Interior as opposed to Exterior Fillets

[ONAWHIM]

My exterior fin fillets need work.

Without getting into that technique, does anyone on the forum fillet their fins on the inside of the airframe only?

It seems to me that the exterior fin to airframe joint would be cleaner in appearance without a fillet.

Is there a structural disadvantage doing it this way?

I am referring to mid power rockets.

William

 

[rokitflite]

I did that on a concept scale model I built for NARAM last year... It worked out very well.

 

[powderburner]

Is there a structural disadvantage doing it this way?

The best structure you are going to build will be a snug fit between the fin and the edges of the body tube slot. No gaps, no crooked cuts. Same thing applies to the root edge of the fin root tab where it fits against the motor tube, and the fwd and aft edges of the tabs where they fit against the centering rings. No amount of glue will make up for a bad part fit (kinda like in woodworking).

If the fin material is balsa, basswood, or plywood, you would probably be a bit better off applying adhesive to the inside corners of the joints because the inside surface of the body tube is often better suited to absorbing the liquid. Outside surfaces of many body tubes are pre-semi-finished---some have a thin layer of plastic-like material---this makes it a little more difficult for adhesives to soak in.

If the fin material is fiberglass, graphite, aluminum, or some other super material, best joint adhesive is going to get a bit more quirky. Some glues work better on certain materials than others. There is a lot of experience already posted here on TRF 2 as well as on TRF 1 (see that button up in the blue toolbar for "archives"?). Do a few searches, play with your search terms, and you'll probably have reading material that will keep you busy for days. Fillets on the inside or outside probably aren't nearly as important as just using the right adhesive....and getting a good snug dry fit.

 

[gpoehlein]

My understanding of fin fillets is that they are less about strengthening the joint between the fin and body tube (although they do help some there as well), but more about reducing the drag at the fin/fuselage interface.

 

[THier]

All of my HPR stuff I fillet inside and outside. Overkill? maybe, but it gives me a warm fuzzy feeling. There are many ways of doing fillets, use some tape to dam up where you want them to stop, use fillers, Micro balloons, cabosil, even use saw dust sometimes, and if need be shape with a Dremmel and sanding drum.

Tom

 

[powderburner]

My understanding of fin fillets is that they are less about strengthening the joint between the fin and body tube (although they do help some there as well), but more about reducing the drag at the fin/fuselage interface.

Well-shaped fillets might make a small reduction in drag but I doubt that it is significant.

If you leave the fin edges square, I wouldn't bother worrying about fillet drag.

If you leave the body tube spirals unfilled, I wouldn't worry about fillets

And if you use a launch lug (or buttons) I wouldn't worry about fillets

From what I have seen of 99% of construction techniques, fillets are made way too big. And when they are made by glopping on mass quantities of epoxy, the added weight kills performance far faster than any fillet drag benefits.

Fillets (small ones) are OK for structural reinforcement. Fillets are just fine if you like how your rocket looks with them. Fillet drag improvements would almost be unmeasureable with a wind tunnel test. Fillets (big curvy poured epoxy ones) are just extra construction steps (IMHO).

 

[SuperSlim]

I build my rockets as shown in this thread. I think exterior fillets look nice, but I think you get your strength inside.

https://www.rocketryforum.com/showthread.php?t=4905

 

[Handeman]

Well-shaped fillets might make a small reduction in drag but I doubt that it is significant.

If you leave the fin edges square, I wouldn't bother worrying about fillet drag.

If you leave the body tube spirals unfilled, I wouldn't worry about fillets

And if you use a launch lug (or buttons) I wouldn't worry about fillets

From what I have seen of 99% of construction techniques, fillets are made way too big. And when they are made by glopping on mass quantities of epoxy, the added weight kills performance far faster than any fillet drag benefits.

Fillets (small ones) are OK for structural reinforcement. Fillets are just fine if you like how your rocket looks with them. Fillet drag improvements would almost be unmeasurable with a wind tunnel test. Fillets (big curvy poured epoxy ones) are just extra construction steps (IMHO).

I agree with powderburner, kind of.

Drag reduction is an accumulation of a lot of different things, as he mentioned. Not doing some other things, doesn't negate the benefit of doing fin fillets. Granted, doing all the drag reduction will make a significant difference, but any one helps, including fillets.

I completely agree that putting large epoxy fillets on a rocket will reduce performance because the excessive weight of the epoxy will more then overcome drag reduction benefits of the fillets. This may be more "break even" when we are talking mach+ rockets, but it's hard to get past mach when your rocket weighs too much.

As for strength, I don't believe you need large fillets on the fin to MMT joint, and if you use external fillets, I don't think you really need the internal fin to BT fillets too.



Let the overbuilding flames begin :neener:

 

[MarkII]

https://en.wikipedia.org/wiki/Fillet_(mechanics)

https://history.nasa.gov/SP-367/chapt4.htm

[93] Generally, interference drag will add to the component drags but in a few cases, for example, adding tip tanks to a wing, total drag will be less than the sum of the two component drags because of reduced induced drag.

Interference drag can be minimized by proper fairing and filleting which induces smooth mixing of air past the components. Figure 71 shows a Grumman F9F Panther Jet with a large degree of filleting. No adequate theoretical method will predict interference drag; thus, wind-tunnel or flight- test measurements are required. For rough computational purposes a figure of 5 percent to 10 percent can be attributed to interference drag on a total aircraft.

Figure 71.- Wing fillets.
MarkII

 

[Micromeister]

The best structure you are going to build will be a snug fit between the fin and the edges of the body tube slot. No gaps, no crooked cuts. Same thing applies to the root edge of the fin root tab where it fits against the motor tube, and the fwd and aft edges of the tabs where they fit against the centering rings. No amount of glue will make up for a bad part fit (kinda like in woodworking).

If the fin material is balsa, basswood, or plywood, you would probably be a bit better off applying adhesive to the inside corners of the joints because the inside surface of the body tube is often better suited to absorbing the liquid. Outside surfaces of many body tubes are pre-semi-finished---some have a thin layer of plastic-like material---this makes it a little more difficult for adhesives to soak in.

If the fin material is fiberglass, graphite, aluminum, or some other super material, best joint adhesive is going to get a bit more quirky. Some glues work better on certain materials than others. There is a lot of experience already posted here on TRF 2 as well as on TRF 1 (see that button up in the blue toolbar for "archives"?). Do a few searches, play with your search terms, and you'll probably have reading material that will keep you busy for days. Fillets on the inside or outside probably aren't nearly as important as just using the right adhesive....and getting a good snug dry fit.

ditto Powerburners comments: couldn't have put it better. Great post powder!

 

[ONAWHIM]

From what I'm reading here, there is no structural disadvantage to using interior in lieu of exterior.
Its the attachment technique that matters most.

Many thanks to all for taking time to comment.
You folks are a good bunch.

Wm.