I've been modifying the default 737-800. All is well, but even with the up-rated engines, the aircraft still requires a very high thrust (45%+ N1) to get rolling, and throttling down to just 35% N1 will make the aircraft come to a halt (at 130,000lbs, a typical weight). This is also an issue with some addon aircraft I have. I'm wondering what parameter in the aircraft.cfg or air file I can change to perhaps lighten up the ground resistance a bit, so the aircraft will roll with less throttle.

Just to note I have updated the moment of inertias for pitch, roll and yaw, all of which were higher than default, but I'm unsure if lowering these right down would help at all.

I have to admit I don't know anything about modifying air files but I see that Flight1 have a product available called AirWrench which enables the user to "generate AIR file coefficients to create a realistic flight model from the ground up". As Flight1 products are usually pretty good, I considered getting it to modify the poor flight dynamics on a few of my addon aircraft but in the screenshots it looked very complex and potentially quite time consuming so I abandoned the idea!! However as you seem to have the requisite knowledge and patience to modify various parameters in some depth so you might find this product useful.

Bill

Ground resistance problems are a known issue in FSX and P3D.  Addon developers use a trick to adjust this, and if memory serves it is to lessen the weight that goes to ground resistance but not to in flight parameters.  You can probably find more by searching FSDEVELOPER.COM.

Best wishes.

 

OK I've signed up to FSdeveloper. Just awaiting approval. I'll post any updates and the fix if I find it. 

You either bump up low RPM thrust in the .air file to ease breakaway and taxiing, at the cost of too much thrust in flight idle or you use the FSUIPC friction fix, which will affect all installed aircraft. The latter comes in two flavors, one for the unregistered (free) version and one for the registered (payware) version.

 

See here:

http://forum.mistymoorings.com/index.php?topic=1498.0

Hi all, back again. In the end I bought a registration for Fsuipc so I could edit the LUA, rather than settle for the sim1.dll. I believe the default friction for concrete was 0.6 and Johann's setting was 0.3. I tested this and the difference was noticeable. I lowered it again to 0.2, and now I can throttle up to 40%, gain speed and then throttle down to 30% to maintain around 18 knots, but high enough that the aircraft won't roll forward on idle. Now I don't have to worry about inertias anymore, as I thought that was where I was going wrong! 

FSUIPC features two flavors of LUA files. One has lower friction values all around and the other switches back to FSX' frictions above 30 knots GS as to not impair takeoff and landing performance for aircraft tuned for the default values.

On 8/25/2017 at 8:19 PM, ShaunSAP said:

I've been modifying the default 737-800. All is well, but even with the up-rated engines, the aircraft still requires a very high thrust (45%+ N1) to get rolling, and throttling down to just 35% N1 will make the aircraft come to a halt (at 130,000lbs, a typical weight). This is also an issue with some addon aircraft I have. I'm wondering what parameter in the aircraft.cfg or air file I can change to perhaps lighten up the ground resistance a bit, so the aircraft will roll with less throttle.

Just to note I have updated the moment of inertias for pitch, roll and yaw, all of which were higher than default, but I'm unsure if lowering these right down would help at all.

I would think some of this is fairly real-world accurate.  Depending on the payload, temp, concrete surface, etc, will depend on how much thrust it will require to set it in motion. and then some level of momentum takes over.  This is how it works in the real world from what I've seen from personal experience.  

I'm now on to flight dynamics and editing the air files. Using the PMDG 737 as a guide. I bought it years ago but alas, the fps hurts and setting up every flight is annoying, and sometimes I just want a good fly around. I can see that there's a lot of parameters they didn't change, so I'm guessing ACES got some things right at least... I'm  not going to be anywhere near as extensive as PMDG, but I would like to add a few more parameters to each file as at the moment there's too few and the parameters are very linear. Currently I'm using AircraftAirFileManager; it's good because I can see the shape of lines I edit, but I can't seem to add nodes so I'm looking for another software. What I have noticed is that idle thrust seems to be somewhat low, which may be causing me to blame ground friction. We'll have to see.

1 hour ago, ShaunSAP said:

I can see that there's a lot of parameters they didn't change, so I'm guessing ACES got some things right at least...

Some .air file tables are pretty negligible or applicable to most aircraft.

But now I'm curious: Are the engine related tables (1502 to 1507) and drag related tables (430 or 154a) any different to the default 737?

The 430 drag table has slight changes, nothing too crazy. The 1502-1507 are a different story... so much for the nice exponential curve I've been working on... my 1506 looks like a nicely done exponential curve, while PMDG's is almost like a cubed curve. I'm guessing this may be to overcome the ground resistance at lower N1s. Also, the default 1506 only uses 2 mach indexes; 0 and 0.9. PMDG use 5 indexes and it is crazy... I'm never going that in-depth!!! Honestly though, at mach 0, PMDG 737 has like no thrust difference between 50% and 65% N1; it's bizarre... This bump reduces with mach but it's still there. Also... the fuel flow is also crazy.

If, by fuel flow, you mean table 1505, note that the description in AAM is misleading. Fuel flow is controlled by the thrust_specific... (or that other) parameter in the aircraft.cfg along with the thrust related tables and parameters (1506 and aircraft.cfg), while 1505 controls engine spooling speed (in conjuction with fuel_flow_gain in the aircraft.cfg).

Thanks for investigating anyway.

If the aircraft flies pretty much on the numbers, it's futile to complain, but from an external point of view and PMDG's reputation, I expected a bit more, well...custom content in the .air file.

Bjoern if that's correct, then all I have to do is find some info or calculations on TSFC. I have already found the TSFC at full thrust, which is 0.373. Putting this into the aircraft.cfg surprisingly gave me a very close fuel flow at idle, max static and cruise thrust (600pph,10,500pph and 2,500pph respectively), and this was before I even got into editing air files. Interestingly, I can't using the PMDG 1505 table as a reference because they haven't used TSFC at all the the CFG... infact they've even upped the fuel flow scalar, so right now I'm rather proud of having correct fuel flows. I would however like to find some real world fuel flows for the mid-range n1s.

Also, I'm having issues correcting my 1506 table. The N1s I have are just calculated static but haven't been corrected. As a result, I'm missing a constant 5% thrust when test the craft. (Sea level airport, 59F, 1013/2992, no weather, wind etc.

Also Bjoern, the way I explained PMDGs air files may have been a little vague. They have edited LOADs of parameters, and added loads of extra 'nodes' so that they performance is very smooth rather than jumpy.

Could I ask that someone please send me the default 737-800 air file and aircraft.Cfg? I've messed up somewhere... it would seem the aircraft randomly pitches up on approach and the autopilot does nothing to stop it. Other than that the aircraft is running smoothly... 

17 hours ago, ShaunSAP said:

Bjoern if that's correct, then all I have to do is find some info or calculations on TSFC. I have already found the TSFC at full thrust, which is 0.373. Putting this into the aircraft.cfg surprisingly gave me a very close fuel flow at idle, max static and cruise thrust (600pph,10,500pph and 2,500pph respectively), and this was before I even got into editing air files. Interestingly, I can't using the PMDG 1505 table as a reference because they haven't used TSFC at all the the CFG... infact they've even upped the fuel flow scalar, so right now I'm rather proud of having correct fuel flows. I would however like to find some real world fuel flows for the mid-range n1s.

Also, I'm having issues correcting my 1506 table. The N1s I have are just calculated static but haven't been corrected. As a result, I'm missing a constant 5% thrust when test the craft. (Sea level airport, 59F, 1013/2992, no weather, wind etc.

Also Bjoern, the way I explained PMDGs air files may have been a little vague. They have edited LOADs of parameters, and added loads of extra 'nodes' so that they performance is very smooth rather than jumpy.

I always set TSFC for cruise performance since that's where you'll be spending most of your time.

Normally, the thrust book values are for ISA conditions, so make sure that the sim weather reflects this ("Clear" scenario or whatever it's called).

Now that sounds more like a professional add-on maker!

 

8 hours ago, ShaunSAP said:

Could I ask that someone please send me the default 737-800 air file and aircraft.Cfg? I've messed up somewhere... it would seem the aircraft randomly pitches up on approach and the autopilot does nothing to stop it. Other than that the aircraft is running smoothly... 

Check the CG values.

 

No joy on the .air file since AirUpdate refuses to work on my laptop. Relevant aircraft.cfg sections:

	[WEIGHT_AND_BALANCE]
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
	max_number_of_stations = 50
	station_load.0 =   170.0,  46.3, -1.5,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)        
station_load.1 =   170.0,  46.3,  1.5,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.2 =   510.0,   0.0,  0.0,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.3 =  1360.0,  33.4,  0.0,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.4 =  5440.0,  16.1,  0.0,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.5 =  5440.0,  -9.7,  0.0,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.6 =  4760.0, -28.0,  0.0,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.7 =  4000.0,  24.7,  0.0,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.8 =  1000.0, -34.4,  0.0,    0.0    //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
	station_name.0 = "Pilot"            
station_name.1 = "Co-Pilot"         
station_name.2 = "Crew"             
station_name.3 = "First Class"      
station_name.4 = "Coach 3-10"       
station_name.5 = "Coach 11-18"      
station_name.6 = "Coach 19-25"      
station_name.7 = "Forward Baggage"  
station_name.8 = "Aft Baggage"             
	;Moments of Inertia
empty_weight_pitch_MOI   = 1600655
empty_weight_roll_MOI    =  588050
empty_weight_yaw_MOI     = 2148751
empty_weight_coupled_MOI = 0
	[flight_tuning]
cruise_lift_scalar     = 1.0
parasite_drag_scalar   = 1.0
induced_drag_scalar    = 1.0
elevator_effectiveness = 1.0
aileron_effectiveness  = 1.0
rudder_effectiveness   = 1.0
pitch_stability        = 1.0
roll_stability         = 1.0
yaw_stability          = 1.0
elevator_trim_effectiveness = 1.0
aileron_trim_effectiveness  = 1.0
rudder_trim_effectiveness   = 1.0
	[GeneralEngineData]
engine_type = 1                                 //0=Piston, 1=Jet, 2=None, 3=Helo-Turbine, 4=Rocket, 5=Turboprop
Engine.0 = 0, -16.1, -3.2                    //(feet) longitudinal, lateral, vertical distance from reference datum
Engine.1 = 0,  16.1, -3.2                    //(feet) longitudinal, lateral, vertical distance from reference datum
fuel_flow_scalar = 1                            //Scalar for fuel flow efficiency
min_throttle_limit = -0.25                      //Minimum percent throttle.  Generally negative for turbine reverser
max_contrail_temperature = -30                  //Temperature (deg C) below which contrails may appear
	[TurbineEngineData]
fuel_flow_gain = 0.002                          //Gain on fuel flow
inlet_area = 19.6                               //Square Feet, engine nacelle inlet area
rated_N2_rpm = 29920                            //RPM, second stage compressor rated value
static_thrust = 24200                           //Lbs, max rated static thrust at Sea Level
afterburner_available = 0                       //Afterburner available?
reverser_available = 1                          //Thrust reverser available?
ThrustSpecificFuelConsumption = 0.6             //Thrust specific fuel consumption (Jets)
AfterBurnThrustSpecificFuelConsumption = 0      //TSFC with afterburn/reheat engaged
	[jet_engine]
thrust_scalar = 1.0
	[electrical]
max_battery_voltage = 24.0
generator_alternator_voltage = 30.0
max_generator_alternator_amps = 400.0
	        //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]
point.0=1,  40.85,   0.00, -10.20,  1600, 0, 1.442, 55.92, 0.6, 2.5, 0.9, 4.0, 4.0, 0, 220.0, 250.0
point.1=1, -11.85,  -9.38, -10.80,  1600, 1, 1.442,  0.00, 1.0, 2.5, 0.9, 6.9, 6.9, 2, 220.0, 250.0
point.2=1, -11.55,   9.38, -10.80,  1600, 2, 1.442,  0.00, 1.0, 2.5, 0.9, 7.1, 7.1, 3, 220.0, 250.0
point.3=2, -29.00, -60.00,   4.00,  1000, 0, 0.000,  0.00, 0.0, 0.0, 0.0, 0.0, 0.0, 5, 0.0, 0.0
point.4=2, -29.00,  60.00,   4.00,  1000, 0, 0.000,  0.00, 0.0, 0.0, 0.0, 0.0, 0.0, 6, 0.0, 0.0
point.5=2, -43.00,   0.00,  -3.80,  1000, 0, 0.000,  0.00, 0.0, 0.0, 0.0, 0.0, 0.0, 9, 0.0, 0.0
point.6=2, -71.00,   0.00,  31.00,  1000, 0, 0.000,  0.00, 0.0, 0.0, 0.0, 0.0, 0.0, 9, 0.0, 0.0
point.7=2,  44.00,   0.00,  -4.50,  1000, 0, 0.000,  0.00, 0.0, 0.0, 0.0, 0.0, 0.0, 4, 0.0, 0.0
	max_number_of_points = 21
	static_pitch=-0.7       //degrees, pitch when at rest on the ground (+=Up, -=Dn)
static_cg_height=9.67   //feet, altitude of CG when at rest on the ground
gear_system_type=1      //Hydraulic
	[gear_warning_system]
gear_warning_available = 1    //Normal
pct_throttle_limit =  0.1     //Percent throttle that activates warning
flap_limit_idle    =  5.0     //Flap angle that activates warning at idle
flap_limit_power   = 25.5     //Flap angle that activates warning at above idle
	[brakes]
parking_brake = 1         //Parking brake available
toe_brakes_scale = 0.885  //Brake scalar
auto_brakes = 4
hydraulic_system_scalar = 1  //On brakes dependant on the hydraulic system, ratio hyd system to max brake hyd pressure
	[hydraulic_system]
normal_pressure = 3000.0    //PSI
electric_pumps  = 1         //electric pump
engine_map      = 1,1,0,0   //pumps on Eng1,2
	[flaps.0]       //Trailing Edge Flaps
type             = 1                                    // 1 - tail, 2 - lead
span-outboard    = 0.8                                  // 0.0 .. 1.0
extending-time   = 20                                   // seconds
flaps-position.0 = 0                                    // degrees
flaps-position.1 = 1                                    // degrees
flaps-position.2 = 2                                    // degrees
flaps-position.3 = 5                                    // degrees
flaps-position.4 = 10                                   // degrees
flaps-position.5 = 15                                   // degrees
flaps-position.6 = 25                                   // degrees
flaps-position.7 = 30                                   // degrees
flaps-position.8 = 40                                   // degrees
damaging-speed   = 250                                  // KIAS
blowout-speed    = 300                                  // KIAS
lift_scalar = 1.0
drag_scalar = 1.0
pitch_scalar= 1.0
system_type = 1                                         //Hydraulic
	[flaps.1]       //Inboard Leading Edge Flaps
type             = 2                                    // 1 - tail, 2 - lead
span-outboard    = 0.8                                  // 0.0 .. 1.0
extending-time   = 2                                    // seconds
flaps-position.0 = 0                                    //
flaps-position.1 = 1.0                                  //
damaging-speed   = 250                                  // KIAS
blowout-speed    = 300                                  // KIAS
lift_scalar = 1.0
drag_scalar = 1.0
pitch_scalar= 1.0
system_type = 1                                         //Hydraulic
	
[flaps.2]       //Outboard Leading Edge Slat
type             = 2                                    // 1 - tail, 2 - lead
span-outboard    = 0.8                                  // 0.0 .. 1.0
extending-time   = 3                                    // seconds
flaps-position.0 = 0                                    //
flaps-position.1 = 0.5                                  //
flaps-position.4 = 1.0                                  //
damaging-speed   = 250                                  // KIAS
blowout-speed    = 300                                  // KIAS
lift_scalar = 1.0
drag_scalar = 1.0
pitch_scalar= 1.0
system_type = 1                                         //Hydraulic
	[vacuum_system]
max_pressure=5.15   // PSI
vacuum_type=2       //Type: 0=None, 1=Engine pump (default), 2=Pneumatic, 3=Venturi
	[pneumatic_system]
max_pressure=18.000000
bleed_air_scalar=1.000000
	[autopilot]
autopilot_available=1
flight_director_available=1
default_vertical_speed=1800
autothrottle_available=1
autothrottle_arming_required=1
autothrottle_max_rpm = 90
autothrottle_takeoff_ga=1
pitch_takeoff_ga = 8;
max_pitch=10.0
max_pitch_acceleration=1.0
max_pitch_velocity_lo_alt=2.0
max_pitch_velocity_hi_alt=1.5
max_pitch_velocity_lo_alt_breakpoint=20000.0
max_pitch_velocity_hi_alt_breakpoint=28000.0
max_bank=30,25,20,15,10
max_bank_acceleration=1.8
max_bank_velocity=3.00
max_throttle_rate=0.10
nav_proportional_control=12.00
nav_integrator_control=0.25
nav_derivative_control=0.00
nav_integrator_boundary=2.50
nav_derivative_boundary=0.00
gs_proportional_control=25.0
gs_integrator_control=0.53
gs_derivative_control=0.00
gs_integrator_boundary=0.70
gs_derivative_boundary=0.00
yaw_damper_gain = 1.0
	[fuel]
Center1   = -5.9,   0.0, -3.2, 4299, 0          //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
LeftMain  = -9.7, -17.2, -3.2, 1288, 0          //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
RightMain = -9.7,  17.2, -3.2, 1288, 0          //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
fuel_type = 2                                   //Fuel type: 1 = Avgas, 2 = JetA
number_of_tank_selectors = 1                    
electric_pump=1
fuel_dump_rate = 0.0167                        //Percent of max quantity per second, i.e. about 1 minute for full fuel
	[airplane_geometry]
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
	htail_area              = 373.6                 //Square feet
htail_span              = 47.1                  //Feet
htail_pos_lon           = -63.5                 //Feet, longitudinal distance from reference point, negative going aft
htail_pos_vert          = 5.4                   //Feet, vertical distance from reference point, positive going up
htail_incidence         = 0.0                   //Degrees
htail_sweep             = 38.2                  //Degrees, horizontal tail leading edge
	vtail_area              = 323.0                 //Square feet
vtail_span              = 27.0                  //Feet, tip to body
vtail_sweep             = 36.3                  //Degrees, vertical tail leading edge
vtail_pos_lon           = -56.0                 //Feet, longitudinal distance from reference point, negative going aft
vtail_pos_vert          = 12.9                  //Feet, vertical distance from reference point, positive going up
	elevator_area           = 70.5                  //Square feet
aileron_area            = 26.9                  //Square feet
rudder_area             = 56.2                  //Square feet
elevator_up_limit       = 22.5                  //Degrees
elevator_down_limit     = 19.5                  //Degrees
aileron_up_limit        = 20.0                  //Degrees
aileron_down_limit      = 20.0                  //Degrees
rudder_limit            = 26.0                  //Degrees
elevator_trim_limit     = 20.0                  //Degrees
spoiler_limit           = 60.0                  //Degrees
spoiler_extension_time  = 2.0            //Seconds
spoilerons_available    = 1                     //Spoilerons Available?
aileron_to_spoileron_gain = 3                   //Aileron to spoileron gain
min_ailerons_for_spoilerons = 10                //Degrees
min_flaps_for_spoilerons = 0                    //Minimum flap handle position when spoilerons activate
auto_spoiler_available   = 1
positive_g_limit_flaps_up  =  4.0               //Design G load tolerance (positive, flaps up)
positive_g_limit_flaps_down=  3.0               //Design G load tolerance (positive, flaps down)
negative_g_limit_flaps_up  = -3.0               //Design G load tolerance (negative, flaps up)
negative_g_limit_flaps_down= -2.0               //Design G load tolerance (negative, flaps down)
load_safety_factor         =  1.5               //Design G load safety factor
	[deice_system]
structural_deice_type=1     //0 = None, 1 = Heated Leading Edge, 2 = Bleed Air Boots, 3 = Eng Pump Boots