3" fiberglass build, need some fin help

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RKeller

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I'm working on a DIY 3" glass kit and could use some help with the fin design. the kit I got came with 3/32 fin stock. I'm starting to wish I had upgraded to 1/8". nothing is cut yet so now is the time if I need to. basic specs,

Nose - 3" 5:1 Ogive with Al tip.
Payload - 16"
Booster - 36"
MMT - 54mm 12"
I have 6" x 36" 3/32" thick fin stock

I'm wanting to build a nice all around flyer, which is why I originally went with the 3/32 but open rocket is showing right around mach 1 with a K motor. I've started messing with finsim and so far I'm showing flutter right around mach 1 with both designs. not sure I'm using finsim correctly yet either. here are my current fin designs although I'm favoring the 2nd one at the moment. I'm looking for recommendations on how to design the fins so they won't flutter as well as recommendations on stability for transonic flight.
1st design, 3 fins
VuSd1FAv4LbVuEcQX87h8aHg4VcVV_0oYsafdNtJojo=w1051-h265-no


2nd design, 4 fins
byok6.JPG
 
Riley,

You're getting flutter due to the "cranks" in the design. Think of the fins as trapezoids with pieces cut out of them, essentially weakening them.


RileyFins.jpg

These designs aren't conducive to high velocity flight in this thickness of material. If you're set on these designs, look to increase the thickness of the G10 or laminate with composite. I may have some Carbon that I'm not using and could donate to your cause, I'd have to check to see if it's enough.
 
Even when i run a basic trapezoid 6" root, 3" tip, 3" span I get flutter with 3/32 stock. Maybe I'm doing something wrong...
 
Visually I like fin A much better. I'd move up to 1/8" G10. I built a 5" Exocet with big swept 1/8" G10 fins about 15 years ago and they held up to motors like an M2500 just fine.
 
The faster the rocket will be traveling the smaller the fin you can get away with to an extent....


I also use 1/8" G10 or so I thought....
It appears that my G10 ( from McMaster) is different (thicker) than RW G10 1/8".
I tried to fit my pre-cut fins into the slots and they don't fit.
They might be 4mm ( .157" ) vs 1/8 (.125) ???

Checked the thickness of the G10 sheet supplied with the kit and it does measure 1/8".

My 4" rocket has 1/8" G10 fins and has flown on Mid to full L motors without issue.
It has done mach with no troubles ( knock on wood).


JD
 
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The faster the rocket will be traveling the smaller the fin you can get away with to an extent....


I also use 1/8" G10 or so I thought....
It appears that my G10 ( from McMaster) is different (thicker) than RW G10 1/8".
I tried to fit my pre-cut fins into the slots and they don't fit.
They might be 4mm ( .157" ) vs 1/8 (.125) ???

Checked the thickness of the G10 sheet supplied with the kit and it does measure 1/8".

My 4" rocket has 1/8" G10 fins and has flown on Mid to full L motors without issue.
It has done mach with no troubles ( knock on wood).


JD

Using a basic clipped delta fin profile, I built 3" and 4" min dia rockets with 1/8" unreinforced G10 fin stock from McMaster. Flew both to thru Mach 2 (3" on a 7600ns M motor, 4" on a 17500ns N motor) with no layups and no issue.

https://www.rocketryforum.com/showthread.php?66313-3-quot-2-Stage-Alpine-Style&highlight=alpine+style

https://www.rocketryforum.com/showthread.php?55965-Sweaty-Balls-Project-Keep-the-Change&highlight=sweaty+balls
 
thanks for all the input. Dan, thanks for the offer! let me know what you find... so far it looks like I will want to do something to thicken the fins. 3/32 would probably be ok but I think i'll go with 1/8". now, my options are to buy some 1/8" G10, laminate the 3/32 I have with glass or carbon, or build it and add tip to tip glass or carbon. since this isn't a min diameter build and will not be designed to be an absolute speed demon I think the 1st 2 options are best. I can get a 6 x 24" piece of G10/FG4 from mcmaster for $15, I can usually get free shipping through work so that's not bad at all. I could also laminate the stock I have with carbon, which would be pretty easy and if it looked good I could incorporate the carbon into the paint job. hmmm decisions decisions...
 
so how big of a deal is fin flutter? obviously it isn't good, but I see lots of people flying 54mm and 2.6" rockets with 1/16" G10 fins over mach. those fins have to be fluttering. like the Madcow Arcas HV for example.
 
Here you go.

Art Upton's Bruiser:
[video=youtube_share;M_FCQ550770]https://youtu.be/M_FCQ550770[/video]

JD
 
how long you're in transition will matter too. Blast through, not as big a deal as a slow accel past mach.
 
This rocket is virtually identical in dimensions to yours....It was made from a project pack 10 yrs ago.[maybe same as yours, It had 3/32 sheet same size.] It has over 100 flights on it from I-161's to K-250 [15,300ft]. I has flown M1.9 on CTI 6 grain every things! [even L's]
Weighs 6.4lbs loaded for flight with 1 altimeter for extreme flying I use 2. I have a 60in SpheraChute in it equally as old.

The most versatile rocket I own. Put the nose cone on the fin can, leave payload and av-bay off..it will fly on H's. This was the 2nd lawn-dart...it's done 3! Shorter by 2in after all of them.

Use a close/similar fin design with the supplied 3/32 you will be fine. I can't kill it. There are not any fillets [external] just wiped some epoxy on joint/slot to seal for paint. You will be fine......I am.


Only the fin can is buried, I forgot the quick link, top end came in under chute. Everything survived just fine.

100_2796.jpg
 
thanks for the info Jim, that is exactly what I want with this rocket. I want it to fly well on whatever fits in it. I do like the look of your fins, I don't have my mind set on anything yet so we'll see where it goes.
 
If you end up wanting to do fins like this...I will do a trace/outline, put the measurements on it, take a pic and post it here for you to duplicate.
 
You want a really small fin if the rocket is to exceed mach... just look at the original Machbuster if you want to know what I mean.
 
If you end up wanting to do fins like this...I will do a trace/outline, put the measurements on it, take a pic and post it here for you to duplicate.


Jim, I don't think I want an exact duplicate but I would be interested in seeing the rough dimensions. I'm not 100% sure how much stability I want for this kit. I know I want enough to fly it on smaller motors, but not too much. not sure where the sweet spot is.
 
how many calibers would you recommend for stability in this rocket? I'm looking for a good all around flyer. not going for top speed or altitude, just want something I can put whatever motor I want into and send it up. here is another fin design, showing stability with drastically different motors. I'm up to fin design number 12 :D
dyok9I223.JPG

dyok9L1400.JPG
 
Here is my fin trace for you to compare. It does not show tab depth as I just traced fin from body tube outward.
There are no fillets to speak of just enough to seal the joint for painting purposes.

I have 1.9 calipers with a 6xl motor in, so I would think your fine.


Good luck ...have fun....fly high!


Oops the fin tip is 1-1/4 not 3-1/4 as shown.... Looks like a pretty good all-round design, should work well for you, I'd go with it.

DSCN3798.jpg
 
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If those are your extremes, I would say you are more then ready to go. You have over to 2 caliber of stability with the largest motor. I guess I don't understand what the concern is.

I would use those fins or maybe something smaller and less likely to flutter at mach speeds
 
Here is my fin trace for you to compare. It does not show tab depth as I just traced fin from body tube outward.
There are no fillets to speak of just enough to seal the joint for painting purposes.

I have 1.9 calipers with a 6xl motor in, so I would think your fine.


Good luck ...have fun....fly high!


Oops the fin tip is 1-1/4 not 3-1/4 as shown.... Looks like a pretty good all-round design, should work well for you, I'd go with it.

thanks for the help, I really appreciate it.
 
If those are your extremes, I would say you are more then ready to go. You have over to 2 caliber of stability with the largest motor. I guess I don't understand what the concern is.

I would use those fins or maybe something smaller and less likely to flutter at mach speeds

I guess my concern is just that this is the largest build I've done and will be using it for my L2 hopefully by LDRS. Just want to make sure I'm on the right track is all. I did realize I forgot to add the mass of the epoxy in the back so that will pull the CG back a little.
 
figured I would give an update... I did end up going with 1/8". I know 3/32 would have been fine but a square foot of G10 was only $15. I figured why not do that and use the 3/32 for ebay sleds and fins on something else. now I just have to decide how I want to bevel them. I could use the mill but I might build a dedicated beveling jig using my rotozip. thanks again for the help.
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0204151021a.jpg
 
I like those fins. A thought. As you mount them to the airframe do it in such a way that you can pull out the motor tube when the fins are glued in place. That way you have room to bond the internal root to the inside of the airframe with glass cloth.

The way I did it with my Mad Cow Arcas HV was to wrap masking tape around the MT for a snug fit, then attach the forward CR to it. Apply a thin bead of glue inside the airframe where the forward CR goes. Rocketpoxy would be good for this because it is thick and tends not to run. Slide the MT with the CR to that point and make sureTape 2 thin strips of electrical tape to the aft CR (180 degrees from each other) so that they faces the inside of the airframe. Leave about an inch of electrical tape sticking out. Slide it onto the motor tube and into the airframe leaving the tape tabs sticking out. Let the glue set with the airframe in a vertical position so that the glue does not run onto the motor tube.

Next, slide the fins into the slot until they are the thickness of the airframe away from being in place. Apply a line of glue onto both sides of the fins, and slide them the rest of the way in. Align them. Make sure none of the glue touches any part of the CRs.

Once all the glue sets, Pull on the tape tabs of the lower CR to remove it. Pull out the motor tube. You now have fins and forward CR glued in place. apply epoxy onto a stick and glue the forward fin roots to the forward CR. Cut strips of fiberglass about 1" wide by the length of your fin root minus about 1/2" away from the aft ends of the roots (This decreases the likelihood that fibers will extend beyond where the aft CR is supposed to rest). In this instance it is easier to apply the glue to the FG then press into place on both sides of the fin roots. make sure nothing sticks to or beyond the bottom of the roots.

After all this sets, apply a very liberal amount of glue to the motor tube in the areas where the fins and forward CR will touch it, then slide into place. Once that sets, apply glue to the MT and the inside of the airframe and glue in the aft CR.

At this point the only way those fins are going to come off is if you break them off.

Fins Installed.jpg Interior Fiberglass.jpg Completed Fin Can.jpg
 
so that's how you keep those fins on at mach+ even when the fillets are failing. not sure what method I'm going to use on this one. I'll use rocketpoxy for externals. I might inject internal fillets, we'll see. I still have to slot the body tube and bevel the fins.
 
I have heard of people gluing sticks where the internal fillet goes, but I am not sure how strong that is compared to fiberglass
 
so that's how you keep those fins on at mach+ even when the fillets are failing. not sure what method I'm going to use on this one. I'll use rocketpoxy for externals. I might inject internal fillets, we'll see. I still have to slot the body tube and bevel the fins.

When I ordered the kit, I downloaded the Rocksim file, looked at it via Open Rocket, and realized it would be a great machbuster if I could keep the fins on. It was going to require strong internal fillets and I didn't think foam was going to be enough.

This was its first flight. Both ground-level and onboard video.

[YOUTUBE]NGm8yza18g4[/YOUTUBE]
 
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I decided to just bevel them on the mill. I flycut the 2x6 so there was a little lip along the edge then clamped it in the vise at 10 degrees. made sure it was level and square with the mill then clamped the fins on and cut the bevels. I meant to leave a small flat on the edge, .020 or so but made my first cut a little deep so they're razor sharp. I'll break the edges a little so they don't chip too easily.
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slots! used the mill again. I located a 3"x 3" internal collet so I started with an index head to get the spacing. the 36" tube was too long to put the index head on the front end and support the rear so I cut the slots most of the way through on the index head then clamped the tube down finish them. I did punch a hole through each end of the slots so I could make sure it was lined up for the finish cut. I also added .003" to each side of the 1/8" slots because the G10 measured .130". the fit is perfect.
0206151413.jpg

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Dry fit together next to the scarab 54
002.JPG

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The fins look awesome. With a big 6-grain+ motor you could do Mach 1.6+. Use Rocketpoxy or something similar to make sure you have a strong bond.
 
So how do I bevel G12 fins if I don't have any of the fancy machines these guys have? I don't think I can use hand planes on fiberglass.
 
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