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.
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.