Leading and trailing edge treatment for split fins

[jqavins]

Why cut each fin twice and then risk misalignment? Instead, make sheets of the balsa two-ply and then cut the fins. Since the "raw" sheets are longer than they are wide, you'll want to cut one into squares and turn all the squares' grains perpendicular to a whole sheet.

If you're using the best tool I don't own, a laser cutter, you'll have to turn the laser's power or duty cycle up or reduce the cutting rate (compared to what you'd use for the same thickness of plain balsa) because the glue is actually a little harder to cut through than the wood. (I learned this the hard way. Well, actually the slightly annoying way; it's not really a big deal to finish off with a knife.)

 

[neil_w]

Thanks for the interesting discussion. I have thought about it and decided that in this instance It Doesn't Matter.

1. The fins will be papered basswood (glued paper, not label) and will be quite strong in general
2. These fins will not be taking much or any landing impact, thanks (?) to the pods.
3. These fins are small and, as Nytrunner correctly points out, I'm not exactly going to be pushing Mach with this rocket or anything.

And so, leading edge it is. Cross-laminated balsa is great but it is also overkill for this application (and I say this as someone who generally embraces overkill. )

Heck I'd align the leading fin's grain to the trailing edge for the same reason.

The problem with cutting the leading fin like that (or strake-like fins in general is that it becomes extremely delicate to handle. A piece of balsa cut like that would be tough to mount before it breaks in your hands. I do agree that it would be very strong once mounted.

It seems to me that in general, the #1 thing to avoid is grain parallel to root, which is obviously asking for breakage. Ideally all grains should have one end at the root so it can be properly anchored. Beyond that, it seems like there is some room for debate about how to do it best in some special situations. For this rocket, I'll be fine.

 

[neil_w]

The problem with cutting the leading fin like that (or strake-like fins in general is that it becomes extremely delicate to handle. A piece of balsa cut like that would be tough to mount before it breaks in your hands.

As if on cue, today on Chris's blog he shows some parts from the new Low-Boom SST that are almost exactly like this: https://modelrocketbuilding.blogspot.com/2020/02/estes-low-boom-sst-7289-build-part-2.html

There are two important differences:
1) The parts shown in the blog post don't come to a point. In my experience, the points are what tend to break very readily when the grain is perpendicular
2) The parts are laser-cut, so they're not subject to much stress in the cutting process.

 

[ThreeJsDad]

It's funny how Chris's models are hosted on an old friends web page. Wayne and I once flew together a lot. Man did Wayne and I build some crazy tiny rockets.

 

[lakeroadster]

......
There are two important differences:
1) The parts shown in the blog post don't come to a point. In my experience, the points are what tend to break very readily when the grain is perpendicular
2) The parts are laser-cut, so they're not subject to much stress in the cutting process.

You could paper the basswood stock before cutting the fins. I did that on the centering rings I scratch built... makes cutting delicate parts easier since the material becomes more durable.

Just be sure to use a pencil to mark the wood grain pattern.

 

[jqavins]

As if on cue, today on Chris's blog he shows some parts from the new Low-Boom SST that are almost exactly like this:

Tangentially, that's a nice looking (rocket) plane but not nearly as nice looking as the real thing.

 

[mbeels]

Tangentially, that's a nice looking (rocket) plane but not nearly as nice looking as the real thing.
View attachment 407450

Wowza, now that's a (rocket) plane I'd love to model.

 

[neil_w]

Yeah, I have noticed in the past how beautiful that is... lots of custom composite work to do, or else some extremely clever work with standard parts.

 

[jqavins]

Or carving from balsa blocks.

 

[neil_w]

Or carving from balsa blocks.

Have fun!

 

[kuririn]

or else some extremely clever work with standard parts.

Hey, it's 2020. Haven't you guys heard of 3D printing? You can print practically anything!
And by the way, it's not the Low Boom SST, it's the Low Boom SST.