Fin alignment

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My first thread: I was wondering if all kinds of fin aligning seems to deal with tube in slot. Why can't you just take a angled piece and put a slot for the fin in the slot. Put the angle piece on the tube and fit the fin through the slot. Just a idea. I'm just getting back into rocketry.
 

techrat

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I can't exactly make out what you're asking, but I'm going to guess that the answer is that through-slot into the motor tube contact is the best construction method, as building the "fin can" makes the motor mount and the fins support the airframe, which is being pushed into the sky via the Motor Mount/Motor, and the transfer of forces keeps the whole thing from ripping itself apart because the fins are attached to both the motor mount tube and the airframe, and help transfer the load.
 
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My first thread: I was wondering if all kinds of fin aligning seems to deal with tube in slot. Why can't you just take a angled piece and put a slot for the fin in the slot. Put the angle piece on the tube and fit the fin through the slot. Just a idea. I'm just getting back into rocketry.
yea, its been done a long time ago...
Roger Wilfong showed me this back in 1985.
They only work for one fin thickness, but otherwise they work fine.
I use a Harrison fin jig for all models up to 24mm motor mount and that fit on the jig.20220915_131421.jpg
 

samb

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Jowayen

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1663693514295.png
I prefer to glue/epoxy all 3 (or 4) fins at the same time with a guide like the one in the above pic.
I usually make and use 2 identical guides at a time so that when I attach the fins, the airframe can set level to my worktable. Using 2 alignment guides also lets me keep the fins parallel to each other and the airframe, keeps the fins at the correct angle from each other, and keeps the fins at the same height from the airframe tube. I have used this technique from 18mm min diameter rocket tube with 3ea .093" fins to my level2 98mm diameter rocket tube with 4ea .250" fins. Each rocket I have built with this method gets compliments after they fly of how straight up they fly without any rolling or arcing over.
I have one rocket that consistently backslides before apogee ejection.
 

mh9162013

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That is the type of fin guide I use. Templates can be made from here:

Print then glue onto poster boadr, mat board, foam board. Let dry and cut out BT hole and fin slots.
The problem with this method is that it still has the potential for human error during the cutting process.

In my opinion, the whole point of a jig or fin guide is to produce results that are even better than what can be done with the naked eye. In other words, I need a tool that can satiate my OCD tendencies. But using a jig I know I cut out myself will still leave that "doubt" in my head that my fins are a little off (even if they aren't).

TL;DR: you buy a fin jig not to obtain better results but for reasurances that your fins are as perfect as they're going to get. And for me, knowing I did something myself doesn't provide the necessary reassurances.

Maybe it's just me...
 

techrat

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The problem with this method is that it still has the potential for human error during the cutting process.

In my opinion, the whole point of a jig or fin guide is to produce results that are even better than what can be done with the naked eye. In other words, I need a tool that can satiate my OCD tendencies. But using a jig I know I cut out myself will still leave that "doubt" in my head that my fins are a little off (even if they aren't).

TL;DR: you buy a fin jig not to obtain better results but for reasurances that your fins are as perfect as they're going to get. And for me, knowing I did something myself doesn't provide the necessary reassurances.

Maybe it's just me...

Cut with a new sharp blade and a straight edge.
If you're still convinced that it's off, then yes, it's you.
Frankly, I'm doing a lot "by eye" and when I check it later, it's pretty darn close to exact.
Unless you're flying in optimum conditions, (no wind, perfect humidity, etc.), your rocket's going to fly a little off regardless.
 

techrat

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I want to clarify my last statement, because I'm just thinking about this: in HPR, yes, it's much more critical that your fins are perfect. With a $200 rocket, what's another $15 for some fin guides? Most of what I am doing is LPR and MPR, so I can be a degree off, and get away with it. A lot of the higher end kits for HPR though either come with an alignment guide or provide enough in the fin-can construction to keep things straight (notched centering rings and so forth.
 

Arnie

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Would that 4 in fin alignment work with a PML 4 in Quantum tube?
 

waltr

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Another source for nice fin/build guides.
 

Jowayen

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Mine are CNC so I know they're better - much better - than I could eyeball.
They work great on HPR rockets that I've scratch built.

Those without the means, the Dragon Rocketry and similar are great tools.
 

Markw

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I had these fin guides made they accept 1/4" and if thinner material is used I shim it on the sides. This works on any diameter of tubes.
Shim example, cardboard from the back of a yellow lined paper tab.
 

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waltr

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Some notes of fin alignment:
There are two alignments to considered.
1- parallel to the BT center.
2- Perpendicular to the BT tangent.

If #1 if off this causes the rocket to spin and if real bad to wobble or cone. Both take energy and reduces to altitude.
If #2 is off then no effect I can think of during flight.

Best way to get #1 correct is to slot the BT for TTW fins. Slots or easy to cut correctly/accuraly with an piece of angle with the inside dimension of a leg near to half the BT diameter. Any rocket I have built with TTW fins not not spin much. Most I've measured is one turn (360°) from launch to apogee. This was probably due to slight warp in a fin rather than misalignment.

Since #2 is not as critical, any of the fin guides work well. Even the payloadbay printed and hand cut ones. Even eye-ball alignment would work.

Without TTW fin slots #1 is hard to get the fins on straight and also hard to design and build fixture/fin guide for this alignment. And if you sand an airfoil on the fins even the best fixture can get the fins off parallel to the BT.
 

hobie1dog

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Another source for nice fin/build guides.
It's a shame they stop at 3" body tubes.:(
 

Off Grid Gecko

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Some notes of fin alignment:
There are two alignments to considered.
1- parallel to the BT center.
2- Perpendicular to the BT tangent.

If #1 if off this causes the rocket to spin and if real bad to wobble or cone. Both take energy and reduces to altitude.
If #2 is off then no effect I can think of during flight.

Best way to get #1 correct is to slot the BT for TTW fins. Slots or easy to cut correctly/accuraly with an piece of angle with the inside dimension of a leg near to half the BT diameter. Any rocket I have built with TTW fins not not spin much. Most I've measured is one turn (360°) from launch to apogee. This was probably due to slight warp in a fin rather than misalignment.

Since #2 is not as critical, any of the fin guides work well. Even the payloadbay printed and hand cut ones. Even eye-ball alignment would work.

Without TTW fin slots #1 is hard to get the fins on straight and also hard to design and build fixture/fin guide for this alignment. And if you sand an airfoil on the fins even the best fixture can get the fins off parallel to the BT.
for surface mount to BT I typically draw my lines on the BT with a pencil and angle and then line up the fin against the marking on the tube. Seems to get them pretty straight but can be a little annoying on really small tubes
For anything 2" + my little aluminum 90deg piece of stock works just fine.
 

dr wogz

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My method:

Jump to Post #19

 

hobie1dog

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4" are clearly on their website.
This is on the website for me:
Badass Build Guides
$5.00Price

Body Tube Size
Select
Fin Count
Select
Fin Size
Select
Quantity

Add to Cart


  • Body Tube Sizes​


    Body Tube Sizes:

    BT20 = 19.0 mm
    BT50 = 25.0 mm
    BT55 = 34.2 mm
    BT60 = 41.0 mm
    2.2" = 57 mm
    2.6" = 68 mm
    3" = 78.75 mm
These guides are modeled off Mach1 fiberglass tubing. The size of the tubes these were made to fit are below. These are meant to fit snug against the tube. Please make sure your tube is within 1mm of the sizes below.
Body Tube Sizes:

BT20 = 19.0 mm
BT50 = 25.0 mm
BT55 = 34.2 mm
BT60 = 41.0 mm
2.2" = 57 mm
2.6" = 68 mm
3" = 78.75 mm

 
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jqavins

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The problem with this method is that it still has the potential for human error during the cutting process.

In my opinion, the whole point of a jig or fin guide is to produce results that are even better than what can be done with the naked eye. In other words, I need a tool that can satiate my OCD tendencies. But using a jig I know I cut out myself will still leave that "doubt" in my head that my fins are a little off (even if they aren't).
Amen, brother. Also, I don't just have a bad feeling that I may have got something wrong, I have a long track record of it; I suck at this if I don't have a jig.


Some notes of fin alignment:
There are two alignments to considered.
1- parallel to the BT center.
2- Perpendicular to the BT tangent.
3- Indexing around the tube circumference.
4- Vertical alignment, i.e. height on the tube.

Both of these are handled by TTW construction IF the slots are perfect.
Slots or easy to cut correctly/accuraly with an piece of angle with the inside dimension of a leg near to half the BT diameter.
That get's #1. As you noted it doesn't get #2, and it also doesn't get #3 or #4.

#4 can be done by hand even to my unnecessarily high standards by a band around the tube where the bottom of the slot should be (or the bottom of the fin for surface mounting). The slot can be cut (or the line drawn, the fin positioned) to the band, and if a slot ends up a little too long I can fill the gap with, um, filler. If the tab is (intentionally) a little shorter than the fin root, then any such gap disappears under the fillets. In all honesty, I haven't cut slots. I've used the same methods to align surface mount fins, and I've covered up overly long slots in kit tubes.

But #3 I will somehow manage to get wrong every damn time. Yes, I know, there are numerous ways to mark the tube before cutting, and I have managed to screw up every one of them. As I said above, I suck.

Tubes can be cut and jigs made with ideal precision by one or another of laser cutting, CNC routing, and 3D printing. Or by purchasing them from someone who uses such tools.
 

BABAR

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Welcome to the party Airborne 🎉 . Based on your description I was envisioning something like what @Al_delaIglesia pictured in post #3. Estes sells a similar widget:
https://estesrockets.com/product-category/accessories-parts/tools-supplies/page/2/

View attachment 537771

Build your own custom fin jigs with the template makers available here:
https://www.payloadbay.com/page-Tools.html

that definitely helps you get the line straight.

for NON-Thru-The-Wall fins the next trick is making sure you fin aligns with the line. I have been using 1/16” “balsa fillets”, basically 1/16” balsa sheet to start, measure a piece 1/16” wide by the root chord length of your fin. Definitions here courtesy @RandyT0001 see post 3



align one side of the fillet with the line, use single or double glue joint technique (I like double, gives a fast tack). Make sure all your fillets are the SAME side (clockwise or counterclockwise) to the lines. Allow to dry. Because these are so light, the go in place easy and don‘t have a tendency to drift while drying. I may put a rubber band around the ends if the moisture of the glue causes it to curl.

once dry, attach your fins to the OPPOSITE side of the line (counterclockwise or clockwis, depending on how you attached the fillet) so your fin root edge is aligned with the LINE you drew (so if your balsa fillets May range from 1/14” to 1/18”, doesn’t matter, you aren’t using the “far” side anyway.)

you still need a means to prevent lateral tilt, although for the most part lateral tilt of a few degrees shouldn’t affect flight.

advantage of the balsa fillet is not just alignment, because it provides for surface area the fin “tacks” faster with wood glue (although @kuririn cheats and uses a touch of CA for initial tack.). perhaps more importantly, it provides a stronger fin-tube attachment again due to larger surface area. Usually the balsa 1/16” balsa fillet can be hidden by an overlying fillet Once dry. If symmetry is a big thing for you, after the fin is adequately tacked you can add another balsa fillet on the other side for symmetry and even more strength.

this is a nice trick especially for minimum diameter birds where Thru-The-Wall fins aren’t an option.

also something to think about for Cub Scout or other youth group builds, since it is so easy to do you can “prep” the rocket tubes with the balsa fillets prior to the group build (doing 12 or 24 tubes should take you less than an hour) since I think fin attachment is probably the toughest part of a group build.
 
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