Strengthening airfoil fins for L1 rocket

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jlabrasca

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I am building a BT80 WAC Corporal for my L1 attempt. The plan is to launch at the end of July.

For no good reason other than access to a CNC and CAD/CAM software, I've decided to airfoil the fins, and taper them root to edge. The plan is to mill each fin in two halves, then clamshell them together. I was going to used 3/32 aircraft ply. Staring at the model, the razor-edged finished fin is a credible facsimile of the fin on the actual WAC Corporal. It also looks too fragile.

Before I change the design, I'd be glad of any advice on how to toughen up the fins.

I am thinking that wrapping the fins would defeat the purpose of milling it down to a sharp edge. I am wondering if there is something I could sandwich between the clam-shell halves that would support the thin edge?

65mmWAC_C_airfoiled_shaded.png65mmWAC_C_airfoiled_wireframe.pngWac-fin-256-airfiol-profiles1.png
 
If you leave 1/10" of flat at the trailing end, they'll be stiffer and the drag difference is negligible.
 
As someone suggested, 1/32" laminate between the sheets would make the sharp edges a bit more robust. G10 fiberglass would be my material of choice. I think carbon fiber would be overkill, as little is gained in stiffness since it is on the centeroid of the bending. Almost all the stiffness is provided by the plywood.

A little flat on the edges, rather than a knife edge, is much more robust. Aerodynamically a slightly flat trailing edge (about 1% of chord IIRC) can be beneficial in some circumstances (supersonic I think).
 
I wouldn't try to put plate between them, I would use a layer or two of 8oz - 10oz cloth and fiberglass resin to epoxy the two halves together. Vacuum bag them of course.
 
I wouldn't try to put plate between them, I would use a layer or two of 8oz - 10oz cloth and fiberglass resin to epoxy the two halves together. Vacuum bag them of course.

Thanks. Unfortunately beyond my current skill level and beyond the limits on my time and budget for this model.

If you leave 1/10" of flat at the trailing end, they'll be stiffer and the drag difference is negligible.

A little flat on the edges, rather than a knife edge, is much more robust. Aerodynamically a slightly flat trailing edge (about 1% of chord IIRC) can be beneficial in some circumstances (supersonic I think).

Yeah, I think that's where I am headed. I like the idea of putting a composite between the fin halves, but these are all new materials for me. This is a complexity I will add to a later design. I am working up a new profile now, to be cut from 3 mm stock. So far I'm leaving 1.5 mm of flat on the trailing edge (0.75 mm per half) and putting a 1.5 mm radius on the leading edge and fin tips. These are constraints imposed by the end mills I have on hand and the way I can hold the work piece to the spoiler board - but I might be able to finesse this a little.

The not-quite-6 mm cord, 6mm thick fin tabs, and the 6 mm slots in the body tube all look really huge to me. Hope to get the pieces on the mill tomorrow.
 
Just mill the fins out of solid G10 or G12. Leave the edge not so sharp; a sharp edge is a weak edge.

Its an interesting idea. My experience machining fiberglass is exactly the same as my experience wrapping stuff in fiberglass.

I am -- frankly -- afraid to try it. I have read on various CNC/Carbide3D/Shapeoko forums that Garolite eats expensive endmills, and that the swarf is known to be a significant health hazard as well as a damage/accelerated wear-risk to the moving parts of the mill and spindle.

I might have used the wet-saw to cut out a silhouette to sandwich between the fin halves, but I don't have great dust control on my mill. My shop is not airtight or under negative pressure, and there 9 year old lungs breathing in the house that sits above it.
 
Here is how I bevel fin edges
Jig.JPG
Router table is a cheap one from Aldi. Jig was made from some 6mm aluminium I had laying around. A plate was made to accept the front thread on the Dremel.

I use a vacuum cleaner when I am milling or turning or whatever with F/G or CF. Seems to keep it under control.
 
Here is how I bevel fin edges ... I use a vacuum cleaner when I am milling or turning or whatever with F/G or CF. Seems to keep it under control.

Probably a better way to go than the silliness I am attempting. It is, after all, not a scale model of the Smithsonian specimen WAC Corporal. It amounts to an upscale Estes K-11 WAC Corporal. The original plan was to do as you would do on your set-up-- bevel the fins to a double diamond profile. There are actually already two sets of fins completed. One beveled on the disc sander (with a jig not too different from yours) and one beveled on the CNC. The first CNC set with bevels only on one side so that the rocket would spin.

This exercise is because the CNC makes it possible to produce a repeatable elliptical airfoil and because modeling the airfoil in Fusion 360 gives me a reason to play with the loft and sculpting tools.

6 contour operations and 18 cutting operations. Depending on stock thickness, I'll have to remove about 50 or 60 cubic centimeters of material. That's a lot of epoxy dust and glass fibers in the air, even with the dust shoe around the spindle and a HEPA filter in the Shop Vac. I will stick with the slightly less noxious plywood for now.
 
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This exercise is because the CNC makes it possible to produce a repeatable elliptical airfoil and because modeling the airfoil in Fusion 360 gives me a reason to play with the loft and sculpting tools.
If I had a CNC mill (I only have manual) then I would probably do the same :)
 
If I had a CNC mill (I only have manual) then I would probably do the same :)

Its a great tool, but its getting so that "everything looks like a nail". I leapt from doing most things with hand-held tools -- mostly unpowered -- to having this robot in my shop. It isn't super fast, but it is precise and repeatable. The temptation is to use it for things that can be accomplished more quickly by other means, just because I know that if the pieces fit together on the drawing they will fit together when they come off the spoiler board.

For instance, I cloned a Semroc ejection baffle for this rocket. A downloaded picture of the unseparated laser-cut sheet -- and some screwing around in Photoshop and Inkscape and Fusion 360 -- was enough to create the tool paths. I could could have fabricated a perfectly serviceable baffle with a coping saw and power dill in less time than it took me to figure out how to do that, but now I've done it.

I didn't get to the fins yesterday, and I won't get to them today. I'll post some pictures when I have one finished.
 
I will just mention that my Velocirpator has the original plywood fins and after many flights (typically L motors) and being carted around the East coast of Australia there is almost no damage to the bevelled fin edges. I left about 1mm flat on the edge.

I guess you could soak the edges in epoxy to add a little strength if you thought it was needed too.
 
What is that small forward facing tab on the main fin tab for?

notchedCRfingeredFIN.png
As dhbarr wisely observed:

If it's worth doing, it's worth overdoing :-D

Another thing I wouldn't have (over)done if I was cutting centering rings with a jeweler's saw.

It was an idea I had after watching this video from John Coker, where he shows a CR with dados cut to receive the through-wall fin tabs.

https://youtu.be/tOvG0n1-NOg

I was trying to work out how to do through-wall fins for a 3X Gyroc upscale, where there is no motor mount to anchor the fin tab. In that design, the fin tabs are notched to receive a piece of coupler at the top and bottom, to anchor the fin (just as dhbarr says)

Here its more along the lines of integrated alignment fixture that also gives torsional mechanical support to the fin (in my imagination, anyway). ^_^
 
I will just mention that my Velocirpator has the original plywood fins and after many flights (typically L motors) and being carted around the East coast of Australia there is almost no damage to the bevelled fin edges. I left about 1mm flat on the edge.

I guess you could soak the edges in epoxy to add a little strength if you thought it was needed too.

I didn't do a good job explaining whats got me worried. I am guessing the L1 certifiers won't disqualify me if a fin edge gets nicked or dented. I am more worried about the root-to-tip taper. I was thinking of something sandwiched between the fin faces to stiffen the fin and to hardened it against a break somewhere along its height (semi-span? -- its hard to keep the naming conventions straight for a three dimensional object). Here is the view from the leading edge, parallel to the long axis of the body tube.

leadingedge.png

I somehow screwed up my revised model in F360. Starting over.
 
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Stiffness is affected most by the outermost layers of material. A wood core with thin fiberglass outer layers is much stiffer than a thin fiberglass core with wood outer layers.



Steve Shannon
 
Stiffness is affected most by the outermost layers of material. A wood core with thin fiberglass outer layers is much stiffer than a thin fiberglass core with wood outer layers.

Thanks Steve -

Yes. I understand. I am not however trying to optimize the stiffness of the fin. I am trying to improve the stiffness -- and more importantly raise the flexural strength -- of a (frankly) whimsical design within the constraints of my expertise, time, and budget.

Thinning the plywood towards the tip will compromise its strength, removing the advantage of crossed grains as it approaches a single ply. Adding a layer of composite to the middle might stop the propagation of a fracture if the fin is bent by flutter, or by impact on landing.

I'm working on a modified design, which preserves two full plies all the way to the tip.

I also spotted a seller on eBay with 0.02" G9 sheets. I am still thinking about that.

I am planning to fly this rocket at the end of July. Between now and then I have just a few days I can devote to this. I want a little bit of a challenge, and I want something that will make the folks on the flight line notice and appreciate my craft and cleverness <grin>, but I don't want to take the time to figure out how to glass fins for this rocket. I'll save that for L2.
 
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Ahh, now I can see why you are worried with the taper of the fin. Could end up damaged, particularly on landing.

Stiffness is affected most by the outermost layers of material. A wood core with thin fiberglass outer layers is much stiffer than a thin fiberglass core with wood outer layers.
Correct. But casting the problem in light of strengthening the tapered fin, the FG (or CF) core would be a significant fraction of the thickness towards the end of the span and add to the overall strength and robustness of the composite assembly. Skinning the outer may still be a better way to go, but the distinction is becoming much less significant.

If you are worried about bonding to the FG or CF, have a read of the sticky on the HPR forum if you haven't already. Good advice there.
 
The WAC corporal used a different airfoil than the one shown in your drawing. I'm not sure what the appropriate name would be, but it had a bevel on all three sides and was not tapered (from root to tip) otherwise.
 
The WAC corporal used a different airfoil than the one shown in your drawing. I'm not sure what the appropriate name would be, but it had a bevel on all three sides and was not tapered (from root to tip) otherwise.

Yeah, my design is a bit of a chimera. The Estes K11 model's fins were beveled, as are the fins on the various ASP models.

The actual sounding rocket's fins were air-foiled and tapered (at least in the later rounds).

You can see the profile in this image of the Smithsonian A & S exhibit

https://airandspace.si.edu/multimedia-gallery/2005-17752hjpg

and here is a picture of the exhibit in the White Sands Range Museum

WAC_2.jpg


and here is Peter Alway's drawing by way of John Coker's website.

https://www.jcrocket.com/images/waccorporal/rotwdrawing.jpg

I am not really doing a scaled WAC Corporal/WAC B fin or an upscaled K11 fin. Because I don't want to remove material from the fin tabs going into the body tube my profile transitions from rectangle with filets at the leading and trailing edges at the fin root to an elliptical airfoil started at about 10% of the span. I also increased the rate of taper at the tip. AND I am cutting the trailing edge tip radius after the airfoil profile is cut so it will be squared-off and a little thicker in the likeliest place for the fin to hit the ground as the rocket comes down. It'll look like the Smithsonian WAC fin, if you don't get too close.
airfoilprofiles.png
I still haven't started to cut. I am having trouble setting up the tool paths in Fusion 360. I am learning a lot about the loft and sweep tools, and setting up offset planes, and how to use the Project>Intersect tool to establish construction points for the profiles... but I am running into problems setting up the 2D contour cut around the fin tab. I can't get it to cut around the outside of the the tab without cutting the tab away from the fin. I suspect I am going to have to set up another fin silhouette sketch in the model to use to use as a tool path for that cut.

WAC-Clam-Shell-Tollpathes-2.png

A friend gave me a couple sheets of 0.5 mm and 1mm G10 scrap, so I'll be ready to try that out as soon as I figure out how to cut the plywood (sigh).
 
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I made a test cut on some "cheap" 3mm by 3-ply craft plywood.

I somehow set the z-zero too low by about 0.5 mm Even so, holding the pieces in my hand it is clear that I need to go with thicker stock.

SAM_0919.png

According to Peter Alway's drawing, the fin on the WAC Corporal was 1.875" thick at its widest point. This scales to about 10 mm for a BT80 body tube. So two halves cut from of 3/16" stock with a .5 mm composite core would be just about to scale. I'll have to stop at the hobby shop and see what sizes of aircraft ply they have on hand.
 
I've got the plywood off the CNC.

SAM_0955.png
SAM_0948.png

I am working on the tool paths for the G10 core.

coreprofile1.png

I am reading up on epoxies. I wonder if it might be a good idea to bore a series of holes around the edge of the G10 core -- to act as vias for the epoxy to adhere the plywood to the plywood?
 
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