Hello,

 

I have a question about CG % MAC. The CG stuff makes me crazy :-).

I have many variables for the CG Calculation from aircraft.cfg:

 

max_gross_weight=155500                      // (pounds)

empty_weight=85710                           // (pounds)

 

reference_datum_position=0, 0, 0         // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)

empty_weight_CG_position=-9, 0, 0         // (feet) longitudinal, lateral, vertical distance from specified datum

 

wing_area=1344.0                //Square feet

wing_span=117.42                //Feet

wing_root_chord=21.0                  //Feet

wing_dihedral=5.5                   //Degrees

wing_incidence=1.0                   //Degrees

wing_twist=-0.5                  //Degrees

oswald_efficiency_factor=0.75                  //Measure of lift effeciency of wing

wing_winglets_flag=1                     //Are winglets available?

wing_sweep=27.2                  //Degrees, wing leading edge

wing_pos_apex_lon=6.96                  //Feet, longitudinal distance from reference point, negative going aft

wing_pos_apex_vert=-3.8                  //Feet, vertical distance from reference point, positive going up

 

and the caculated Total Weight and Total Arm for the B737-800 (FSX default).

 

Now the question is, what is the fsx ( or AirWrench ) internal formula for the CG % MAC value? I have tried a lot of formula find via Google. But I never have the same resulat as FSX or AirWrench ( B737-800 with EmptyWeight 35.95 % CG MAC).

 

Thanks in advance for your help !

 

 

Sebastian

i'm not surprised you're confused - many of those variables are nothing to do with CG.

 

Look for fs_flight_dynamics_v1.0.zip, which should be in the AVSIM library. Section 3 Weight & Balance is what you want.

Hello,

 

I have a question about CG % MAC. The CG stuff makes me crazy :-).

I have many variables for the CG Calculation from aircraft.cfg:

 

max_gross_weight=155500                      // (pounds)

empty_weight=85710                           // (pounds)

 

reference_datum_position=0, 0, 0         // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)

empty_weight_CG_position=-9, 0, 0         // (feet) longitudinal, lateral, vertical distance from specified datum

 

wing_area=1344.0                //Square feet

wing_span=117.42                //Feet

wing_root_chord=21.0                  //Feet

wing_dihedral=5.5                   //Degrees

wing_incidence=1.0                   //Degrees

wing_twist=-0.5                  //Degrees

oswald_efficiency_factor=0.75                  //Measure of lift effeciency of wing

wing_winglets_flag=1                     //Are winglets available?

wing_sweep=27.2                  //Degrees, wing leading edge

wing_pos_apex_lon=6.96                  //Feet, longitudinal distance from reference point, negative going aft

wing_pos_apex_vert=-3.8                  //Feet, vertical distance from reference point, positive going up

 

and the caculated Total Weight and Total Arm for the B737-800 (FSX default).

 

Now the question is, what is the fsx ( or AirWrench ) internal formula for the CG % MAC value? I have tried a lot of formula find via Google. But I never have the same resulat as FSX or AirWrench ( B737-800 with EmptyWeight 35.95 % CG MAC).

 

Thanks in advance for your help !

 

 

Sebastian

Basically this is all about the weight and arm of an aircraft and its contents. For example, the basic weight from one of my G550s is 49232 with a moment of 295670. Think of the CG formula as W,A,M. Weight, ARM, and Moments. ARM is the inches from the ref datum. Usually its just off the nose after removal of test equipment. Weight times the flight station/ARM gives you the moment. So as items are added to the aircraft, its weight is multiplied times the location from the ref datum. So in my jet, the pilots are located at station 114. 114 inches from the ref datum. We count the pilots at 195 pounds a piece so its 390 pounds times 114 divided by the simplifier of 100. (390 x 114)/100= 444.6 so we round to 445. we will calculate with no fuel just pilots.

 

First we add 390 to the basic weight. 49232 + 390 = 49622. Next we add the moment of 445 to the aircraft's moment. 295670 + 445 = 296115. This is the op weight. Now we do M/W to get the basic arm. 296115/49622 * 100 = 596.74. Now we add this to the Gulfstream 5/550 formula. (ARM_________ - 524.74) ÷ 1.7119 = (596.74 - 524.74)/1.7119 = 42.05 or 42.1% CG.

 

This might not help you since its real world but it gives you an idea how its calculated. Every thing in an aircraft is weighed and given a moment based on the ARM. Even fuel. Once all the weights and moments are added up, you divide moment by the weight to get the arm and then place it in the aircraft's specific formula to get the percent of MAC.

Dear mgh, dear all,

 

thanks for the comment. I have try this formula for the calculation, but not wirh the same result as FSX or AirWrench.

 

As I understood in the "flight dynamic document" from avsim is the challange to get the LEMAC ( leading edge MAC ). The basic formula for the CG % is:

 

Distance to CG - Distance to LEMAC = Distance CG to LEMAC

 

and

 

(Distance CG to LEMAC / MAC) * 100 = CG % MAC

 

For Example Default B737-800:

 

Variables given:

wing_span = 117.42 ft

wing_sweep = 27.2°

wing_root_chord = 21.0 ft

wing_area = 1344.0 ft

MAC 14.10 ft ( calculated, same result as AirWrench )

 

The Calculation:

 

OK 14,41 ft LEMAC, then I will calculate the CG % MAC from empty weight of the B737-800

 

Arm = 9 ft

Then the formula: 9 ft - 14,41 ft = -5,41 ft

And: (-5,41 ft / 14,10 ft) * 100

= 38,64 CG % MAC

 

This result is not the same as FSX and AirWrench empty weight CG ( 35.95 % ).

 

 

Can anyone see the mistake in the calculation ?

 

 

Best regards

Sebastian