Any volunteers to run AeroFinSim on my rocket design?

[KevinT]

I could use some help. I would like to run AeroFinSim on my L3 design, but I do not have a copy, and I am curious to the flutter velocity it would calculate. Attached is the rocksim file. Fin Material is FR4, 3/16" thick. Motor will be an M1939W and if it holds together, a later launch on an O3400. Let me know if more info is needed. Thank you for your support.

 

[user 30299]

Kevin,
Attached is the FinSim anaylsis for your fin design. I marked the two velocities that you need to consider; Theoretical Velocity, and Velocity with a 25% Safety Factor. The G10 material is your FR4. I used 3,000 ft. as the max. velocity altitude. Sorry, but I got a little lazy on checking what altitude the max. velocity is at per your RockSim and RASAero simulations.

Theoretical Velocity : 1191 ft/sec

Velocity with 25% Safety Factor : 953 ft/sec

This analysis is from John Cipolla's Excel spreadsheet version of his fancier FinSim program. The fancy version and the spreadsheet give you all the same numbers. Please note that I used 2,700,000 psi for the "E" of the G10. This is per industry specs. If you have specs on your material that shows a higher "E" value, then please post it. If you know the "G" value - that's even better. G is Shear Modulus, sometimes called Modulus of Rigidity.

If you were to go 1/4" on the fin thickness then the Theoretical Velocity goes to 1834 ft/sec, and the 25% Margin goes to 1467 ft/sec.

 

[KevinT]

That is excellent data. Thank you so much. I agree with you that 1/4" would be better. I was hoping for a little more margin to avoid making a change, but it's not too late. I machined some custom parts to accept 3/16" material but they can be modified. Here's a picture of my thrust plate.



The rocket is in it's final stages of completion and with this data I think I'm good to go. Here is a pick of the rocket to date.

 

[thzero]

Wow, that is very nice machining!

 

[KevinT]

Wow, that is very nice machining!

Thank you

 

[UhClem]

That is excellent data. Thank you so much. I agree with you that 1/4" would be better. I was hoping for a little more margin to avoid making a change, but it's not too late.

Thicker is better but it does not have to be solid. Solid G10 is heavy.

Fins are kind of like I beams in that material on the surface is far more important than material at the core. You can make heck of a fin by putting carbon fiber over a balsa core. The downside is that measuring or even estimating the overall material properties (stiffness is key) is difficult.

Rather than make thicker solid fins, add a layer of balsa topped with CF. Or just plain old fiberglass if CF is too spendy for your tastes.

 

[KevinT]

Thicker is better but it does not have to be solid. Solid G10 is heavy.

Fins are kind of like I beams in that material on the surface is far more important than material at the core. You can make heck of a fin by putting carbon fiber over a balsa core. The downside is that measuring or even estimating the overall material properties (stiffness is key) is difficult.

Rather than make thicker solid fins, add a layer of balsa topped with CF. Or just plain old fiberglass if CF is too spendy for your tastes.

That is definitely something to think about. Thank you,