Back in 2003, this was posted at Avsim Design forum to correct some of teh FDE changes between the 2 sims.
As Arne mentioned, the INCIDENCE_ALPHA variable reports the aircraft BODY angle of attack (which is the angle between the longitudinal fuselage axis and relative wind, ie TAS vector). Although it is of course a major factor for lift, we know wing lift depends on WING angle of attack which is simply: AOAw = AOAb + wing incidence + 0.5*wing twist (at least in FS8) ; FS9 discards any wing incidence and twist data in the aircraft.cfg so AOAw always equals AOAb..not true in FS8.
Wing lift is thereafter calculated from AOAw and relevant data of air file table #404 (CL vs AOAw) or Table #1545 if it exists
According the way this table is build, positive lift can be generated even for small negative AOA angles..It solely depends on the table values
Hope this will clarify
Hervé Sors
Setting Proper CoG in FS9
The key is to set the CoG at 25% Root Chord, 1/4 of the chord length back from the leading edge of the wing.
Aircraft.cfg Parameters that are key ...
wing_area=242.500 (Must be correct for lift/drag equations)
wing_span=44.058 (Must be correct for lift/drag equations, and to calculate correct MAC)
wing_root_chord=6.25 (Make this the MAC length, not root length)
wing_incidence=3.000 (Ignored by FS9 for now)
wing_twist=-4.000 (Ignored by FS9 for now)
The .air file parameters that are key to this ...
Table 404 to shift data if necessary for lift and AoA differences from MSFS' assumed incidence-twist. (Did not have to change for the Commander)
Table 1101 for handling characteristics and ...
Use cmo_Pitch Moment Coeff at AoA=0 (Trim os) to zero the trim at cruise
Table 1204 to set critical main wing parameters (area, span, MAC, Induced drag, and CoL)
Table 1515 Wing Planform - MAC/CoG to ensure MAC length, MAC to LE and root chord is correct.
The rest is normal FD tweaking for numbers and feel.
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The following was also added in that thread ... unfortunately I did not capture the author:
DETAILS OF THE FS2004 FDE CONVERSION
Load the plane at least once into FS2004 first, to get the new sections of the aircraft.cfg file added.
1. Using AirEd (
http://members.dsl-only.net/~eagle/), open the AIR file in the default FS2002 737-400 folder. Copy the Section 1534 from there (or any other plane with that section) by right clicking on the "1534 button" and choosing Copy to Clipboard. Then open your plane's AIR file and right click on any button - choose Add to Airfile (can't remember exact wording).
2. Click on the 1534 button to open the section, and click on any line that isn't 0.00. Edit the number to 0.00, and then press Enter. Do this for any non-zero line. This fixes the nose heavy trim.
3. Now leave AirEd running and open the plane's aircraft.cfg file by double clicking it. Go down to the Aircraft Geometry section and write down the values for wing_incidence and wing_twist. Now calculate the number: wing_incidence + wing_twist/2 divided by 57.3.
Example: wing_incidence = 5, wing_twist = -2
5 + -2/2 / 57.3 = 5 + -1 / 57.3 = 4/57.3 = 0.0698 (0.07).
4. Now back in AirEd click on the 404 button. A graph will open up. Press the right arrow key twice to put you on the third data point (we will ignore the first two and last two data points). Now press the x key. A number to edit will pop up. We need to subtract the number we calculated in #3 from this number (since this data point is negative, we add the number to it - example: data point = -.16, so -.16 + -0.07 = -0.23). Press enter. Then press the right arrow and x keys to bring up the next data point. Once the data points become positive (or zero) you will need to subtract the value instead. Continue this process until the antipenultimate point is reached (the third from the last) - you don't need to change the last two points. This fixes the pitch (too nose-up).
5. Now click on the 1101 button, and look for the Drag section. At the top of that section is Zero Lift drag. Reduce this value by a first guess of 16-19%, based on how much too slow the plane is in FS2004 vs FS2002 in mid level cruise at middle weights. We will fine tune this value later to fix the drag (plane is too slow).
6. Save the file in AirEd, and quit the program.
7. Now, back to the aircraft.cfg file we opened in step 3 (or double click it again). Go down to the Piston Engine section and increase the critical_altitude value by about 3500 ft (i.e. change 5000 ft to 8500 ft). You can fine tune this number by checking in FS2002 during a climb at what altitude the MAP starts to drop. Change this variable's value until it is the same in FS2004. This fixes the problem of piston planes losing power at too low an altitude (piston only).
8. 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.
9. In the General Engine section, change the min_throttle_limit to around 0.1. This will fix the piston engines dying at idle (piston only).
10. Edit the station load lines of the Weight and Balance section to change to the new FS2004 format. See one of my update files for the new format. It's best if you leave the default weights to be under the MTOW when at full fuel, and in balance (i.e. equal weight ahead and behind the center of gravity).
11. Check the new max_gross_weight line and make sure it matches with what it should be.
12. Include details of this new weight and balance in the checklist file (if any) - see my new CV-340 checklist file for details.
13. Flight testing. Check the following:
a. Cruise at a mid level (12-17,000 ft) and check the cruise speeds vs when the plane is flown in FS2002. Vary the Zero Lift Drag number in AirEd to fine tune this number.
b. Check the takeoff behavior, and make sure that you change the listed trim setting for takeoff in the Reference file, if necessary (I had to change one, out of 5 so far). The takeoff trim value should give a smooth takeoff at the proper rotation speed as given in the Reference file, and minimum elevator force needed for the takeoff and initial climb (but not too nose up on final climb).
c. Check the landing behavior, and make sure with an AP landing (set on APR mode) that you don't run out of trim before touchdown (the plane will nose into the ground). Also check the braking behavior - you should be able to stop within the distance usually described in the Reference file. In any case, check the landing distance in FS2002 and FS2004. If different, change the toe_brake_scalar (?) in the new braking section of the aircraft.cfg file until it's the same. I had to increase most of mine in the larger aircraft.
That's it!
8/2003
Then, Bob Scott offered this version:
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 to
dump 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 * Kdi
So 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 ft
or the 0.7 Oswald number to:
6.4 / (3.14 * 52.1 * .0468) = 0.836
of the 6.4 ft wing_root_chord to:
3.14 * 52.1 * 0.7 * .0468 = 5.4 ft
I 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.
Cheers
Bob Scott
ATP IMEL Gulfstream II-III-IV-V L-300
Washington, D.C.
