I decided to check out FinSim before committing to the fins of my new build.
Looking into it, I'm finding a very counter-intuitve result that makes me very skeptical about its answers.
My current design looks like this:
and I get results for divergence of Mach 2.51 and flutter at Mach 2.31 using the NACA TN-4197 method.
But if I drastically shorten the root chord, which should reduce the torsional stiffness, the critical velocities went up 3.28 and 5.53 for the divergence Mach and the the flutter threshold. Really? The fin dimensions below are is less flutter prone than the fins above?
Playing with this some more, increasing the root chord length consistently decreases the critical velocity. Increasing the tip chord consistently increases the critical velocity. Both of those have to be incorrect, unless I'm completely missing something. Is it possible he got those two geometry values swapped?
Looking at the additional results, the bending natural frequency is constant regardless of tip chord and root chord, which is clearly wrong since it should be a function of both. If the tip chord is larger than the root chord, the natural frequency would be much lower than if the fin is the other way around. Also, the torsional natural frequency goes down with increasing tip chord (correct) and also goes down with increasing root chord (incorrect).
Am I missing something or are the stiffness calculations needed to find the critical speeds in FinSim just fundamentally broken?
Looking into it, I'm finding a very counter-intuitve result that makes me very skeptical about its answers.
My current design looks like this:
and I get results for divergence of Mach 2.51 and flutter at Mach 2.31 using the NACA TN-4197 method.
But if I drastically shorten the root chord, which should reduce the torsional stiffness, the critical velocities went up 3.28 and 5.53 for the divergence Mach and the the flutter threshold. Really? The fin dimensions below are is less flutter prone than the fins above?
Playing with this some more, increasing the root chord length consistently decreases the critical velocity. Increasing the tip chord consistently increases the critical velocity. Both of those have to be incorrect, unless I'm completely missing something. Is it possible he got those two geometry values swapped?
Looking at the additional results, the bending natural frequency is constant regardless of tip chord and root chord, which is clearly wrong since it should be a function of both. If the tip chord is larger than the root chord, the natural frequency would be much lower than if the fin is the other way around. Also, the torsional natural frequency goes down with increasing tip chord (correct) and also goes down with increasing root chord (incorrect).
Am I missing something or are the stiffness calculations needed to find the critical speeds in FinSim just fundamentally broken?