AeroFinSim questions

I've been playing with the latest free download version of AeroFinSim, what is the general opinion of this software? with any analysis software your results are only as good as your inputs, so I'm trying to understand what I'm doing before I trust any results.

is there a quick way to determine the E.A. and C.G. for a fin design?

is there a way to account for TTW fins and fillets?

Well, I have only used early versions. That being said, it is good for typical flat fins. If you start throwing tip to tip on it I think that will mess up the sims as it changes, in a some what unpredictable manner, the qualities of the fin.

markkoelsch said:

Well, I have only used early versions. That being said, it is good for typical flat fins. If you start throwing tip to tip on it I think that will mess up the sims as it changes, in a some what unpredictable manner, the qualities of the fin.

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I would suspect that tip to tip would improve ones chances that the fins survive higher speeds. I wonder if a single layer of whatever was being used could
make it worse? There's always the 1/3rd, 2/3rds and full span layering that can be tried that will increase the thickness of the fin root over that of the tip.
I'll find out in a minimum diameter 38mm project in the spring. Kurt

Kurt, you misunderstand. I think the tip to tip will improve your chances as well by decreasing the flutter of the fin. I was saying that due to the different materials with unknown adhesion that it becomes very difficult to simulate.

markkoelsch said:

Kurt, you misunderstand. I think the tip to tip will improve your chances as well by decreasing the flutter of the fin. I was saying that due to the different materials with unknown adhesion that it becomes very difficult to simulate.

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Right Mark, I understand. Anything that can increase the "stiffness" would change the qualities and likely increase resistance to flutter and destruction. The degree to which that resistance would be "increased" would be hard to simulate.

Say do a single layer and that would help of course. Do the 1/3rd, 2/3rds and full span laminating in layers with the middle layer fibers rotated 45 degrees would likely change the resonance and strength further. Trying to simulate that without real world
data is impossible. I say do a sim with materials as close to one is using. If the speed is in the "danger zone" figure out something else to reinforce the fins or use a different material. It's a best guess at best.:wink: Kurt

I have the older version of FinSim but suspect C.G. and E.A. haven’t changed. I have spent some time trying to understand FinSim like you but have not built or flown any rockets with designed fins. So there is no “proof in the pudding” from my stand point. Sims of my rocket design indicate max velocities of Mach 1.2 – 1.5. So to give myself some safety margin my design point for the fins is Mach 2.0 for both divergence velocity and flutter velocity.

For Center of Gravity I use the following;

https://www.engineeringtoolbox.com/center-gravity-d_1310.html . To use, assumptions would be that the fin is a uniform thickness and a uniform material or at least symmetrical material from flat side to flat side. In other words you could have a base material of fiberglass then have 2 layers of carbon fiber on each side of the fin. Hope that makes sense. I draw my fin then geometrically determine the C.G. In my case I used the Trapezoid version. And as far as the thickness varies, which will affect C.G, I have ignored it in my sims.


Elastic Axis is a horse of a different color. If the fin has the tip chord parallel to the root chord, and the fin is a uniform thickness, and symmetrical materials side-to side, the E.A. is half the distance of the root chord. If one is going to vary the thickness leading edge to trailing edge (as in an airfoil shape) then E.A. would be different. My reading indicates you almost have to build it then test it. As I intend for my fins to be airfoil in shape I “assume” the E.A. will be along the thickest part of the wing with E.A. measured at the root. The percent used will be consistent from the leading edge. If not you have another variable to account for that I don’t think FinSim handles.


You account for TTW fins and fillets in this input screen. You specify fin mounting type, Butt or TTW, specify Fin-to Airframe fillet radius, and the Fin Body Adhesive. I didn’t run across any reference to tip-to-tip laminating. My assumption is that it is better than just a layer of reinforcement.


Hope that helps ;>)
StanO

RKeller said:

I've been playing with the latest free download version of AeroFinSim, what is the general opinion of this software? with any analysis software your results are only as good as your inputs, so I'm trying to understand what I'm doing before I trust any results.

is there a quick way to determine the E.A. and C.G. for a fin design?

is there a way to account for TTW fins and fillets?

Click to expand...