Easy To Build Fin Jig

[ftlrockets]

Hello everyone.

Recently I made a fin alignment jig inspired by Air Command. I use 4mm corrugated plastic in white for my fins.
The jig is made up from a couple pieces of timbre laying around the workshop. 2 pieces of old skirting board which was cute to lengths of around 600mm then trimmed thickness so that it wasn't too bulky. There is 1 piece of 22mm pvc pipe running through the spliced bottle which is perfectly aligned with the timbre above.
By measuring the thickness of the fins I worked out how I was going to make it. The 2 pieces of timbre and perfectly 4mm apart this was made from just adding washers which were 1mm thick. So far works great.

Thanks

-Harry

[Blenderite]

The only thing that I can see that could potentially limit you in the future, is that you can't use it on any bottles longer than a spliced pair.

Other than that, it is a excellent design!

[ftlrockets]

Hello. Thank you and if it came to needing them on longer spliced bottles. I would simply replace the pvc pipe and the 2 timbers.

Thanks

[U.S. Water Rockets]

That looks like it would do the job. As mentioned before, it looks like it would only be useful on short rockets. Perhaps you could use it to make up some removable fin cans from a separate bottle. That way you could move the fins from one rocket to another or replace them as a unit if damaged. It seems like fixing them permanently to the rocket would mean a damaged fin would ruin your launch day because it would not be field repairable.

Also, why do you mount the corriflute with the corrugations going perpendicular to the air flow? You are presenting the edges with the most air resistance to the wind, so the air cannot pass through the channels. Also, the waves on the outer surface of the corrugated plastic are perpendicular to the air as well, which leads to drag from the wavy skin of the fins. If you put the lines the other way there's a lot less drag on the fins.

[ftlrockets]

Hello. US Water Rockets.

I didn't mention this is the post as it was for the fin jig but should have gone more in depth. The fins are glued onto a fin can which is then placed onto a spliced pair. I did this for a reason to keep the fins aligned the fin can. This gave me a solid platform to work on. If I didn't use a spliced pair I wouldn't have been able to run my 22mm PVC pipe through.

The fins are made of 4mm corriflute the reason for mounting them this way is so the PL Premium glue gets "soaked" into the fin channels giving me a stronger and better join. I simply stopped air travelling into this by using drinking straws and sticking them on with tape.

Thanks

[Blenderite]

Hello. US Water Rockets.

I didn't mention this is the post as it was for the fin jig but should have gone more in depth. The fins are glued onto a fin can which is then placed onto a spliced pair. I did this for a reason to keep the fins aligned the fin can. This gave me a solid platform to work on. If I didn't use a spliced pair I wouldn't have been able to run my 22mm PVC pipe through.

The fins are made of 4mm corriflute the reason for mounting them this way is so the PL Premium glue gets "soaked" into the fin channels giving me a stronger and better join. I simply stopped air travelling into this by using drinking straws and sticking them on with tape.

Thanks

Ok that makes sense. The spliced pair shown is just a holder for the fin sleeve. That makes more sense now!

[U.S. Water Rockets]

Hello. US Water Rockets.

I didn't mention this is the post as it was for the fin jig but should have gone more in depth. The fins are glued onto a fin can which is then placed onto a spliced pair. I did this for a reason to keep the fins aligned the fin can. This gave me a solid platform to work on. If I didn't use a spliced pair I wouldn't have been able to run my 22mm PVC pipe through.

The fins are made of 4mm corriflute the reason for mounting them this way is so the PL Premium glue gets "soaked" into the fin channels giving me a stronger and better join. I simply stopped air travelling into this by using drinking straws and sticking them on with tape.

Thanks

Ok, that makes sense. It's been a while since anyone used that technique and you didn't show the fin can or the straws in your previous images, so it wasn't clear that you had done it intentionally. it makes sense now. You have put some thought into it, and have found some solutions to those issues. We try to avoid making the corrugations run horizontally for the reasons mentioned above as well as the unfortunate tendency of corriflute to curve along that direction over time. In that orientation, the root of the fin will hold one edge straight and the tip will curl and cause the fin to warp along the direction of air flow. That causes spinning.

By mounting the corrugations vertically, the fin can curl up, but the curled edge will still be parallel to the direction of flight (think of it like a partial ring fin of a sort). You'll have to keep an eye on your fins for warping, and you should be ok.

[Herb77]

You can also just tape a fishing line or kite line on both ends of the bottle for a straight line. That way your fins are nice and straight.

[Blenderite]

I believe that point of this jig is to make sure the fins are perfectly vertical to the rocket, not aligned horizontally, though it helps with that too.