BeamDyn + UA_Mod=4 in dev-tc requires DT <= 0.001s

[deslaughter]

Bug description <A clear and concise description of the bug.>

When using UA_Mod=4 with BeamDyn in the dev-tc branch, the time step (DT) needs to be 0.001s or smaller; otherwise, the model will become unstable and the aerodynamic forces will oscillate wildly. The generalized-alpha solver in the tight-coupling algorithm can still make progress, but the runtime increases significantly due to the instability.

To Reproduce <Update the following list with your specific information.> Steps to reproduce the behavior:

1. Compile the dev-tc branch with any compiler.
2. Run the IEA-22MW OpenFAST model with a DT of 0.01s
3. Note the long runtime and frequent Jacobian updates
4. Open output inspect AeroDyn tip node Cl and Cd to find unusual oscillations.

Expected behavior <A clear and concise description of what you expected to happen.>

These nodal values should not oscillate depending on the time step.

Screenshots, if applicable <Add screenshots to help explain your problem.>

OpenFAST Version <Please provide as much detail as possible including git commit. The best information is a screenshot of the OpenFAST system description that prints when running OpenFAST:>

Build OpenFAST from dev-tc branch.

 **************************************************************************************************
 OpenFAST

 Copyright (C) 2025 National Renewable Energy Laboratory
 Copyright (C) 2025 Envision Energy USA LTD

 This program is licensed under Apache License Version 2.0 and comes with ABSOLUTELY NO WARRANTY.
 See the "LICENSE" file distributed with this software for details.
 **************************************************************************************************

 OpenFAST-v4.0.3-659-gec9b169a-dirty
 Compile Info:
  - Compiler: GCC version 14.2.0
  - Architecture: 64 bit
  - Precision: double
  - OpenMP: No
  - Date: May 14 2025
  - Time: 11:32:48
 Execution Info:
  - Date: 05/15/2025
  - Time: 10:33:05-0400

System Information (please complete the following information):

• OS: macOS 15.4.1
• Compiler: GFortran 14..2.0
• Compiler settings: default

[bjonkman]

I have noticed that the integration method in the continuous UA models sometimes needs a smaller timestep. I assume tight coupling algorithm doesn't include the AD/UA states?

[jjonkman]

@bjonkman -- Correct; the tight coupling only collectively integrates the states of the structural modules (BeamDyn, ElastoDyn, and SubDyn). AeroDyn remains loosely coupled.