Alternative to Internal Fillets

[dr wogz]

Fin cans are inherently strong with minimal adhesives. Over building fin cans is one of the more comical activities in this hobby.

and cue my comment from post #10 ....

 

[dr wogz]

Good Post. Odd that this didn't show up yesterday.

thank you! I tend to be long-winded, but I do a fair amount of technical writing in my every-day life!

 

[MetricRocketeer]

I prefer to use thin epoxy.
Before I place the rocket horizontally to dry, I like to run/pour/inject the epoxy down the root where the fillets will be.
I'll do 4 at a time, let dry, rotate 90 degrees and run epoxy down 4 more fillets... until completed.
I'm not extremely concerned about a "perfect" all-matching radii. It's just not that critical.
The purpose is to widen the bonding width of the adhesion surface.

Preparation of the surface is by far the most important step of the internal fillets.

View attachment 459097

Hi Jowayen,

I have this question about your method, please.

Does your thin epoxy, which you illustrate by means of the red dots, replace the fillets, or do you use the thin epoxy and then apply thicker epoxy to further build up the fillets?

Thank you.

Stanley

 

[dr wogz]

Metric,

I have done the same thing a few times. Some epoxies are just runnier than others (West Systems for example). And using a longer cure epoxy allows the 'fillet' to run & "level out" over it's working time. Do check it, as it can run off what / where you want it to stay! Tip it slightly one way to get it to run the length, then tip it the other to have it run back, then tip it.. (We have all had one rocket, where the last fin doesn't seat right, as the epoxy has run around the MMT!!)

I also tend to just glob it on, at say, every inch or two, then let gravity flow it into one big long fillet..

As long as you get an Epoxy seam / the joint is filled, you are generally good to go.

Epoxy use & epoxy fillet takes a bit of skill, and a fair amount of practice!

 

[Great Hobbies]

In RC airplane builds, I have done the dowel method in the motor box joints, but used carbon rods. It's an effective way to increase surface area for the adhesives and also to add bracing.

My current plane uses small carbon rods along the entire length of the fuselage and it adds a lot of stiffness.

If I was going to use this method on filets I would probably go with carbon tube to save weight, and I also would not use excessive amounts of epoxy.

Tyler P

 

[jd2cylman]

Hi Jim! Thanks for your comment. Based on my experience, internal fillets are the first to give way, which would seem to indicate that they are a key stress point. I've been flying rockets for more than 65 years. Wildman voids his warranty if you don't have internal fillets. I'll keep the internal fillets.

The fillet Jim is referring to is (I believe) the middle set on the inside of the body tube to fin. Those are not as necessary. And NOT in the Wildman instructions. Internal fillet to the motor mount and external fillet on the outside of the body.

 

[OverTheTop]

I used 6mm triangular beading for the internal fillets on my Velociraptor. Seemed to work well, but it is the only bird I have done that on. I just wanted more surface area for the epoxy on the cardboard motor mount tube.

 

[Jowayen]

Hi Jowayen,

I have this question about your method, please.

Does your thin epoxy, which you illustrate by means of the red dots, replace the fillets, or do you use the thin epoxy and then apply thicker epoxy to further build up the fillets?

Thank you.

Stanley

The red dots were just a quick sketch from power point showing the location of the 4 places I epoxy at a time. I do pretty much exactly what DR WOGZ describes in post #34. Get some "thin" epoxy. Apply to each of the 4 places shown. Tip the rocket back and forth a few times until the epoxy starts to level out and begins to set up. I don't go back over them at all. It's just enough to make the epoxy joint/bond joint wider. I'm really not interested in a perfect radii. I try to use the same amount in each area for peace of mind with regards to balance. Also, on mid power rockets, I'll use nothing but tight bond 3. High power gets epoxy.

I save the exterior fillets for last and I make these fillets with 5 minute epoxy for high power rockets. I use glass marbles of about 1/2" diameter - sometimes larger depending on the size of the fillet I want. I rub the marble back and forth in the root. This scores the can and the fin. I put masking tape right on the lines I just scored. Mix up the epoxy, apply it, then wait a few minutes. I'll have a paper towel ready with rubbing alcohol to wet the marble. Then, using one pass only, press the marble down and run it thru the epoxy. Clean and wipe the marble with alcohol before each pass. After 2 passes, or 3 passes at most if I screwed up, set the rocket aside about 10 minutes, then repeat for each external fillet.

The marble is wet with alcohol when it touches the epoxy. This leaves a finish that needs minimal sanding before paint. YMMV

 

[firemanup]

Yeah, this is a perennial question: how much is too much overbuilding? Since we don't know the exact properties of the materials we use nor the exact stresses they will encounter, we're always using our judgement. To me, skipping fillets to save time isn't not overbuilding, just taking a shortcut.

I inject internals, and i overbuild, that way when a coupling un does itself and my booster free falls from 800 feet i can just pick it ip and fly it again...

 

[firemanup]

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