Surface mounted ply fins on blue tube

Hi all,

I'm planning out a scratch build for my level 1 cert which will also serve as practice for some of the design features on a future level 2 plan I have in mind.

I'm interested to know what experience folk have with surface mounting ply fins (say 3/16") on a 54mm Blue Tube body and what kind of abuse they can take. My current designs see speeds of 400-700 ft/s and max acceleration around 20G's.

Also - does it make much difference between Titebond and epoxy with these materials?

Thanks

Zebedee said:

Hi all,

I'm planning out a scratch build for my level 1 cert which will also serve as practice for some of the design features on a future level 2 plan I have in mind.

I'm interested to know what experience folk have with surface mounting ply fins (say 3/16") on a 54mm Blue Tube body and what kind of abuse they can take. My current designs see speeds of 400-700 ft/s and max acceleration around 20G's.

Also - does it make much difference between Titebond and epoxy with these materials?

Thanks

Click to expand...

If you are expecting 20G's, I would do tip to tip fiberglassing the fins, or you risk getting them ripped off, as far as epoxy, people have used high temp epoxy, I can't think of the epoxy name but I'm sure others will chime in and give you some more advice on you project.

Your flight is far from stressful, but I would heavily reinforce the joints anyway.

I have twice had surface-mounted fins come cleanly off of Blue Tube, both on landing. Both were big, fat Aeropoxy fillets (one had lots of CF too), and the fins were not big at all, making the failure surprising.

I would wrap the tube once with fiberglass before surface mounting. You can then fiberglass the joint, or full tip-to-tip if you feel like.

Titebond will be entirely inadequate.

It's possible that I am incompetent, but I have never been able to get a wood glue joint between any two types of wood or paper product to be very strong-everyone always says wood glue bonds are stronger than the wood around them, but I have never once managed that, no matter how I did the surface prep. Cheap epoxies are even worse, though. Good structural epoxies like Aeropoxy (~$45 for a 2-quart set) are just hands-down better, in my experience.

Plywood fins are relatively stiff, so I wouldn't think ~mach.8 or so would be a problem. Fiberglassing the blue tube first will help (CarVac's broken fins looked like the epoxy didn't soak into the blue tube, causing the paper to peel off the top; an FG wrap will distribute the load more). It might not be necessary for flight, but it will help landing; all of our broken fins came from perfect recovery landings on hard lakebed surfaces; landing in a field I'm sure they would have been fine.

Thanks for the feedback guys.

My level 2 will most likely be all FG with tip to tip as well but I was hoping to keep things a little simpler (and cheaper) for the level 1 stuff. E.g. Single deploy on a small H to get the cert, then add a payload tube and an altimeter to get comfortable with DD before moving up to FG and Level 2 cert. Maybe the compromise for this rocket is going with TTW fins instead to make sure they don't pop off. I think I can still do that and keep the flexibility of 29mm and 38mm motor options.

Carvac/CCotner - do you think the issue was with BT specifically or just surface mounting cardboard/wood in general with no tip to tip support?

Zebedee said:

Thanks for the feedback guys.

My level 2 will most likely be all FG with tip to tip as well but I was hoping to keep things a little simpler (and cheaper) for the level 1 stuff. E.g. Single deploy on a small H to get the cert, then add a payload tube and an altimeter to get comfortable with DD before moving up to FG and Level 2 cert. Maybe the compromise for this rocket is going with TTW fins instead to make sure they don't pop off. I think I can still do that and keep the flexibility of 29mm and 38mm motor options.

Carvac/CCotner - do you think the issue was with BT specifically or just surface mounting cardboard/wood in general with no tip to tip support?

Click to expand...

Blue Tube in specific. First of all, it wasn't plywood or cardboard: one was CF/foam sandwich and the other was G10/FR4. Secondly, unlike what CCotner said, on one of the failures it popped off the surface of the Blue Tube without even stripping the first layer. I had sanded the surface well beforehand.

Don't be afraid of fiberglassing. It's not like it's an additional risk or anything for L1.

I'm currently building a blue tube 29mm minimum diameter with 1/8" plywood fins. I drilled rows of small holes in tube where the fins attach, spaced 1/4" apart, to hopefully give the epoxy a better grip on the tube. We'll see what happens when I fly it. It sims at 37G and 768 mph on a G80.

Nathan said:

I'm currently building a blue tube 29mm minimum diameter with 1/8" plywood fins. I drilled rows of small holes in tube where the fins attach, spaced 1/4" apart, to hopefully give the epoxy a better grip on the tube. We'll see what happens when I fly it. It sims at 37G and 768 mph on a G80.

Click to expand...

Once again, you'll have no issue in flight but the problem may be on landing. It's probably not an issue on something that small on soft ground, but one of my failures was a 38mm MD. You can see the size decently well despite the distortion in the video:

[video=youtube;nYVKN0ehHxM]https://www.youtube.com/watch?v=nYVKN0ehHxM[/video]

Wood glue (aliphatic resin) is plenty strong, for the right kind of joints, but doesn't work well for fillets.

Wood glue has poor gap-filling properties and should be used in tight-fitting joints and clamped. Epoxy on the other hand has good gap-filling properties and should not be clamped (which will squeeze it out of the joint).

So, it's not so much the strength of the adhesive per se, but the appropriate application.