Slight fin warp

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AlexBruccoleri

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I have been building a few kits this winter with plywood fins and noticed a slight warp in the wood. About half the thickness of the wood. I am not too worried since these are for low altitude rockets. Does anyone have any experience with this? Or suggestions/concerns from experience?
 
I didn't time it, but not very long.
I used a steam kettle.
Perhaps the water vapor also expanded the fibers on the concave side.
When I saw that it had straightened out, I stopped and pressed the fins under some heavy books and allowed them to dry overnight.
Will glue them on before it has a chance to warp again. ;)
 
I think steam rising from a pot of water is mild compared to steam coming out from a whistling tea kettle. Anyway, worked for me. Good luck.
 
I have not heard of that. What was the exact product you diluted?

Regarding a kettle. The steam flux is indeed higher than a pot, but I am not sure how much. Did you move the fin around or just hit one spot? The total "dose" from the pot is a lot, albeit the local concentration is lower. I may order a kettle to give it a try. Do you recall when you did it, if it was for a quick moment, or long enough to walk away and come back? Guestimate of the time...

Also one rocket is done with slightly warped fins. I am curious what others have found regarding flight characteristics. If it is a slight rotation, I am not worried.
 
I have not heard of that. What was the exact product you diluted?

Regarding a kettle. The steam flux is indeed higher than a pot, but I am not sure how much. Did you move the fin around or just hit one spot? The total "dose" from the pot is a lot, albeit the local concentration is lower. I may order a kettle to give it a try. Do you recall when you did it, if it was for a quick moment, or long enough to walk away and come back? Guestimate of the time...

Also one rocket is done with slightly warped fins. I am curious what others have found regarding flight characteristics. If it is a slight rotation, I am not worried.
The helicopter blades were bent with a 50/50 solution of bottled ammonia from the drug store and water. So I would imagine your fins could be straightened with the same solution and pressing under heavy books. But I haven't tried that for straightening, just bending balsa.
As far as the steam technique, I put the bent portion in the flow, moving it back and forth for a couple of minutes. When it visibly straightened out I removed it from the plume and pressed it under heavy books overnight. YMMV.
 
I tried steaming/clamping and it didn't help. Any industry that uses steaming to soften wood for shaping will steam the wood for a good amount of time, not just a few seconds. This was probably my mistake. I also thought about using a few layers of material to build a reverse curve into the clamping so hopefully it would come out straight after unclamping. This could be dangerous and you could end up warping the other direction. At best it might require a little bit of trial and error.
As for the warps that I had, wetting and clamping helped a little bit. What I ultimately did was clamp them and glue to the airframe while clamped.
 
Any industry that uses steaming to soften wood for shaping will steam the wood for a good amount of time, not just a few seconds.
For beams, boards, and planks yes.
We are talking about 1/8" thin plywood.
Before
1674404962857.png

After
1674405020942.png
The results speak for themselves.
Like I said, YMMV.
 
One way to do this (assuming you have not already installed the fins) is to liberally paint each side of the fin with laminating epoxy, sandwich it between mylar (or wax paper) and then sandwich all that between two plates (I use 1/2" acrylic plates for smaller fins and 1/4" thick aluminum plates for big fins) and weight it down with something heavy (I like to use about 30 pounds of diving shot weight bags). Once cured, take it out and sand it flat.

But, frankly, these days, if I get a messed up plywood fin, I just cut a new one out of known good stock. And then laminate it anyway. Or glass it.
 
I have been building a few kits this winter with plywood fins and noticed a slight warp in the wood. About half the thickness of the wood. I am not too worried since these are for low altitude rockets. Does anyone have any experience with this? Or suggestions/concerns from experience?
I had one FINISHED rocket (3" LOC BBX) warp its fins last weekend DURING the trip to the site. 6 hour drive and then an overnight stay in freezing conditions (-4°C). I started to unpack and examine the rockets for prep when I noticed my BBX had 2 warped fins. 1 moderate, 1 severe (delamination occurred). The fins were absolutely fine when they were loaded into the car. I wrapped loosely with moving blankets. There was NO weight on the fins. It was simply the cold air that warped them. Obviously, I didn't fly and I just put back in the vehicle since I had no method of repairing on site.

Back at home, and back in room temperature conditions - the warpage has started to ease a bit. I've got the severe one clamped up now to help continue the de-warping process. Later I will wick some thin CA into the laminations and re-clamp. I'm hoping that will suffice and make them ready for the next flight.

I've also had a 4" Phoenix incur some slight warping of the main fins (3 of the 4) during the priming stage. No delaminations. I am going to try to clamp them as well to eliminate the warp or at least minimize it.
 
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