MSFS20 Need Help Tweaking Default DA40NG Plane - Datum Reference

[Alexismmef]

My First Post in this Forum, after receiving the suggestion by my Great Friend Jcomm

I have started to learn how to Tweak Flight Models as a Hobby, using the excitement that this new Simulation gave me to get back to flying like I haven't in a long time. Since I have so many material regarding the DA40NG, this is the Aircraft I have choosen to tweak and Learn.

One of the issues I am experiencing right now is to Position the Datum Reference Point, in according to the real DA40NG Manual. In MFS, it is clearly not in correct position, as it should be ahead of the Nose Cone, and in MFS it's behind it. I have tried positioning in according to the AFM, which states "The Datum Plane is located 2.194 meter (86.38 in) forward of the most forward point of the root rib on the stub wing."



Since MFS now shows me visually the Datum point in Dev Mode. I have tried several methods to position the Datum on a reference surface, and then calculate the correct Distance to Datum. It never ends up in correct position. I have tried to start from the Root Rib on the Stub Wing, and also from the Most Forward Point in the Nose Cone, to do the calculations.

Either I am missunderstanding how this works, or the 3d Model has wrong Dimensions?

Is there any other way I could verify where I may be making a mistake, or something is wrong with the model?

Best Regards!

Alexis

 

[jcomm]

Good point Alexis.

We do not know what is the so called VMO ( "Visual Model Origin" ) in MFS I guess ( ? )

By definition, and from Guillaume's PDF:

Three offset points from the VMO:
 2. Reference_datum_position in aircraft.cfg affecting
 Weight and fuel distribution (CG position)
 Moments of Inertia
 Contact points
 Engine location
 Lights
 …
 3. Wing Aerodynamic Center
 Set via R1534 (if existing) or
 set by MSFS internally (if R1534 missing)
 Only affects pitch moments (but not roll and yaw moments)
 4. Center of Gravity (CG)
 Calculated based on empty weight, current payload and current fuel load

 

[Roy Holmes]

The visual reference point is chosen as a conveniently acquired position to measure moments from when the airplane is weighed. The airplane CG will have to be within defined limit distances from that point.
In other words it is only used when the aircraft CG is being calculated.
As such it plays no part in the aerodynamic performance of the airplane.
Roy

 

[WarpD]

The reference datum is based on the center of the visual object. The visual object center is 0,0,0... and in FSX it was (by default) aft of the LEMAC by 25% of the MAC

 

[Alexismmef]

THank you both. My issue is that I can't confirm if Asobo did put this point at 25% of LEMAC. Also, I have just found out, manually positioning the Datum to the visual model to measure the distance ,that the real DA40NG has a Length of 8.06 Meters, while the one Asobo Modeled has 8.46Meters. So this is why I can set the Datum to the proper position, and everything goes wrong afterwards. It's just too big :/

 

[jcomm]

ASOBO is Big about everything

They did Big Scenery, Big Weather ( depiction, because the physics are still unfinished... ) and now we know, Big Aircraft too

But the approach you followed, and I had the opportunity to follow thanks to the images you sent me was the correct one. You made in the CFG the "z" coordinate of the Datum = 0, so we could see the so called absolute reference point ( VMO on Guillaume's PDF ), which indeed appears to be situated a bit aft of the wing leading edge, but ahead of what we would say it's 25% MAC.

The other problem, specially for a guy like me where close vision becomes a "hard" problem ( and I think this modern LED monitors have contributed to worsen it ... ) is the interesting approach of making it possible to visualize the various elements of the 3d model represented by MFS to infer the various derivatives, against the scenery. It would be much easier if there was no background scenery... It was quite difficult for me to glimpse the position of the "cross" marked Datum point...

Anyway, and above all, more than 13 years after having dropped FSX for good, and 8 after the biggest disappointment on my simmer life when MS killed MS FLIGHT, I'm back to the thrill of "Aircraft CFG Editing"

 

[Alexismmef]

ASOBO is Big about everything

They did Big Scenery, Big Weather ( depiction, because the physics are still unfinished... ) and now we know, Big Aircraft too

But the approach you followed, and I had the opportunity to follow thanks to the images you sent me was the correct one. You made in the CFG the "z" coordinate of the Datum = 0, so we could see the so called absolute reference point ( VMO on Guillaume's PDF ), which indeed appears to be situated a bit aft of the wing leading edge, but ahead of what we would say it's 25% MAC.

The other problem, specially for a guy like me where close vision becomes a "hard" problem ( and I think this modern LED monitors have contributed to worsen it ... ) is the interesting approach of making it possible to visualize the various elements of the 3d model represented by MFS to infer the various derivatives, against the scenery. It would be much easier if there was no background scenery... It was quite difficult for me to glimpse the position of the "cross" marked Datum point...

Anyway, and above all, more than 13 years after having dropped FSX for good, and 8 after the biggest disappointment on my simmer life when MS killed MS FLIGHT, I'm back to the thrill of "Aircraft CFG Editing"

That was my plan all along, bring Jcomm back to the "Flight Simming Tweak until you can't anymore" attitude

 

[Roy Holmes]

In a side elevation view you can only see the root chord of the wing, you need to also consider if there is taper such that the wing tip chord is smaller than the root chord. That would indicate that mean (average) chord is shorter than root chord. And while not the case here, if your wings are swept the mean chord will be quite a way aft of where that of an unswept wing would be.
The data for the reference datum and empty weight CG positions for your airplane is in Official/Onestore/asobo-aircraft-da40-ng/SimObjects/Airplanes/Asobo_DA40_NG/ flight_model.cfg
[WEIGHT_AND_BALANCE]
reference_datum_position = 7.2, 0, 0 ; Position of reference datum relative to FS(0,0,0) (FEET), z, x, y
empty_weight_CG_position = -7.5, 0, -0.5 ; Position of airplane empty weight CG relative to reference datum (FEET), z, x, y
So you have 3 positions.
FS(0,0,0) which is the model origin.
reference datum position 7.2 feet forward of the origin which looks to be ahead of the spinner.
empty weight CG position, 7.5 feet aft of the reference point and therefore 0.3 feet aft of the origin.
So in this case, and it is not uncommon, the CG is not 25% of the MAC behind the MAC leading edge
As I said above, the visual reference as such plays no part in the sim aerodynamics, it is the CG position relative to where the lift vector is that has a pretty significant effect on pitch stability and trim needed for level flight. If the CG is at or behind the lift vector the beast will be unstable in pitch and if too far forward it will be relatively unresponsive.
Roy

 

[jcomm]

Yes Roy,

all good points!

I believe Alexis has a good DA 40 blueprint from the aircraft manual, with lot's of measures. Probably ASOBO used that exact same document because we can see in that blueprint that the reference datum is indeed slight ahead of the tip of the spinner.

The problem is that the 3d model of the aircraft is most probably not in sync with the data in the CFG files, so, when we use the in-sim visualization tools the references ( which are relative to the CFG measures and geometry ) appear displaced...

Alexis told me the aircraft tend to cruise nose down ( I don't have MFS installed right now... ).

Just as an additional side note I find a bit of a deception that having ASOBO taken the flight model a step ahead, using a reference 3d sketch for the base of stability derivate calculations they didn't include features like it being possible to define a cant for the engine thrust line, the vertical stab, multiple wings, at least allowing for the definition of biplanes, etc...

We ended up with something that appears to be more powerful than what we had until MS FLIGHT, but still not as detailed of what, for instance, X-Plane and Aerofly FS 2 allow for, not to mention DCS and, I guess IL2 and Condorsoaring, etc...

 

[bstolle]

All MSFS Diamond aircraft are apparently using a 'conventional' NACA 2412 like airfoil instead of the correct Wortmann FX glider airfoils.
That's why the pitch attitude during all phases of flight is noticeable too high.

 

[jcomm]

@bstolle, interesting... Where did you get that info from Bernt ?

Also noticed today for the very first time that sometime ago, around Nov 19, FSX:SE got an update and most of the .AIR files vars were brought to the aircraft,cfg.

 

[bstolle]

I simply compared the MSFS attitudes with my RW experience and youtube videos (to confirm my findings). Stall speeds, constant speed prop behavior etc etc. there's presently a lot seriously wrong with the Diamond FDMs and the MSFS base FDM.

 

[Alexismmef]

All MSFS Diamond aircraft are apparently using a 'conventional' NACA 2412 like airfoil instead of the correct Wortmann FX glider airfoils.
That's why the pitch attitude during all phases of flight is noticeable too high.

Hey Bernt! Nice to see you here

I replaced the lift_coef_aoa_table with information from this Airfoil, that the DA40NG says is the one used.

WORTMANN FX 63-137 AIRFOIL (fx63137-il)

Details of airfoil (aerofoil)(fx63137-il) WORTMANN FX 63-137 AIRFOIL Wortmann FX 63-137 human power aircraft airfoil (Liver Puffin) airfoil

airfoiltools.com

Actually, the AFM says the Airfoil is Wortmann FX 63-137/20 - W4 . I haven't found any info regarding this 20 - W4? Maybe it's a variation made for Diamond Aircraft.

But after I did that, and replaced some default values with more realistic info, the aircraft has now a Cruise AoA of -6. So something is clearly wrong, the aircraft is Pitching Down too much.

I couldn't locate the DATUM also with reference from the AFM, the Asobo model doesn't seem to have the same Length as the real one, at least my Measument didn't match the numbers, so it's impossible for me to locate the Empty CG in the same position as the real airplane?

I also don't know what's the correlation between "cruise_lift_scalar" which seems to limit the Slope of CL vs Alpha. Also "lift_coef_horizontal_incidence = 0 ; The change in lift per change in horizontal incidence angle" . I don't understand what it does. Seems to be about the same as cruise lift scalar? Reduce the Slope of CL curve?

Regards!

 

[Roy Holmes]

I opened the Wortman airfoil page. If you look at the chart for CL versus Alpha you can see that the AOA for CL=0 is minus 8. That means when flying straight and level at cruising speed you will have an AOA of about -6. So the attitude using Wortman is correct. I would suspect that an airfoil of that type would be set at around 5 degrees or so nose up with respect to the fore and aft axis. In the sim AOA is always measured with respect to the fore/aft axis and that needs to be taken into consideration when displaying lift and AOA.
If you want to set the airplane so it cruises more or less fuselage level, then what you do is use the graph of its lift versus AOA but adjust the AOA axis so it has zero CL at about zero AOA rather than -8. That equates to tilting it up as would happen in a real airplane. Just the way you adapt the sim geometry to correspond to reality.
The empty CG position can and will vary according to load and it is not desperately critical. I have seen stock airplanes with the CG in front of the leading edge, they still fly but need a lot of trim compensation.

Roy

 

[jcomm]

@Roy, good suggestion, but since MFS brought back that "wing_incidence" variable that was lost between fs9 and fsx ( because they went from wing AoA into aircraft AoA ) and it's set at:

wing_incidence = 0.75 ; Wing incidence (DEGREES)

in the DA 40, maybe simply setting that to what it is IRL will do the job ? I don't know what the root wind incidence is for the Katana though :-/

P.S.: Forget: Alexis just reminded me that the DA 40 has 0 wing incidence...

 

[Alexismmef]

I opened the Wortman airfoil page. If you look at the chart for CL versus Alpha you can see that the AOA for CL=0 is minus 8. That means when flying straight and level at cruising speed you will have an AOA of about -6. So the attitude using Wortman is correct. I would suspect that an airfoil of that type would be set at around 5 degrees or so nose up with respect to the fore and aft axis. In the sim AOA is always measured with respect to the fore/aft axis and that needs to be taken into consideration when displaying lift and AOA.
If you want to set the airplane so it cruises more or less fuselage level, then what you do is use the graph of its lift versus AOA but adjust the AOA axis so it has zero CL at about zero AOA rather than -8. That equates to tilting it up as would happen in a real airplane. Just the way you adapt the sim geometry to correspond to reality.
The empty CG position can and will vary according to load and it is not desperately critical. I have seen stock airplanes with the CG in front of the leading edge, they still fly but need a lot of trim compensation.

Roy

It really makes sense Roy. I think the issue for me is that I'm using the Wortmann FX 63-137 Data, but the real DA40NG uses the Wortmann FX 63-137/20 - W4 . This must have different ClMax and CL0, as they would never design an airplane to be a Cirrus Alternative that didn't perform well at cruise. I have searched all over Google, and there's no information on this Airfoil. I can get access to some footage of the DA40NG in a range of speeds, with Flaps Up and Down, but I don't know if that alone would be sufficient for me to make a realistic FM without any CL data.

 

[bstolle]

I can get access to some footage of the DA40NG in a range of speeds, with Flaps Up and Down, but I don't know if that alone would be sufficient for me to make a realistic FM without any CL data.

It is IMO. Especially if it's about getting the DA40 at least into a realistic attitude range. Don't know how much washout, if any, the DA40 wing has.
Haven't tested lately, but the last time I've tested the DA62 the stall speed was way too high as well, which contributes to the too high pitch attitude.
Another serious problem is the completely wrong constant speed prop simulation in the MSFS DA40 and 62.