HPR Fin Thickness?
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rharshberger
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I will be watching this thread,to see the more experienced builder/fliers responses. I know for my L2 rocket, a clone of the Madcow Frenzy XL 3" I used 3mm Baltic Birch ply laminated each side with a layer of 7oz and a layer of 3oz fiberglass and it is really amazing how the fins went from fairly flexible to stiff.
Unfortunately, the answer is "it depends." Some of the factors that need to be considered:
- How fast do you plan to push the rocket?
- What is the fin planform? (Root chord, tip, planform)
- what are the materials used on the other parts of the rocket? (pointless to have super stiff fins attached to Estes BT55 cardboard tubing)
For reinforcing moderately sized Estes rockets, I've used 0.74oz fiberglass over 1/16" balsa. This is actually overkill. Simple paper layer is sufficient. On the flip side, I have a 6" rocket with a 98mm motor mount. I used 1/2" plywood cores with a layer of 7oz FG.
- How fast do you plan to push the rocket?
- What is the fin planform? (Root chord, tip, planform)
- what are the materials used on the other parts of the rocket? (pointless to have super stiff fins attached to Estes BT55 cardboard tubing)
For reinforcing moderately sized Estes rockets, I've used 0.74oz fiberglass over 1/16" balsa. This is actually overkill. Simple paper layer is sufficient. On the flip side, I have a 6" rocket with a 98mm motor mount. I used 1/2" plywood cores with a layer of 7oz FG.
This would be for a 4" rocket with 75mm motor mount. The construction of the rocket is going to be phenolic with fiberglass laminate. The fins will be surface mounted with tip to tip layup, so I don't think they will have to be too thick. The only wood I have right now is 1/8" balsa, so Im trying to figure out how many layers of 5oz cloth Ill need on each side. The rocket is also projected to go a little over mach 1.
If I were doing your project I would go with 3/16 inch birch ply, and put one layer of 6 oz glass on each side. If you only plan to go around mach 1 and a bit over I wouldn't bother with the tip to tip. I have a 2.6" Cherokee-D upscale (the shape look terrible for flight stress) built as I dedcribed and it has gone a touch over Mach 1 with no signs of a problem. Those fins are through the wall though.
Again, this is what I would do. It is probably overkill, but that rocket landed on a concrete runway and didn't break a fin.
In any event, good luck on your project.
Again, this is what I would do. It is probably overkill, but that rocket landed on a concrete runway and didn't break a fin.
In any event, good luck on your project.
SMR
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+1. If the trailing edge of the fin sweeps forward (i.e. on recovery, it will hit the nozzle first) you can get by with weaker fins, but with an aft swept fin, the fin hits first and will need to be stronger. I've never fluttered a fin off in flight, but have broken a few on normal, under fully open chute landings.
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As a guide, you can always look at what's already been produced. Compare what you want to do with what's out there in similar shape, size, placement, material, and thickness.
rharshberger
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You can use AeroFinSim to get an idea of what your basic wood fin will survive. I usually try and get a rough idea from that, say if a 1/4" ply fin will survive the speeds simulated then I can back it down to 3/16" or 1/8" plywood with a fiberglass lamination. Not real scientific but its worked so far.
I'm a little surprised you are asking this. You're flying a 4" rocket with a 75mm MMT which will be capable of L3 flights to over 20K ft and Mach 2. A little more detail on what type of design info you are looking for would be helpful. Even if you are going to limit the 75mm motors to L2 levels, you are still pushing this way beyond balsa.
Laminated balsa based fins may work, but I would recommend that you use a through the wall assembly method through the 4" BT and filleted to the 75mm MMT. You may say it is limited to Mach 1, but I've found that sooner or later, I always put the biggest motor that will fit in the rocket and pushed the limits. A Pro75 6GXL M2245 Imax would be really cool but if it isn't built for that, it could be very costly.
I was not planning on using just balsa, but I wanted people opinion on what a good base for the lamination would be. The largest motor that will fit is a 4G, so I'm not super worried about the M2245 temptation. I had also planned on doing tip to tip lamination (good for practice even if its over building). I also machined a thrust plate to take the force of the motor, so none of it is being transferred to the fins.
dixontj93060
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End grain balsa can be a fabulous core material for laminating: https://www.acpsales.com/Carbon-Fiber-2-Ply-Plain-Weave-Balsa.html.
Big +1
I've been doing some testing with it and a couple layers of carbon fiber over end grain balsa, and the stuff is amazing. I'll get some pictures up in my V2 thread in the next few days. The moral is I cut the center of the fin out leaving only a 1" ring of plywood (about 60% of mass removed IIRC) and filled the center with end-grain balsa (from FibraGlast) and laminated that with 2 layers of 3K 195 GSM CF (45 deg rotation between layers). Today I spanned the fin between two bricks and stood all 220 lbs of me on one foot in the center...not even a creak, much less a snap. I'm considering making another test fin with no plywood at all. The stuff is light too, the end fin I described was 15% lighter than just the plywood I started with.
I would consider going a balsa sandwich with FG core and FG laminate. Basically put 2 layers on each side of the 1/8" balsa, and then glue 2 of these together. This gives you: 2ply FG, 1/8" balsa, 4ply FG, 1/8" balsa, and finally 2ply FG. You can drop the middle layer down to 1-2 ply instead of 4. The outside skin is where the strength is, not the core, but making the parts identical makes it easier to glue together depending on how you actually do the layup. This will give you ~.3" thick fins, and with that much separation between the outer skins, that will give quite a lot of stiffness. You can also run the balsa grain 90 degrees apart in the 2 layers.
On top of this, I would also put a FG "tape" layer over the leading edge so that the laminate isn't directly exposed to the airstream, just as another precaution.
On top of this, I would also put a FG "tape" layer over the leading edge so that the laminate isn't directly exposed to the airstream, just as another precaution.
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Binder Design
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FWIW, I've flown 1/4" 12 ply with no glass on M's to above mach 2.
astronwolf
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That's because you're probably using real aircraft plywood. Good stuff.
Binder Design
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Yes, the highest grade.
rharshberger
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Lowes and Home Depot wont carry a good high grade ply. What you are looking for is Baltic Birch, sheets of which are usually 5'x5' or longer. Aircraft Spruce should have what you are looking for.
terryg
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Are the sheets of 12 ply from Aircraft Spruce non-warped (i.e. can they be used for larger fins without problems) ?
Are they well enough packaged to survive the abuse that UPS shipment will subject them to? When I buy cheaper plywood at Michael's I have to examine 20 sheets to find one that is non-warped enough to be usable.
Are they well enough packaged to survive the abuse that UPS shipment will subject them to? When I buy cheaper plywood at Michael's I have to examine 20 sheets to find one that is non-warped enough to be usable.
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Just a heads up, most of the Midwest brand that I've been getting for RC airplanes lately from local suppliers that turn over stock pretty regularly, has been junk. Voids, warped, poor grade wood.
I've been getting 1/8" 5 ply from some of the rocket guys, and it's spot on! If you're planning on testing the limits of your materials, start with the highest grade you can afford, even if that means Ramen for dinner a few times!
I've been getting 1/8" 5 ply from some of the rocket guys, and it's spot on! If you're planning on testing the limits of your materials, start with the highest grade you can afford, even if that means Ramen for dinner a few times!
If you have a Woodcraft store nearby you will find excellent Baltic Birch ply. Finish ply too, but that stuff is real spendy.
rharshberger
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Not all Baltic Birch plys are created equal. Below is a picture of 3 types of plywood from left to right: 3/8" AC construction grade Plywood (3 ply), 5mm Luan (3ply+2very thin cosmetic veneers), 6mm Baltic Birch (5ply), 12mm Baltic Birch (9ply), and 17mm Baltic Birch (13ply). Not shown because I don't have any is Finnish Birch Aircraft Plywood which is available in 1/4" 12-ply, some people refer to this as Baltic Birch which it is, but its far more expensive than standard BB, just as BB is more expensive per sq. ft and better quality than asian and domestic birch plywoods. I use regular Baltic Birch for most of my rocketry, but when I start building my L3 I will invest in the 1/4" 12ply Finnish Aircraft Ply similar to what Aircraft Spruce sells, and probably still laminate it will FG and inner cutouts filled with end-grain balsa to lighten. I never trust Luan ply for anything except MPR performance rockets, and never use regular construction grade plywood for rockets it has too many defects. If you look closely at the right side of the Luan you can see where it is delaminating on the cosmetic veneers. The hardwood plywoods from Home Depot and Lowes are okay for cabinets and stuff like that (they have many voids and internal delaminations), but Baltic Birch is the minimum IN MY OPINION for rockets.
Would these birch plywoods work? Midwest Products is doing a 10% off and free shipping special it looks like, so I'd much rather order from them than Aircraft Spruce.
rharshberger
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Midwest ply is craft plywood, not really sure. Aircraft Spruce is the real stuff it evan meets certain government standards.
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Notes above in red. YMMV. I've sworn off Midwest until I see a reason to trust it for rockets again.
