large scale aircraft COG balancing

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richard armstrong
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large scale aircraft COG balancing

Postby richard armstrong » July 31st, 2013, 11:42 am

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
Last edited by richard armstrong on July 31st, 2013, 7:09 pm, edited 1 time in total.
Richard "the rocket" Armstrong

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David Jones
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Re: large scacel aircraft COG balancing

Postby David Jones » July 31st, 2013, 2:59 pm

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.

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Re: large scacel aircraft COG balancing

Postby John Rickett » July 31st, 2013, 5:57 pm

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.

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Re: large scacel aircraft COG balancing

Postby Chris Lane » July 31st, 2013, 6:00 pm

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
Attachments
CofG2.jpg
CofG2.jpg (15.39 KiB) Viewed 3854 times
Last edited by Chris Lane on July 31st, 2013, 9:15 pm, edited 3 times in total.

richard armstrong
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Re: large scacel aircraft COG balancing

Postby richard armstrong » July 31st, 2013, 7:07 pm

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
Richard "the rocket" Armstrong

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Re: large scale aircraft COG balancing

Postby Chris Lane » July 31st, 2013, 7:35 pm

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
Last edited by Chris Lane on July 31st, 2013, 9:16 pm, edited 2 times in total.

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Re: large scale aircraft COG balancing

Postby Dave Parry » July 31st, 2013, 8:27 pm

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

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Re: large scale aircraft COG balancing

Postby Chris Lane » July 31st, 2013, 8:45 pm

Thanks Dave - that worked! Chris

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Re: large scale aircraft COG balancing

Postby sean smith » August 1st, 2013, 7:08 am

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

balancer-1.jpg
balancer-1.jpg (9.01 KiB) Viewed 3831 times


balancer-2.jpg
balancer-2.jpg (10.51 KiB) Viewed 3831 times


balancer-3.jpg
balancer-3.jpg (10.06 KiB) Viewed 3831 times


Sean.

richard armstrong
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Re: large scale aircraft COG balancing

Postby richard armstrong » August 1st, 2013, 6:12 pm

sean smith wrote: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
Richard "the rocket" Armstrong

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Ron Pearman
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Re: large scale aircraft COG balancing

Postby Ron Pearman » August 2nd, 2013, 8:07 am

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

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Re: large scale aircraft COG balancing

Postby sean smith » August 2nd, 2013, 12:31 pm

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.

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Re: large scale aircraft COG balancing

Postby richard armstrong » August 8th, 2013, 1:31 pm

sean smith wrote: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
Richard "the rocket" Armstrong

"Fly me to the moon" then crash and burn BABY!


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