Steel rule to align fins - what can go wrong?
 

Ok modelers, I'm puzzled by how my current build turned out. I'd like to figure out what's wrong with my technique, so I don't do it again.

I'm kitbashing a 'Ventris SLV' - inspired by the new Estes kit and the Ventris in the box in my closet.

I put a pod (basically a Baby Bertha) between the fin and Ventris body. I trimmed the Ventris fintab and made a new fintab for the other side. There's a 24mm motor mount in the middle.



I added fin pockets to the Ventris 29mm motor mount. They seemed to align well with the precut slots in the body tube.



And I used a steel rule clamped across the bottom of the fins to align pairs. I first did this for a Wildman V2. Lines up a pair really well.



The problem is that I got an 'X' rather than a '+' - and I don't understand why.

I'm not totally sure what you mean by X vs +, and I'm gleaning a bit from the similar post you made on the other site, so I may be missing the exact problem.

Nonetheless, I'll take a stab at it, based on things I've screwed up in the past.

When you placed the fin pockets on the center tube or on the pocket endcaps, I'm assuming you made 4 marks to align against. Did you center the pockets on these marks, or did you put one side of the pocket on the mark, and then place the other side of the pocket offset from this. If you did the former, then your pockets would be radially aligned to the center of the tube. If you did the latter, each of the pockets would aim off center. This would not allow your fins to line up through the center of the rocket.

I mean the pairs of opposite fins aren't at right angles to each other. Close, but the gaps aren't even all the way around.



I did the fin pockets by marking 4 lines on the mount like for fins. Then put the pocket sides spaced on either side of that line, using some scrap 3mm plywood as a spacer. The pockets looked good when test fit and lined up with the body tube slots.

My guess is that it comes down to handcrafting tolerances. In theory, the pieces of fin on one side of the pod would be in perfect alignment with the plywood mount on the other side - for all four pods. But, that would take robotic skills, I think, or a jig specifically for that.

Then, the center of the pockets would be exactly 90 degrees apart, and the edges of the pockets would be perfectly aligned with the pockets on the other side. A jig might be needed for that too.

Any variation in those placements would adjust either where the tips of the fins end up, or the direction the fins travel getting from the tip to the body tube.

My recommendations, based on the little that I see, would be
1) Change how you clamp the fins. Clamp at the tips, like you did, as well as in the middles with at least 2 clamps on each fin. This would force the fins to align to the steel rule, even if there ends up being some misalignment in the pods due to handcrafting tolerances.
2) Reconsider the use of the pockets. You might want them for strength. But, if not, they are forcing the base of the fins to fit a specific location, and I think you are wanting the fins to have a specific location. Let any small handcrafting misalignment end up inside the tube by letting the base end up wherever it does, just to get a better alignment of the fins. Maybe then slide in the sides of the pockets like buttresses for strength.
3) Or, for more precision and time spent on the project, build jigs for each step.

I'm still not exactly clear on the problem... which gaps are uneven?

The Ventris kit comes with a pre-slotted body tube, so the space from one fin tab to the next ought to be the same (unless Estes screwed up).

If so, and if the fins are not at right angles to one another, then that implies one (or more) of three things: The fins are not correctly located on the outboard tubes; the fins may be correctly located but are not coplanar with the tabs; or the fins may be correctly located and may be coplanar with the tabs but the tabs (and hence the fins) are not aligned radially to the center tube.

The jig in your first photo would seem to preclude the first two assuming the jig itself was drawn and cut carefully.

So we're left with option 3. But that means the steel rule and clamps didn't do their job (or, again, that Estes screwed up).

It looks like you only clamped at the tips of the fins. I think that leaves open the possibility of bowing, where the root edges of the fins are not tight against the rule.

Or if the rule isn't rigid, that could cause a problem.

But the fin tabs are being held in place by your pockets. If the pockets are not evenly spaced then the fins would tend to be non radial, and clamping against a rigid rule might make them flex to a radial alignment but when the clamps came off they'd relax to the non radial position again.

Or if the pockets are evenly spaced, but slightly offset from the slots, that might cause it too.

I've never done pockets like those, and for the above reasons I'm not sure I like the idea. You'd have to get them precisely lined up with the slots in order to have a chance to get the fins right, and that seems problematic. Internal fillets are easier.

Anyway, maybe I'd feel differently if I had the rocket in my hands in front of me, but from the photo I can't tell there's a problem at all and as long as the fins aren't actually canted, I have a hard time believing there will be any significant effect on performance. So finish it and fly it!

It's purely cosmetic - in that orientation, I think the top and bottom gaps are wider than the left and right gaps by 1/4" of an inch or so. I'm just surprised it didn't self align better - for all the reasons you noted.

The next time I try it, I think I'll throw a square on the rule/brace and check that the right angle slots are actually right angle - maybe it takes a little twist to adjust for slight mis-alignments adding up?

Here's a pre paint photo.

Very interesting kit bash of the Ventris and a challenge for any builder without a jig for this particular modification. I doubt that my Guillotine Fin Jig would have been much help with this one.

I suspect the fin slots on the central or stock body tube may be the culprit. Did you check them before you began the assembly? I have noticed that the fin spacing gauge cutout on Estes kits are occasionally a tad off so perhaps the slots are not always perfect either.

Perfect is the enemy of good and it looks good to me. I think that a jig made specifically for this project would be the only way to achieve "perfection", and the jig might take longer to make than the actual build.

Don't tell anybody and they will never notice. Ok, one or two might eyeball it and if they say something, tell them you designed a bit of an X into it because you were watching Star Wars as you were building it. ;) :cool:

Fix it with paint. Put diagonals in that screw up the eye's ability to project lines.

Fix it with light bulbs. Put illuminated 300W bulbs in the pods so that everyone has to squint to see it.

Fix it with guy wires that pull the out-of-alignment pieces into alignment. Like spokes on a wheel.

Fix it with the next step in construction because to paraphrase and extrapolate on what was recently stated, "Good enough slays the dragon of perfection"