett]

I used the following method on the last project.

Tape a small spirit level parallel to the datum line, preferably close to the expected CG position to nullify the weight.

Jack up the model with a bottle jack. The jack can be repostioned until the balance point is found. If you are uncertain as to whether the weight of the model can be taken on such a small point, a piece of 1/4" ply or similar will act to spread the load.

The model only needs to be raised just enough for the wheels to leave the ground so if it topples its not going to hurt.

[Chris Lane]

Richard,

suggest weighing it at each main and the tail wheels with an appropriate platform scale. It must be level for each weighing which involves a block the same thickness as the platform scales (for the main wheel not currently being weighed!) plus a deeper block for the tail wheel.

You then need to know the horizontal distance from the main wheels to the tail wheel (in level attitude). The CofG itself is at a currently unknown position "x" behind the main wheels.

Switching the blocks and scales as necessary, weigh the model at each wheel. The total of the three weighings is the weight of the model.

Now take Moments: The sum of the mainwheel weighings times "x" equals the tail wheel weighing times (wheel base minus "x").

"x" = w.(wheelbase - 1)/(W1+W2).

Chris

[richard armstrong]

thanks i thing i going to use a combination of dave and johns and try them on my 2.5 metre glider i just brought and will be bringing to Elvington, need to test fly it after hours or on the Friday flyin

cg on the tubes for nose to tail and and to test the role?

chris method is a little over my head sorry maths was not a strong suit

chris what is the file extension jpg or bmp or any graphic format? or a zip file

[Chris Lane]

Richard - I have tried to upload an image having a .BMP extension. It refused this so I inserted the image into a Word blank page which then had a .docx extension. I then tried the bitmap with the extension reference removed. When I go through the attachment procedure here it all seems to happen until a red message appears saying that extension is not permitted!!

I've had no trouble in the past but now I have Windows 8. . . . . . . .

Chris

[Dave Parry]

Chris try and reduce the file image and attach as a Jpeg that should work.

[Chris Lane]

Thanks Dave - that worked! Chris

[sean smith]

Hi Richard,
I had the same problem so i made my own balancer.

balancer-1.jpg (9.01 KiB) Viewed 9860 times

balancer-2.jpg (10.51 KiB) Viewed 9860 times

balancer-3.jpg (10.06 KiB) Viewed 9860 times

Sean.

[richard armstrong]

Hi Richard,
I had the same problem so i made my own balancer.

balancer-1.jpg

balancer-2.jpg

balancer-3.jpg

Sean.

i like this idea you should post a full blog here, on how you made it, a set of drawings and measurement for the lma to use and add it to the forum as a sticky

please can u pm me the larger photos or send them to my email address so i can have a closer look please

[Ron Pearman]

Google "vanessa rig" simple, cheap and very accurate or try here- http://www.rcuniverse.com/forum/m_708060/tm.htm

[sean smith]

Hi Richard,
I'll take some close-up pictures and send them to you. I used the "vanessa rig" to check the CG on my Meister P47 but it was tricky trying to rig up a 19.5kg model when its fully assembled that's why i made the one in the pictures, you can assemble the model and just place it on the balancer.

Sean.

[richard armstrong]

Hi Richard,
I'll take some close-up pictures and send them to you. I used the "vanessa rig" to check the CG on my Meister P47 but it was tricky trying to rig up a 19.5kg model when its fully assembled that's why i made the one in the pictures, you can assemble the model and just place it on the balancer.

Sean.

cool