Help Analyzing F-15 Stability in OpenVSP

[Adeline Teo]

Hi everyone,

I’m currently analyzing the static stability of an F-15 in straight and level flight using OpenVSP and VSPAero. My goal is to match the stability derivatives to a standard reference list below.

M                        0.5                (Mach Number)
Alt (ft)                 1000               (Altitude in feet)
U (m/s)                  169.559            (True Airspeed)
q (N/m²)                 17113.18747        (Dynamic Pressure)
alpha_trim               0.18105            (Trim Angle of Attack)
CD0                      0.01468            (Zero-lift Drag Coefficient)
CL0                      0.20708            (Zero-lift Lift Coefficient)
CDu                      0.00E+00           (Drag Coefficient with respect to Angle of Attack)
CLu                      3.73E-06           (Lift Coefficient with respect to Angle of Attack)
CMu                      -3.53E-06          (Moment Coefficient with respect to Angle of Attack)
CDalpha                  0.37257            (Drag Coefficient with respect to Angle of Attack)
CLalpha                  4.8706             (Lift Coefficient with respect to Angle of Attack)
CMalpha                  -0.268819          (Moment Coefficient with respect to Angle of Attack)
CZ_alfa_dot              -17.2322           (Pitching Moment Coefficient due to Alpha Dot)
CM_alfa_dot              -11.887            (Pitching Moment Coefficient due to Alpha Dot)
CZq                      17.2322            (Pitching Moment Coefficient due to Pitch Rate)
CMq                      3.8953             (Moment Coefficient due to Pitch Rate)
CY_beta                  -0.97403           (Side Force Coefficient with respect to Sideslip Angle)
CL_beta                  -0.13345           (Lift Coefficient with respect to Sideslip Angle)
CN_beta                  0.12996            (Normal Force Coefficient with respect to Sideslip Angle)
CY_p                     0                  (Side Force Coefficient with respect to Roll Rate)
CL_p                     -0.2               (Lift Coefficient with respect to Roll Rate)
CN_p                     -0.033721          (Normal Force Coefficient with respect to Roll Rate)
CY_r                     0                  (Side Force Coefficient with respect to Yaw Rate)
CL_r                     0.1099             (Lift Coefficient with respect to Yaw Rate)
CN_r                     -0.40471           (Normal Force Coefficient with respect to Yaw Rate)


From my VSPAero analysis, I’ve identified similar stability variables from the .flt output file, but the values seem significantly off from the expected reference values. I’m trying to determine whether this discrepancy is due to incorrect simulation settings, issues with the model, or other factors.

Key Discrepancies
Lift Curve Slope (CL_alpha)
VSP: 22.85
Standard: 4.87
Issue: The lift curve slope from VSPAero is almost 5 times larger than expected, which suggests an overly aggressive lift response. Could this be due to improper panel resolution, incorrect reference area definitions, or missing compressibility effects?

Pitching Moment Slope (CM_alpha)
VSP: -33.82
Standard: -0.27
Issue: The extreme negative CM_alpha suggests an excessively unstable pitching moment response in VSPAero. This might be due to an inaccurate center of pressure location or missing contributions from fuselage aerodynamics.

I’ve attached an Excel file of the VSPAero values to the standard reference stability values for comparison, in addition my F-15 VSP file, and all VSPAero output files.

I’d really appreciate any insights or suggestions on potential causes and how to improve the accuracy of the results.

Thanks in advance for your help!

Best regards,
Adeline