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FSX Nose up issue at cruise speed and high altitude

Fdd_fr

Fdd_fr

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france
Hello,

I have some problems with my last panel adaptation on the Project Airbus A321.

Aircraft has nose too up when it cruise at its flight level.
Attitude is 2.5/3°, at FL300/390 with cruise speed of M0.77.

In real life, attitude must be close to 1.8/2°.

Also, I think that nose up too much when aircraft turns...

I did some change on engines settiong in AIR file with success in the past, but here, I don't know which parameter changes, and I amm a little afraid to do things that I don't undestand.

I tried to increase "Cruis_lift_scalar" but without success....

I assume that must be the AoA of wings, but if someone can help me to get a more realistic behavior for this bird ?


Thank you

François
 
Hi Franqois,

While it might not be what you want - Jerry's Air Wrench available on Flight1 has a field to change the pitch in cruise flight - it's a great tool and makes it a bit easier for beginners like myself to adjust parameters... I don't know what it is specifically changing though...

Regards,
Scott


Sent from my iPad using Tapatalk
 
Cruise AOA is set by table 404 (CL vs alpha) in the .air file.

Keep in mind that the table is in radians. To convert radians to degrees multiply the value by 180 and divide by pi (or use 57.2958).

You need to associate your cruise coefficient of lift (C_L) with your desired attitude of 1.8° or 0.0314 radians.

Find the cruise C_L via AFSD.
Then, open table 404 in AircraftAirfileManager. C_L follows the curve displayed there, so in order to get a lower cruise pitch, you need to lower the slope of the curve in the right upper quadrant.
While you're at it, also check that the stall AoA is okay for an A321 and then model the curve so that it stays smoothly between AoA=0 and stall AoA.

Reload the aircraft in FSX and check your cruise AoA. You might need to fine-tune via trial&error until you have your desired result.
 
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I don't understand this table. Curve is very strange....isn't it ?

442588Image2.jpg



Value of my table 404:
Record: 0404 Wing Cl vs AoA Table
Points: 17
-3.141600 0.000000
-2.748900 0.359900
-2.356200 0.500000
-1.963500 0.359900
-0.785400 -0.666500
-0.290000 -1.323136
-0.011000 0.000000
0.000000 0.030000
0.035000 0.301889
0.054000 0.460290
0.107500 1.053787
0.185000 1.919648
0.218000 2.157750
0.240000 2.162000
0.261700 1.205096
0.785400 0.377209
1.570800 0.000000


What are the explanation of first and second columns ?
Cl = Coeficient lift ?
 
The X axis is AOA from -180 to +180 degrees but measured in radians. Your left point is-3.14 radians = -180. Normally the scale extends to the right and +3.142 radians but yours ends at about 90 degrees, no problem just unusual
The Y values are Lift coefficient. For example at 0.24 radians you have a CL of 2.16 which is awfully high for a swept wing airplane,especially at only 13.4 degrees. At a guess I'd say it was made up by someone not entirely proficient at the task
For more explanation try Yves Guillame's excellent document.
Roy
 
Yes, C_L is the coefficient of lift.

What's the C_L during a typical cruise situation for an A321?

Yves' document is on Avsim (fs_flight_dynamics_v1.0.zip).
 
François,
Here is the Math.
In the cruise lift=weight. Let's use 150,000 pounds
Wing Area =1320 sq ft =S
At 0.77Mach and 30,000ft, True airspeed V is 766 feet/second,Air Density RHO is 0.000891 slugs/cuft
We need CL
Lift= CL*0.5*RHO*V^2*S.
CL=Lift/(0.5*RHO*V^2*S)
CL=0.435

In your 404, 0.035 radians gives 0.302 CL and 0.054 radians gives 0.46 CL, so the radians for 0.435 CL are about 0.051 =+3.09 degrees AOA
Since FSX measures AOA as fuselage AOA then your pitch would be 3.09 degrees, which is about what you stated.
To get the pitch to change to 2 degrees, you need to have the same CL but at 2 degrees AOA, so 404 needs to be redrawn.
This simply involves moving the curve you have so that you obtain a point where CL =0.435 and AOA=0.034(2 degrees).
The stock A321 has a rather more sensible 404 in terms of maximum CL 1.984 at 16.5 AOA. However, the AOA for zero lift is minus 4.6 degrees which is lower than I would have thought even though there seems to be no published airfoil data. Currently it has 0.435 CL at about 0.34° AOA and you want that at 2.0°. So if you were to simply add 1.65 degrees to all the AOA points in the stock A321 record 404 central region, leaving the CL values as is, you would get the 0.435 CL at 2 degrees AOA and it would fly at that pitch angle. Leave the left and right hand data points alone and just add 0.028 radians to the central radian values.
Roy
 
Thanks, but I think that I will need an Aspirin, please :banghead:

I tried the Stock A321 table. Attitude is better, but nose too low limit.

Some test results with default A321 table 404 :

FL390, 0.77M
Fuel 100%: pitch -2.3
Fuel 50%: pitch -1.18
Fuel 25%: pitch: -0.67
Fuel 10%: pitch: -0.27

FL350, 0.77M
Fuel 100%: pitch -1.13
Fuel 50%: pitch -0.27
Fuel 25%: pitch: -0.23
Fuel 10%: pitch: +0.55

FL300, 0.77M
Fuel 100%: pitch -0.16
Fuel 50%: pitch +0.63
Fuel 25%: pitch: +1.02
Fuel 10%: pitch: +1.25


Maybe a little nose too low; Maybe also a problem of tanks location; positive value are not expected (nose below horizon...
 
Francois,
I did not say to use the stock airfile, I suggested using it with modifications so you would get the result you wanted.
I pointed out that you should add 1.65 degrees to the central values which means adding 0.0288 radians.
This will make the plane fly 1.65 degrees more nose up than it does with the stock airfile.
Anyhow, use this modified table.
Record: 0404 Wing Cl vs AoA Table
Points: 13
-3.142 0
-2.356 0.5
-1.571 0
-0.3392 -1.389
-0.0512 0
0.0458 0.531
0.2998 1.905
0.3168 1.984
0.3338 1.905
0.3518 1.389
1.571 0
2.356 -0.5
3.142 0
The first 3 and last 3 rows are the same, the ones in the middle have had 0.028 added to the AOA in Radians column.

Let me know how this works
Roy
 
Last edited:
Thank you very much for your help, Roy.

I will test your table tonight.

Regards

Francois
 
Hello Roy,

Excuse me for the delay, but I had to do many many tests because my aircraft behavior was not correct.

After testing default FSX A321 table 404, aircraft nose was correct to close 0° at cruise level and nose down when high speed at low level.

When I tested your table, things was better, but nose a little too up.... so I did many tests by change values which are different between your table and the default A321, with very small increment or decrement.

I know, this method is not a scientific method, but I arrived to a final behavior which is ok for me.

My final table is the following:

Record: 0404 Wing Cl vs AoA Table
Points: 13
-3.142 0
-2.356 0.5
-1.571 0
-0.3492 -1.389
-0.0712 0
0.0308 0.531
0.2889 1.905
0.3012 1.984
0.3181 1.905
0.3335 1.389
1.571 0
2.356 -0.5
3.142 0


Thanks for your help

Francois
 
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