Fin fillets, and why do you do your that way?

I got to thinking the other day about how we do epoxy fillets for our fins, and I realized that I really had no justification (other than simple convenience and cosmetics) on why I make them one given size or another. Same for what I use to make the fillets.

I've seen all the build threads, and it seems like everyone has an idea that 'works' for them, but I've never seen any real answer or justification for any particular radius or material from an engineering standpoint.

Is a given radius of fillet more suitable to one size or rocket/fin than another?
Is there a point or no return at which a fillet is too wide/too narrow?

What do you do and why?
What material do you use for filleting?
Is it different from what you use for structural attachment?

From the "book"...... 4 to 8% of fin root length, the radius should be.....



Material dépendant on size and purpose. A low power rocket can have them made of White/Yellow glue.

High power usually epoxy, with a body type filler for looks overlay. These can also be epoxy with a plethora of different fillers available, and different sizes used for "looks".
When using fillets on minimum diameter rockets where the fillet itself is not only cosmetic but also totally structural holding the fin on, then a larger fillet is used. [by me]
If through the wall fin attachment is used [fin glued to motor mount tube internally] smaller fillets can be done as there are 2 attachment points for fins, motor mount & airframe.

Epoxies used can be different from structural , can be laminating epoxy with thickening fillers or some type of thick epoxy designed for fillets.

The above are sorta "rules of thumb". Too many requirements dictated by project design to say "one size fits all.
Motor sizes being used, a 4inch airframe may have a small 38 motor mount, another may use a 75mm motor mount. Big difference on stresses between an I motor and a M for instance. So are thickness of fins and fillets required for safe flight.

blackjack2564 said:

From the "book"...... 4 to 8% of fin root length, the radius should be.....

View attachment 272720

Click to expand...

Ahhh, very cool. I would wager that most work within that out of happenstance rather than design.

I thought that the prime purpose of fillets was aerodynamic rather than strength. Admittedly strength is key for low power surface mounted fins. But surely on higher powered rockets where TTW fins are used, adequate strength is gained by epoxying the fins within the body frame? If so I guess that the purpose of fillets is aerodynamic only?

I find the whole process of applying epoxy fillets messy, hit and miss, and more about sealing the slot. I remember when I was an apprentice engineer (many years ago) working for a few months in the company foundry. I was taught pattern making where fillets are necessarily used to allow the smooth withdrawal of the final casting. In those days we used wax fillets that were heat bonded onto the pattern. They came in various radii, I remember at least 2mm to 20mm plus. Easy to apply and firmly bonded on clean wooden(porus) surfaces. Is there anything like that available on the market today?

As far as the longevity of the wax fillets is concerned, I still have a few of the patterns I made, stored in the garden shed. Many, many years later and having gone through some extreme summer and winter temperatures they've remained intact and still good for creating moulds.

Just a thought.

SO.

Fin Fillets in the 4 -8% Fin root cord range add incredible strength to any size model fin. While White or yellow glue can be used for Low and Mid-power Rockets. I've found over the years for speed of application and consistent radii, 2part epoxy makes the best and strongest bond regardless of materials involved while making the Aerodynamic smoothing much simpler with the aid of a simple Sandpaper covered dowel. These smooth transitions greatly reduce 2 or 3 types of Drag created at the fin/body intersections which help overall performance for just about any Model Rocket from Micro-Maxx to 3.3lb LMR's.
Personally I use 5minute epoxy on Micro Fillets, 30minute for LPR & MPR larger models. When maintaining a max of 4-6% Radius on these fillets the Added mass from either 5 or 30minute epoxy is negligible to very minor.