Stability Derivatives

[Thomas Dylewski]

Hello! 

I am back and we are now trying to implement stability derivatives using the SCT to find the different terms with respect to u, alpha, q, and alpha dot. My question mostly lies with the alpha dot terms, as there is no way to set the denominator of the derivatives to be alpha dot (essentially downwash and the associated lag with the tail). I thought to maybe use an unsteady solver but that did not add any additional options.

I would like to take these stability derivatives and do my own dynamic stability analysis and modal analysis but I am just having trouble getting the correct things I will need. 

Thanks!

[daniel.enriquez]

Unfortunately there is not currently a built in way to calculate alphadot. However, we will take your question as feed back to add this capability. You will need to setup a "custom motion" in the unsteady solver (see User Manual for more details), and provide a sinusoidal pitching motion. Extract the forces and calculate derivatives based on this. I found a paper here that describes a methodology for use in CFD that should translate well to FlightStream. 

[Thomas Dylewski]

Thank you!

[Thomas Dylewski]

After looking through several textbooks, I was able to isolate the tail-lift-curve-slope (what the alpha dot derivatives are based on) by iterating the model with a sweep solver over angles of attack and plotting the lift coefficient of the tail. For the u derivatives, I used the sweep solver over a range of different velocities then plotted the non-dimensionalized velocity with the different force and moment coefficients and then found the slope (derivative) of the data. This seems to hold with how the textbooks are written, but if not I will try the forced oscillation.

Thanks!