Flatness tolerance for fins?

[SolarYellow]

I bought some Baltic birch plywood, eight sheets of 1/8x8x12. It’s been on steel oven racks or standing up vertically on edge for a few weeks acclimating to the temperature and humidity in my house. No Pringles, but not as flat as I’d like for fin stock when checked against a straightedge.

Does anyone out there have or use any quantitative metric or rule of thumb for non-flatness of plywood fin stock? I’m thinking along the lines of thousandths of an inch per inch. Obviously, any imperfection has potential to cause arcing in flight, spin, or just excess drag. For a rocket where that matters, I’m thinking G10 or cf is the way to go, but for a sport rocket, I’m just looking for good, straight, nominal flights. Hoping to get an idea how much lack of flatness starts to get in the way of that without intentionally building and flying rockets in which I have low confidence.

 

[Alan15578]

I bought some Baltic birch plywood, eight sheets of 1/8x8x12. It’s been on steel oven racks or standing up vertically on edge for a few weeks acclimating to the temperature and humidity in my house. No Pringles, but not as flat as I’d like for fin stock when checked against a straightedge.

Does anyone out there have or use any quantitative metric or rule of thumb for non-flatness of plywood fin stock? I’m thinking along the lines of thousandths of an inch per inch. Obviously, any imperfection has potential to cause arcing in flight, spin, or just excess drag. For a rocket where that matters, I’m thinking G10 or cf is the way to go, but for a sport rocket, I’m just looking for good, straight, nominal flights. Hoping to get an idea how much lack of flatness starts to get in the way of that without intentionally building and flying rockets in which I have low confidence.

Just cut the fins from the sheet in the same orientation so that any warpage causes the rocket to roll rather than arc.

 

[bjphoenix]

Just cut the fins from the sheet in the same orientation so that any warpage causes the rocket to roll rather than arc.

I think this would help.
I bought some plywood some years ago and it was warped a little bit. I tried steaming it, clamping it flat, etc and made some improvement but was still not flat. I cut my fins out then made a clamp out of 2 strips of wood to put on each side of the fin to make it flat, these are "cauls" in woodworking terminology. With them held flat I glued them to the airframe and put on the first part of the fillets. Being restrained by the airframe when I unclamped them they were straight.

 

[rharshberger]

I bought some Baltic birch plywood, eight sheets of 1/8x8x12. It’s been on steel oven racks or standing up vertically on edge for a few weeks acclimating to the temperature and humidity in my house. No Pringles, but not as flat as I’d like for fin stock when checked against a straightedge.

Does anyone out there have or use any quantitative metric or rule of thumb for non-flatness of plywood fin stock? I’m thinking along the lines of thousandths of an inch per inch. Obviously, any imperfection has potential to cause arcing in flight, spin, or just excess drag. For a rocket where that matters, I’m thinking G10 or cf is the way to go, but for a sport rocket, I’m just looking for good, straight, nominal flights. Hoping to get an idea how much lack of flatness starts to get in the way of that without intentionally building and flying rockets in which I have low confidence.

Plywood should be stored either flat or if in a rack on fully supported inclined "vertical". My plywood storage is a vertical storage rack with the back and bottom tilted off vertical enough for the plywood to stay where I put it, and the backing is a full sheet of ply with vertical stringers to stiffen the ply, still doesn't always prevent thinner ply sheets like 3mm and 6mm from developing slight warps across whole panels (not an issue when cut into small pieces like fins), I also buy my ply by the full sheet from cabinet supply shops and its real Baltic Birch from Finland not the "Baltic Birch" from wherever it comes from that the Dolor Stores sell (yes I did spell it right, look up the word....). Real BB is sold in 5'x5' sheets commonly and can be had in 5'x10' sheets. As for flattening ply, just do the best you can, its not really designed to be shaped beyond basic bends that are intentional.

 

[BABAR]

I bought some Baltic birch plywood, eight sheets of 1/8x8x12. It’s been on steel oven racks or standing up vertically on edge for a few weeks acclimating to the temperature and humidity in my house. No Pringles, but not as flat as I’d like for fin stock when checked against a straightedge.

Does anyone out there have or use any quantitative metric or rule of thumb for non-flatness of plywood fin stock? I’m thinking along the lines of thousandths of an inch per inch. Obviously, any imperfection has potential to cause arcing in flight, spin, or just excess drag. For a rocket where that matters, I’m thinking G10 or cf is the way to go, but for a sport rocket, I’m just looking for good, straight, nominal flights. Hoping to get an idea how much lack of flatness starts to get in the way of that without intentionally building and flying rockets in which I have low confidence.

Unless you are flying cameras, for sport (non-competition), I think the 10 foot rule probably works. If it's not obviously curved when observed from 10 feet away, it will be adequate.

If it's a little off, I tend to think of CorkScrewing as a feature rather than a defect.