What is the optimal size (radius) for a fin fillet to minimize interference drag? There must be a formula based on some combination of airframe diameter, root chord length, fin thickness, fin semi-span, inter-fin spacing or some other basic combination of measurements. I did a little searching amongst NASA technical papers and they were so far over my head I couldn't begin to understand the math. I need something simple like the fin root radius in inches is equal to the airframe radius in parsecs blah-blah-blah the length of the root chord in cubits blah-blah-blah the interfin spacing in furlongs blah-blah-blah over a constant Z or something like that. Something practical I can apply to specific build scenarios rather than a theoretical understanding - I'm a software and electrical engineer by training, not an aerodynamicist and have no training in fluid dynamics.
Another question relates to filleting the junction of fin leading and trailing edges to airframe. What do people do about this? Fillet them? Leave them clean? Any experimental or theoretical data to back up the preference?
This is all from the perspective of minimizing interference drag for an altitude project.
Warren
Warren, for my part I have never seen anything on the most prime fillet profiles.
Regarding fillets above and behind the leading and trailing edges - I *do* lay fillets there, but that is because I am minimum diameter and I figure it doesn't matter how many layers of what I have on the fins -- if something bad happens at the tippy top, the party is over. I am generous with my epoxy right about there...
I want to re-iterate my request for folks to give me some input on fin fillets and fin fillet sizing with respect to interference drag. I received a reply on the arocket list from the inestimable Henry Spencer that quotes a ratio of approximately 6%-8% of fin span as an appropriate radius? (or is that % of fin span?) as the "best" size for a fin fillet. Anthony Cesaroni sent me some pictures of a semi-ITAR controlled project that Cesaroni did for a military application that looks to be in the 10% size range at a guess. (I can share these pics, but only if I kill ya'... 👿 )
In any event, ANY of these above rules of thumb or anecdotal examples are FAR larger than the fillets I see on most people's rockets. What do you think about this?
I'd like folks to comment on this if they have any thoughts at all on this topic. I think this could be a productive discussion.
Warren
If you're curious enough, do your own test and use a Parrot to measure the drag differences. Since you're measuring the drag force rather directly, the results won't get affected my motor impulse variation, weathercocking, etc. I'm planning to do this to compare a conical 29mm nosecone with an Apogee nosecone on a couple of G80 flights.
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