Fin alignment

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dr wogz

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Hi All,

1st post - blush- :D

How / what do you all use to align your fins? to get them strainght, to get them 'perpendicular' ( to teh BT) etc.. I mostly build the smaller kits, typically Estes & such..

I have always done the 'by eye' method, but there's always one fin that seems to 'lean' to one side of teh other..

Estes used to have a 'fin alignment & glueing' stand, but don't seem to make it anymore. Their new one; the 'V' ruler with the pencil holder works, but I wouldn't say it' is any better than 'by eye'..

Plans, ideas, tips, anything?!
 

powderburner

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I use the old eyeballs also. It's easiest to do a four-fin bird because you can sight across the BT to see if opposite fins are lined up and flat.

For a three-fin design I sight along the fin toward an imaginary line through the center of the BT. I also only tack-glue the fins until all three are in place, when you can check alignment again: set the rocket (gently) on its side on a table or flat surface, and the third fin should be pointing straight up.

You can make yourself an easy alignment tool by cutting a stiff piece of cardboard with a hole in the center for the BT and thin slots for the fins. Slip it over the BT while the fins are drying and you will get great results every time.

For sport-fliers and even for competition rockets, it suprises many people to learn that the last one or two degrees of fin alignment really doesn't make much difference. For scale models, of course it makes a big difference. Mostly it is just the 'craftsman' in all of us who strives for perfection in fin orientation.
 

dr wogz

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Thanks for the reply!

I guess I fall into 'that' catagory that requires a bit of fuss, if only on my part, for only my benefit!! :D:D
 

Silverleaf

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You could find a Este's Fin Alignment Guide on Ebay for around 50 to 60 bucks, yeah, pricey, but it certainly makes low and mid power fin attachment much easier.

Many take and make a mandril out of wood so all you do is lay the tube in the depression, and then lay the fin on the angled plane - then glue it in place.

For a cheap and easy method - you can buy the Este's Rocket Builder's Tube Marking Guide for about 7 bucks anywhere online. It does work for fins up to 1/8th thickness. Only 1 fin can be glued on at a time, but it does work quite well.

See my mini-review here:

https://scale-rocketry.tripod.com/id19.htm

Cheers,
 

BlueNinja

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I'll post a pic of my preferred method tomorrow.

I also use body tube lines.
 

Hospital_Rocket

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I have both of the Estes tools, and found the fin marking guide far easier to use. I've even used it on .093 G10 fins by PML.

Soon as I can make friends with a machinist I plan to take a piesce of angle iron an make one foe the larfer fins.

BTW for multi-stage, I build the sustainer first and then mate the booster BT. Then I use popsicle sticks and clothespins to line up the booster fins.

For my Sudden Rush, I'm trying the template approach. I'm using 1" styrofoam and used the BT to cut the main hole. Then I made a template for the fins using Autocad and cut them with a knife.

I used 6 min epoxy to tack the fins to the MMT and will fillet everythig else with 30min.

A


A
 

powderburner

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Those pix are the perfect illustrations for what I was trying to describe. I think it is really worth the time to make these tools, and they are waaay worth it in terms of reduced frustration. Cut your initial cardboard sheets into 10 inch x 10 inch squares (I use my templates for low-power stuff) and they will even store nicely in a file cabinet.

Perhaps the 1/8 inch masonite is a bit of overkill for low-power . . .
 

dr wogz

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thanks all,

that is one direction I was thingking of..

And, I think I might jsut do this this afternoon! Two for each tube size, and fin arangement..

Ahh, to have Autocad, and a plotter...
 

lalligood

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Originally posted by powderburner
Perhaps the 1/8 inch masonite is a bit of overkill for low-power . . .
I've made fin template guides out of foamboard. It's cheap (20" x 30" piece for ~$2), is fairly rugged, & easy to cut.
 

Hospital_Rocket

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Originally posted by Dr Wogz


Ahh, to have Autocad, and a plotter...
I'm trying to unload a licensed copy of Autocad LT97 (I upgraded to the full product - The LT is legit as I had to buy a full copy of ACAD14)

If you, or any other reader, are interested....:)
 

kkooch

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Originally posted by powderburner
"For sport-fliers and even for competition rockets, it suprises many people to learn that the last one or two degrees of fin alignment really doesn't make much difference. "

In high level competition, especially altitude, it could be the deciding factor that separates the loosers from the winners. I would place an equal emphasis on having dead straight fin alignment for the duration events also; if you fly higher, even slighltly, you'll take longer to come down, however so slight.

For that matter, you don't *have* to build precisely at all to fly competition. However, precisely built models will always be more competitive and fortunately, this is one area the modeller can control.

Kevin Kuczek
 

powderburner

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Yes, perfect is usually best.

However, if we are talking about competition models here, then there are some practical real-world considerations to think about too. Many guys don't do a perfect job of getting the same airfoil shape sanded onto both sides of their fins, or even the same airfoil shape from fin to fin (if they bother to put any airfoil at all on their fins). Many guys don't spend all the time they should to get a well-shaped final surface on their nose cones, or don't even select an optimum subsonic NC profile to begin with. Many guys don't get a good mirror finish on the NC, and don't get a good, tight ('invisible') fit between the NC and the front of the BT, or a good smooth finish on the BT or fins.

So, IMO, if a fin is off by a degree or so, it's not going to make much difference. Yes, it might cost you a foot or two, but there are many other aspects to final finish and prep that will make a much bigger difference.
 

kkooch

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Originally posted by powderburner

"However, if we are talking about competition models here, then there are some practical real-world considerations to think about too."

Absolutely; it's the whole picture. Respectfully, I disagree with your assesment. Fin alignment is one area thats easy to control with a decent jig. You claim a few degrees will affect the ultimate alt. by one or two feet. Consider that a misaligned fin will also cause the rest of the model to track at a higher aoa. The drag will go up more than you think, certainly enough to cause more than just a few feet reduction as you claim. Also consider that a misaligned fin will cause a model not to go where you want it to go; this might be important when picking air in the duration events. I've always been in the money when flying altitude and took the silver altitude medal at the world space modeling champsionships in 00'. Make no mistake in thinking that fin alignment is something that you can just eyeball, especially when competing at higher levels.

Ok to eyeball for flight points though :)

Kevin K.
 

powderburner

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While not doubting your expertise at the WSM-level, I point out that for an airfoil section such as the 63A-series (yes, it's an old NACA airfoil but it is still representative), you cannot even measure a difference in Cd off of the drag polars for an angle of attack of one degree (and larger)--the plot is essentially flat across that range at the bottom of the bucket.

When you consider that the fin drag is only one component of the overall rocket vehicle, any +delta Cd that results from small mis-alignments is even more negligible.

Yes, of course it will make a small difference, but not hundreds of feet or anything like that. It would be extremely easy to have any number of other drag influences completely overshadow the small decrement caused by a small fin mis-alignment.
 

kkooch

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Originally posted by powderburner
"Yes, of course it will make a small difference, but not hundreds of feet or anything like that."


I agree. Let's assume that you and I are flying nearly identical altitude models. Both are mass optimized, both have exquisite finishes, both are molded using the latest and greatest air cheating shapes, both use motors that have been tested and shown to be within 1% of the total impulse allowed for that event.

The only difference is that you eyeballed your fin alignment and I jigged mine.

What is your competitive advantage then?

A lot of assumptions, I know. But typically the very top placers are all within 10-20' of each other and sometimes one is way out front (hot motor). Their models look very, very much the same. My point is that if you're not doing everything in your power to make that "perfect model", you will end up on the loosing side most of the time and only luck or a hot motor will send you to the top.

When competing, I always look at what competitive advantage my models have over the others and if one cannot point to a single item that sets their model apart from other top flyers, it's going to be a crap shoot. It is more than luck that enables some to be consistent in their winnings. And that is the key, consistency. People that consistently land in the winners circle would certainly acknowledge luck was a bit of a factor, but bigger is the fact that they did everything possible to make the perfect model.

True, the drag differences of a misaligned fin by itself may be less than a percent. However, I would bet the overall drag of the model due to a misalignment is more than a percent. I have seen many models track that have had their fins eyeballed vs. jigged. And the trajectories are very different. Models whose fins that were eyeballed tend to spin on the way up and some even wag their tails. The smoke trails are very telling. Ones with dead straight alignment have trails that look like thin straws; no wobbling, no spinning etc. They are on rails. And before the actual results are even posted, you know that those flights are going to be up at the top. I'll stick with my observations too ;)

Kevin Kuczek
 
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