Results 1 to 25 of 25
  1. #1
    Join Date
    31st October 2016
    Posts
    27

    Strengthening airfoil fins for L1 rocket

    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?

    Click image for larger version. 

Name:	65mmWAC_C_airfoiled_shaded.png 
Views:	55 
Size:	23.8 KB 
ID:	322313Click image for larger version. 

Name:	65mmWAC_C_airfoiled_wireframe.png 
Views:	56 
Size:	26.7 KB 
ID:	322314Click image for larger version. 

Name:	Wac-fin-256-airfiol-profiles1.png 
Views:	53 
Size:	11.1 KB 
ID:	322315


  2. #2
    Join Date
    30th January 2016
    Location
    US > OK > NE
    Posts
    2,492
    1/32" carbon fiber or fiberglass plate should fit the bill.


  3. #3
    Join Date
    15th October 2016
    Location
    Huntsville AL
    Posts
    973
    If you leave 1/10" of flat at the trailing end, they'll be stiffer and the drag difference is negligible.

  4. #4
    Join Date
    5th August 2015
    Posts
    1,180

  5. #5
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    665
    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).
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  6. #6
    Join Date
    19th January 2009
    Location
    Stafford VA
    Posts
    6,798
    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.
    Handeman

    TRA #09903 L3 :D 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

  7. #7
    Join Date
    31st October 2016
    Posts
    27
    Quote Originally Posted by Handeman View Post
    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.

    Quote Originally Posted by Nytrunner View Post
    If you leave 1/10" of flat at the trailing end, they'll be stiffer and the drag difference is negligible.
    Quote Originally Posted by OverTheTop View Post
    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.

  8. #8
    Join Date
    23rd July 2011
    Location
    Butte, MT
    Posts
    1,480
    Just mill the fins out of solid G10 or G12. Leave the edge not so sharp; a sharp edge is a weak edge.
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  9. #9
    Join Date
    31st October 2016
    Posts
    27
    Quote Originally Posted by Steve Shannon View Post
    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.

  10. #10
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    665
    Here is how I bevel fin edges
    Click image for larger version. 

Name:	Jig.JPG 
Views:	40 
Size:	98.2 KB 
ID:	322409
    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.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  11. #11
    Join Date
    31st October 2016
    Posts
    27
    Quote Originally Posted by OverTheTop View Post
    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.
    Last edited by jlabrasca; 19th June 2017 at 02:21 PM. Reason: to reduce the self-congratulation

  12. #12
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    665
    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
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  13. #13
    Join Date
    31st October 2016
    Posts
    27
    Quote Originally Posted by OverTheTop View Post
    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.

  14. #14
    Join Date
    30th January 2016
    Location
    US > OK > NE
    Posts
    2,492
    I'm not sure anybody got into rocketry to be particularly -practical- :-D

  15. #15
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    665
    I'm not sure anybody got into rocketry to be particularly -practical- :-D
    I know I am guilty of doing something just because I can. I am usually pragmatic, but sometimes I just have to go overkill...
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  16. #16
    Join Date
    30th January 2016
    Location
    US > OK > NE
    Posts
    2,492
    Quote Originally Posted by OverTheTop View Post
    I know I am guilty of doing something just because I can. I am usually pragmatic, but sometimes I just have to go overkill...
    If it's worth doing, it's worth overdoing :-D

  17. #17
    Join Date
    11th September 2013
    Posts
    198
    What is that small forward facing tab on the main fin tab for?
    Gregg
    NAR #97216

  18. #18
    Join Date
    30th January 2016
    Location
    US > OK > NE
    Posts
    2,492
    Quote Originally Posted by Swissyhawk View Post
    What is that small forward facing tab on the main fin tab for?
    I'm not the OP, but I've seen this to grab into either a CR ( nub in the middle ) or a coupler ( nub at the MMT ). This looks to me like the latter.

  19. #19
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    665
    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.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  20. #20
    Join Date
    31st October 2016
    Posts
    27
    Quote Originally Posted by Swissyhawk View Post
    What is that small forward facing tab on the main fin tab for?
    Click image for larger version. 

Name:	notchedCRfingeredFIN.png 
Views:	27 
Size:	18.9 KB 
ID:	322558
    As dhbarr wisely observed:

    Quote Originally Posted by dhbarr View Post
    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). ^_^

  21. #21
    Join Date
    31st October 2016
    Posts
    27
    Quote Originally Posted by OverTheTop View Post
    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.

    Click image for larger version. 

Name:	leadingedge.png 
Views:	34 
Size:	7.3 KB 
ID:	322581

    I somehow screwed up my revised model in F360. Starting over.
    Last edited by jlabrasca; 21st June 2017 at 04:10 PM. Reason: typos typos typos

  22. #22
    Join Date
    23rd July 2011
    Location
    Butte, MT
    Posts
    1,480
    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
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  23. #23
    Join Date
    31st October 2016
    Posts
    27
    Quote Originally Posted by Steve Shannon View Post
    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.
    Last edited by jlabrasca; 21st June 2017 at 11:00 PM. Reason: to fix the Tamaranian syntax

  24. #24
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    665
    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.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  25. #25
    Join Date
    31st October 2016
    Posts
    27
    Quote Originally Posted by OverTheTop View Post
    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.
    Looking now. Thnx!


Similar Threads

  1. Large balsa fins -help with strengthening
    By sed6 in forum Low Power Rocketry (LPR)
    Replies: 15
    Last Post: 17th June 2016, 02:14 PM
  2. Strengthening Plywood Fins
    By Pull_and_Twist in forum Techniques
    Replies: 16
    Last Post: 12th July 2015, 06:57 PM
  3. Strengthening fins
    By Pippen in forum Techniques
    Replies: 9
    Last Post: 15th July 2009, 04:47 AM
  4. Strengthening attached fins
    By Motörhead in forum Techniques
    Replies: 5
    Last Post: 27th October 2006, 07:21 PM
  5. Strengthening fins on a Quantum airframe?
    By Westy in forum High Power Rocketry (HPR)
    Replies: 9
    Last Post: 19th March 2005, 06:33 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •