Why tip-to-tip? Why not wrap fiberglass around the leading edge of the fin?

finspin

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We performed a practice tip-to-tip layup and the results were fairly ugly at the leading edge since the fiberglass was quite tough to cut with the dremel. We were hoping for a nice rounded edge to achieve an airfoil, since our rocket is subsonic (M < 0.77). We could work on our technique to grind/sand down an airfoil shape there, but we were thinking, why not just wrap the fiberglass around the leading edge of the fin? We are already airfoiling the wooden core of the fin after all.

The idea is, start with one sheet of fiberglass on the body tube, halfway between two fins, have it cover the face of the fin, wrap around the leading edge, and then end halfway between two fins on the other end. Yet everything I can find online only talks about a tip-to-tip. Just curious if there are any issues with what I'm thinking.
 

rharshberger

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People do tip to tip because they’re reinforcing the connection of the fin to the body tube. That’s usually done with fairly heavy fabric. Heavy fabric doesn’t wrap around sharp edges well. You should try it.
In addition to Steves statement, the best time to trim FG layups is while the epoxy is at the leather stage (still flexible and either
not tacky feeling or slightly tacky) using a sharp razor blade. After the epoxy cures it takes a lot more work to trim it back. I keep a small swatch of the current layup on my work bench as a test piece and once it hits leather stage, I remove the layup from the vacuum bag or other setup (several different methods) and trim.
 

Sandy H.

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In addition to Steves statement, the best time to trim FG layups is while the epoxy is at the leather stage (still flexible and either
not tacky feeling or slightly tacky) using a sharp razor blade. After the epoxy cures it takes a lot more work to trim it back. I keep a small swatch of the current layup on my work bench as a test piece and once it hits leather stage, I remove the layup from the vacuum bag or other setup (several different methods) and trim.

I wholeheartedly agree. I also keep 4 or 5 new razor blades on the bench when doing it, so if it starts trimming poorly, grab a new blade instead of trying to 'work at it.' If you do it all with one blade, fine, but if you wished you had a spare in the middle of the job, its a pain to take off gloves and get more out of the toolbox, then put gloves back on etc. You can always put unused ones back.

Sandy.
 

rharshberger

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I wholeheartedly agree. I also keep 4 or 5 new razor blades on the bench when doing it, so if it starts trimming poorly, grab a new blade instead of trying to 'work at it.' If you do it all with one blade, fine, but if you wished you had a spare in the middle of the job, its a pain to take off gloves and get more out of the toolbox, then put gloves back on etc. You can always put unused ones back.

Sandy.
First rule of composites should be: Before mixing any epoxy make sure you have everything you MIGHT need ready and within reach.
 

Sandy H.

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First rule of composites should be: Before mixing any epoxy make sure you have everything you MIGHT need ready and within reach.

I agree with your rule #1 for sure.

Second rule might be: Make sure the significant other knows you're going to be unavailable for twice as long as you think the task may take.

Third rule might be: Make sure that same individual knows that the cat can't come out in the garage during that time either!

Sandy.
 

DragonRocketry

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I agree with your rule #1 for sure.

Second rule might be: Make sure the significant other knows you're going to be unavailable for twice as long as you think the task may take.

Third rule might be: Make sure that same individual knows that the cat can't come out in the garage during that time either!

Sandy.
I lock the shop door so no one can come in...LOL Never fail someone wants to stop in and shoot the breeze when I am laying up tubes.
 

Handeman

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People do tip to tip because they’re reinforcing the connection of the fin to the body tube. That’s usually done with fairly heavy fabric. Heavy fabric doesn’t wrap around sharp edges well. You should try it.

Steve is right. The reason for tip to tip is to reinforce the BT to fin joint. That is almost always to prevent losing a fin to flutter when approaching or exceeding Mach.

Since the OP's rocket is going to be below 0.77 Mach, I don't understand why they would need tip to tip or fiberglass the leading edge, except to follow all the TRF recommendations to overbuild everything.
 

AlexBruccoleri

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I recommend using a little cloth right at the joint and then putting a fillet over that. Unless the fins themselves are too thin, that should be plenty strong.
 

finspin

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Update, we did it and it worked pretty well. Though it was pretty ugly at the center of the body tube where the sheets connected, so we smoothed it out with sanding and Bondo. There were some cases where the sheets didn't meet eachother all the way, but that could be improved in the future by cutting and managing the shapes better. Figuring out how to cut the shapes out was a little more challenging than the tip-to-tip; at the top of the leading edge, the sheet actually needs to have an approximately 70-90 degree angle in order to get the sheets to wrap along the direction of the circumference of the body tube. I'm very pleased with the airfoil's leading edge using this method. I'm surprised it hasn't seen more use
 

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rharshberger

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What did you gain over tip to tip...nothing, and its more difficult to do, especially if using a vacuum bag to get the best composite properties. The reason more of us don't use that method is its all pain with little or no gain. By using traditional tip to tip we can also vary the thickness of the fin (single layer at tip and two or more layers at base) to help dampen fin flutter. Done properly traditional tip to tip has little sanding and filling.
Update, we did it and it worked pretty well. Though it was pretty ugly at the center of the body tube where the sheets connected, so we smoothed it out with sanding and Bondo. There were some cases where the sheets didn't meet eachother all the way, but that could be improved in the future by cutting and managing the shapes better. Figuring out how to cut the shapes out was a little more challenging than the tip-to-tip; at the top of the leading edge, the sheet actually needs to have an approximately 70-90 degree angle in order to get the sheets to wrap along the direction of the circumference of the body tube. I'm very pleased with the airfoil's leading edge using this method. I'm surprised it hasn't seen more use
 

plugger

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That is almost always to prevent losing a fin to flutter when approaching or exceeding Mach.
I'm not sure I'd agree with the above statement. Most often fin flutter is mitigated by making your fin thick enough to not flutter at max V. Sure, you can calculate the required fin thickness, shave a bit off, and the 'build that back up' with t2t. But that's a lot of work.
I think t2t is best for ensuring your fins stay on during landing when you can easily pop a fin when you hit the ground.
 

Sandy H.

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A clean leading edge. The goal with this was a clean airfoil.

I've only done fiberglass reinforcement a few times and each was different, as I was both learning and had different goals.

I appreciate seeing what you did, as it appears to me you spent a good bit of time figuring out where the glass goes and even did hand stitching to make things work out. I like the creativity and the effort to accomplish your specific goal, even if some of the other pro's have different opinions. I never thought about stitching fiberglass to hold it in place and will keep that concept in my mind if nothing else.

I believe (not fact, but from what I remember reading) that if you layer fiberglass at the proper time you can still get a chemical bond and not just a mechanical bond. I'd research it more, in case I'm totally mixing memories, but my thought is you could possibly do a standard tip to tip up to about 1/8-1/4" from the leading edge and let it go green, then put another layer over the leading edge and overlap to the tip-to-tip. You'd only have a bit of a bump and maybe the mismatch would be easily blended with Bondo etc. Again, please do your own research on the right methods for joining multiple layers of fiberglass, as I would hate to lead you down a wrong path.

Thanks again for posting your method. While not conventional, I found the idea interesting and it might help to give someone another idea to try as well. Doing the same thing the same way all the time will lead to stagnation. Entropy leads to failure - new ideas add energy to the system, even if it is not conventional.

Sandy.
 

jderimig

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The stress load on a fin is maximum at the root and goes to zero at the edges. The fg reinforcement should follow that pattern. Otherwise you are adding weight, resin and work for no benefit.
 

G_T

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A fin is just a small wing. I'm stealing a picture from one of my wing designs, because it is handy. But the concept and the pattern is the same. Look at the bending moment. Stiffness should follow a similar pattern for best efficiency - and likely for best resistance to flutter.

Just look at the middle of the graph as if it were the body tube, and the left and right edges as if they are the fin tips. See how quickly the bending moment decreases?

So how should the thickness taper?

Stiffness at a chord station is proportional to the cube of the thickness at that station, times the length of the chord. Halfway out you need about 20% of the stiffness. The chord halfway out is a little over half the root chord on a typical fin. Anyway, somewhere between half and full root chord, depending on shape. For example, let's say the tip is a third the chord of the root. So halfway out the chord is 2/3 the root chord. So the thickness needs to provide 20% / 2/3 or 0.3 of the stiffness. The cube root of that gives 2/3 the thickness of the root. That's halfway out.

One could be lazy and just make the tips about 1/3 the thickness of the roots, use a linear thickness taper, and call it a large improvement over the typical constant thickness.

Hopefully one can also see that just changing the layers in T-T only makes a dent in the right direction. It just makes a person put in more layers to help prevent ripping off a suboptimally profiled fin. IMHO, of course!

With the appropriate thickness taper for the profile, then any reinforcement need extend only a little beyond the fillet. Just enough to securely bond the fin and reinforce the fillet. The fiberglass or carbon added on top the fillet then just becomes a structural member.

BTW, different subject, fabric over leading edges. Try Kevlar or layers of light fiberglass, they conform better. The fabric angle for just that part should be at 45 degrees to the leading edge. It will bend better, and all fibers go over the edge.

Prepare material by laying down a sheet of wax paper on your bench, and spraying 3M77 from about two to three feet back, aiming along a line a foot or more above the wax paper. You are making a light fog that will settle on the wax paper. It is a very light mist coat.

Carefully - you get one shot - lay on a section of light fiberglass or light Kevlar. Preferrably on a bias, but most fins are so short one can cut at an angle and still have enough length.

Flip it over, and you can lay out your pieces. You want strips at least half an inch wide. You could go somewhat wider if you want. Cut out with scissors.

How dexterous do you feel?

Fold back a little wax paper at one end.

Spray some 3M77 on the fabric side. Let it fairly well dry.

Put centerline of sticky biased fabric strip along edge of fin. Rip off the wax paper. Ripping it off can work. Pulling slowly never will. Then smooth it down with your fingers, leading edge to trailing edge direction.

The minus of this approach for fins is the 3M77 blocks epoxy uptake.

So for rocket fins, perhaps instead try this:

Lay out a strip of drop cloth. Lay out some fabric on the drop cloth. Wet out the fabric with rollers and epoxy. Dry it down so it doesn't have puddles, and is uniformly coated with no bubbles. Lay another piece of drop cloth on top. Keep a sample of the wetted fabric separate. When the sample is starting to get firm, but is still sticky, roll the drop cloth sandwiched piece and put in freezer to stop the cure.

You've just made some prepreg.

Now do your fin work, and when the fin fabric is still a bit tacky but firming up, pull your prepreg out of the freezer. Cut a strip, pull off plastic from one side, and smooth it down where you want it. Then pull off the other plastic - or just wait until it is more cured or fully cured. The plastic will probably pull off even then.

Hopefully I've given people some ideas.

Gerald
 

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