.air file record 1515 explained

[Wingplanner]

The following is an explanation of .air file record 1515.

1515=Wing Planform - MAC/CoG ect,record
1515h=Sets FS variables for CoG gauge, etc.||FS2002: Sweep, Root Chord, Taper||This data (except MAC length) is overruled by parameters in [airplane_geometry] section of Aircraft.cfg||FS9: used only by Cessna 182 and 208, Extra 300, Mooney Bravo, Learjet, DH Dash-8, Concorde||CFS3: not used.

[1515]
1=00,double,MAC Length (ft)
1h=Not overruled by anything if greater than 0.000000. FS uses this figure as the MAC length when calculating the position of the CG Ball in the Fuel and Payload Planner regardless of how unreal the figure might be (FS7 Concorde 90.39)||If the real MAC length is known it should be used here||FS9-FSX MD83 beta 13.17 would require a wing_root_chord length of 19.325086 ft.

2=08,double,MAC Leading Edge position (ft)
2h=Mean Aerodynamic Chord Leading Edge longitudinal distance from FS_Reference_Position||overruled by wing_pos_apex_lon in [airplane_geometry] section of Aircraft.cfg.

3=10,double,wing_sweep (rad)
3h=should be the equivalent in radians of, but overruled by, wing_sweep (deg) in [airplane_geometry] section of Aircraft.cfg||FS will accept negative wing_sweep for wing leading edges that sweep forward instead of back.

4=18,double,wing_root_chord length (ft)
4h=Measured at aircraft's longitudinal centreline, not at its usual position at the junction of wing and fuselage||should be the same as, but overruled by, wing_root_chord in [airplane_geometry] section of Aircraft.cfg||FS will accept a wing_root_chord length that is shorter than the Standard Mean Chord (SMC) length but if the wing_root_chord in the Aircraft.cfg is longer than twice the SMC length, FS overrules it and uses the SMC length as the wing_root_chord length||FS9-FSX MD83 beta 23.1 is too long as MD83 beta maximum wing_root_chord length is 22.430427 ft, according to 1 above and 5 below should be about 19.33.

5=20,double,Taper Ratio
5h=wing_tip_chord length/wing_root_chord length||wing_tip_chord length=(SMC length*2)-wing_root_chord length||appears to have no effect||FS9-FSX MD83 beta 0.16 would require a wing_root_chord length of 19.336570 ft.

[mgh]

While it is included in, for example, the FS9 C182 .air file , that doesn't mean it's actually used. FS9 itself calculates MAC for the vaues in the [airplane_geometry] section of the .cfg file.

It doesn't appear to be included in any default FSX aircraft.

[Wingplanner]

I set up this trace by copying the record 1515 info from aam.ini then set about editing it. Unfortunately, it would seem, I omitted to remove the info in 1515h about which aircraft has record 1515 in its .air file. The information is basically irrelevant and was put there years ago, presumably by someone who was trying to figure out what 1515 was all about.

I am aware that FS calculates MAC length internally. I know how to calculate FS MAC lengths mathematically as well as FS MAC leading edge positions and FS MAC lateral positions.

All I am trying to say is - if the real MAC length is known it can be used in record 1515 as the 1515 MAC length overrules the MAC length calculated internally by FS.

The MAC length of a B727 as calculated by FS is about 17 feet, the MAC length published in the aircraft's C 0f A is 15.058333 feet (180.7 inches) which is actually shorter than the SMC length.

The only other parameter in 1515 that is of any real use is MAC Leading Edge position. If wing_pos_apex_lon is switched off in the aircraft.cfg file (by placing a semi-colon at the beginning of the wing_pos_apex_lon line) this 1515 MAC Leading Edge position can be used (provided it is not 0.000000 or negative) to get FS to tell you where wing_pos_apex_lon has to be (by rewriting the wing_pos_apex_lon line in the aircraft.cfg) if you want the MAC Leading Edge to be in a specific/published position. I use this to check that my own calculations regarding the position of wing_pos_apex_lon relative to the MAC Leading Edge position given in 1515 agree with the calculations for same made internally by FS (which they do).

[mgh]

You are becoming over elaborate

If you want to change the MAC to match Boeing's values in the interests accuracy then you should also change the wing plan to match Boeing's values. but that can't be done in FS. Incidentally,do you know what effect changing MAC has in Flight Simulator?

In FSX, as I said, Record 1515 is not used in FSX defalut aircraft. Also in FSX wing_pos_apex has no effec,t nor do many of the others in the [aircraft_geometry] section of the .cfg. See this dicussion

[Wingplanner]

1515 has no effect on flight dynamics. I just wanted to clarify the entries in that section of the .air file for anyone who might be interested.

I have made no claim that 1515 should be included in the .air file or, if it is, that it should be used.

Also (in FSX)

.air file record 1513 appears to be:

1513=[pneumatic_system],record
1513h=was FS2K Vacuum Loss Related|Could set vacuum loss in C208.

[1513]
1=00,double,bleed_air_scalar
2=08,double,max_pressure

as the figures in the [pneumatic_system] section of the FSX default C208B aircraft.cfg agree with the record 1513 figures in the FS9 default C208B .air file.

And (in FSX)

.air file record 1519 appears to be:

1519=[deice_system] structural_deice_type,int32
1519h=0=None|1=Heated Leading Edge|2=Bleed Air Boots|3=Eng Pump Boots

the emphasis in both of the above being on "appears to be". I have not found any FSX default aircraft that has record 1513 or 1519 in its .air file.

[mgh]

1515 has no effect on flight dynamics.

If that's the case, why were you explaining how to use it to "correct" MAC?

[Wingplanner]

I create my own 727 load and trim sheets, I fly the aircraft using various combinations of fuel and payload, and I don't fly the aircraft if it's out of trim.

The attachment is my current trim sheet and uses the published MAC length to determine %MAC. If I use the FS internally calculated MAC length the ramp weight %MAC moves to 12.056% which is just out of trim forward.

In this case it makes no difference to the flight dynamics which MAC length is used because the physical CG position at ramp weight in terms of feet from the datum remains unchanged. Only the %MAC changes because of the different MAC lengths used in calculating %MAC.

Some people other than myself, Vistaliners being one, do sometimes use published MAC lengths in record 1515. I have never indicated that it is mandatory to do so.

I guess it depends how "elaborate" you want to be about %MAC. User discretion, personal preference, call it what you like.

[mgh]

In this case it makes no difference to the flight dynamics which MAC length is used because the physical CG position at ramp weight in terms of feet from the datum remains unchanged

.

Just so.