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mnmon

CoG on the nose

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I've got a plane where the CoG, as shown on the fuel and payload diagram is sitting on the nose. I've done tons of reading and at this point I'm guessing that the model reference point does not agree with the cfg reference datum or the empty Cg point (both set to 0, 0, 0) This is in FS9. How do I open up the mdl file to see where the model ref point is set. It's a non-gmax model? Or am I just going about this all wrong? Mike

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Model reference point is always (0,0,0). The reference datum point however can be whereever it needs to be as long as all aircraft.cfg entries are referenced to it.The problem you're having isn't in the .mdl file, that file has absolutely no bearing on weight and balance. All that is in the aircraft.cfg file.

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Ok, so much for that theory. :-) Why would the CG show up on the nose then if I have set the ref. datum and the empty CG both at 0, 0, 0? Another little bit of evidence: changing the fuel will shift the CG around on the diagram, but changing the load will not. This is an airliner, so even getting rid of all the passengers and bags seems to not budge the CG. It's as if the payload stations are not being read, but they look fine in the cfg. Is there something in the air file I should look at? I get get the CG to the right spot by adjusting the wing_apex_long from the current 8 to 30, but that would put the wing too far forward. Mike

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>> Ok, so much for that theory. :-) Why would the CG show up>on the nose then if I have set the ref. datum and the empty CG>both at 0, 0, 0?The model should be positioned in Max so the CG is at (0, 0, 0). The point on the nose on weight and balance diagrams is a reference point to make ARM calculations easier, and not the CG. You can adjust that in the aircraft.cfg Not sure about the rest.

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I'd have to look at the aircraft's files to be certain what was wrong. I know it's not in the .mdl file.

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I'll post the cfg file later today. The station load positions look fine and as far as I can tell, so does the rest of the plane geometry. This is the HJG B707 (freeware). Most of the users don't seem concerned about the CG resting on the nose, but it's a mystery I'd like to solve. There was some discussion in a HJG thread last year about the CG location and the "fix" of moving the wing_apex_long position, but it seems to me to not be the right solution. I'm learning a lot about FDE's in the process which is fun. Mike

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Well, the entries for the fuel tanks are interesting. Apparently they sit outside the wing.

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You noticed that too! I changed their positions, but no effect. Ed apparently has the cfg file, but I'll list it here also.Mike[fltsim.0] title=HJG B707-320B American Airlines 1978sim=B707-320Bmodel=323Bpanel=sound=texture=american_1978_323b_n8433kb_checklists=kb_reference=visual_damage=1description=HJG Boeing 707-323B repaint by Yannick CHARLANDui_manufacturer=Boeingui_type=707-320Bui_variation=American Airlines (1978)atc_id=N8433atc_airline=AMERICANatc_flight_number=760atc_heavy=0atc_parking_codes=AALatc_parking_types=GATE,CARGO,RAMP[General]performance=Length: 152.9 ftnWing span: 142.5 ftnMGTOW: 335,000 lbs.nPower: (4) P&W JT3D-3 @ 18,000 lbs eanPassengers: 121 Range: 5,350 nmnOriginal Design: Gary Carlsonnatc_type=BOEING AIRCRAFTatc_model=B703[WEIGHT_AND_BALANCE]reference_datum_position = 0.0000, 0.0000, 0.0000 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)empty_weight_CG_position=10.000, 0.000, 0.000max_number_of_stations = 50station_load.0 = "170.0, 59.0, -1.5, 0.0, Pilot" station_load.1 = "170.0, 59.0, 1.5, 0.0, Co-Pilot"station_load.2 = "170.0, 55.0, 1.5, 0.0, Engineer" station_load.3 = "20.0, 0.0, 0.0, 0.0, Crew"station_load.4 = "20.0, 46.0, 0.0, 0.0, Fwd Galley A"station_load.5 = "510.0, 40.0, 0.0, 0.0, Fwd Galley B" station_load.6 = "1360.0, 29.5, 0.0, 0.0, First Class 1-8" station_load.7 = "6120.0, 2.0, 0.0, 0.0, Coach 9-15" station_load.8 = "10200.0, -24.5, 0.0, 0.0, Coach 16-26"station_load.9 = "35.0, -39.0, 0.0, 0.0, Aft Galley A"station_load.10= "30.0, -45.5, 0.0, 0.0, Aft Galley B" station_load.11= "10000.0, 30.5, 0.0, 0.0, Forward Baggage" station_load.12= "4000.0, -25.5, 0.0, 0.0, Aft Baggage" empty_weight=140525.000empty_weight_roll_MOI=7837897.000empty_weight_pitch_MOI=15313596.000empty_weight_yaw_MOI=14379742.000empty_weight_coupled_MOI=1300.000max_gross_weight=335000.797CG_forward_limit=10.000CG_aft_limit=1.000[fuel]LeftMain= 0.000, -57.575, 9.056, 439.000, 0.000 //#1 ReserveLeftAux= 0.000, -40.600, 7.306, 2323.000, 0.000 //#1 MainCenter2= 0.000, -19.250, 4.681, 4069.000, 0.000 //#2 MainCenter1= 0.000, 0.000, 2.691, 10193.000, 0.000 //Ctr WingCenter3=0.000, 19.250, 4.681, 4069.000, 0.000 //#3 MainRightAux= 0.000, 40.600, 7.306, 2323.000, 0.000 //#4 MainRightMain= 0.000, 57.575, 9.056, 439.000, 0.000 //#4 Reservefuel_type=2.000000number_of_tank_selectors=1electric_pump=0[airplane_geometry]wing_area=3050.000000wing_span=145.750000wing_root_chord=20.667wing_dihedral=6.998wing_incidence=1.000wing_twist=-0.500oswald_efficiency_factor=0.75wing_winglets_flag=0wing_sweep=35.000wing_pos_apex_lon=8.000wing_pos_apex_vert=0.000htail_area=776.000htail_span=45.750htail_pos_lon=-76.917htail_pos_vert=0.000htail_incidence=0.000htail_sweep=35.000vtail_area=430.000vtail_span=23.643vtail_sweep=35.000vtail_pos_lon=-70.250vtail_pos_vert=5.833elevator_area=194.000aileron_area=120.600rudder_area=73.100elevator_up_limit=15elevator_down_limit=10aileron_up_limit=19.481aileron_down_limit=14.897rudder_limit=23.491elevator_trim_limit=15.481spoiler_limit=60.000spoilerons_available=1aileron_to_spoileron_gain=2.000min_ailerons_for_spoilerons=5.000min_flaps_for_spoilerons=0.000//0 Class <0=none,1=wheel, 2=scrape, 3=float>//1 Longitudinal Position (feet)//2 Lateral Position (feet)//3 Vertical Position (feet)//4 Impact Damage Threshold (Feet Per Minute)//5 Brake Map (0=None, 1=Left, 2=Right)//6 Wheel Radius (feet)//7 Steer Angle (degrees)//8 Static Compression (feet) (0 if rigid)//9 Max/Static Compression Ratio//10 Damping Ratio (0=Undamped, 1=Critically Damped)//11 Extension Time (seconds)//12 Retraction Time (seconds)//13 Sound Type//14 Airspeed limit for retraction (KIAS)//15 Airspeed that gear gets damage at (KIAS)[contact_points]//landing gearpoint.0=1.000, 54.586, 0.000, -4.400, 1874.803, 0.000, 2.136, 40.000, 0.897, 0.800, 0.500, 5.000, 6.000, 0.000, 0.000, 0.000point.1=1.000, -3.083, -10.017, -6.400, 1874.803, 1.000, 3.536, 0.000, 1.500, 0.800, 0.580, 8.200, 9.200, 2.000, 275.000, 300.000point.2=1.000, -3.083, 10.017, -6.400, 1874.803, 2.000, 3.536, 0.000, 1.500, 0.800, 0.580, 8.000, 9.000, 3.000, 275.000, 300.000//scrapepoint.3=2.000, -27.750, -65.833, 9.833, 1574.803, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 6.000, 0.000, 0.000point.4=2.000, -27.750, 65.833, 9.833, 1574.803, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 6.000, 0.000, 0.000point.5=2.000, -54.250, 0.000, 5.167, 1574.803, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 9.000, 0.000, 0.000point.6=2.000, 65.167, 0.000, 4.500, 1574.803, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 4.000, 0.000, 0.000point.7=5.000, 12.167, 47.957, 7.642, 1600, 0, 0.00, 50.0, 0.0, 0.0, 1.00, 1.0, 1.2, 0, 0.0, 0.0point.8=5.000, 12.167, -47.957, 7.642, 1600, 0, 0.00, 50.0, 0.0, 0.0, 1.00, 1.1, 1.4, 0, 0.0, 0.0//CoGstatic_pitch = -0.9 //degrees (pitch when loaded on ground)static_cg_height=4.843 //feet, altitude of CG when at rest on the groundgear_system_type=0[Flaps.0]type=1span-outboard=0.500extending-time=11.000system_type=0damaging-speed=250.000blowout-speed=300.000lift_scalar=0.000drag_scalar=0.000pitch_scalar=1.000flaps-position.0=0.000, 0.000flaps-position.1=14.000, 0.000flaps-position.2=25.000, 0.000flaps-position.3=40.000, 0.000flaps-position.4=50.000, 0.000[Flaps.1]type=1span-outboard=0.500extending-time=11.000system_type=0damaging-speed=250.000blowout-speed=300.000lift_scalar=0.500drag_scalar=0.000pitch_scalar=-0.400flaps-position.0=0.000, 0.000flaps-position.1=26.000, 0.000flaps-position.2=37.000, 0.000flaps-position.3=44.200, 0.000flaps-position.4=50.000, 0.000[Flaps.2]type=1span-outboard=0.500extending-time=11.000system_type=0damaging-speed=250.000blowout-speed=300.000lift_scalar=0.000drag_scalar=1.000pitch_scalar=0.000flaps-position.0=0.000, 0.000flaps-position.1=5.000, 0.000flaps-position.2=10.000, 0.000flaps-position.3=30.000, 0.000flaps-position.4=50.000, 0.000[GeneralEngineData]engine_type=1Engine.0=-17.416667, -46.500000, -2.916667Engine.1=-1.916667, -28.083333, -3.833333Engine.2=-1.916667, 28.083333, -3.833333Engine.3=-17.416667, 46.500000, -2.916667fuel_flow_scalar=1.0600000min_throttle_limit=-0.280029[TurbineEngineData]fuel_flow_gain=0.002000inlet_area=18.000000rated_N2_rpm=29920.000000static_thrust=18000.000000afterburner_available=0reverser_available=1[Reference Speeds]flaps_up_stall_speed=160.000000full_flaps_stall_speed=128.000000cruise_speed=478.698930max_indicated_speed=370.0max_mach=0.889000[keyboard_response]elevator=150.000000,250.000000aileron=150.000000,250.000000rudder=150.000000,250.000000[Views]eyepoint=61.297880,-1.563525,9.739993[exits]number_of_exits=2exit_rate.0=0.4 ;Percent per secondexit_rate.1=0.2 ;Percent per second[autopilot]autopilot_available=1flight_director_available=1default_vertical_speed=1500.000000autothrottle_available=0autothrottle_arming_required=0autothrottle_takeoff_ga=0autothrottle_max_rpm=90.000000pitch_takeoff_ga=8.000000use_no_default_pitch = 1use_no_default_bank = 1default_pitch_mode =0default_bank_mode =0max_pitch=10.000000max_pitch_acceleration=1.000000max_pitch_velocity_lo_alt=2.000000max_pitch_velocity_hi_alt=1.500000max_pitch_velocity_lo_alt_breakpoint=20000.000000max_pitch_velocity_hi_alt_breakpoint=28000.000000max_bank=25.000000max_bank_acceleration=1.800000max_bank_velocity=3.000000max_throttle_rate=0.100000nav_proportional_control=9.000000nav_integrator_control=0.250000nav_derivative_control=0.000000nav_integrator_boundary=2.500000nav_derivative_boundary=0.000000gs_proportional_control=9.520000gs_integrator_control=0.260000gs_derivative_control=0.000000gs_integrator_boundary=0.700000gs_derivative_boundary=0.000000yaw_damper_gain=1.000000[direction_indicators]direction_indicator.0=3,0[sMOKESYSTEM]smoke.0=-2.91, -17.416667, -46.500000, fx_hjg_b707smoke.1=-3.83, -1.916667, -28.083333, fx_hjg_b707smoke.2=-3.83, -1.916667, 28.083333, fx_hjg_b707smoke.3=-2.91, -17.416667, 46.500000, fx_hjg_b707[LIGHTS] //Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landinglight.0=1, 0.881, 0, -0.201, fx_beaconb.fx ,light.1=1, 13.546, 0, 14.317, fx_beaconb.fx ,light.2=3, -31.285, -70.811, 9.110, fx_navredm ,light.3=3, -31.285, 70.811, 9.110, fx_navgrem ,light.4=3, -77.087, 0, 11.110, fx_navwhi ,[brakes]toe_brakes_scale=1.000031parking_brake=1[gear_warning_system]gear_warning_available=1pct_throttle_limit=0.099976flap_limit_power=35.00000flap_limit_idle=12.00000[hydraulic_system]electric_pumps=0engine_map=1,1,1,1normal_pressure=3000.000000[stall_warning]type=1[attitude_indicators]attitude_indicator.0=1[turn_indicators]turn_indicator.0=1,0[Radios]Audio.1 = 1, 0Com.1 = 1, 0Com.2 = 1, 0Nav.1 = 1, 0, 1Nav.2 = 1, 0, 0Adf.1 = 1Transponder.1 = 1Marker.1 = 1

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Hi,When I was converting FS2002 models to FS2004, it was discovered that:In the Aircraft Geometry section, change the wing_pos_apex_lon value to wing_root_chord/4. Example: if wing_root_chord = 8.6, then set wing_pos_apex_lon to 2.15. This fixes the position of the CG "datum" (black and white circle) in the Fuel and Payload diagram.Perhaps that might help???Take care,--Tom GibsonCal Classic Propliner Page: http://www.calclassic.comFreeflight Design Shop: http://www.freeflightdesign.comDrop by! ___x_x_(")_x_x___

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Yesterday, I got from the Boeing site some scale drawings and went to work ;-). First thing I discovered is that the wing_apex_long was wrong, but not for some of the reasons given. Since FS defines the center of the model as the "geographic" center this has a BIG effect if we have a swept wing aircraft such as the 707. If the ref datum point is set to 0, 0, 0, then it coincides with the geographic center. The CG is then set off the ref. datum point, now also the geographic center. The model center does have an effect if you set the ref. datum to 0. First I found the geographic center, extended the leading eges of the wings into the fuselage and then measured the distance the apex was from the geographic center which is also the ref. datum point. Guess what?... 34 feet in front! I then set the wing_apex_long value to 34. Next I located the MAC (1/4 chord from leading edge) and then set the CG at 25% of MAC as given in the FAA cert sheet for the 707, which was about 12 feet in front of the geographic/ref datum center. Fired up FS and VOILA!, the CG symbol was now sitting where it should be! In effect the original file had the 707's wings about 25 feet too far aft! If the plane did not have swept wings, this wouldn't be such a problem. Tweaked some other locations such as the vstab and hstab, and engines in the air file to conform with the measurements from the drawings, but were mainly minor changes. I still can't get the station loads to affect the CG, (fuel does) but I'm working on that now. Fun learning experience with a payoff! Mike

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