Sandwich Fin Layup Questions

[kjhambrick]

All --

I have been making Diamond Airfoil Fins for my all my mach-capable HP Rockets -- usually from sandwiches of Balsa + ( FiberGlass -or-Kevlar ) Cloth.

I am working on a design for a 38mm scale model Vulcanite -- La Pequeña Vulcanita and after reading here on TRF, I realize how little I know about the best direction to align the fabric weave in the sandwiches.

Back in 1998 or so, I purchased 5-yards of 1.7 oz Kevlar Cloth and 5-yards of Cabon Fiber Cloth of a similar thickness ( don't recall the weight ) for my 98mm Min Diameter Level 3 Project ( El Mui Grande Vulcanite ).

I bought plenty of Kevlar and Carbon Fabric to spare so I could experiment with techniques.

Anyhow, I plan on making either a Carbon Fiber or Kevlar Core followed by step-wise layers of 1/32 Balsa and more fabric.

EDIT: I should add that I shoot for a max thickness ratio of 20::1 from root to tip or in this case from 0.1575 inch at the root to 0.0788 inch at the tip.

Q1: Kevlar or Carbon Fiber Core ?

Q2: Should I align the fabric weave with the leading edge or should it be at some other angle ?

A rough, but good enough sketch of the fins for La Pequeña Vulcanita are below.

Any and all opinions are welcome !

Thanks !

-- kjh

 

[kalsow]

You can probably skip the internal carbon/Kevlar layer(s). Assuming the interior is strong enough to transfer forces from one side to the other without being deformed, the strength of the structure comes from the external layers.

https://en.wikipedia.org/wiki/Sandwich-structured_composite

 

[kjhambrick]

Thanks Kaslow.

Great link and great advice !

-- kjh

 

[rharshberger]

If you are using sheet balsa you are not getting the benefits of true composite construction, you need to be using end grain balsa so that the fibers run between the two faces of the fin or between the layers of reinforcement. Balsa fibers in tension are much stronger than when laminated with the grain (balsa has poor peel resistance qualities).

 

[JohnCoker]

Here's how I do it:
http://www.jcrocket.com/nike-custom-fin.shtml

 

[OKTurbo]

This thread shows my first journey into homemade composite fins. This rocket is still one of my favorites to fly.
https://www.rocketryforum.com/threads/high-powered-flis-praetor-will-the-fins-stay-on.149293/

 

[kjhambrick]

Thank you rharshburger for the thoughts on end-grain -vs- sheet balsa.

I will be making a test fin or two and I believe I will try out skinned, end-grain balsa -- I like the sound of that, especially after reading the info on Kalsow's Wikipedia link.

JohnCoker and OKTurbo --

Wow !

I knew I had a lot to learn and now I've also got a lot of good material to study and ponder.

Thank you !

Instead of a min-diameter 38mm rocket with thin-wall fiberglass tubes for my first endeavor, I've decided to go with a glassed Apogee BT-55 tube with a 29 mm motor mount because I have all the parts on hand.

The scaled down Vulcanite Fins for the BT-55 ( 1.325 inch ) rocket will be:

Cr = 2.404 inch
Ct = 1.202
Span = 1.713
Sweep = 1.082

Pretty tiny for my fat fingers and hand tools

Based on my 48mm, 54mm and 73mm Vulcanites, the BT-55 version should be able to handle all the motors that fit.

I'll follow up after my Level 2 attempt on July 15 when I'll be able to focus on I this build.

I am probably over-engineering this rocket but materials and methods have changed so much in the past 25-years that I wanted to look before I leapt and maybe try something new ( for me ).

Thanks again all'Y'all !

-- kjh

 

[kjhambrick]

All --

My fins are too small and my hands are too fat to do anything too fancy so I ended up using my old method from 25 years ago, except I went with 1/32 inch basswood not balsa ( the 1/32 inch balsa at Hobby Lobby was pretty bad ).

I first made a 0.75 oz fiberglass sandwich between two sheets of basswood. These are the basswood and fiberglass materials. The whisker below the bottom pair of basswood is a strand of Kevlar thread that I embedded along the mid-chord on the outside of each piece of basswood.



I first made a glass sandwich with basswood bread:



I aligned the wood carefully and made sure there was a little extra glass all-around the perimeter and then I squashed them between two 12-inch Bathroom Tiles until the 30-min epoxy was leathery and then, I trimmed off the extra glass and let them cure.

Then I tacked a strand of Kevlar thread with two dots of CA to the mid-chord of each fin on both sides. This makes a handy sanding-stop for 'later' ( sorry, no picture ).

Then I cut out 1/32 inch basswood 'steps' to apply to each side of the fin sandwiches. I cut the steps along the mid-chord and I beveled the underside of the mid-chord cuts so that the Kevlar mid-chord spines would not interfere with the fit of the steps and I epoxied the on the steps and squashed them between the tiles until the 30-min epoxy hardened. At that point I trimmed the kevlar spines along the root and tip chords ( more step-wise pictures next time ):

:

Finally, I sanded them down to diamond airfoils ( a tad heavy at 18-grams but not too bad, I suppose ).



One nice thing about the sandwich structure is that it is pretty simple to ensure symmetry on each fin and I can also see that the airfoils on each of the three fins are consistent.

I've now got three fins with diamond airfoils where chord::thickness is about 20::1 all the way from root-to-tip.

Still a bit of touch-up to do and I've not decided whether to apply an outer layer of 0.75 oz glass before I attach them to my beefed up BT-55 or to try tip-to-tip without turning the diamonds to symetrical NACA ogive airfoils ( I know how to glass the fins before installing them but not sure I can keep a diamond profile with tip-to-tip ).

Anyhow ... thanks for the input.

I will definitely try some of the suggestions here when I have larger fins to build !

-- kjh

 

[cls]

Those fins look great!

 

[cbrarick]

you've done a lot of good work, however,
why?
just change your material to Baltic birch. my vulconate is bone stock, only epoxy was the retainer (jb weld). it's flown on the J600 red. No problems. there's no bigger motor for it to push it further.

It's really a myth that you need composites for mach 1

 

[ljwilley]

You can achieve Mach 1 on a G motor with an apogee aspire built with only wood glue. You are right it is a myth. Unless you are going to carbon fiber the outside of the rocket because it looks cool. For that, "why not" is enough of a reason for me.

 

[kjhambrick]

you've done a lot of good work, however,
why?
just change your material to Baltic birch. my vulconate is bone stock, only epoxy was the retainer (jb weld). it's flown on the J600 red. No problems. there's no bigger motor for it to push it further.

It's really a myth that you need composites for mach 1

Thanks @cbrarick and @cls

These fins took two days of clock time and a few hours of actual work ( mostly ... ick ... sanding ).

I agree: it is a myth that cardboard can't break Mach 1.

And Baltic Birch Plywood will show the same layer effect that my hand laid-up sandwich does when sanding a diamond airfoil.

But landing on rough terrain is sometimes the most destructive phase of the flight that we have to contend with ( especially without a chute see below )

I too have flown my 25 year-old more-or-less stock Vulcanite ( Spock's Johnson ) on a J570 several times back when I was testing AltAccs.

I don't remember if there was a J510 back in 2000 -- I don't think so -- but I would have loved to try that one in him too

Spock's Johnson is only more-or-less a stock Vulcanite any more because all that remains of my original kit is the lower 12-inches of the FinCan due to a core-sample when I forgot to arm my AltAcc with a J570 in the motor mount ( did you know a Vulcanite whistles just a little when it comes in balistic ? )

It was long ago repaired with plain-old LOC 54mm Tubing and I just re-certified level 1 on this same Spock's Johnson in June on an H128.

If AARG had the Launch Site for it, I wouldn't hesitate to load it with a J570 to go for my Level 2 in it.

But we don't have the right site for a 10k flight this time of year.

The Vulcanite is one of my all-time favorite kits: it flys great and speaking of fins and Mach Numbers, the Vulcan'te's fin sweep angle is good up to about Mach 1.88

I've never even gotten close to Mach 1.88, even in my 73mm scale Vulcanite -- Nocturnal Missions

I do want to fly an L1090-W at the right site -- I don't recall that motor either back in 2000.

Nocturnal Missions should hit Mach 1.5 and 12K feet or so on an L1090-W, depending on the local temperature but the LE of the Fins will still be inside the Mach Cone ...

My fastest flight is Mach 1.4 and 11K feet in this rocket on a cold day ( 40 F or so ) in Ocotillo, CA on a K700.

Anyhow ... I build fins like these because I like fins, not so much because I like composites

IOW, I build them this-a-way for fun.

These specific fins are scale Fins for a BT-55 Vulcanite -- La Pequeña Vulcanita ( 34mm AF -- 0.63::1.00 scale )

I built these fins the way I did because on each of my existing Vulcanite Scale Models also have 20:1 diamond airfoil fins.

It didn't take many flights to learn that the nekked plywood fins on Spock's Johnson looked pretty sad after a few flights in the desert -- the leading and trailing edges have more than a few dings due to rock piles on the ground and maybe even some due to recovery hardware flying around in the air.

The LE and TE of Spock's Johnson's fins are also pimpled with CA + Glass bandaide spot welds -- it will be a sad day when have to finally retire him because the fins are eventually damaged beyond repair

OTOH, the fins on my 48mm Vulcanette with a glass core and glass LE's and TE's are still in fine shape, even after flying all the AT 38mm motors that will fit intp her 24-inch Fin Can.

I also plan to work my way 'up' from La Pequeña Vulcanita's BT-55 ( 34mm ) AF to a new 38mm min diameter rocket ( name TBD ) and then down to 29mm min diameter also unchristened as yet.

I only do this because the Blue Raven will fit comfortably in these Air Frame so I can gather data for the flights and eject a Drogue at apogee.

All of these scale models will have the same fins, scaled up or down so maybe someday I might 'see' patterns in the data ( like, for example induced drag when the rocket pitches and yaws ( aka wiggles ) or when it weather-cocks ).

I've also got all the parts, including the specific materials I need to make similar 20::1 Chord::Thickness fins for my Level 3 rocket, designed around an M1939 in 4-inch Glass Tubing with Carbon Fiber for a little stiffness and a Kevlar jacket to prevent road-rash ( this is the spare composite fabric I mentioned in my OP above ).

I want to at least get the parts out of the cartons and build that one some day soon, even if I never fly it on an M1939 ...

Anyhow, I guess the answer to why ? because I like FINS

-- kjh

 

[cbrarick]

ok, cool.

I'm a bit lazy about rockets, they're nothing but motor holders :>

g10 for me, no muss or fuss. My current build is a 11.5 PM Bullpuppy that was stretched and the motor hole enlarged to 6" for a P motor.
I have new fins, thicker to support this, g10 as well.....

but, if you like to process, go for it! As we say in hiking, hike your own hike!

 

[G_T]

Instead of tip-to-tip, think instead perhaps of a composite fillet with sufficient bonding area for the tube and the fin.

Gerald

 

[kjhambrick]

@G_T --

Yes, I've decided on this approach as well.

The fins themselves are extremely stiff as they are with the existing layers.

I have decided to coat them with a thin layer of thinned epoxy and then do proper filets after I complete a single wrap of 0.75 oz glass on the BT-55 AF this weelend.
.
The AF::Fin bonds will be plenty strong with proper filets and I don't want to spoil the Diamond Airfoils with the tip-to-tip layers.

Thanks for the input !

-- kjh

 

[SolarYellow]

If you are using sheet balsa you are not getting the benefits of true composite construction, you need to be using end grain balsa so that the fibers run between the two faces of the fin or between the layers of reinforcement. Balsa fibers in tension are much stronger than when laminated with the grain (balsa has poor peel resistance qualities).

Do you have a preferred supplier for end-grain balsa material?

 

[Rschub]

Do you have a preferred supplier for end-grain balsa material?

In theory the comment about end grain balsa is correct, but in practice I don't believe there is anyone selling "sheets" of 1/8' thick end grain, although I haven't looked.
The best you can do is something like this where the balsa is attached to a light fabric to hold it together.
https://www.corelitecomposites.com/balsasud-core
This stuff is designed to work with curved molds as a core material, and there are different geometries depending on the extent of the curvature.
They also make foam and I believe honeycomb versions.

 

[bjphoenix]

In theory the comment about end grain balsa is correct, but in practice I don't believe there is anyone selling "sheets" of 1/8' thick end grain, although I haven't looked.

From a strength of materials standpoint- a sandwich panel has to transfer shear through the core to link the outer skins together. And pure shear in the core translates to diagonal tension. So theoretically you need the grain oriented at a 45 degree angle, depending on the direction of bending. Since fins can bend both directions you would need half the core oriented one way and half the core oriented the other way as if you cut diagonal strips out of a sheet of plywood. This is starting to get a bit extreme for model rocket fins. In reality the shear stress is probably pretty low and so balsa grain oriented the normal way is probably ok. Plus it's pretty hard to glue to end grain of any wood and develop much strength. It's easy enough to calculate the shear stress, assuming you know the load on the fins, thickness of the skins, etc.

 

[SolarYellow]

As I understand it, the main difficulty in gluing end grain is that the glue is likely to soak into the pores of the wood and away from the joint. So you just need to keep the joint fed with glue/resin.

The material is typically sold for boat hulls or other similar uses. I know the C5 Corvette used it in the floor and that the sandwich was significantly better than any other available option. Don't know if they still do that or when they stopped if they did.

I did some quick searching and did find end grain balsa panels supplied down to quite thin thicknesses, like 1/16-inch. The price went up as you got thinner, below around 1/4 inch or so if I remember.

Another factor is that a thicker panel of balsa could be easily shaved with power tools or put in a CNC router and carved into an airfoil. If 3/8 thick happens to be the cheapest per square foot and I only need to make 0.30-inch thick fins, I'm still probably going to just buy that. I have had success with early experiments with power tools that make the necessary material removal not a problem.

I've flagged a plan to mess around with this down the road. Won't be right away, but it's definitely on the list of things I want to play with.

 

[Rschub]

As I understand it, the main difficulty in gluing end grain is that the glue is likely to soak into the pores of the wood and away from the joint. So you just need to keep the joint fed with glue/resin.

The material is typically sold for boat hulls or other similar uses. I know the C5 Corvette used it in the floor and that the sandwich was significantly better than any other available option. Don't know if they still do that or when they stopped if they did.

I did some quick searching and did find end grain balsa panels supplied down to quite thin thicknesses, like 1/16-inch. The price went up as you got thinner, below around 1/4 inch or so if I remember.

Another factor is that a thicker panel of balsa could be easily shaved with power tools or put in a CNC router and carved into an airfoil. If 3/8 thick happens to be the cheapest per square foot and I only need to make 0.30-inch thick fins, I'm still probably going to just buy that. I have had success with early experiments with power tools that make the necessary material removal not a problem.

I've flagged a plan to mess around with this down the road. Won't be right away, but it's definitely on the list of things I want to play with.

The reason end grain is used is not just to purely take up shear loading, it is also very good for impact and point loads, which are non trivial design considerations.

a sandwich panel has to transfer shear through the core to link the outer skins together. And pure shear in the core translates to diagonal tension

This may be true in a truss, but I am not sure it applies to a sandwich panel. The function of the core is to keep the facings apart so stiffness in that direction is paramount. Since we know the direction of the load, it makes sense to use an isotropic material there. This is one reason why nomex honeycomb is used as a high performance core material. Also, there are compressive loads in the core [and a truss] as well as tension.

 

[kjhambrick]

Hmmm ... I beieve I bought some end grain Balsa for my 4-inch Level 3 Vulcanite upscale.

But I bought my parts 25 years ago and they've been in a set of boxes all this time and my memories are pretty foggy now.

I'll try to get the boxes out of the attic and take inventory along with some pictures.

I do recall having Al at Hawk Mountain cut some relatively thin fiberglass core plates and there were lots of balsa bits that I was going to glue to the cores and shape the same diamond airfoils as I made for the BT-55 model above.

I reread what I wrote about the Vulcanite fins above and I said that the leading edge is a Mach 1.88 angle.

The 32.2 degree sweep angle on the Vulcanite fins is actually a Mach 1.18 fin !

-- kjh

 

[Rschub]

For your application using the balsa to get a nice fin shape is of course perfectly fine, in case anyone thinks I am being critical.

 

[kjhambrick]

For your application using the balsa to get a nice fin shape is of course perfectly fine, in case anyone thinks I am being critical.

No worries, @Rschub ... I didn't take what you were saying that way at all.

As I said above, I like fins so I spend a lot of time on them

-- kjh

 

[Rschub]

Fins are where it's at, the rest is just payload.

 

[bjphoenix]

This may be true in a truss, but I am not sure it applies to a sandwich panel.

Any element in bending has to do this- there has to be shear transfer to get the tension and compression forces into the outer surfaces.

 

[Mick Kelly]

Divinycell high density foam is still my preferred core material skinned in carbon. using a carbon bar stock frame to house the foam. In my humble opinion makes the strongest lightest fins possible

 

[2-0turbo]

From a strength of materials standpoint- a sandwich panel has to transfer shear through the core to link the outer skins together. And pure shear in the core translates to diagonal tension. So theoretically you need the grain oriented at a 45 degree angle, depending on the direction of bending. .

This is not correct. Shear in the core (or web, since we're really talking a beam in bending) is along the same axis as the outer plys--actually, inline with the neutral axis. For a symmetrical fin, that is the center of the structure.

Have you done any RC airplanes? Think of the upper and lower spars that join the ribs. To strengthen the D-shaped structure on the front of the wing, you install balsa "shear webs" with the grain oriented up-down vertically. That puts the max shear stress (at the neutral axis) interacting with wood plies at 90-degrees to the maximum shear. Wood is typically most resistant to shear cross-grain. Not to say that your 2-plies at 45-deg approach wouldn't work. You're really getting into discussion about materials that are an-isotropic, materials that have dissimilar strengths in different directions, like wood and composites. Metals are typically isotropic and have the same properties in all directions.

One of the posters above also mentioned the compressive capacity needed, which favors vertically oriented core materials, or core materials that have improved compressive capacity. In this case, end-grain balsa is better than balsa parallel to the grain, but for the loads that we could see in flight, standard sheet balsa may be sufficient. It all depends on how much load and how thick the structure is. A 1/4" thick fin is 8X stiffer than a 1/8" fin, all other dimensions being kept equal.

 

[Rschub]

In the case of @kjhambrick, his sandwich is not the classic 2 slices of bread with peanut butter in between, but rather one slice of bread, with peanut butter coating both sides, because he likes the taste of peanut butter.

 

[lr64]

If regular balsa is good enough, there's not much point in using end-grain unless you can get it in 3 lb density. And yes, I have seen 3 lb. density. I've even seen 3.5 lb density that was usable, though probably not for a fin unless you used it end-grain wise. A regular balsa cored fin with the right amount of composite material on the surface is already going to be lighter, stiffer, and more flutter resistant than plywood or, especially, G10. If you are making it to slam into the ground, then maybe G10 is better. For flutter resistance, a lighter fin doesn't have to be as stiff. Especially if the weight it has is close to 25 percent MAC. (i.e. closer to the leading edge than the trailing edge). This is gross oversimplification, which also describes my understanding of flutter.

 

[bjphoenix]

This is not correct. Shear in the core (or web, since we're really talking a beam in bending) is along the same axis as the outer plys--actually, inline with the neutral axis. For a symmetrical fin, that is the center of the structure.

Neutral axis relates to flexural stresses, not shear stresses. In a solid material the shear stress varies slightly through the thickness of the material because there is varying tension in the material all the way through the thickness. In a composite material with relatively weak core the shear stress would be relatively constant through the height of the core. Internal shear stress is calculated by VQ/I and Q would be different for a solid material vs. for a sandwich panel, for a sandwich panel it is essentially a constant. Shear in a beam such as an I-beam is relatively constant throughout the height of the web. Because the web is thin compared to the width of the flange it is relatively weak, Q does vary over the height of the web but not very much and calculated shear stress is almost constant.

Have you done any RC airplanes? Think of the upper and lower spars that join the ribs. To strengthen the D-shaped structure on the front of the wing, you install balsa "shear webs" with the grain oriented up-down vertically. That puts the max shear stress (at the neutral axis) interacting with wood plies at 90-degrees to the maximum shear. Wood is typically most resistant to shear cross-grain. Not to say that your 2-plies at 45-deg approach wouldn't work. You're really getting into discussion about materials that are an-isotropic, materials that have dissimilar strengths in different directions, like wood and composites. Metals are typically isotropic and have the same properties in all directions.

I don't know that much about airplane wings but I suspect most of the load is in one direction hence the reason you might angle the grain one direction. A rocket fin would have to be designed to carry bending either direction hence the 2 plies at 45 degrees approach.

I'm a structural engineer so I know more than a little bit about stress analysis. The primary shear in the core is induced perpendicular to the skin of the fin because it comes from air pressure on the fin. If you start there and do a Mohr's circle analysis of the stresses that's where you find that shear stress is the same perpendicular and horizontal, and if you use Mohr's circle to determine the main axis of primary tension/compression stress is oriented at 45 degrees.

Somebody mentioned impacts on the surface of the sandwich and that is a pretty good reason to orient the grain perpendicular to the axis of the sandwich.