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FSX Aircraft geometry; affect pitch.

capt_x

capt_x

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588
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germany
Hello everyone :)

I have adapted the Lear45 aircraft.cfg for my bizjet and I am running into some FDE issues. The plane behaves like a super-heavy, very slow on pitch movement. Also, take-off field length is way too much.

I have changed:
reference_datum_position
empty_weight_CG_position
contact points (landing gear)
fuel tanks CG
wing data
htail data
vtail data
to reflect the aircraft geometry of my aircraft. But behavior hasn't changed. I have searched the forums and found some useful tips, but pitch is still way too stiff. Is there anything I missed in the aircraft.cfg that could affect pitch? I want this thing to rotate at VR, not after runway end!

Thanks for any help :)
 
Thanks for the reply, Roy. From the thread that you referenced, I understand that positional data of the wings in aircraft.cfg has little to no importance at all. Here are excerpts of my aircraft config (the parts that differ from the default Lear45):
Code: 
[WEIGHT_AND_BALANCE]
max_gross_weight=18078.0                      // (pounds)
empty_weight=11582.0                          // (pounds)

reference_datum_position=24.00,  0, 0    // (feet) distance from FlightSim Reference position, which is: (1/4 chord, centerline, waterline)
empty_weight_CG_position=-25.70, 0, 0    // (feet) longitudinal, lateral, vertical distance from specified datum

[contact_points]
point.0=1,  -5.76,  0.00, -5.12, 1500, 0, 0.68, 57.0, 0.25, 3.68, 0.8, 7.0, 7.1, 0
point.1=1, -26.81, -4.53, -5.92, 3500, 1, 1.01,  0.0, 0.90, 1.71, 0.7, 9.1, 8.9, 2
point.2=1, -26.81,  4.53, -5.92, 3500, 2, 1.01,  0.0, 0.90, 1.71, 0.7, 8.9, 9.1, 3

static_pitch=-1.45
static_cg_height=3.33

[fuel]
LeftMain=-23.97, -6.0, -1.3, 399.49, 7.50     //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
RightMain=-23.97,  6.0, -1.3, 399.49, 7.50     //Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)

[airplane_geometry]
wing_area=306.8                 //Square feet
wing_span=53.2                  //Feet
wing_root_chord=9.2                   //Feet
wing_dihedral=2.44                   //Degrees
wing_incidence=2.5                   //Degrees
wing_twist=-3.0                  //Degrees
oswald_efficiency_factor=0.8                   //Measure of lift effeciency of wing
wing_winglets_flag=1                     //Are winglets available?
wing_sweep=27.52                  //Degrees, wing leading edge
wing_pos_apex_lon=-17.9                 //Feet, longitudinal distance from reference point, negative going aft
wing_pos_apex_vert=-1.52                   //Feet, vertical distance from reference point, positive going up
htail_area=80.0                  //Square feet
htail_span=19.6                  //Feet
htail_pos_lon=-43.7                 //Feet, longitudinal distance from reference point, negative going aft
htail_pos_vert=10.7                   //Feet, vertical distance from reference point, positive going up
htail_incidence=0.0                   //Degrees
htail_sweep=28.0                  //Degrees, horizontal tail leading edge
vtail_area=43.6                  //Square feet
vtail_span=6.4                   //Feet, tip to body
vtail_sweep=40.0                  //Degrees, vertical tail leading edge
vtail_pos_lon=-38.5                 //Feet, longitudinal distance from reference point, negative going aft
vtail_pos_vert=4.3                   //Feet, vertical distance from reference point, positive going up
All these values are derived either from the TCDS or the model in Blender. I didn't make any changes to the .air file yet. It is stock default Lear45 and in the "Fuel and Payload" menu, the CG is shown at approx. 25% MAC. Yet VR is at about 170 kts IAS, while VR of the default Lear45 is 108 kts IAS. Why this difference in take-off performance? I'm puzzled... The aircraft can't be too nose heavy, can it?

And one thing I can confirm: wing_pos_apex_lon definitely affects weight and balance. You can see its changes reflect directly in the CG position indicated in the Fuel and Payload menu.

Regards :)
 
Felix
Try setting more nose-up trim for take-off. I would also look at the amount of elevator movement you have in the config file. Elevator effectiveness is dependent on elevator deflection, though not elevator area (area is taken care of in the air file coefficients).
If the elevator is a bit ineffective, aft position of the main gear can delay VR. Yours is not too far aft, so I doubt that is the issue.
I would leave the air file alone for the moment.
And, yes, wing_pos_apex_lon does affect CG
Roy
 
Roy, you were right about pitch trim. Thanks. How could I miss that?! Takes off at VR as per the book! However, I wonder if the real aircraft would be so much affected by pitch mistrim... Because the Lear45 isn't so much affected by pitch mistrim. So elevator effectiveness is definitely a clue here. Where do I set that?

Is there a tutorial on how to setup aircraft geometry and flight characteristics from scratch?
 
There isn't a tutorial about FDEs at all... even Microsoft doesn't know all of the details of the .air file. I know... I asked. So... there's a lot of educated guessing going on. :)
 
So... there's a lot of educated guessing going on
Click to expand...

Maybe so, but when they revealed the air files with ESP, the only surprises I saw were the ground effect and that the autopilot data was only for AI.
The rest stacked with what was published by the likes of Jerry Beckwith.
Roy
 
I checked my .air file in Airwrench and it tells me CoL is at -24.00 from reference datum. Empty CoG is at -25.70, which is also at 34.83% MAC. MAC itself is 6.45 ft. This gives me leading edge of MAC at -23.45 and CoL at 8.5% MAC. Can this work? Is there a way to change CoL in the .air file or aircraft.cfg?
 
CoL is defined as 1/4 chord. Your reference point is 24' forward of the CoL... cuz that's the visual model center as well. Based on your values posted above:

LEMAC is 17.9' aft of the reference point. That's the 0% MAC point. 25.7' - 17.9' = 7.8'. If your MAC is 6.45'... you're actually outside your MAC.
 
17.9 aft of reference datum is wing_pos_apex_lon, which I understood (from here) is the intersection point of the prolonged leading edge and the centerline/plane. LEMAC lies behind that, and to my understanding, that would be 23.45 aft of reference datum.

If empty CoG is at 34,83% MAC (says Airwrench) and MAC is 6.45, then CoG is 2.25 aft of LEMAC. But empty CoG is also 25.70 aft of reference datum, so LEMAC is: -25.70 + 2.25 = -23.45.

edit:
AirwrenchP300_zpsaf2d01ef.png

Note that CoL almost coincides with LEMAC (left end of the bold red line segment). Is that a problem?
 
Last edited:
Per 3.1.4 on page 13 of 60 from "Flight Dynamics in MSFS V1.0.pdf" by Yves Guillaume: "The placing of MAC leading edge to the correct position along the airplane body is important for a correct CG indication in gauges and is set in wing_pos_apex_lon." He also states it does not affect flight dynamics... but I do not agree with that statement.
 
Thanks for reminding of that document, Ed. It also says that pitch moment is set in record 1534, which is missing from the default Lear45.air. How can I add that?
 
LOL... you're not ready for that. Your weight/balance is completely off.

This is for the Phenom 300, correct?
 
Correct. What do you mean "completely off"? Here is a top view from Airwrench (I've tuned wing data once more):
AirwrenchP300_1_zps883b3821.png

As you can see, all CoG positions are within 25-35% MAC.
 
Ok... visual model center is also the CoL. This is also referenced as 1/4 MAC. This is FS.

From the TCDS (A60CE_Rev_8) the CoG is at 284.69" at 25% MAC. That means that 25% MAC is at 23.72'. Assuming your MAC length is correct... then wing_apex_pos_lon should be 22.11'. Also, based on the TCDS... your fuel tank position is about 1' aft of where it should be.
 
You are quite right about LEMAC position. From EASA TCDS, it is actually 22.05'. But if you look at the following pic (Blender top view superimposed on Airwrench top view), my wing_pos_apex_lon must be correct... The wing planform calculated by Airwrench fits the Blender model almost perfectly and reference datum is exactly at the nose.
P300_geometry_zpsd5ae58de.png


Anyway, I've managed to insert 1534 in my .air file and I am now doing some tests moving CoL around...
 
Trust me... your approach is incorrect. You are trusting AirWrench and it is not flawlessly perfect.

I have done a lot of FDE work... I can assure you that despite what AirWrench is showing... my calculations are correct... it is wrong.

You asked for help... it was given. You've chosen to ignore what knowledge was offered. At this point, I'm out.
 
Hi Mr. Felix!

I ran into the same problems that you are having with my aircraft. I started off using the mooney's air file for my project. I had all kinds of problems with how the plane performed. I tried everything that I could think of. I also use AirWrench and it was having horrible problems with the mooney's air file. So I decided that I would start with a clean air file. I used the PistonSample.asm file from Lockhead Martin SDK. Then I used AirWrench to setup all the data for my aircraft. After that everything worked much better. I am steel having a couple issues that a couple of real pilots told me about. The main problem they told me that I have is that the ailerons do not work correctly. They told me that the ailerons have the same roll rate no matter what the air speed. I can live with that! Because most of the people that will fly my aircraft will probable not be real pilots. I just want a plane that is stable and fun to fly.

So if you can not fix the problem with using the air file that you are currently using. I would try using one of Lockhead Martin's sample air files.

Thanks Your Friend, Kris:)
 
Ed.
Per 3.1.4 on page 13 of 60 from "Flight Dynamics in MSFS V1.0.pdf" by Yves Guillaume: "The placing of MAC leading edge to the correct position along the airplane body is important for a correct CG indication in gauges and is set in wing_pos_apex_lon."
Click to expand...

LEMAC and wing_pos_apex_lon only coincide on a wing with an unswept leading edge. When the wing has sweep the LEMAC position is aft of wing_pos_apex_lon by a distance equal to the sweep offset. Sweep offset is the distance from the leading edge of the root chord to the leading edge of MAC

For example, I'm working on a jet with 45 degree leading edge sweep. MAC is 8.51. CG and COL coincide at 0,0,0 which is 24.95% MAC. CG readout from A:CG PERCENT, percent is 24.94.
Wing_pos_apex_lon is 10.74. LEMAC is at 2.127.
The MAC, CG and COL data are from Airwrench. CG percent is from the A:Var. FS and AirWrench are in agreement.
Roy
 
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