Fillet Location and Strength

In another thread a fellow TRFer posted that external fillets were mostly for aerodynamics and that internal fillets were stronger. I expressed skepticism and asked why. However, I then realized that this sidebar could become a distraction from a really nice build thread that Dottore has going. So, rather than hijack that thread I thought I would pick it up over here.

My suspicion (based on intuition, but not a good analysis) is that the external fillet is also important in resisting aerodynamic loading and thus strengthening the fin structure. I believe that if you are going to use TTW you are best off with good strong fillets in all three glue joints along each side of the fin: At the root, where it anchors to the motor mount tube (MMT), at the interior of the body tube (BT), and at the exterior of the BT.

First, I would like to consider the stresses on each glue joint as well as the stresses that affect the fin as a result:
At the motor mount tube, there are compression and tension stresses on either side of the fin root. Typically the root of the fin is not anchored into a slot in the MMT, so the glue joint has to withstand the entire force. Because the root is some distance away from where the fin passes through the BT, the glue joint here has more of an effect countering the torque caused by the dynamic load on the fin. Having a fillet here makes a lot of sense. However, having a single fillet here, but nowhere else would result in the stress on the fin being concentrated along the sharp edge of the fin slot. The MMT fillet is superior at counteracting the moment caused to the fin by dynamic loading.

Second, having a single fillet along the interior of the BT. In this case the edge of the BT pressing against the fin whenever the fin is in loading still causes a concentration of stress right where the sharp edge of the fin slotted BT and the fin meet. For compression, tension, and torsional stress the internal fillet would have almost the same cross sectional area as would an external fillet. Thus, I don't believe the internal BT fillet offers any advantage over the external fillet with the possible exception that it would be more difficult for an internally filleted fin to be yanked out of the fin slot.

Finally, having a single fillet along the exterior of the BT. In this case, that fillet causes the stresses that would otherwise affect the fin (due to its being cantilevered out from the BT) to be distributed. The tapered cross-section of the fillet helps prevent sharp edge stresses as it reduced the lever arm that aerodynamic loading would otherwise have. Also, the shape of the fillet allows it to work like a strut propping up the fin.

So, I guess I think that having all three fillets is very important. The BT fillets helps prevent the fin from rotating around the axis formed by the fin against the fin slot. The internal fillet helps strengthen the body tube and keeps the fin from being pulled away from the MMT. Finally, the external fillet helps reduce the moment that lateral forces would cause against the fin. Discarding that fillet as less important than the other two fillets might be a mistake.

Seems like a logical analysis. The argument can be made that large external fillets can/will cause interference drag, so the edges should be smooth to the BT. Probably true, but I suspect the external fillets provide a very necessary structural support worthy of some extra drag. I would rather sacrifice a few hundred or thousand feet before sacrificing a fin!

Cory said:

The argument can be made that large external fillets can/will cause interference drag, so the edges should be smooth to the BT.

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Isn't an external fillet by definition smoothing the edge between the fin and the body tube?

Interference drag comes about because of air moving around two parts of an object being forced together into a restricted space (like at the fin root on a rocket, or the wing root on an airplane). Once that joint is smoothed out, interference drag should drop off fairly dramatically. Of course, there may be some increased form drag, but that should be far outweighed by the reduced interference drag.

I guess this explains the upper bound on the efficient fin fillet radius as a function of the fin's root chord. At some point, the increased form drag overcomes the reduced interference drag, and total drag starts increasing.

Steve Shannon said:

Finally, having a single fillet along the exterior of the BT. In this case, that fillet causes the stresses that would otherwise affect the fin (due to its being cantilevered out from the BT) to be distributed. The tapered cross-section of the fillet helps prevent sharp edge stresses as it reduced the lever arm that aerodynamic loading would otherwise have. Also, the shape of the fillet allows it to work like a strut propping up the fin.

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Thanks for the analysis!

You touched on something here that I think is really important about external fillets: they move the effective anchor point of the fin further towards the fin tip. This should make them quite a bit more resistant to flutter, since it is much harder to flutter something that is shorter relative to something that is longer.

I'm in the process of grinding out and replacing the old external fillets on my 6" Tomahawk. (One got cracked when the payload section collided with a fin on its last flight and so my OCD has me redoing them all.) Each fin is attached at the motor mount with internal fillets and carbon fiber cloth. They're way "twangier" without the fillets than with them, i.e., I can pluck them and they hum. The re-filleted fins don't do this. They just make a thud sound.

So I agree: external fillets are important!

dottore said:

Isn't an external fillet by definition smoothing the edge between the fin and the body tube?

Interference drag comes about because of air moving around two parts of an object being forced together into a restricted space (like at the fin root on a rocket, or the wing root on an airplane). Once that joint is smoothed out, interference drag should drop off fairly dramatically. Of course, there may be some increased form drag, but that should be far outweighed by the reduced interference drag.

I guess this explains the upper bound on the efficient fin fillet radius as a function of the fin's root chord. At some point, the increased form drag overcomes the reduced interference drag, and total drag starts increasing.

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I agree, referring to large external fillets like I tend to create.

I am hesitant to sand down the edge of the fillet for fear of causing an asymetrical finish or getting into the BT fibers. Regardless, the point of creating a fin with less flutter potential seems worth the small amount of induced drag.

I think we are on the same page Steve, I wasn't saying that external fillets weren't important. I've seen fins ripped off a bird because it didn't have exterior fillets, but I just think that 3 sets off fillets (as you were saying) are far more effective than only external. By me saying that external fillets are mainly for aerodynamics may have been a mistake on my part but what I meant was that exterior fillets are not quite as strong (or take the load of) the fillets on the MMT, therefore I said they were mainly for aerodynamics.

Thanks Cody.


Steve Shannon
Prefect - Tropoli Montana
L3CC, NAR Montana.

Lately I've been doing some Builds with surface mounted Fins, so I'de like to beleive the Fillets add strength. The Stiffness of the Fins definitely improves after Fillets.

One thing that comes to mind here. From my old shop classes, I am certain that smaller is stronger for the glue joint. And certainly filler isn't helping the strength. It does help the aerodynamics.

So I am wondering if anyone has tried making a minimal glue joint along the fin/but joint and then using a lighter material for the aerodynamic reasons. I would think that for strength, a small filet inside the bt would provide more to hold the fin in place then outside the bt. (Realizing that there are other reasons for that filet outside.)


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1tree said:

One thing that comes to mind here. From my old shop classes, I am certain that smaller is stronger for the glue joint. And certainly filler isn't helping the strength. It does help the aerodynamics.

So I am wondering if anyone has tried making a minimal glue joint along the fin/but joint and then using a lighter material for the aerodynamic reasons. I would think that for strength, a small filet inside the bt would provide more to hold the fin in place then outside the bt. (Realizing that there are other reasons for that filet outside.)


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Click to expand...

This may have been true for wood glue but not so much for filled paste adhesives. The load is being spread through the fillet the larger the radius the stronger it will be. You are effectively shortening the lever arm the fin has to break its sell off you tubular flying object.

Keeping in mind there is an optimum bond line thickness and a thicker bond line is not better. But a bond line and a fillet are not the same thing. If you do some reading of the reference material in the bonding sticky it shows fillet shapes and stress distributions.

Nice discussion; I often like to put in all 3 fillets as well.

Sometimes the strength of the materials is more of an issue however. Case in point: I just repaired a rocket with a snapped fin, due to a combination of multiple landings on the tip & car bashing during transport. The fin was 1/8" 5 ply baltic birch; the rocket was not a performance buster - but a converted 4.25" crayon bank. It did see its share of vmax & blue motors however, but surely never broke mach 1. In this case there was no external fillet at all because of looks and the cheesy plastic bit the fins went through. The fin did not bust at the joint, but away from it a bit - above where a fillet would have ended. It may be true that a fillet would have helped some, but I don't think much. The fin was weaker than the internal structure already (fillet at motor tube, plus foam in the can).

I don't do traditional fillets anymore inside; I purchase 1", 1.5" and larger roll of fiberglass clothes and I use a strip of it soak in Aeropoxy 6209 to make the fillet, it's way more clean. Outside I do fins to fins layers, so the rocket is build like a tank. Take note that build only composite rockets.

Steve, in general I agree. I tend to do it with fillets on all three joints in situations where I can do it.

Building smaller stuff, say 54mm tube and below making the assumption that we are not talking minimum diameter, I have changed tactics a bit. Fins go in with JB Weld on root edge to hold against the motor mount. After that sets up, I fill the space with 4 or 8 pound urethane foam. Then, I do external fillets. Seems to work well.

On rockets where I have room to work I do a fillet on each joint. All epoxy has amendment added- either Kevlar pulp or chopped carbon. I make the structural fillets very thick- probably close to 50/50 epoxy to amendment. Pack it in and shape it. Let it set. Then a do a make it pretty fillet over the top with Superfil.



Mark Koelsch
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