Higher Percent Drag for Elliptical Fins vs. Rectangular in Rocksim

[Anonymous]

I've been conducting some simulations in Rocksim to optimize fin designs for a rocket project and encountered an intriguing result that I hope to get your insights on. I tested two different fin shapes for the same rocket design: elliptical and rectangular fins. To my surprise, the elliptical fins contributed to 68% of the total drag, whereas the rectangular fins only contributed 46% at the same airspeed. I made sure all the other variables (e.g: shape of the edge, thickness, etc.) were constant

Given the conventional wisdom that elliptical fins are often more aerodynamically efficient due to their shape promoting smoother airflow and potentially reducing drag, this result was unexpected. I'm trying to understand why the elliptical fins showed a higher percentage contribution to total drag compared to the rectangular ones under these conditions, perhaps some simplification that rocksim makes?

Any insights would be greatly appreciated!

P.S Below the first image is for the elliptical and the second one is for the rectangular

 

[neil_w]

1) Did you size the fins such that fin area was the same?
2) I don't believe that calculations that Rocksim or OpenRocket do with regard to fin *shape* are sophisticated enough reproduce the "conventional wisdom" about elliptical fins being most efficient at subsonic speeds. Others may correct me if I'm wrong on this.

 

[Anonymous]

1) Did you size the fins such that fin area was the same?
2) I don't believe that calculations that Rocksim or OpenRocket do with regard to fin *shape* are sophisticated enough reproduce the "conventional wisdom" about elliptical fins being most efficient at subsonic speeds. Others may correct me if I'm wrong on this.

No, the area was different. Seeing as, I maintained the number of fins and the semi span on both replicas. The thing is, then the elliptical would supposedly have less area (just going off the 2D diagram), since they get cut off earlier and don't fill out like the rectangular ones, but this is another contradiction, since less area should produce less drag. Or am I wrong in my assumtpions?

 

[SolarYellow]

The advantage of elliptical fins is when generating lift, due to minimizing the generation of tip vortex. At zero lift, that advantage doesn't exist.

At zero lift, given the same span and thickness, they have the same frontal area. Given the same airfoil shape assumptions, the form drag should be the same. Less area should lead to friction drag. So the elliptical as you describe having constructed it should indeed have less drag.

I would suggest reading all the published info about how RS calculates drag and, if you can't figure it out from that, pose your question to Tim.