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I'm not an aerospace engineer, so don't know if I'm asking this correctly. I've been using the new XFLR5 with stability analysis. Just don't know what values are good.

What are typical stability analysis dampening values for GA aircraft for:

Lateral modes:

  • roll control
  • dutch 1
  • dutch 2
  • spin

Longitudinal modes:

  • short period 1
  • short period 2
  • phugoid 1
  • phugoid 2
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For the eigenvalues:

  • The Roll Subsidience should be negative.

  • The Dutch Roll values should have negative real parts, and they will be complex conjugates unless something is really wrong with your model.

  • The Spin (spiral) mode should ideally have a negative sign but there is some leeway to it being positive if the modulus is small.

  • The Longitudinal modes should both have negative real parts, especially the short period mode (the one with the highest frequency, in the 10Hz range where the pilot cannot react in time to damp it)

Assuming the eigenmodes are convergent, the dampening coefficients need to be large enough to kill any oscillation in a reasonable time frame. What is reasonable is often up to the particular design case; for example fast enough to prevent resonance with some structural mode, or far away from the pilot´s natural feedback loop to avoid PIOs .

For a brief primer on stability, see here: https://courses.cit.cornell.edu/mae5070/DynamicStability.pdf

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