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CG'ing A Large Jet

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Old 09-23-2012, 07:58 PM
  #1
bcovish
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Default CG'ing A Large Jet

Need to help someone CG a large Jet. Need some ideas on how to do it. The plane is a little over 11 ft long and weighs in the 50+ pound range.
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Old 09-23-2012, 08:05 PM
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Default RE: CG'ing A Large Jet

Bob,
Normally I just weight the models and do some math Calc's per full-scale weight and balance methods to get the CG. I have sort of a "beta" spread-sheet I have been workin on for a few months that I can send you if you PM me your email address.

Thomas
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Old 09-23-2012, 08:11 PM
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Default RE: CG'ing A Large Jet

pm sent
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Old 09-23-2012, 10:12 PM
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Default RE: CG'ing A Large Jet

She's a beauty, love that paint job - perfect for it!
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Old 09-23-2012, 10:25 PM
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Default RE: CG'ing A Large Jet

I would string it up from a pulley (fore aft). Drop a plumb bob straight down from the pulley. when you tilt the plane to a level 0-deg incidence attitude, the plumb bob will be directly over the CG ... adjust weight distribution from there until you can get the plumb bob to hang directly over the CG ..


all the best,


~V~
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Old 09-24-2012, 12:30 AM
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Default RE: CG'ing A Large Jet

Hi

Does it matters where you string it in the for and rear position? Also I guess the lenght of the cable should be the same in both parts, right?

The plane is beatifull. Is it a Airworld F5?

Regards

Nuno
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Old 09-24-2012, 12:50 AM
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Default RE: CG'ing A Large Jet

It's from Tomahawk
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Old 09-24-2012, 01:14 AM
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Henke Torphammar
 
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Default RE: CG'ing A Large Jet

You can build a jig like this one in any size, or buy the finnished product in any size from HAB Electronic.

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Old 09-24-2012, 03:10 AM
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Default RE: CG'ing A Large Jet

Bob, check the JPO site at http://www.jetpilots.org/index.htm Go to members/weight and balance.
Keith developed a spread sheet for use with scales complete with instructions. In addition, there are
spreadsheets for some popular models already filled in with the distance measurements.

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Old 09-24-2012, 04:32 AM
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Default RE: CG'ing A Large Jet


Quote:
ORIGINAL: bcovish

Need to help someone CG a large Jet. Need some ideas on how to do it. The plane is a little over 11 ft long and weighs in the 50+ pound range.
I did an artical in Contrails some years ago on how to do this using a scale. The artical even has pictures of this being done using the scales I borrowed from you. :-)

Steven
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Old 09-24-2012, 04:59 AM
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Default RE: CG'ing A Large Jet

Bob,

I have had good luck with this.A little pricey,but worth it in the long run.Good luck,

Erik


http://dreamworksrc.com/catalog/ez-b...ii-p-1415.html
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Old 09-24-2012, 05:00 AM
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Default RE: CG'ing A Large Jet

[link=http://dreamworksrc.com/catalog/ez-balancer-ii-p-1415.html]THIS[/link] makes it a breeze!
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Old 09-24-2012, 06:07 AM
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Default RE: CG'ing A Large Jet

Thanks guys. Will give it go on trying to get it done.

To answer a couple of the pm's that I received and as most of you probably know the F5 belongs to Scott Marr. Its a good thing that he has a 31ft trailer to haul things around. We will have it full for the trip to Best in the West next month. It's a good thing that I wasn't planning on taking my Boomerang to the event. Let's just say that the Boomer had 503 flights on it but there will not be flight number 504
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Old 09-24-2012, 06:22 PM
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Default RE: CG'ing A Large Jet

I take the mfg recommended CG and put a mark on the side of the fuse where the wing joins
this is typically at the front of the main spar for example. Then I get a strong cord and drape it under the fuse
and up across the marks so it sticks up when the wings are attached. Then by lifting this cord with a simple set of pullys,I can determine
exactly what weights etc has to be added and where it has to be added to make the plane sit exactly level while suspended on the strings.
This only costs the price of the string and 2 small pulleys. MOST of the time CG is achieved by careful placement of running gear and batteries
Sometimes small weights have to be added however. This method is very accurate.
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Old 09-24-2012, 08:15 PM
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Default RE: CG'ing A Large Jet


Quote:
ORIGINAL: jetnuno

Hi

Does it matters where you string it in the for and rear position? Also I guess the lenght of the cable should be the same in both parts, right?

The plane is beatifull. Is it a Airworld F5?

Regards

Nuno
it wont matter where its strung up, when level it will always hang directly under its CG .. with its weight wanting to find center, this will end up always directly under the pulley. The plumb bob is just a pointed that's always paralel with gravity


~V~
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Old 09-24-2012, 11:29 PM
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Default RE: CG'ing A Large Jet

most "realtime" and ACCURATE way: hung it with cables from the wing tube,or where cg should be .Look it from the sides and add weight till it gets level .You are done..

LOW WING AIRCRAFTS MUST BE BALANCED UPSIDE DOWN (L39 photo)

my 2c

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Old 09-25-2012, 12:12 AM
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Default RE: CG'ing A Large Jet

Easy and accurate C of G calculation:

Place model nose against wall, measure the distance of each wheel axle, main and nose, to wall. (you can use any other datum point you wish but a wall is often convenient) Place scales under each main wheel in turn and pack up nose so that the machine is level, note weights.

Then measure weight on nosewheel.

Multiply TOTAL weight on main wheels by distance from wall or other chosen datum. Moment A.
Multiply weight on nose gear times distance from wall or datum. Moment B.

Add A + B to obtain total moments.

Divide value of A+B by TOTAL weight and that will give you exact distance of C of G from wall or chosen datum. eg nose.

To adjust, I tape a paper cup to nose or tail and add fishing weights to get C of G exactly correct.

Easy , quick, accurate and free, can easily be done by one person, and prevents hernias. Works on 747s too, (which are NEVER inverted !) honest !
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Old 09-25-2012, 10:12 AM
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Default RE: CG'ing A Large Jet

Here is another method similar to Mr. Gladwins method.

Remember that if you have to add weight to achieve balance, it would be an interative process.

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Old 09-25-2012, 11:00 AM
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Default RE: CG'ing A Large Jet

As some have elluded to, with full size aircraft it is done with scales and not outside, but in a hangar. You dont want wind messing up the readings. However it is important to set the attitude correctly to get an accurate measure. Sometimes the manufacturer will describe a wedge be created and placed on the fuselage with which you use to level by placing blocks under nose or tail wheel.
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