Are external fillets on TTW fins necessary?

[BigBlueDart]

Just thinking out loud here, but I'm wondering if fillets are necessary externally for TTW fins. The fins would be filleted to the MMT, and they can be filleted to the inside of the BT, but with all that, would fillets on the outside really be necessary? Could they be done without and would this help reduce any drag or weight? Would adding the fillets just be overkill?

 

[cjl]

Are they necessary? Probably not. However, you'll actually increase the drag by not using them. Fillets reduce the drag by making the 90 degree transition from the body tube to the fin more gradual. Plus, I think fillets look nice

 

[SAC of MMMSClub]

I was thinking of the semi-span and total fin length from root of the fin to tip, if every time you fillet you shorten the semi-span, except for the one holding it to the MMT. This would make the fin stiffer.
I know that radius corners are SOP for strength too.

Scott

 

[bjphoenix]

For strength purposes I think it probably doesn't matter so much- fillet on the outside is about the same as fillet on the inside, and with fins that go to the motor mount you have a whole lot more strength to work with anyway. It seems to me to be harder to get a good fillet on the inside but if you can do that then you could treat the outside however you like. You would have to do something to finish it off but you could end up with tiny little fillets out of wood glue even. I might even try this myself sometime because I can make tiny little fillets out of wood glue that would look better than my bid fillets out of epoxy. And it would be something different too.

 

[cbrarick]

I've done a few rockets with MMT fillets and internal body tube fillets. Makes a cool, smooth finish....just as strong (IMHO) as external fillets.....after all it's only a few tenths from the external fillets...

 

[Lamp]

The only possible difference really might be the load spread on the impact at landing. Even though epoxy seems rigid it will flex a little and this spreads the impact forces compared to the rigid straight line of the body tube, this might be enough to prevent a fin breaking but it would be very marginal.

I have recently built a 2.6" Madcow Patriot without the external fillets for aesthetic reasons. I figure just the full internal fillets will be more than enough for its intended launches.

 

[Rocketjunkie]

I prefer outside fillets for aerodynamics and appearance. However, internal fillets are plenty strong enough (stronger than birch plywood fins). For my 7.5" Nike Smoke, for scale appearance, there were only internal fillets and the fin can was all that survived a serious crash.

 

[jms]

As I'm about to start my Mad Cow Super DX 3, I was looking at it and thinking that I'd do internal fillets on the MMT and the inside of the body tube. Externally my thought was two passes of carpenter's glue, and then using some filler of any kind (Elmers/Lepage) to get a little bit of a smoother edge, simply for aerodynamics and aesthetics.

That being said (and somewhat related). Does anyone have tips for leaving off the aft CR and doing the fillets and how to get the CR in afterwards and properly secured?

 

[SteelyEyed]

As I'm about to start my Mad Cow Super DX 3, I was looking at it and thinking that I'd do internal fillets on the MMT and the inside of the body tube. Externally my thought was two passes of carpenter's glue, and then using some filler of any kind (Elmers/Lepage) to get a little bit of a smoother edge, simply for aerodynamics and aesthetics.

That being said (and somewhat related). Does anyone have tips for leaving off the aft CR and doing the fillets and how to get the CR in afterwards and properly secured?

I'm not familiar with that kit, but one way I've done that is to install the forward CR and MMT and then insert a temporary rear CR with screws sticking out of it so I can pull it back out. Glue the fins through the wall to the MMT making sure not to get glue on the temporary CR. After the glue sets up, pull the CR out and do your internal fillets. Then put a ring of glue (or epoxy) on the MMT and inside the aft airframe tube and insert the permanent CR.

Bret

 

[RedRocket]

Rocketroar:

I too am building the Super DX3 which I will use for my Level One Certification Flight at this years LDRS XXX.

What I have done with the aft CR is:

When installing the motor mount, epoxy in the forward CR as you normally would. Apply fillet to the top of the forward CR from the top of the air frame. But just slide the aft CR onto the tube at its marked position.
This will keep the motor mount tube centered at both ends while the forward epoxy sets.

Once the forward CR has cured, I use a thin "popcicle stick" and lightly poke it through the TTW slots forcing the aft CR off the MM tube and out of the air frame. I only move the aft CR a little bit with each push requiring that I work my way around the air frame many times. The main MM tube will remain in the center giving you full access to all areas around the fins for attachment and fillets.

It's not "the" way, but, it's "A" way.

Hope this helps...

 

[BigBlueDart]

Thanks for the feedback. Some interesting discussion going on here.

One of the reasons I ask, by the way, is I'm about to start a scale build of an Iris and I thought that minimal or no external fillets would help make it look slightly more authentic. The original actually had the fins bolted on to rings on the outside and there was a gap between the body and the fins. I don't want to go that far, but figured leaving the fillets out might help the look a little.

 

[jms]

I'm not familiar with that kit, but one way I've done that is to install the forward CR and MMT and then insert a temporary rear CR with screws sticking out of it so I can pull it back out. Glue the fins through the wall to the MMT making sure not to get glue on the temporary CR. After the glue sets up, pull the CR out and do your internal fillets. Then put a ring of glue (or epoxy) on the MMT and inside the aft airframe tube and insert the permanent CR.

Bret

Awesome tips on the CR's guys, thanks!

 

[Pantherjon]

Are they necessary? Probably not. However, you'll actually increase the drag by not using them. Fillets reduce the drag by making the 90 degree transition from the body tube to the fin more gradual. Plus, I think fillets look nice

I agree completely..Plus there are times when the fin slots are a bit wider and/or longer then the fin tang and the external fillets help fill those gaps enhancing the appearance..

 

[Scott Evil]

As I'm about to start my Mad Cow Super DX 3, I was looking at it and thinking that I'd do internal fillets on the MMT and the inside of the body tube. Externally my thought was two passes of carpenter's glue, and then using some filler of any kind (Elmers/Lepage) to get a little bit of a smoother edge, simply for aerodynamics and aesthetics.

That being said (and somewhat related). Does anyone have tips for leaving off the aft CR and doing the fillets and how to get the CR in afterwards and properly secured?

For my Crapien II build ( A modified PML Endeavour), I wrapped 2 loops of electrical tape through the aft centering ring. This kept the MMT straight while the epoxy set up. I was then able to pull the aft centering ring out and apply epoxy to the aft internal fin fillets, then adhere the aft CR back in when I was done.

It's kinda tricky with a six fin rocket but pretty simple with 3 or 4 fins as you can get at everything from the back.

Here's a link to the PML instructions. See Page 5 for a general idea.

For whatever it's worth. :sigh:

EDIT: BTW, I also put external fillets on all the fins

 

[Dlee]

In my opinion... that fillet is the most important one applied to the fins.

The one the inside of the airframe.

That is the fillet that does the most to keep your fins on.

 

[GregGleason]

For LPRs and MPRs using TTW methods, then in most cases fillets would not be necessary. However, on a large-span fin profiles like the IRIS, just make sure they are anchored well on the inside. Especially so if you think the motors will be in the greater than 10 to 1 thrust/weight category.

Greg

 

[edwinshap1]

i had a friend who never filleted his fins externally, and instead of cracking the body tube or mmt, the fin popped off and was easily repaired.

if you're not going high or fast, external fillets are more asthetic than anything else. and if you don't want to use epoxy, you can use wood filler or something