How-to: FS2002 to FS2004 FDE conversions

[Bob Scott 7,720]

Folks; Here's a few nuggets on converting FS02 FDE profiles (airfile+aircraft.cfg) to FS2004/ACoF: FS2004 ignores several potentially important parameters that were used by FS2002 FDE designers to parameterize aircraft. Among them are wing_incidence, wing_twist, Body AoA at min drag, and induced_drag_constant. I've been hip deep in the FS2004 model the last week, and believe the following is a good means of converting your FS2002 FDEs over to FS2004 so they'll fly the same way. FS2004 ignores wing angle of incidence, although it reads it from both the airfile and aircraft.cfg. This is of most consequence in aircraft that have significant "net" AoIs of perhaps 0.5 deg or more. Net AoI in the model is the wing_incidence plus half the wing_twist value. Pay attention to signs...most aircraft have wash-out (incidence of the tip is less than the root) which is a negative value. In many cases the twist completely or nearly completely cancels out the wing AoI. If the wing_incidence plus half the twist is less than 0.25 deg, it may not be worth messing with except to purists (like me!). There are two critical parameters that need a look for the update. The first is the AoA vs CL curve in table 404 of the airfile, and the second is a combination of the wing_span, wing_root_chord, and oswald_efficiency_factor parameters.BEFORE YOU START MAKE A SAFE COPY OF YOUR .AIR FILE AND AIRCRAFT.CFG!!First, calculate the net AoI in the FS2002 model (using the aircraft.cfg numbers)...for example with +1.0 AoI and twist of -1.2, the net AoI is 1+(-0.6) = +0.4. For a positive net AoI value, you will need to shift the points in table 404 left by that amount converted to radians (divide by 53.7). I leave the two left and right endpoints of the curve alone (they're way outside any normal flight regime) and modify the other values between them. You'll need to use an airfile editor like AirEd or Airfile Manager (both good freeware tools). Another good tool is AirUpdate, which allows you todump the airfile into text format, then change it and write the changes back to the airfile. In any event, what you need to do is subtract 0.0074 (0.4/53.7) from the x coordinate in each of the points in the table...should result in each of the points moving left (not right or up/down). This in effect rotates the AoA curve table so that lookups yield the proper value taking AoI into account.The second parameter is wing aspect ratio (AR), defined roughly as wing span divided by mean aerodynamic chord length. FS2004 uses wing_span and wing_root_chord for this calculation. FS2002 reads an induced drag constant from table 1204 of the airfile (main wing). FS2004 ignores this parameter--instead it computes this constant as Kdi = 1/(Pi * aspect ratio * Oswald Efficiency factor)What we need to do is make sure the AR figure (or alternatively the Oswald number, found in the airfile and aircraft.cfg) is right to yield the same drag constant, or the drag performance of the airplane in FS2004 will be off, affecting power settings and fuel flows. Find the FS2002 drag constant value Kdi (in AirEd it'll be a whole number...the actual constant is that number divided by 65536...in Airfile Manager it gives you the already converted value). It's the seventh entry in Table 1204. If you will use wing_span to control the value: wing_span = wing_root_chord / (Pi * Oswald No * Kdi)Or alternatively, using Oswald No as the control variable: Oswald Efficiency No = wing_root_chord /(Pi * wing_span * Kdi)Or for wing_root_chord as control variable: wing_root_chord = Pi * wing_span * Oswald no * KdiSo for the real-world case of the FS2002 Cessna 208B, where span=52.1 ft, chord=6.4 ft, Oswald No = 0.7, and induced_drag_constant=0.0468 (3069/65536) we'd reset the 52.1 ft wing_span value in aircraft.cfg to: 6.4 / (3.14 * 0.7 * 0.0468) = 62.2 ftor the 0.7 Oswald number to: 6.4 / (3.14 * 52.1 * .0468) = 0.836of the 6.4 ft wing_root_chord to:3.14 * 52.1 * 0.7 * .0468 = 5.4 ftI do not know what other consequences may come from messing with the Oswald number...grossly large or small wing span values could effect roll performance due to change in the wing MOI, and shifts in the chord have potential for even more complicated changes in aerodynamic performance. But for most airplanes the correction required will be a reasonably minor adjustment.I've made this mod to several airplanes now...works really well. Without it, the Cessna 208, for example, showed noticeable nose-down pitch and additional induced drag effects.For the technically inclined, there's a good ongoing discussion in the Avsim MSFS Aircraft and Panel Design Forum.CheersBob ScottATP IMEL Gulfstream II-III-IV-V L-300Washington, D.C.

[Avcomware 1]

Hi, Bob.Thank you for the information. I will implement these changes and see how close we can get. I had a combination of .air and .cfg that performed reasonably well in 2k2, including Slow flight, quasi Spin, Control sensitivity and Stability. In fs9 has developed many deficiencies, which would imply that there may be other changes that M$ does not tell us about, other than Incidence and Twist.Has anyone been able to get the original 172SP to do a slow flight, Straight and Level with Stall horn on? It would seem that you start loosing altitude before the Stall horn comes on, which may point to another parameter that will have to be changed.Thanks again to all of you guys that really care about this product. It would seem that most are satisfied with just about anything, as long as it comes from a specific Co. TV

[Guest PhilipManwaring]

Thank you for posting this very helpful summary. I was able to update Charles Fox's excellent 727 .air file by shifting TBL 404 as you indicated. It took less than an hour for a complete novice using AirUpdate and she now flies beautifully! The math on the drag equations didn't quite compute for this a/c (e =1.2 !) but by reducing root_chord to reflect the mean wing chord as you did in your Cesna example, drag seems to be as it was before in FS2k2 with no apparent loss of lift (I was able to ake off at MTOW and climb to FL250 without incident). I'll give this a try on my 314 Boeing Clipper, DC-6B, and PSS Dash 8 too, since all have suffered from somewhat increased drag and increase required body AoA due to the wing-incidence issue. Thanks again!