Visit Captain Sim web site  
  Welcome, Guest. Please Login or Register

 

Page Index Toggle Pages: [1] 2  Send TopicPrint
 10 How to calculate "CG%" (Read 32988 times)
DAL712
New Member
Offline



Posts: 1
Joined: Jul 24th, 2008
How to calculate "CG%"
Jul 24th, 2008 at 6:33pm
Print Post  
Hello,

I just purchased this aircraft and WOW, it was beyond my expectations.

Anyways, I'm having trouble finding out the CG% value on the "TAKEOFF" page of the FMC.  I've tried looking on FS9's Fuel/Load Manager and the ACE but it doesn't show the CG%.

Would anyone care to shed some light on this?
  
Back to top
 
IP Logged
 
Neal
Full Member
*
Offline



Posts: 53
Joined: Jul 24th, 2008
Re: How to calculate "CG%"
Reply #1 - Jul 24th, 2008 at 9:32pm
Print Post  
I have FSX and there is no CG% there either. I use 27% as found on the second Takeoff Ref Page. I dont have any problems.
  

Back to top
IP Logged
 
Neal
Full Member
*
Offline



Posts: 53
Joined: Jul 24th, 2008
Re: How to calculate "CG%"
Reply #2 - Aug 13th, 2008 at 10:46pm
Print Post  
Ok,
I found a neat calculator that should fix your CG Problem.
http://www.eflite.com/software/wb_demos/301/1063/
  

Back to top
IP Logged
 
SEAN ZARELLA
Full Member
*
Offline



Posts: 36
Joined: Sep 18th, 2013
Re: How to calculate "CG%"
Reply #3 - Oct 1st, 2013 at 9:23pm
Print Post  


this is the actual 757-200 load sheet in real life, and math is heavily used

you need to know the balance ARM  figures.
  
Back to top
 
IP Logged
 
Blover
Full Member
*
Offline



Posts: 165
Location: Fort Lauderdale FL, USA
Joined: May 10th, 2011
Gender: Male
Re: How to calculate "CG%"
Reply #4 - Oct 12th, 2013 at 4:26pm
Print Post  
Hi, the CG of airliners ( with tapered wings ) is called the CG of MAC
Mean Aerodynamic Chord. factory like Boeing establiswhes a datum line. the pitch axis of the aircraft. Then using a mathematical formula, they define the leading edge of MAC and the trailing edge of MAC of an equivalent straight wing. Then the factory defines a Neutral Point on that chord, They build a safety Margin in front of the NP - 5 to 15% of the MAC chord.
Then the factory establishes the empty weight CG, they also provide CG for various fuel load. - the CG creeps forward as the fuel burns in the wing tanks because of the shape.

The operator is responsible to load the aircraft within the CG of MAC
as established by the factory. i.e. from 12.5 to 35% of MAC

A low static margin from the NP, will make the aircraft tail heavy and susceptible to stall.
To much static margin  will make the aircraft nose heavy, reduces the effectiveness of the stab, and promotes very hard landing with a risk of stall at TO>

The factory provides all her numbers in the flight manual.

Happy flying Grin

Raymond   Boeing or nothing
KFLL  South florida
  
Back to top
 
IP Logged
 
Blover
Full Member
*
Offline



Posts: 165
Location: Fort Lauderdale FL, USA
Joined: May 10th, 2011
Gender: Male
Re: How to calculate "CG%"
Reply #5 - Oct 12th, 2013 at 8:14pm
Print Post  
Dal, if you still want to determine the CG of MAC for your loaded aircraft, follow my guideline, it is pretty easy, you need a little time to do it. The formula is in the internet.  Google " CG of MAC "

First find the chord of MAC for the airplane you are working with.
Use a Boeing graphic to find the lenght of wing panels, position of stab
for calculation. blow up the graphic and scale measure the distances.

Next the datum line, go to the aircraft.cfg find 'ref_datum_position"
the numbers could be 0.0, in such case Microsoft in its definition of the file says 0.0 is equal to the front edge of the leading edge of the wing.
Each weight added sits a a distance from the datum line ( the arm)
watch also the lateral distance to load equal on each side. You could fly with a low wing.
Work the moment of each item.  and including the aircraft empty weight
moment and the distance to the datum. Fuel !
remember that the weight of passenger varies with country of departure and season.
add up all the moments + or minus / total weight of aircraft.
this will give you the CG. Transfer it to the MAC chord and see if you are within the range.
I dont do that !
I rather fly !
This is what I do. take the 57, add your payload. Make the cargo a little light at the back. Take a little over 9000lbs of fuel - your expected landing fuel - the reserve.
Take off and come back to land. In full landing config at the treshold at the proper Vref speed, the attitude of the plane should be 1 or 2 degrees nose up. Your CG is set right. If not, move cargo weight until you get it.
Recently I though my 67 when landing, the nose wheel was coming down a little too quick, I added 300 lbs in my bulk cargo at the back'
Now I get 2 to 3 seconds before the nose come down.

Dal, Happy flying  Wink

Raymond   boeing or nothing
KFLL   South Florida  
  
Back to top
 
IP Logged
 
Blover
Full Member
*
Offline



Posts: 165
Location: Fort Lauderdale FL, USA
Joined: May 10th, 2011
Gender: Male
Re: How to calculate "CG%"
Reply #6 - Oct 24th, 2013 at 12:19am
Print Post  
It might also be of interest that the airliners' 757 fly with a CG of Mac around 26 or 27. Both AA and Delta fly domestic around these numbers.
The CG is with one decimal like 26.8
The CI is around 56 -60.
Then there is a section of the impact is the CI was 0 or 300.
I cannot speak of International flights - I have none for the 57.

Raymond    Boeing or nothing
KFLL   South Florida
  
Back to top
 
IP Logged
 
DAL191
Senior Member
*
Offline



Posts: 388
Location: Tampa FL USA
Joined: Nov 26th, 2009
Gender: Male
Re: How to calculate "CG%"
Reply #7 - Oct 24th, 2013 at 1:04pm
Print Post  
Forget all the calculations concerning center of gravity because the CS752 is not sophisticated enough to recognize any changes. Just use 30 all the time and you will be alright.

Michael Cubine
  

Cooler Master HAF 932 Case, Corsair 1200W PS, Corsair H50 Liquid Cooler,I7-2700K 4.6GHz, ASRock Z68 ProfessionalGen3 MB, 8GB G.Skill RipJaws DDR3 1866 MHz 2-300GB VelociRaptor 10000RPM SATA 16MB Cache, NVIDIA GeForce GTX 780ti, 3072MB, GDDR5, PCIe, ASUS 27" LED Monitor, Windows7 64bit, FSX Gold
Back to top
 
IP Logged
 
FabioL
Full Member
*
Offline



Posts: 99
Joined: Nov 11th, 2012
Re: How to calculate "CG%"
Reply #8 - Oct 24th, 2013 at 10:40pm
Print Post  
If you install v-one for the CS757, it will provide spoken v-speeds, spoken flaps, slats and spoilers, and CofG.

Also, the default FSX load manager can provide a CofG % indication (from memory, so do please check: count lines from top down to your present CofG, and divide this by 32, then x100).

I completed an immaculate flight in my CS752 tonight, VNAV and LNAV from start to finish. Brilliant!
  

Captain Sim C-130, 727, 737, 757, 767, 777, L1011, CS WX Radar & CS Weapon
Back to top
 
IP Logged
 
Markoz
CS Team
*
Offline



Posts: 12349
Location: Victoria, Australia
Joined: Apr 24th, 2009
Gender: Male
Re: How to calculate "CG%"
Reply #9 - Oct 25th, 2013 at 12:14am
Print Post  
DAL191 wrote on Oct 24th, 2013 at 1:04pm:
Forget all the calculations concerning center of gravity because the CS752 is not sophisticated enough to recognize any changes. Just use 30 all the time and you will be alright.

That's what I do, and it works fine every time. Wink Grin
  

Mark Fletcher



PC: i7 10700K @3.8/5.1GHz | 64GB DDR4 3200 | 8GB RTX 2070 | 32" LCD Monitor | 1TB SSD & 2x2TB SSD | Win 11 Pro - FSX/FSX-SE/P3D3/P3D4/P3D5/P3D6/MSFS2020
15.6" Gaming Laptop: i7 7700HQ | 32GB DDR4 | 6GB GTX 1060 | 256GB SSD & 1TB HDD | Win 10 Pro 64bit - FSX-SE/P3D4
Back to top
 
IP Logged
 
Blover
Full Member
*
Offline



Posts: 165
Location: Fort Lauderdale FL, USA
Joined: May 10th, 2011
Gender: Male
Re: How to calculate "CG%"
Reply #10 - Oct 25th, 2013 at 2:48am
Print Post  
My friends ! If I read this correctly, you are saying that the weight and balance of the aircraft.cfg is baloney and you overide it in the FMC.

My 757 is reading the aircraft.cfg,  Tongue  Tongue  and all my planes.

This is how I tune my airplanes, in the aircraft.cfg.
And they fly great !

Raymond   Boeing or nothing
KFLL   South Florida
  
Back to top
 
IP Logged
 
DAL191
Senior Member
*
Offline



Posts: 388
Location: Tampa FL USA
Joined: Nov 26th, 2009
Gender: Male
Re: How to calculate "CG%"
Reply #11 - Oct 25th, 2013 at 5:22pm
Print Post  
Blover wrote on Oct 25th, 2013 at 2:48am:
My friends ! If I read this correctly, you are saying that the weight and balance of the aircraft.cfg is baloney and you overide it in the FMC.

Raymond   Boeing or nothing
KFLL   South Florida


Raymond
It is not baloney in the RW but this is not the real world. I don't think I would put 10 or 90 in the CG but 30 will handle it every time.
Michael Cubine
  

Cooler Master HAF 932 Case, Corsair 1200W PS, Corsair H50 Liquid Cooler,I7-2700K 4.6GHz, ASRock Z68 ProfessionalGen3 MB, 8GB G.Skill RipJaws DDR3 1866 MHz 2-300GB VelociRaptor 10000RPM SATA 16MB Cache, NVIDIA GeForce GTX 780ti, 3072MB, GDDR5, PCIe, ASUS 27" LED Monitor, Windows7 64bit, FSX Gold
Back to top
 
IP Logged
 
Blover
Full Member
*
Offline



Posts: 165
Location: Fort Lauderdale FL, USA
Joined: May 10th, 2011
Gender: Male
Re: How to calculate "CG%"
Reply #12 - Oct 26th, 2013 at 2:46am
Print Post  
Michael, I just did my test flight from KJAX to KMIA under these conditions.  First I did not touch the payload of my 57, because it is perfect as is.
When I did my preflight in the FMC, I keyed in a CG of 39%
Then I pulled my genuine flight manual to set my stab trim,
at my weight of 160.0 , 39% CG is 2.0. I set it for 25% - roughly where I fly.   at 3.5.  The take off was normal at V2.

In flight my angle of attack was normal- under 2 deg.

I landed on RW 9, in a light crosswing. Vref30 was 118kts. I added 2kts for Vref30  120.

Over the treshold, 2 deg of nose up.  After touch down, a good delay of 3 sec and the nose wheel was lightly down. Like it is always.

The 39% does not do anything except to show the aircraft pop up with a rear CG. The 57 flies with the weight & balance of the aircraft.cfg.
not with the FMC number.

If the 39 was active , my airplane would have taken off at 100kts with a stab trim at 3.5 instead of the required 2.0 at a 39% CG<

It is a game Michael, not RW, but honest CS does a great job.
This little flight now. Real weather. some IFR patches, Miami open but clouds over me. A little turbulence close to the clouds.  Pretty good.
To get the weather, I look out my windows!   Smiley

Raymond    Boeing or nothing
KFLL   South Florida
  
Back to top
 
IP Logged
 
DAL191
Senior Member
*
Offline



Posts: 388
Location: Tampa FL USA
Joined: Nov 26th, 2009
Gender: Male
Re: How to calculate "CG%"
Reply #13 - Oct 26th, 2013 at 5:09am
Print Post  
Blover wrote on Oct 26th, 2013 at 2:46am:
I just did my test flight from KJAX to KMIA under these conditions.  First I did not touch the payload of my 57, because it is perfect as is.

Raymond    Boeing or nothing
KFLL   South Florida


Raymond
What fuel, number of passengers, and cargo weight are you using?
Thank you
Michael Cubine
  

Cooler Master HAF 932 Case, Corsair 1200W PS, Corsair H50 Liquid Cooler,I7-2700K 4.6GHz, ASRock Z68 ProfessionalGen3 MB, 8GB G.Skill RipJaws DDR3 1866 MHz 2-300GB VelociRaptor 10000RPM SATA 16MB Cache, NVIDIA GeForce GTX 780ti, 3072MB, GDDR5, PCIe, ASUS 27" LED Monitor, Windows7 64bit, FSX Gold
Back to top
 
IP Logged
 
Blover
Full Member
*
Offline



Posts: 165
Location: Fort Lauderdale FL, USA
Joined: May 10th, 2011
Gender: Male
Re: How to calculate "CG%"
Reply #14 - Oct 26th, 2013 at 12:43pm
Print Post  
Michael, you are early on the apron !

Payload is 23,150 - 119 pax and 5300lbs of cargo.
This being a 37 minutes flight, I took on 10500lbs of fuel.
10000, in the wings.

In aircraft.cfg I did my own weight and balance, I load my pax in section.

My CRZ was FL190,  CI 60  distance 279, I also benefited with a northern wind of around 30 - 32kts all the way , GS around 425

My rate of dsc in the GS under 1000 was -600 -700, very stable in landing cfg at Vref30 120  Cool Boeing says to come in over the treshold with 1 or 2 degress nose up. It is my only goal when I tune my CG.

At the end of a flight, I normally land with 9000lbs of fuel after a straight in approach. My reserve. Not in this flight.

Raymond   Boeing or nothing
KFLL   South Florida
  
Back to top
 
IP Logged
 
Page Index Toggle Pages: [1] 2 
Send TopicPrint
 
  « Board Index ‹ Board  ^Top