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May 14, 2019, 04:26 PM
Keith
kknox's Avatar
Thread OP
Quote:
Originally Posted by AndyKunz
The math is simple, and a spreadsheet is attached to make it even easier for you. The math is in a single field in the sheet. Enter the data into the red boxes. The blue box is the answer.

Quality of results depends upon quality of scale and quality of measurement of distances. In other words, garbage in, garbage out (GIGO).

Easiest way to do this is on a table top with tape marking the locations in a T shape, with cal marks on the tape. Set the main wheels on the cross-bar of the T, and call that the 0 reference. Measure the distance to the nose/tail wheel. Enter that into the distance box. Put each wheel on a balance (you can do this all at once, or one at a time) and record the entries in the appropriate boxes.

Andy
Hey Andy, long time no talk to...... I have been gone for a couple years = moving from Utah to Indiana to Texas plus new jobs........ plural and life. Glad to see you are still here........... Thanks much for the help........... Still loving my Spektrum DX 18 and all the goodies.......... I have a new plane - B25 I am working on plus a P-40 that is almost finished. That is why I had my Vanessa rig out and trying to remember all the ins and outs............

Have a good one......

Keith
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May 14, 2019, 04:34 PM
Keith
kknox's Avatar
Thread OP
Quote:
Originally Posted by 2michaely
I am virtually positive that the proprietary Xicoy System uses the "three scale" system both Soarmark and AndyKunz have referenced; the Xicoy presumeably has a manual and software to guide one on setup and working thru to the results. Apart from AndyKunz's Excel encoded software there are YouTube videos, etc. online, if you search for them... I've used three generic kitchen digital scales (the $10-$15 ones all seem to have weight limit of 11 pounds and have a "sensitivity" to a gram, good enough for planes weighing up to 25 to 30 pounds at least).... to successfully use this method..... I think you can use just one kitchen scale, provided you move it around to the other two legs of landing gear without disturbing the setup.


Michael in Ontario, Canada
I checked and you can get Kitchen scales around $ 15..... much cheaper than $350............

Thanks
May 14, 2019, 05:54 PM
Pro Hoarder
turbonut's Avatar
With CG you want the plane is level flight attitude..what ever that is...I use small scales to weight check the cg as Andy is talking about..Same way I do full scale..
Weight X arm = Moment
Add up the weights and total moment. Then divide the moment by the total weight to get the CG number from your datum point. You need to do some pretty accurate measurement of the airframe.
There is some posts about the exact way to this all..It can be confusing at first but after you have done it you wont go back.
I have 3 of the low cost scales..You could buy just one expensive scale and use blocks of the same height on the other wheels. You just have to move to each wheel to get the separate weights
Latest blog entry: In flight
May 15, 2019, 12:44 PM
Mark LSF # 3792
As, Andy mentioned it is GIGO using the three point system. I fly large aerotow gliders, so a three point measuring system, or two point, would be difficult to use. The aircraft will tip to the lower or heavier wing tip, using the two point system, squeueing the reading. The vanessa rig works well for my application with those aircraft sometimes weighing close to 25 pounds. I can suspend it and add and delete weight as needed.
May 15, 2019, 01:31 PM
Pro Hoarder
turbonut's Avatar
GIGO is true of most anything..Like saying I CG my plane nose low...that just means it is at some random number nose heavy
Three point is easy on any plane. Even no landing gear. You just need 3 small stands one for each scale. Just have to use a specific location on the plane to support the weight that is easy to measure. I have done this on planes from 2 lbs to 3000 lbs
Last edited by turbonut; May 15, 2019 at 01:42 PM.
May 15, 2019, 01:44 PM
AndyKunz's Avatar
Quote:
Originally Posted by Soarmark
As, Andy mentioned it is GIGO using the three point system. I fly large aerotow gliders, so a three point measuring system, or two point, would be difficult to use. The aircraft will tip to the lower or heavier wing tip, using the two point system, squeueing the reading. The vanessa rig works well for my application with those aircraft sometimes weighing close to 25 pounds. I can suspend it and add and delete weight as needed.
Actually, that makes it very easy! You roll your glider's main wheel onto the scale and take a reading. You can have somebody keep the wing tip up for you if you can't do it from the root easily enough.

You then roll it off, carry the scale to the tailwheel, and put it on (probably by lifting by hand - it's how I did it with a full-scale Krosno and 2-33). Take the reading.

It's NOT HARD and with a plane as large as you have, an inch difference on the wheel position isn't going to move the CG but a small fraction of an inch.

The bigger the plane, the easier it is to get an accurate measurement. NO EXCUSES! Give it a try!

Andy
May 15, 2019, 03:28 PM
Registered User
ace4rc's Avatar
I have a number of airframes that weight in excess of 40lbs. The Vanessa is out of the question because of this weight sitting on such a small area of the wing, it would break the sheeting or crack the ribs. So I went to the three point weight method and have never looked back. The first time is the worst being to anal on how and where with measuring but it does become quite easy after a couple more times. I bought one quality scale, a postal scale, and made an equal to the scale block, and an adjustable tail support.
The pic is the first time I tried it, a 52lbs Pitts, at a friends house. Needless to say the scale and custom blocks came after this first time, but it flew perfectly.
May 16, 2019, 06:33 AM
Mark LSF # 3792
Everybody has some reason for using their desired approach to CG their aircraft. I am single and I don't think either of my two cats would hold a wing tip for me for very long if I wanted to use the scale method.

On my lighter ships I use a bearing supported balance beam CG device. The heavier 1/4 scale ships are all composite construction and no damage will occur to them using a vanessa rig. So, use what works best for you. Any of these methods discussed have merit if used correctly.

On a side note I saw a nice guy recently bring a new (to him) 6 meter + Minimoa out for it's first flight after he acquired it. One of the guys asked if it had been CG'd. The owner just shrugged his shoulders and smiled. They ended taping wrenches and other tools to the nose to get it close....
May 16, 2019, 07:19 AM
Keith
kknox's Avatar
Thread OP
Question- if I use 3 scales, picked up three from Wally World yesterday, do i have to level the plane while it is on the scales? In my case with the P-40 I would have to raise the tail and put blocks under the tail wheel.......... is that correct?
May 16, 2019, 10:04 AM
AndyKunz's Avatar
Yes, you want it in flying attitude.

The CG will be along a vertical line above the calculated spot. If the plane is tail down, the CG will be above the calculated point, but that will make you measurement make it look tail heavy. If it's nose down, it will make it appear nose heavy. (HINT- you can use this to find the actual CG vertically). So putting it in flying attitude gives the balance point you are looking for.

EXCELLENT question, btw.

Andy
May 16, 2019, 10:05 AM
'Douglas' to his friends.
I think the 'plane has best to be in its normal flying attitude, both during weighing and taking measurements.

Edit: Ah, too late..!
May 17, 2019, 09:00 PM
Keith
kknox's Avatar
Thread OP
Thank you Andy and Merci Douglas
May 17, 2019, 10:19 PM
Pro Hoarder
turbonut's Avatar
Yep Just like Ace has in his photo of the Pitts
Latest blog entry: In flight
May 22, 2019, 07:16 PM
Keith
kknox's Avatar
Thread OP
I tried the 3 scale setup and found that my plane actually weighs more than I thought - 27 lbs or so. I couldn't use the kitchen scales i purchased as they only go to 11 pounds so I ordered two more 22 pound scales. That should solve my problem and make it much easier to get CG compare to Vanessa. She is a great GAL but very hard to operate with large planes...... in my opinion...... Medium sized planes I was okay with but the P-40 has been a pain to get balanced. I think I have added way too much nose weight so I will see and be able to compare to the Vanessa rig.
May 23, 2019, 06:51 AM
Registered User
for over 50 years, i've just used my fingers (or mine and a helpers) to check CG. never had a problem, probably never will. over thinking makes my head hurt


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