Types of Misaligned Fins and Their Effects?

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mccordmw

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I've been slapping together a bunch of Chuter 2 rockets with the kids lately for them to have drag races at local launches. I explained the details of how to build them and offered an extra pair of hands, but I'm letting them handle much of the build. I explained to them how fins work for stabilization and showed them the concept with a weather vane for the CG - CP and how the fins steer into the wind.

They asked me how misaligned fins affect the whole thing. Now I now that canted fins will induce spin. But what about all the other types of misalignments that can happen?

Good alignment = straight flight

fin_good.jpg

Canted fin = spin

fin_bad1.jpg

Fins at uneven levels? The CP will not be the same all around, so would the rocket snake a bit in the air? Any spin might also make it corkscrew.

fin_bad2.jpg

But how about a fin that isn't perpendicular to the body while others are? What flights does that cause?
fin_bad3.jpg

And the youngest bumped his rocket when attaching the fins. Causing it to no longer be centered over the alignment guide. So his last fin, while perpendicular, what shifted a bit. Maybe this will have the same effect as above.

fin_bad4.jpg

I figure with LPR, none of these will be very bad. But as I look to pushing mach with my L2, I'm thinking what these will do at those higher airspeeds and tighter tolerance requirements.

Hmm...
 

rstaff3

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Generally, for LPR up through mild HPR, if there is enough fin area to keep stability in every axis, all but the canted fin example are likely be fine. As you pointed out, if one fin is set forward, you could have a stability issue. However, I have done this several time on purpose. Of course, I checked stability on those designs. The canted fin example IMO is more iffy. It may just cause a spin but it could cause the rocket to go unstable. Gotta look into all that dynamic stability stuff.

If you accelerate really fast or approach mach, what I said also may not hold.

I think the biggest issue, unless the misalignment is crazy bad, is to make sure all the fins are well attached. I've seen more scout rockets go awry by losing fins, MMTs not glued in, recovery harness not tied on, etc.
 
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EXPjawa

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Another thing to think about, I suppose, is that if two fins 180 degrees apart are canted in the same direction (i.e. both to the left side of the assembly), they will rudder the rocket in that direction. Or, if one has a bit more cant than the other, you could get a roll about an offset axis (not on the rocket's centerline). It would corkscrew.
 

AlfaBrewer

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One fin not perpendicular doesn't have much apparent impact on LPR. I've tested this on at least 2 dozen rockets of different fin configuration, size, shape, etc. I have an amazing ability to get all but one fin on perfectly even when using a fin guide.
 

rstaff3

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On 99 percent of mt rockets, they are aligned by eyeball.
 

RocketFeller

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I run an after-school rocket club for elementary students at my school. I have seen fins misaligned in every conceivable way, and for the most part it makes little to no difference in the flight trajectory. Most LPR rockets are built with a high stability margin and will go straight up even with messed up fins. They may cone or corkscrew a bit, but you have to get pretty lucky (unlucky?) to have a LP rocket arc over.
 

adrian

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Ever seen the Far Side cartoon showing a rocket which is probably in worse shape than anything you'll see produced by the kids and probably worse than anything you'll see in your nightmares?

Here's the kit of that rocket:
https://www.fliskits.com/products/rocketkits/kit_detail/acme.htm

If that flies straight (and it does, I've seen it), then minor misalignments by your kids aren't likely to do much harm.
 

BABAR

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I've been slapping together a bunch of Chuter 2 rockets with the kids lately for them to have drag races at local launches. I explained the details of how to build them and offered an extra pair of hands, but I'm letting them handle much of the build. I explained to them how fins work for stabilization and showed them the concept with a weather vane for the CG - CP and how the fins steer into the wind.

They asked me how misaligned fins affect the whole thing. Now I now that canted fins will induce spin. But what about all the other types of misalignments that can happen?

Good alignment = straight flight

View attachment 301068

Canted fin = spin

View attachment 301064

Fins at uneven levels? The CP will not be the same all around, so would the rocket snake a bit in the air? Any spin might also make it corkscrew.

View attachment 301065

But how about a fin that isn't perpendicular to the body while others are? What flights does that cause?
View attachment 301066

And the youngest bumped his rocket when attaching the fins. Causing it to no longer be centered over the alignment guide. So his last fin, while perpendicular, what shifted a bit. Maybe this will have the same effect as above.

View attachment 301067

I figure with LPR, none of these will be very bad. But as I look to pushing mach with my L2, I'm thinking what these will do at those higher airspeeds and tighter tolerance requirements.

Hmm...
As you said, probably at MACH gonna have some problems (in fact, I suspect if there was significant misalignment wouldn't make mach anyway.
Not sure if dividing by HPR and LPR and MPR really is the issue, I think it is more a velocity issue.
Canted is very different from eccentric. Rockets can tolerate a lot of eccentricity in fin position so long as the longitudinal alignment of the fin matches that of the body of the rocket (and of course the other fins.)
So your pics 3 4 and 5 should be fine.

On Scratchers today I posted a bunch of intentionally asymmetric fin rockets. As in, no kid would accidentally put the fins on THIS far off. I work pretty hard to make sure the LONGITUDINAL alignment is dead on. Usually they will do some corkscrewing (surprisingly, once in a while with the right motor they have a typical flight path), but so long as they have enough fin area the ascent vector is straight. So I am not sure that "corkscrewing" is equivalent to unstable.
So the kids that don't get the fins straight and even will have rockets that probably won't go quite as high as the others, but as long as they aren't canted off too far, they should be stable.
A suggestion to help them get the fins on straight (and faster.) Take a 1/16" sheet of balsa, and cut some 1/16"x1/16" balsa "fillets", the length of the root edge of the fin. Mark your fin lines on the body tubes (obviously THESE have to be perfectly positioned, otherwise you are hosed from the get go.) Glue the balsa "fillets" right over the lines (can be done ahead of time, before the kids even get there. Then all they have to do is match up the root edge of the fin with the lateral edge of the fillet. Guarantees the fin will be on straight (may be tilted to the side a few degrees, like your pic 4, that part you need to eyeball, but that won't significantly affect the flight path.) It also gives more surface area for the fin to "grip."
 
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lcorinth

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The CP is a single, theoretical point, so even if the fins are a uneven levels, the CP won't be at different places on the rocket body depending on which side you're looking at.

It will be at a slightly different location than it would be if all fins were properly placed, but it will be in one spot. (It can technically move forward or backward a little depending on the angle of attack, but that's normal.)

Having taught rocket camp several times, I can say that kids can really screw up fin placement, and most of the basic kits you buy in educator's bulk packs will probably be fine. As long as they're not too far forward. But a little crooked is usually OK.
 

BDB

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Does anyone have some insight on a split fin rocket with a single misaligned fin? A "friend of mine" may be interested. "My friend" predicts that it may corkscrew, but won't know for sure until it flies again (after the wonky repair) sometime this fall. :facepalm:

ImageUploadedByRocketry Forum1467668287.938738.jpg
 
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Banzai88

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As long as there's no off angle cant to the fin, and it's only that one fin, it should fly fine. Not as efficient as it would be if they were 100% in alignment, but I don't think it'll be to radical in any case. How did it fly before your 'friend' made the repair?

I have an NCR SA14 Archer that has 1/4 inch thick side strakes that are about 2-3 degrees off and it launches and perfectly spirals like it's been shot from a rifled barrel!
 

BDB

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How did it fly before your 'friend' made the repair?
It flew perfectly straight before I...I mean, my friend...crashed it and snapped one fin off. The fins were aligned during the repair until I foamed the fin can to provide some extra strength. The expanding foam pushed one of the fins out of alignment.
 

Exactimator

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Does anyone have some insight on a split fin rocket with a single misaligned fin. A "friend of mine" may be interested. "My friend" predicts that it may corkscrew, but won't know for sure until it flies again (after the wonky repair) sometime this fall. :facepalm:

View attachment 301580
I did a fin repair of an aft fin on a 4" Phoenix that ended up looking like yours.

I mean your friend's.

It flew fine after. I was keeping an eye out for any wobbling or spinning, but didn't notice anything serious. I wasn't pushing for performance (speed or alt) just launching on H VMax and White Thunders to mid 1000 feet.

I'd think unless you're planning some extreme flight, you should be fine. And even then, you might still not have an issue.

Sorry, I mean your friend. Your friend's rocket won't have an issue.
 

SpaceManMat

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Large spin rates going Mach will cause serious issues.
For the fins that are not perpendicular or off center, these will have higher stress loadings on the fillets.
 

rstaff3

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Luckily, the OP's kids won't be going anywhere near mach 1....at least for now :dark:
 

rstaff3

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My 13 yo wants to try for Jr. L1 next year when he's 14. He wants to bust mach.


*shudder*
Good for him! I assume by then he will have had the proper training and can get da' fins on straight ;)
 

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