Easy fin repair?

[billdz]

The bottom of a fin on my 3" fiberglass Tomahawk somehow broke off a couple of months ago. I attempted to repair by epoxying on a new piece, but the new piece did not survive the next flight, which had a rather hard landing because the chute did not fully deploy.

Is there any easy repair? I suppose the proper thing to do is to remove the fin entirely, gouge out the remnants from the fin slot on the airframe, then install a completely new fin. But I do not have the tools or skill to do that.

Could the rocket be safely flown with the damaged fin as-is?

 

[waltr]

My guess, note this is a guess.

If the stability was more than enough with stock fins (no breaks) then the missing fin piece probably will not be an issue with stability. Look at rockets that have different fin sizes, etc. They fly fine.

I can not tell from the photo:
What size rocket (motor size) is this?
Weight?
Motor typically used and approx. top velocity?

 

[tim cubbedge]

Do what Brian & I told you. Remove paint from fin & butt up a piece of glass same thickness and then add a layer or 2 of glass over the fin on both sides. Once cured, you will need to cut/sand fin to shape of others.

 

[afadeev]

The bottom of a fin on my 3" fiberglass Tomahawk somehow broke off a couple of months ago.

Wow - I've NEVER seen a fiberglass fin break like that.
It almost looks like the original piece had a straight cut / defect in it, which became a guide for a clear and perpendicular break line.
Or was it a 2-piece fin to begin with?

I attempted to repair by epoxying on a new piece, but the new piece did not survive the next flight, which had a rather hard landing because the chute did not fully deploy.
Is there any easy repair? I suppose the proper thing to do is to remove the fin entirely, gouge out the remnants from the fin slot on the airframe, then install a completely new fin. But I do not have the tools or skill to do that.

Did you anchor the new fin piece down to the motor mount (TTW)? Re-laid fin fillets to provide extra bonding surface near the airframe?

The only sure-fire ways of fixing it for good are:
1). Remove the broken fin and replace - you've already ruled that out.
2). Do what timbucktoo said: tip-2-tip glassing over the two fin pieces (survivor and the replacement). Maybe even glass all 4 fins, while you are at it. Are you comfortable with glassing?
3). Use Dremel to cut the other 3 fins to match the shape of the broken one. I would run OR first, to make sure you will not excessively impact the stability.

It looks like the fins are pretty big, and extend pretty far aft. At least 2 of them will take an impact upon landing, always.

Could the rocket be safely flown with the damaged fin as-is?

Two ways to find out:
1). Model it in OR, with variable fin shapes. I have no idea what results will pop out, but expect some coning.
2). Fly and find out. This can be less than safe, so I would go with option #1.
3). Fix it !

"Easy fin repair?"

No, it will be a bit of a project.

HTH,
a

 

[billdz]

I can not tell from the photo:
What size rocket (motor size) is this?
Weight?
Motor typically used and approx. top velocity?

It takes a 38mm motor, weighs 2400 grams, and is 6' tall. I've been flying it on I and J motors with a max velocity around 600mph.

 

[billdz]

Do what Brian & I told you. Remove paint from fin & butt up a piece of glass same thickness and then add a layer or 2 of glass over the fin on both sides. Once cured, you will need to cut/sand fin to shape of others.

Wow - I've NEVER seen a fiberglass fin break like that.
It almost looks like the original piece had a straight cut / defect in it, which became a guide for a clear and perpendicular break line.
Or was it a 2-piece fin to begin with?
Did you anchor the new fin piece down to the motor mount (TTW)? Re-laid fin fillets to provide extra bonding surface near the airframe?
The only sure-fire ways of fixing it for good are:
1). Remove the broken fin and replace - you've already ruled that out.
2). Do what timbucktoo said: tip-2-tip glassing over the two fin pieces (survivor and the replacement). Maybe even glass all 4 fins, while you are at it. Are you comfortable with glassing?
3). Use Dremel to cut the other 3 fins to match the shape of the broken one. I would run OR first, to make sure you will not excessively impact the stability.
It looks like the fins are pretty big, and extend pretty far aft. At least 2 of them will take an impact upon landing, always.

Thanks for the replies. I've never glassed anything, not sure I'd be comfortable with it. You're right, the break is unusual, but it was never a 2-piece fin. The odd thing is that the break occurred on a perfect flight, soft landing on soft ground. I could not find the broken off piece, perhaps the break did not occur at impact.

Think I'll just retire this rocket at least temporarily.

 

[tim cubbedge]

Think I'll just retire this rocket at least temporarily.

Ask Brian if he will fix it.

 

[dhbarr]

Amend your sim to see what it would be like if you dremelled the others to match?

 

[BABAR]

Sand it smooth at the break, paint it a contrasting color, and fly it. When people ask, just tell them you designed it that way. I think the other three fins will more than make up for the missing feathers.

 

[Steve Shannon]

I think you have more problems than one broken fin. That looks like your fins had a bad case of flutter, which might be why your fin broke also.
See how there’s cracking at the aft end of your other fin fillets and what might be a crack or bond failure parallel to the root of the broken fin? I’ve got a rocket that I repeatedly stressed that has very similar signs of stress.

 

[jqavins]

I see what appears to be a crack along the body tube, a good size chip out of the next fin closer to the camera at the trailing end of the root edge, and numerous other small chips and scratches.

Think I'll just retire this rocket at least temporarily.

It looks to me like this old bird has earned its rest. Maybe put an H motor in it, with enough thrust for safe rail exit speed, as a farewell flight. (Check the stability in OR/RS first, of course, but I bet it's fine.)