23" x 10.85" Elliptical Fins - Will they snap off?

[RocketFeller]

We are working on the design for the fins on our 4.9X Upscale Dragonfly. The fins are relatively long with a short root, so fin-flutter is a major concern...

Some basic information on the rocket:

Airframe - 12.75" x 54" carbon fiber tube, 1/16" wall thickness (~4 pounds)
Nosecone - 5:1 ogive, 12.75" x 73.75" including 10" shoulder. Foam core with carbon fiber lay-up. (~12.75 pounds)
Centering rings and thrust plate - 1/8" carbon fiber plate (~3 pounds)
Motor - 98mm, K-M impulse (ideally)

The fins are about 23" x 10.85", not including the fin tab.

The plan is to taper a BB plywood core from full thickness (1/2" or 5/8") at the root to 1/8" thick at the tip. A local cabinet shop will be planning the core blanks and then the fins will be cut out on a CNC. The plywood cores will be sandwiched between two 1.3mm carbon fiber sheets which are also being cut on a CNC. The fin tabs will have cut-outs to reduce weight. We will use Aeropoxy laminating resin to attach the CF sheets to the plywood. We may wrap the plywood edge with CF cloth, but I'm not sure if it is necessary.

I have heard that the tapered shape will help to reduce fin-flutter by breaking up the harmonics that cause flutter (I can't say this is true, but I tend to believe what I've been told on this). I am assuming that a thicker blank (and therefore a more pronounced taper) would increase the resistance to fin-flutter? Obviously, at some point the added weight becomes problematic. I am leaning towards 5/8" as it will create a truer upscale and more taper, but I have also considered 1/2".

Opinions? Experience? Ideas?

Thanks in advance for any and all replies!

 

[mikec]

Are we talking about the Binder Design Dragonfly? If so, this seems like an odd choice for a lightweight high-performance rocket that will go fast. The design seems more suited for a heavy low-and-slow upscale. What maximum velocity are you expecting?

I've never found a satisfactory way to quantify fin flutter for a composite layup like this short of full-up finite element modeling and a lot of materials testing, so I can't offer any guidance there. No shortage of opinions about it but just how based in reality I can't say.

 

[Mainless]

If you are concerned about fin flutter, do the calculations to figure out if they will (Written by NACA):

https://www.mediafire.com/download/2z9z99g92jddqcs/NACA_TN_4197_Flutter_Preliminary_Design.pdf

 

[RocketFeller]

Are we talking about the Binder Design Dragonfly? If so, this seems like an odd choice for a lightweight high-performance rocket that will go fast. The design seems more suited for a heavy low-and-slow upscale. What maximum velocity are you expecting?

I've never found a satisfactory way to quantify fin flutter for a composite layup like this short of full-up finite element modeling and a lot of materials testing, so I can't offer any guidance there. No shortage of opinions about it but just how based in reality I can't say.

Thanks for the reply!

Yep, the Binder Design Dragonfly! I agree, it really is a low-and-slow rocket. The carbon fiber construction is mostly to keep things light enough to fly on smaller motors, rather than to increase performance. Also, the company that built the tube for us, Innovative Composite Engineering, only works in carbon fiber.

The weight was originally estimated at about 35-40 pounds without motor, but it is looking like it will be closer to 45-50 pounds when it's all said and done. The tube is built, the carbon fiber plate for the fins and motor mount is being cut, and the nosecone is on its way from Python Rocketry. The tapered plywood blanks for the fin cores is on order from a local cabinet shop. Once we have all the components we will be able to get a closer estimate of the final weight which will allow us to do some simulations and find the max V.

 

[RocketFeller]

If you are concerned about fin flutter, do the calculations to figure out if they will (Written by NACA):

https://www.mediafire.com/download/2z9z99g92jddqcs/NACA_TN_4197_Flutter_Preliminary_Design.pdf

I will take a look!

 

[bobkrech]

The strength and stiffness of the fin is in the carbon fiber, not the plywood. Scrap the plywood and laminate CF over the foam. Alternative use mat CF sheet bonded over the foam and then handlayup 2 layers of think CF cloth over the mat sheet. Final thickness should be ~ 1/16".

A 1/16" wall x 12" diameter CF tube may not be that stiff. You may need some internal rings to stiffen the skin at the attachment point.

The fins should be at least 1/2" thick and you could probably go up to 1" with a foam core without getting heavy. You would epoxy the fins (contoured to match the airframe OD and then use 3 tapered gusset layers to bond the fins smoothly to the airframe instead of an epoxy fillet.

The fins would be similar to a Spitfire wing so a properly constructed fin should be safe for airspeed to ~700 fps and may be to 800 fps velocity.

Bob

 

[mikec]

If you are concerned about fin flutter, do the calculations to figure out if they will (Written by NACA):

I've looked at this document a bunch of times, but I've never been able to make much use of it. First, it uses a bunch of variables that are only defined in other references, and second and more important, you need to know the material properties (like modulus) of the stuff the fins are made of. If they were metal you might have a chance, but the properties of plywood, fiberglass layups, CF, and other composites are really hard to quantify and wildly variable.

Given that I assume you're subsonic with this rocket, I think you have a good chance of making it work with construction like what you suggested. Bob's suggestion of laminating foam will reduce weight, but maybe that's not what you want since lighter will go faster and so have more risk to flutter.

 

[RocketFeller]

The strength and stiffness of the fin is in the carbon fiber, not the plywood. Scrap the plywood and laminate CF over the foam. Alternative use mat CF sheet bonded over the foam and then handlayup 2 layers of think CF cloth over the mat sheet. Final thickness should be ~ 1/16".

A 1/16" wall x 12" diameter CF tube may not be that stiff. You may need some internal rings to stiffen the skin at the attachment point.

The fins should be at least 1/2" thick and you could probably go up to 1" with a foam core without getting heavy. You would epoxy the fins (contoured to match the airframe OD and then use 3 tapered gusset layers to bond the fins smoothly to the airframe instead of an epoxy fillet.

The fins would be similar to a Spitfire wing so a properly constructed fin should be safe for airspeed to ~700 fps and may be to 800 fps velocity.

Bob

Thanks for the input!

The CF sheet for the fins is 1.3mm, (~.05") thick with a glossy side and a textured matte side for bonding. I am a little hesitant to do a true layup with cloth and resin as I am working with students and things could get messy. Also, I am not sure how "pretty" it would turn out and the rest of the rocket is going to be so darned pretty!

The current plan is to do TTW fins with the centering rings for the motor mount bonded to the fin tabs. The tube is thin but it is quite stiff - it is built of a layer of twill, three layers of directional material (one longitudinal and the other two at opposing 45 degree angles), and then another layer of twill. I was planning to use some scraps (the tube was built longer than necessary) to reinforce above and below the fin slots. We are also planning to use some CF cloth to reinforce the tube-fin junction from the inside.

I had originally considered a foam core, but I wasn't sure about planing/machining the tapered profile - I have a lot more experience with wood. I was also a little worried about finishing the edge of a foam-cored fin. I have also thought about building a plywood skeleton and filling the cutout with foam core. There would definitely be a weight reduction (about two pounds per fin, by my rough calculations) by using foam core. However, seeing as how performance is not a priority I'm not sure it is necessary.

 

[dixontj93060]

This sounds like a really fun project! Bob is right, foam core with CF skin is probably optimal, but a light wood core or end grain balsa will work. Main thing is to analyze the design and fly within it's limits. Do you have FinSim? If not, I'd be happy to do some sims for you on flutter to give you an idea on your thrust/velocity limits.

 

[3stoogesrocketry]

Here is one of my fins for my current project . It is hot wired 1 inch blue foam with 3 layers of fiberglass per side. The leading and training edges are 3/8 oak dowel . It is incredibly stiff , but yet weighs just over 1.2 pounds .

Eric

 

[RocketFeller]

I've looked at this document a bunch of times, but I've never been able to make much use of it. First, it uses a bunch of variables that are only defined in other references, and second and more important, you need to know the material properties (like modulus) of the stuff the fins are made of. If they were metal you might have a chance, but the properties of plywood, fiberglass layups, CF, and other composites are really hard to quantify and wildly variable.

Given that I assume you're subsonic with this rocket, I think you have a good chance of making it work with construction like what you suggested. Bob's suggestion of laminating foam will reduce weight, but maybe that's not what you want since lighter will go faster and so have more risk to flutter.

Thanks for the reply!

I agree that Bob is right, foam-core would be the optimum construction. However, you are right that while I do want to make it light enough to fly on smaller motors, I also don't mind a little weight to keep it slow.

 

[RocketFeller]

This sounds like a really fun project! Bob is right, foam core with CF skin is probably optimal, but a light wood core or end grain balsa will work. Main thing is to analyze the design and fly within it's limits. Do you have FinSim? If not, I'd be happy to do some sims for you on flutter to give you an idea on your thrust/velocity limits.

Thanks, it really has been a lot of fun! I don't have FinSim - we have been using RockSim to get some basic ideas about thrust/weight/drag, but other than that we haven't done any simulations. I would love to see some simulations - what information do you need?

 

[RocketFeller]

Here is one of my fins for my current project . It is hot wired 1 inch blue foam with 3 layers of fiberglass per side. The leading and training edges are 3/8 oak dowel . It is incredibly stiff , but yet weighs just over 1.2 pounds .

Eric

That is one nice looking fin profile - very nice!

 

[dixontj93060]

Thanks, it really has been a lot of fun! I don't have FinSim - we have been using RockSim to get some basic ideas about thrust/weight/drag, but other than that we haven't done any simulations. I would love to see some simulations - what information do you need?

Give me the core material, the laminate material and thickness, epoxy used with the laminate, the physical dimensions of the fins including final thickness, the diameter of the rocket, the adhesive being used to attach the fins, the fillet size and the thru-the-wall dimensions (tab size, depth, etc.).

 

[Binder Design]

Are we talking about the Binder Design Dragonfly? If so, this seems like an odd choice for a lightweight high-performance rocket that will go fast. The design seems more suited for a heavy low-and-slow upscale. What maximum velocity are you expecting?

Long burn small M would be the biggest motor. About 2/3 mach at most. Lightweight to fly on K's at small fields.

 

[McKailas Dad]

Don't mind me, I'm just lurking and taking notes for a project I'm working on....

Any chance you thought of using wooden dowels?! (Sorry, never mind )

 

[RocketFeller]

Give me the core material, the laminate material and thickness, epoxy used with the laminate, the physical dimensions of the fins including final thickness, the diameter of the rocket, the adhesive being used to attach the fins, the fillet size and the thru-the-wall dimensions (tab size, depth, etc.).

Thanks for the offer! Hopefully this will do:

Core Material: BB Plywood
Laminate: CF Sheet, 1.3mm (.052") thick, glossy outer surface
Epoxy: Aeropoxy Laminating Resin
Physical Dimensions: 23" x 10.86" Elliptical, tapered from .625" thick at the root to .25" thick at the tip (this may be the tricky part to calculate)
Rocket Diameter: 12.75"
Adhesive: Aeropoxy Structural Adhesive
Fillet Size: ~1/2"
TTW Dimensions: Tabs will be 10.86" long by 4.375" deep. They will be .625" thick. Tabs will connect to the 98mm MMT.

 

[RocketFeller]

Long burn small M would be the biggest motor. About 2/3 mach at most. Lightweight to fly on K's at small fields.

I think people might like to see it go up at Perrydale - I've never seen anything there with a six foot long nosecone....

 

[RocketFeller]

Don't mind me, I'm just lurking and taking notes for a project I'm working on....

Any chance you thought of using wooden dowels?! (Sorry, never mind )

No worries, I've been doing the same thing with your 10" paper monster!

 

[McKailas Dad]

No worries, I've been doing the same thing with your 10" paper monster!

Why thank you, I take that as a compliment! Many times for me, the interest generated and/or replies in a thread is what motivates me to continue the build.

I like to try to do things that 'haven't been done' but is difficult to do in this hobby.

 

[bobkrech]

Thanks for the offer! Hopefully this will do:

Core Material: BB Plywood
Laminate: CF Sheet, 1.3mm (.052") thick, glossy outer surface
Epoxy: Aeropoxy Laminating Resin
Physical Dimensions: 23" x 10.86" Elliptical, tapered from .625" thick at the root to .25" thick at the tip (this may be the tricky part to calculate)
Rocket Diameter: 12.75"
Adhesive: Aeropoxy Structural Adhesive
Fillet Size: ~1/2"
TTW Dimensions: Tabs will be 10.86" long by 4.375" deep. They will be .625" thick. Tabs will connect to the 98mm MMT.

You seem set on making this rocket very heavy. I'm not so sure this CF rocket needs to be between 45 to 50 -pounds. If you use foam core instead of plywood as the fin core, you can reduce the aft weight by close to 8 pounds, and eliminate another 8 pounds of ballast from the NC. You could also use a 75 mm motor instead of the 98, and also reduce the size and weight of the recovery system, making the rocket weight ~25 pounds. With these changes can reduce the impulse required by 1 full impulse class if you half the pad weight. That's a 50% cost reduction each time you launch the rocket. Instead of K, L and Ms, the lighter rocket could launch J, K and Ls. It seems a shame to make a carbon rocket so heavy.

Bob

 

[RocketFeller]

You seem set on making this rocket very heavy. I'm not so sure this CF rocket needs to be between 45 to 50 -pounds. If you use foam core instead of plywood as the fin core, you can reduce the aft weight by close to 8 pounds, and eliminate another 8 pounds of ballast from the NC. You could also use a 75 mm motor instead of the 98, and also reduce the size and weight of the recovery system, making the rocket weight ~25 pounds. With these changes can reduce the impulse required by 1 full impulse class if you half the pad weight. That's a 50% cost reduction each time you launch the rocket. Instead of K, L and Ms, the lighter rocket could launch J, K and Ls. It seems a shame to make a carbon rocket so heavy.

Bob

It's not that I'm really set on this, it's just that I have fears...

I had originally planned on using pre-made carbon fiber sandwich panels. However, when discussing the design on the Tripoli facebook page (not a bad source of opinions) I was swayed to the idea that this may not be robust enough. The specific issue is that of fin-flutter, from the way I understand things, a flat profile is subject to harmonic vibrations that can tear a fin apart. Tapering the fin will theoretically break up the harmonics and lessen fin-flutter considerably. Maybe this is not true and I am way over-building the fins - I can't really say from my limited experience with projects of this magnitude.

As far as weight, the rocket is stable without nose weight, so the actual added weight is only the additional weight of the core material (and additional recovery gear), no additional ballast. We are going with 98mm for the simple reason that the sponsor that is supplying the motor asked for 98mm.

So far the weight breakdown is as follows:

Airframe: ~4.5 pounds
Nosecone: ~12.75 pounds
Fins: ~14 pounds
MM and CR: ~3 pounds
Recovery Gear ~6 pounds

It comes up to about 40 pounds. While I could see shaving off eight or ten pounds at the most, I have a hard time seeing how I could halve the weight.

I would like this to be a true featherweight, but if we come in under forty pounds I think we are doing pretty well for a 12" rocket.

 

[Binder Design]

We can tailor the burn profile to the build, so if you do go lighter weight foam core fins we'll go for a slower flight. I've got a slow formula and good liners so if we can get some solid finsim numbers, we'll keep it sane.

 

[RocketFeller]

We can tailor the burn profile to the build, so if you do go lighter weight foam core fins we'll go for a slower flight. I've got a slow formula and good liners so if we can get some solid finsim numbers, we'll keep it sane.

Sounds good. I am still thinking that a skeletonized fin with the void filled with foam (or maybe just air?) might work as well. Our carbon fiber supplier said that he could also supply foam core material, so there are options.

If it were a simple triangular or quadrilateral fin I would have been more likely to go with a sandwich panel. Edging the elliptical fins in a simple, strong, and aesthetic manner seems daunting, to say the least.

 

[dixontj93060]

Thanks for the offer! Hopefully this will do:

Core Material: BB Plywood
Laminate: CF Sheet, 1.3mm (.052") thick, glossy outer surface
Epoxy: Aeropoxy Laminating Resin
Physical Dimensions: 23" x 10.86" Elliptical, tapered from .625" thick at the root to .25" thick at the tip (this may be the tricky part to calculate)
Rocket Diameter: 12.75"
Adhesive: Aeropoxy Structural Adhesive
Fillet Size: ~1/2"
TTW Dimensions: Tabs will be 10.86" long by 4.375" deep. They will be .625" thick. Tabs will connect to the 98mm MMT.

I'm getting a max velocity of ~500 ft/s to guard against flutter. PM me your email and I can send you the results.