Leading and trailing edge treatment for split fins

This is one of the dumber questions I've asked in a while, but whatever.

Given a split fin design, do you give the edges of the fins around the split any special treatment? I'm wondering if you would tend to leave them square to make the split more.... splitty, I dunno.

I have this fin arrangement:

Should I be thinking of doing something other than my usual rounded leading and trailing edges on all three fins? My main concern here is appearance, although obviously I do not frown on performance.

Hmmm... I hadn't thought about that before. One challenge if you leave the edges around the split square is that you either need to leave the entire edge square, or figure out how to transition from square to not-square. My first inclination would be to do the same rounded edge, but there are other possibilities.

I definitely leave them square. More chance of whistling during flight that way .

Since the criteria is appearance then do what looks best to you.
For low power keeping them square as opposed to rounding and tapering them won't make much of a difference. Maybe a few feet.
The splits will adversely affect performance much more than any edge treatment, since they will disrupt laminar flow over the fin surface.
But if your criteria is getting them to whistle.....
Instructions for my Sky Wolf say to sand a slight taper to the leading edge of the forward fins and leave all the other edges square."With a proper build, she will whistle while she works!".
We shall see!

Der MicroMeister (currently entertaining Angels with MicroMaxx rockets, most likely) frequently reminded me that while airfoiled fins (rounded forward and tapered trail) were most efficient for subsonic flights, even rounding just forward and rear was much better aerodynamically than square.

That said, I have always felt that squared edges, if clean, looked more precise on futuristic rockets. I am likely also biased by an aversion to the endless filling and sanding to get good smooth finish on all surfaces. I do wrap around paper on most of my “finished” rockets, so I have paper on lead, trail, and lateral edges, so I don’t have to mess with tapering or rounding, I just cut fins to shape and paper it all around, except of course for the root edge.

for split fins on your design there is no reason to do anything in the gap. aerodynamically it does nothing, that being said, just knocking off the sharp edge with a couple swipes of sandpaper will help paint adhesion, and is not noticeable.

What about something like this? No transition, just a hard edge along the red line. The sanding would be, um, fun, but it might have a cool visual effect.

Mugs914 said:

What about something like this? The sanding would be, um, fun, but it might have a cool visual effect.

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I thought of that, but apart from the sanding effort (not that bad really) I have no idea what it'll look like. I might sand a test fin like that just to get a visual preview.

I'm kind of leaning towards leaving them mostly square, but just knocking down the edges a bit. If so it will require some intestinal fortitude on my part, because the urge to round all edges is very strong with me.

At the speeds our rockets launch, even smaller stuff, the airflow wont be tripped by the gap or split. Do what looks nice to you because the air flow will not be effected. You could round all the leading edges that make "First Contact" with the air stream. I am presuming the air hitting the fins is clean.

My vote is square..

lakeroadster said:

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I needed that today !! You made my day ! If we ever get to hang out together I have no doubt it will be crazy, wild, fun and probably dangerous. Maybe we should bring Neil and Babar along to keep us from doing something totally insane.

I see a few decent options:
Round all edges
Leave all edges square
The Mugs hybrid round/square
The Boatgeek variation of the Mugs hybrid: square to the red line then a transition to round over say 10% of the fin span. Also round the leading edge of the leading fin and the trailing edge of the trailing fin.

I'd probably do my variation (if I thought I could get them sufficiently even) or rounding all the edges. The nice thing about these choices is that you can progress from mine to all rounded without having to rebuild fins.

boatgeek said:

The Boatgeek variation of the Mugs hybrid:

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Ah yes, one day I'll refer to the "Boatgeek variation of the Mugs hybrid" and at least one other person will know what I'm talking about.

mbeels said:

Ah yes, one day I'll refer to the "Boatgeek variation of the Mugs hybrid" and at least one other person will know what I'm talking about.

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Alternatively, it could be a chamber piece for viola, trombone, bagpipe, and accordion.

boatgeek said:

Alternatively, it could be a chamber piece for viola, trombone, bagpipe, and accordion.

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I like it, and hey, I even play the viola! Dibs on the viola part!

boatgeek said:

Alternatively, it could be a chamber piece for viola, trombone, bagpipe, and accordion.

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That is the most disturbing thing I’ve read on the internet today.

neil_w said:

That is the most disturbing thing I’ve read on the internet today.

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I'll count that as a win!

Of course, it could never work. Everyone knows a viola is an indoor instrument and a bagpipe is an outdoor instrument, so they're incompatible. Cue jokes about locking the piper out of the concert hall.

neil_w said:

That is the most disturbing thing I’ve read on the internet today.

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Nah, you've haven't spent enough time on the internet. All you'll hear is the bagpipe anyway.

boatgeek said:

I'll count that as a win!

Of course, it could never work. Everyone knows a viola is an indoor instrument and a bagpipe is an outdoor instrument, so they're incompatible. Cue jokes about locking the piper out of the concert hall.

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So playing a bagpipe indoors is a rule violation? Sort of like flying MicroMaxx in a gymnasium?

BABAR said:

So playing a bagpipe indoors is a rule violation? Sort of like flying MicroMaxx in a gymnasium?

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MMX IN A GYMNASIUM....COUNT ME IN !!!!

BABAR said:

So playing a bagpipe indoors is a rule violation? Sort of like flying MicroMaxx in a gymnasium?

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One could easily make the argument that playing a bagpipe *anywhere* is a rule violation.

Ooooohhhhh…...…...

"But captain, they said playing the bagpipes was a rule violation!"

BABAR said:

So playing a bagpipe indoors is a rule violation? Sort of like flying MicroMaxx in a gymnasium?

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Not so much a rule violation as a time to need hearing protection. They're ... kinda loud.

neil_w said:

One could easily make the argument that playing a bagpipe *anywhere* is a rule violation.

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The bagpipe *is* the only musical instrument ever banned as a weapon of war, so yeah, probably not too far off.

blackjack2564 said:

[J]ust knocking off the sharp edge with a couple swipes of sandpaper will help paint adhesion, and is not noticeable.

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It will also help prevent chipping.

Because the edges fore and aft of each split are parallel, I imagine taking a single fin, cutting it, and then moving the pieces a little apart. And that image leads me to want the facing edges square.

Personally I'd go with a very slight variant of Mike's idea, going with a short transition from square to rounded. Picture this: go back to my image of cutting a single fin. That fin would be rounded all the way around. As the rounded contour comes to those two inside corners there'd be some corner turning transitional contours. When they are cut through they would become square-to-round transitions on the central piece.

Now here's another idea, slightly wacky: use one sided bevels. Imagine that the single fin had been cut off-normal to the fin face, and in opposite directions on the two cuts.

I'm sure that would have a tendency either to induce whistling or to definitively prevent it, I just don't know which.

I'm gonna post a follow-up question to this thread. Going back to the original image of the fin layout:


Which way should the grain go on the trailing fin? Aligned with leading or trailing edge? If this were all one piece, it'd be trailing edge. If it was a standalone fine, it'd be leading edge. I'm guessing it should be leading edge, but it looks weird.

I'd've guessed trailing, because the proximity of the large middle fin shields the leading edge of the trailing fin both from impact and from air flow. Not as well as being glued together would, but better than the trailing edge is shielded.

That said, I must say this: I totally don't know what the heck I'm talking about.

Maybe this is a good time to make some balsa plywood.

Ditto what Joe said....

Where are you expecting impacts? You don't have fin flutter to deal with, I know your flights

I lean towards leading edge since any impact I can imagine will try and bend it parallel to the grain, whereas rear impacts to trailing edge grain would make it want to "fold over" kinda
Heck I'd align the leading fin's grain to the trailing edge for the same reason.

Looking at DinoRider’s drawings and my experience, the best choice is: BOTH.

weirdly contoured or pointy edged fins do really well with orthogonal balsa ply construction, assuming your acceptable final fin thickness is 1/8” or more, make a card template and cut two 1/16” fins, with grain rotated 90 degrees. Thoroughly coat the appropriate sides with yellow glue, then squeeqee off any excess with a hotel key card. Put the pieces together make sure edges are lined up perfectly, stick in wax paper, and stick in an old book with a weight on it for a night or two (this is a must, the pieces will tend to curl away from each other due to the moisture on on,y one side of each piece. You can also do this with medium or thick CA if you are in a hurry. You get a rock hard fin that can handle stress from any angle, with surprisingly little weight penalty. From a strength standpoint, it’s kind of like papering on steroids.