Reinforcing fin fillet with CF

[Dane Ronnow]

I keep having problems with the fins flexing on ground impact, and cracking the paint on the lower end of the fillet.


I've been repairing these cracks just by filling them with thin CA. That holds, until the next ground impact. Then it's more CA.

I found carbon fiber veneer online at Protech Composites that's just .25mm thick, and I'm thinking of applying 1-inch wide strips to the bottom end of the fillets to add a little rigidity to that joint.



I'd be using 3M 468MP double-sided film adhesive (5.2 mil thickness) to hold the strips in place, per the recommendation from Protech.

I'm keeping the strips short to keep the weight down. The white areas in the illustration above are the extent of the cracks.

Does this seems like a workable solution? I know this won't make that joint absolutely rigid, but every little bit helps.

Thanks for any input/advice.

 

[cls]

Might help...

Are you sure those cracks are from landing?

One of my PML Tomahawks has big fins like that, very thin FR4 glass, like 3/32". They really flex a lot (resonance) at high speeds. In fact, it makes a lovely billow in the exhaust smoke, especially with black jack reloads. And you can hear it, a brrrrrrt sound...

Lotsa cracks in the paint on the fillets, but the fillets holding just fine. I figure the cracks are battle scars. Great for impressing the ladies... (Hasn't worked but I try)

 

[Dane Ronnow]

Are you sure those cracks are from landing?

One of my PML Tomahawks has big fins like that, very thin FR4 glass, like 3/32". They really flex a lot (resonance) at high speeds. In fact, it makes a lovely billow in the exhaust smoke, especially with black jack reloads. And you can hear it, a brrrrrrt sound...

Lotsa cracks in the paint on the fillets, but the fillets holding just fine. I figure the cracks are battle scars. Great for impressing the ladies... (Hasn't worked but I try)

I can't say I'm absolutely certain they're from landing. But the max velocity of this rocket on a G80 is a tad over 400 mph. I wouldn't think that's fast enough to cause flutter with these fins. They're 1/8 inch balsa, papered with 50 lb Avery label paper, and four coats of paint (not to mention three coats of Pledge floor polish). There's practically no flex to them by hand.

I could be wrong, though. (And they are just in the paint. The fillet is rock solid.)

 

[Dane Ronnow]

@cls -- Also, these are TTW, tabs epoxied to the motor tube and CRs. I just can't see them flexing that much from velocity. But, like I said above, I could be way wrong on this.

On the other hand, I prefer not to do this mod if I don't need to. Fact is, I don't mind dribbling CA into those cracks. I just worry that it might get progressively worse if it is impact that's causing it.

 

[Dane Ronnow]

@cls -- I'm starting to think you're right—this could well be due to fin flutter, not ground impact. I just downloaded and ran the spreadsheet from Aerorocket Fin Sim. According to it, 411 mph is the speed at which this fin will flutter. And that spread sheet doesn't take into account the tail end of these fins that hang below the body tube. It only considers root cord, tip cord, thickness and semi-span.

Here's what the fins looked like on the first three launches (when the cracks occurred), before I cut them down:



My guess is that these would flutter a lot earlier than 411, assuming the spread sheet is accurate.

 

[tim cubbedge]

I had similar problem with my black brant cracked filets on landing. I put a couple of layers of CF on them and several flights later , still no signs of cracks.

 

[lakeroadster]

What type of paint are you using? Some tend to handle flexing better than others.

 

[Dane Ronnow]

What type of paint are you using? Some tend to handle flexing better than others.

Krylon. Gray filler primer, followed by white sandable, then top coats of white followed by gray (two light coats, one medium, both colors). After curing, two coats of Pledge floor gloss initially, then two more later on.

 

[Dane Ronnow]

I had similar problem with my black brant cracked filets on landing. I put a couple of layers of CF on them and several flights later , still no signs of cracks.

Is that cloth? What adhesive did you use to lay it?

 

[tim cubbedge]

Is that cloth? What adhesive did you use to lay it?

Yes, carbon fiber cloth. Don’t remember the weight. I used US Composite laminating epoxy.
First I had to remove the cracked filets, then redo. Then I added two layers of CF cloth. It’s held up very well. I didn’t bother repainting.

 

[Dane Ronnow]

First I had to remove the cracked filets, then redo.

I don't want to go that deep. I was thinking of just cleaning the surface, filling the cracks again with CA, then laying strips of the .25mm thick CF veneer using the 3M double-sided adhesive film (5.2 mil), then painting, then Pledge.

But the more I think about it, the less certain I am that the CF veneer will bend sharp enough to cross the fillet (3/8 inch radius) despite how thin it is. I've ordered it; I just don't have it in hand to see how stiff it is.

 

[cls]

Seems like 400mph ought to be achievable with a black jack motor. Want to try an H97? If you can get a good quality video, slow it down, I bet you could see the flutter. Or, maybe an on board camera. Might be fun.

If the fillets are holding, why bother with cosmetics... Battle scars!

 

[tsmith1315]

If the fillets are holding, why bother with cosmetics... Battle scars!

Exactly!

If you really want to strengthen it, follow @timbucktoo's example.

 

[Dane Ronnow]

Seems like 400mph ought to be achievable with a black jack motor.

It is. 408 mph according to FlightSketch, on a G80.

If the fillets are holding, why bother with cosmetics... Battle scars!

Until the paint lifts in large pieces.

 

[Dane Ronnow]

If you really want to strengthen it, follow @timbucktoo's example.

I want to stiffen the fin, but I don't want to add a lot of weight. I've got this rocket exactly where I want it—CG, stability, overall weight, deployment velocity. Adding an ounce of cloth and resin changes all that.

I haven't flown it yet with the shorter fin and angled trailing edge. I'm going to fill the existing cracks with CA, then launch it and see what happens. If I still get cracks, I'll resort to the CF veneer strips with the 3M adhesive. If the veneer won't bend tight enough to lie flat against the fillet, I'll use polystyrene sheet strips, .5mm thick, with the same adhesive.

As far as cosmetics are concerned, I get the whole battle scar thing. But paint lifting off in large pieces is another matter entirely. That's what I want to avoid.

 

[Dane Ronnow]

Okay, I've been trying to educate myself on fin flutter. I downloaded several spreadsheets and plugged in the data. The results varied, which I expected. What I didn't expect was that by changing the thickness of the fin by just .033 inches—from .125 to .158, which is the actual thickness of the finished (papered and painted) fin—the theoretical flutter velocity in the NACA_TN_4197_V2 spreadsheet increased by 42 percent. In David Harris' spreadsheet it doubled. (Harris' let me plug in the fin area. Not sure if that has anything to do with the discrepancy.)

That tells me that the original fin (pictured above, post #5), which suffered the cracks, should be good to around 720 mph, assuming the NACA spreadsheet is reliable. That's 320 mph faster than this rocket can fly on its biggest motor, the G80.

I realize that the spreadsheets aren't bet-your-life-on-it accurate. But even with, say, a 20 percent margin of error (which I think would be huge), there's no way this rocket can reach that kind of velocity.

Anyway, I'm starting to think the cracks are from ground impact, not fin flutter.

 

[John Kemker]

It's a shame Scott Dixon of Vulcan went out of business. I used to have a jar of his CF reinforced epoxy. GREAT stuff for no-crack fillets and repairs.

 

[David Schwantz]

Your thin CF won't do much. The tape you want to use is not the best choice. As Tim said, best to grind out fillet and then add. I have used both glass and CF reinforced epoxy. Hard as hell to break. Just mix up your glue and add chopped fibers to it. Mix well and apply. they will not break again. As you grind out the fillet, you are removing weight. So you might not add much weight, if any.

 

[thzero]

Bigger parachute?

 

[lakeroadster]

Okay, I've been trying to educate myself on fin flutter. I downloaded several spreadsheets and plugged in the data. The results varied, which I expected. What I didn't expect was that by changing the thickness of the fin by just .033 inches—from .125 to .158, which is the actual thickness of the finished (papered and painted) fin—the theoretical flutter velocity in the NACA_TN_4197_V2 spreadsheet increased by 42 percent. In David Harris' spreadsheet it doubled. (Harris' let me plug in the fin area. Not sure if that has anything to do with the discrepancy.)

That tells me that the original fin (pictured above, post #5), which suffered the cracks, should be good to around 720 mph, assuming the NACA spreadsheet is reliable. That's 320 mph faster than this rocket can fly on its biggest motor, the G80.

I realize that the spreadsheets aren't bet-your-life-on-it accurate. But even with, say, a 20 percent margin of error (which I think would be huge), there's no way this rocket can reach that kind of velocity.

Anyway, I'm starting to think the cracks are from ground impact, not fin flutter.

View attachment 518704

Any chance you could provide a link to ^^^ this spreadsheet? I'm curious what the values are for Shear Modulus in Table-1.

The std. fin flutter calc isn't applicable to fins with a shape such as yours. The overhang below the body tube will drastically increase the fins tendency to flutter. If your fin geometry was like the diagram below, then you could have confidence in the fin flutter calc data.

FWIW: I did a very basic strength test of papered basswood and found that the paper increased the bending strength 8X

 

[Dane Ronnow]

Bigger parachute?

It's oversized as it is—36 inch semi-ellipsoidal, with descent mass at 19.3 oz. Descent speed is 10 fps.

 

[Dane Ronnow]

Any chance you could provide a link to ^^^ this spreadsheet? I'm curious what the values are for Shear Modulus in Table-1.

I found this on John Cipolla's Aerorocket FinSim web site. (I was looking for a FinSim download, only to find that he's no longer providing the software.)

Go here - http://www.aerorocket.com/finsim.html. The link is below the boxed item that begins with "POF 291 FLUTTER VELOCITY ERROR: The author of POF 291 claims..." It's the link in the text that is highlighted in the page snippet below.



In the spreadsheet, choose the fin material from TABLE 1, SHEAR MODULUS (G), then select the value from the blue column (G). In my case, it was balsa, 153,704 psi.

This is a link to the Harris spreadsheet I used:

View attachment Fin Flutter Analysis David Harris.xlsx

Interestingly, the Harris spreadsheet has very different values for balsa. In his spreadsheet, I used the value from the NACA spreadsheet (153,704). But it also has a field for fin area. I thought that might give the calculation a better indication of what my fin actually is.

The std. fin flutter calc isn't applicable to fins with a shape such as yours.

Of course it isn't. If it were, this would be easy, and that's just not going to happen.

 

[Dane Ronnow]

Your thin CF won't do much. The tape you want to use is not the best choice. As Tim said, best to grind out fillet and then add.

Imagining the process of grinding the fillets in my head, I picture a ruined body tube. It's thin-wall BT80. If I were going to completely redo the fillets—which means cutting new fins and epoxying them into the motor mount and CRs—I'd probably just replace the entire fin can and join it with a coupler. But that's just me.

 

[David Schwantz]

But you do not have to remove them all. I have fixed fillets juts like yours. Go slow, get as close to the body tube as you can and then just go from there.

 

[Dane Ronnow]

But you do not have to remove them all.

All of them have hairline cracks.

To reiterate, the fillets aren't cracked. The paint is. My intent is to stiffen that area with the CF and adhesive film. I realize that's not going to add significant strength to the joint. But it will stiffen it a little.

 

[David Schwantz]

I was referring to you do not have to remove all of a fillet to fix it. I knew they all had cracks. Good luck with however you go. And just so you know, none of us will believe you fixed anything unless there are lots of pics

 

[lakeroadster]

I found this on John Cipolla's Aerorocket FinSim web site. (I was looking for a FinSim download, only to find that he's no longer providing the software.)

Go here - http://www.aerorocket.com/finsim.html. The link is below the boxed item that begins with "POF 291 FLUTTER VELOCITY ERROR: The author of POF 291 claims..." It's the link in the text that is highlighted in the page snippet below.

View attachment 518773

In the spreadsheet, choose the fin material from TABLE 1, SHEAR MODULUS (G), then select the value from the blue column (G). In my case, it was balsa, 153,704 psi.

This is a link to the Harris spreadsheet I used:

View attachment Fin Flutter Analysis David Harris.xlsx

Interestingly, the Harris spreadsheet has very different values for balsa. In his spreadsheet, I used the value from the NACA spreadsheet (153,704). But it also has a field for fin area. I thought that might give the calculation a better indication of what my fin actually is.


Of course it isn't. If it were, this would be easy, and that's just not going to happen.

Thank you for taking the time to explain link, much appreciated.

 

[Dane Ronnow]

And just so you know, none of us will believe you fixed anything unless there are lots of pics

Hey, I won't believe I fixed anything unless there are lots of pics!

 

[Dane Ronnow]

The CF arrived in today's mail. It's a 4"x4" piece of 3k twill weave veneer, .25mm thick. Protech included a 4"x4" sheet of 3M 468MP adhesive film, 5 mil (.005") thick.



The description on Protech's web page says the .25mm veneer is flexible enough to wrap around a 1" pipe, which is not as tight as I need (3/8" radius on the fillet), but I'm an optimist, so I ordered it anyway, thinking I might be able to get a tighter bend. Besides, it was only $2.99 (plus $3.38 shipping), so I'm not out that much if it doesn't work.

I could get an initial bend of the 4x4 piece down to about 5/8".



Then I cut a piece 2.00" by 1.25"—the size I need for laminating the lower part of the fin—and bent it parallel to the 2.00" length. It snapped at about 1/2" radius.



So the CF veneer won't work.

Now I'm going to try polystyrene sheet. Hobby Lobby has it in .5mm and 1mm thickness. I'd like to get the 1mm sheet to bend that tight. If it won't, I'm pretty sure the .5mm sheet will.

To reiterate what I'm trying to do here:

I'm not trying to increase the strength of the fillet. I'm trying to stiffen the fin in the area where the paint cracking occurs to reduce the amount of flex. Whether the flexing occurs through flutter at max velocity (and I'm not convinced it does), or during ground impact (more likely, I think) doesn't matter.

I think increasing the thickness of the fin in the critical area is the easiest way to accomplish that.

More later.

 

[waltr]

CF or FG cloth would be better to reinforce the fillets.
But someone already suggested this.