Discrepancies in XNP and Cm-alpha results for a Canard UAV Design in Flow5

[AlperenTtl4]

Hi everyone,
I am currently developing a canard-configuration UAV as a personal project to improve my skills in aerodynamic design. The current model is consisting of only the main wing and the canard (no fuselage or vertical fins have been added yet).
I have encountered some inconsistencies in my Flow5 results and would appreciate your insights on a few points:

1. Discrepancies between VLM2 and Panel (TriUniform) Methods
   The first image shows the Cm-alpha polars for different XCG positions. I ran the analysis using the recommended TriUniform-ThinSurface panel method (solid lines) and compared it with the VLM2 method (dashed lines).
   I noticed a significant shift in the trim angles and slopes between the two methods.Is such a large discrepancy normal for canard configurations? Which method should I consider more reliable for establishing the baseline stability?

2. Variation in Neutral Point (XNP) and Center of Pressure (CP)
   I am observing different XNP values depending on the analysis type and method:

• T1 Analysis (TriUniform-ThinSurf): XNP = 147.7 mm
• T1 Analysis (VLM2): XNP = 125.75 mm
• T7 Stability Analysis (TriUniform-ThinSurf): XNP = 137.475 mm (at alpha = -4.416)
• XCG required for horizontal Cm-alpha slope: X = 141.383 mm
  I am concerned that these variations might lead to reliability issues in my stability calculations. Additionally, is it physically normal for the XNP to shift with changes in alpha in a canard setup, or should it theoretically remain constant?

3. Canard-Wing Interaction
   I want to make sure that Flow5 is accurately capturing the canard wake and its aerodynamic effects on the main wing. In the second figure, the pressure distribution and streamlines at α = 0 are shown. How reliable is Flow5 in this regard? For proper modeling of interference drag and downwash effects in a canard–wing interaction, are there any specific settings or mesh refinements that I should pay attention to?

Thank you in advance for your help
Best regards,

[techwinder]

1. Discrepancies between VLM2 and Panel (TriUniform) Methods
   The methods used to evaluate pressure forces are different in VLM and panel methods. In the first case it's the vortex's lift force divided by the panel's area, and in the second case it's derived from the gradient of doublet densities. This means that the results are bound to differ. From experience, this will be amplified in the Cm=f(alpha) curve if the CoG is located backwards with a low static margin. Both methods however are consistent separately in the sense that results are comparable if using the same method, but none should be considered accurate.
2. Variation in Neutral Point (XNP) and Center of Pressure (CP)
   The position of the NP should be considered even less precise than the pitching moment.
3. Canard-Wing Interaction
   I do not have experience of canards, but there is no reason for flow5 to be either more or less accurate in the case of a canard than it is in the case of a classic configuration - providing that the wake panels shed by upstream wings do not intersect downstream wings.