How Do They Find the CG?  
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How Do They Find the CG? - 10/17/2006 3:43:52 AM   
Ferocious Frankie



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From: San Jose, CA, USA
Status: offline 		Hi,

I never posted in this forum, but I have some questions for you guys. How do they find the CG of full scale planes?How do they balance them? How do they determine the control surface throws?

Just some questions that I've been wondering about.



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RE: How Do They Find the CG? - 10/17/2006 4:39:58 AM   
Wayne22



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From: Edmonton, AB, CANADA
Status: offline 		on something like airliners, they weigh them. Each landing gear strut supports a certain portion of the aircrafts weight and from that, and the distances between them, they can determine the weight, and cg (it would be called a station, or moment) of that particular airframe. Each indivicual aircraft is different - even though they may seem identical, the differences in installations, equipment, repairs, paint, interiors, etc, when added together can make huge differences. I've seen 2 sister aircraft (they came off Boeing's line one after the other) have a 3000 lb difference in empty weight by time they are fitted out for revenue service.

Nowadays weight and balance is either done online, or on a laptop. In the old days with 727's we were provided a chart which we had to plot positions and weights, which added to the airframes basic weight plus fuel, had to fall withing a clearly defined envelope on the chart. If the result did not fall within that area, the payload had to be re-distributed and a new chart drawn up until it was right. On top of that, we were urged to keep the cg as close to the back edge of the envelope as possible to minimize trim drag which in turn would reduce fuel burn....

< Message edited by nony -- 10/17/2006 4:54:59 AM >

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RE: How Do They Find the CG? - 10/17/2006 7:01:26 AM   
Kweasel


 

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Status: offline 		Aircraft get weighed when they are new and after major modifications to establish a base CG. After that they know how to load the aircraft to get the CG where it needs to be. Each aircraft is unique depending on its equipment and repair history. For instance most 727-200 freighters need extra fuel in the center tank for ballast if they fly with no cargo, each plane requires a different amount. You can make them sit on their tail by moving fuel from the center tank to the wings.

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RE: How Do They Find the CG? - 10/17/2006 7:10:28 AM   
GRANT ED



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Status: offline 		As for control surface throws the manufacture will give a spec of what the throw should be in degrees.

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RE: How Do They Find the CG? - 10/17/2006 2:01:38 PM   
rajul



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From: Missouri City, TX, USA
Status: offline 		Apart from shifting fuel, are there any other methods used to dynamically shift the cg?

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RE: How Do They Find the CG? - 10/17/2006 3:19:42 PM   
normgoyer


 

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From: Apple Valley, CA, USA
Status: offline 		A few years ago I restored my 4th North American SNJ-6 (AT-6) and discovered a huge hunk of lead bolted to the rear fuselage longerons just before the tail. Just like modelers do. I have also had to move battery location to bring a CG into the envelope. Many of WWII trainers had the pilot sitting in the rear cockpit and the passenger (usually the instructor) sitting in front which was usually right on the CG so it wouldn't make any difference if there were a passenger or not. The J-3 also used this method of maintaining CG with or without a passenger. If you flew a J-3 solo from the front cockpit you would probably nose it over. Norm

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RE: How Do They Find the CG? - 10/18/2006 2:12:07 AM   
jjglennon


 

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From: Fiskdale, MA,
Status: offline 		Weight times Arm equals Moment

John

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RE: How Do They Find the CG? - 10/18/2006 2:14:57 AM   
FLYBOY



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Status: offline 		A really big pair of fingers under the wing works too!

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RE: How Do They Find the CG? - 10/18/2006 3:26:22 AM   
mtt3107


 

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From: West Chester, OH, USA
Status: offline 		The control throws are first calculated, and then confirmed during flight test and certification, or modified as required.
Same goes for the CG range, it is firts calculated, and then in test flight, the safe max. forwrd and max. aft CG position is determined during flight testing.

Every airplane which comes off the assembly line is weighed and it's basic empty operating weight and CG are established and recorded.
Basic Empty Operating Weight is the aircraft, with all required gear for operation onboard, but no fuel, payload or crew.
This document is part of the airplane, and has to be onboard at all times.
It is sthe starting point for the weight&balance report, which has to be calculated before each flight, and in which the amount of cargo and it's location, quantity of fuel, and quantity of passengers aboard is taken into account.
Once it is completed and the CG location is established, the flight creew receives it, and takes it into account for the setting of the elevator trim on takeoff.

And the basic empty operating weight and CG has to be established again every time additional gear is installed, or major repairs have been performed, even if the airplane receives a new paint job.

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RE: How Do They Find the CG? - 10/18/2006 9:00:46 AM   
GRANT ED



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quote:

ORIGINAL: normgoyer

A few years ago I restored my 4th North American SNJ-6 (AT-6) and discovered a huge hunk of lead bolted to the rear fuselage longerons just before the tail. Just like modelers do. I have also had to move battery location to bring a CG into the envelope. Many of WWII trainers had the pilot sitting in the rear cockpit and the passenger (usually the instructor) sitting in front which was usually right on the CG so it wouldn't make any difference if there were a passenger or not. The J-3 also used this method of maintaining CG with or without a passenger. If you flew a J-3 solo from the front cockpit you would probably nose it over. Norm


At work we reciently finished a refurb on a bell 206 helicopter. During the reweigh it was found that we needed to add just over 3 lbs of lead to the tail to get the CG within limits. There is a spot for this right on the end of the boom on the bracket the holds the tail rotor gearbox. There is also a spot for lead in the nose of the 206 should it be needed.

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RE: How Do They Find the CG? - 4/19/2008 11:52:39 AM   
corbystarlet11



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Status: offline 		on my old mans plane he had a scale under each wheel

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RE: How Do They Find the CG? - 4/19/2008 10:58:32 PM   
C141FlyGuy



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From: Fairborn, OH, USA
Status: offline 		On large planes like the C-141, C-5, 747 and DC 10 they use the formula length x weight = moment. They add up all of the moments and all of the weght use them in a formula using LEMAC leading edge mean aerodynamic chord and the mean aerodynamic chord or MAC. Then you find the percent of MAC. Then it is checked against a chart with fuel weight with zero fuel weight and estimated landing weight to ensure they stay with in the design limits. The percent of MAC is also used in a chart to find the setting for the stab trim setting for departure.

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RE: How Do They Find the CG? - 4/21/2008 6:17:43 AM   
cjtyped


 

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From: Derby, KS, USA
Status: offline 		actually, the manufacturer isn't required to weigh every plane coming off the assembly line, only about every tenth one ( i am talking about small, general aviation aircraft)

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RE: How Do They Find the CG? - 4/25/2008 8:31:30 PM   
Johng



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From: Deland, FL, USA
Status: offline 		The gen-av planes I worked on(production), each one was weighed in order to find the CG in order to put the specific location for the CG in the POH. Every POH that I've read thru, up to Twin Otter size planes, has the actual CG as measured for that aircraft. Airplanes get different options as far as interior, avionics, paint that can make a difference between planes of the same type.

I know that modern airliners can weigh themselves. The LG struts are instrumented so the weight and CG can be found for each flight. They report to the cockpit computers and the pilot can read it along with many other parameters.

< Message edited by Johng -- 4/25/2008 8:37:05 PM >

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RE: How Do They Find the CG? - 5/7/2008 1:27:46 AM   
cjtyped


 

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Joined: 12/23/2003
From: Derby, KS, USA
Status: offline 		FAR Part 21, 21.327:
"Manufacturers of new, nontransport airplanes...may submit reports