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Old 04-24-2015, 10:39 AM
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Checking center of gravity on one of these large planes

As with helis center of gravity is crucial it appear in a plane (even more so apparently).


So for a small plane I can see a small little gadget that you can balance the wings on works well. Just use a sharpie marker to locate the correct place then place the wings on that spot and all is good.


With an 8' wingspan and 30 lbs on my p47 I am having a hard time thinking of how to just manipulate the plane let alone check the center of gravity.


The kit is so old there does not appear to be any information on where it should be.


With the research I have done and talking with experienced pilots, a good place to start would appear to be a the thickest portion of the wing, likely in the place of the main spar.


What are you guys using or how do you do it?
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Old 04-24-2015, 10:58 AM
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If an unknown "recommendation"... I figure %25 to %33 percent of mean aerodynamic chord from leading edge for starters... %25 for more stable flight and %33 for agile aerobats...Fly... then make adjustments.
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Old 04-24-2015, 11:01 AM
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2 guys and a finger under each wing.
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Old 04-24-2015, 11:02 AM
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For warbirds of this size.........

You have three choices (2 of them include 2 people)

Find the CG reference point like littlecrankshaft said. Use tape to mark the spot on each wing.

1. Lift from the tips at that point (1-2 fingers) each person on a side (this is how I did my 50cc TFP47)
2. Flip the plane upside down and try to lift from closer to the fuse or out at the wingtips
3. Use a EZ balance or similar device built for 50cc-150cc planes, but that costs money.

I balance with my gear retracted because that is where I spend most of my flight. Once I get it close to where I want it I balance again with them retracted to see if there is a huge difference toward the tail. Then I go fly and start adjusting to how I like to fly and how the model reacts.
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Old 04-24-2015, 11:04 AM
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This works good and easy to make
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Old 04-24-2015, 11:05 AM
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Quote:
Originally Posted by Renoone View Post
2 guys and a finger under each wing.
That's how I have done it for many years.
As for finding the correct CG, a little measuring and math will get you there.

Here is a CG calculator I have used http://adamone.rchomepage.com/cg_calc.htm
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Old 04-24-2015, 11:07 AM
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You can get mixed results it you balance it upright so balance it inverted.
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Old 04-24-2015, 11:08 AM
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CG method

Attached is a pretty self explanatory CG spreadsheet that I made. The plane can sit on the floor, and only requires a tape measure and a scale, so is super simple, but accurate. I typically put a small wood block underneath each wheel, then move the scale around to each wheel, and insert it in place of the wood block.
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Old 04-24-2015, 11:09 AM
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Just be carful. Its a WARBIRD!!!
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Old 04-24-2015, 02:18 PM
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Quote:
Originally Posted by dabrandt22 View Post
Attached is a pretty self explanatory CG spreadsheet that I made. The plane can sit on the floor, and only requires a tape measure and a scale, so is super simple, but accurate. I typically put a small wood block underneath each wheel, then move the scale around to each wheel, and insert it in place of the wood block.
I use the scale and tape measure method too with a couple of minor exceptions. First I place a block under the tail wheel to level the airplane before taking any weights. The second difference is that rather than measuring from a wall to the mains and from a wall to the tail, I simply measure the distance from one main wheel axle to the tail wheel axle (on the diagonal) and the width of the main gear. The spread sheet then uses the mains as the origin that the CG is measured from and calculates the straight line distance (altitude of Isosceles triangle formed by mains and tail wheel) between the mains and tail wheel. Not implying there is anything wrong with your spreadsheet, just noting some minor differences that may simplify things even further..

Wayne
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Old 04-24-2015, 02:23 PM
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I taped a small piece of liteply on the bottom of the each wing tip with the "correct" CG in the middle of the ply piece. Then a buddy and I used two 1/2" hardwood dowels to lift the plane up. It took us both two hands on the dowel to check the balance. It worked though and was fairly easy to do/make.


The plane was a 124" Carden PRO.
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Old 04-24-2015, 02:45 PM
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Another option is the Vanessa Rig: google it and you will see how it works.

You do need a hard anchor on the ceiling to hang the rig from. I use the ceiling brackets for my garage door opener.

here is WEB link: http://home.mindspring.com/~the-plum...%20Machine.htm
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Old 04-24-2015, 02:56 PM
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This was discussed at length over at another forum. Take a look here:
http://www.rcuniverse.com/forum/rc-s...-big-boys.html

Rafael
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Old 07-08-2015, 08:03 AM
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Well with the plane ready to fly with all batteries and electronics installed I was able to mount the plane on my stand inverted and using 2 fingers on each hand I measured the current COG.


At the wing root the COG is 7" back from the leading edge of the wing.


The wing is 24" wide at the body and tapers on the trailing edge to the wing tip.


This gives me the COG somewhere between 25% and 33% likely at about 28-29% from the leading edge of the wing. This appears to be right at the location of the fuel tank that was originally in the plane as well as spot on the main spar of the wing.


I think this should work quite well.


My test pilot came over and after discussion we think this in a reasonable range and in all probability very flyable for an experience pilot. Worst case scenario I will go and get some self adhesive tire weights we can use if needed.


Wish me luck guys as Sunday the 12 is test day!!!!!
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Old 07-08-2015, 11:28 AM
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The 92" ziroli P47 is 5.5" from the leading edge at the root. That should be close to what you need. Warbirds are tricky, nose heavy is ok. Tail heavy is a disaster.
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