Fin Thickness?

I see the Estes kits I've assembled have 1/8" balsa fins. (ok, mic'd at .10) I see some of the HPR guys are using 1/2" plywood. Is there a general rule of thumb for how thick the fin should be? Obviously to be thick enough not to flutter, but not too thick for extra weight and drag. I would also figure the smaller the total area of fin, the thinner it could be made?

How do balsa, basswood and polycarbonate(Lexan) strengths compare? Example; Is 1/8 balsa equal to 3/32 basswood? Does 1/8 balsa = .040 Lexan? .060? Is .080 Lexan overkill for LPR? I just found G-10 sheets way down to 3/64! For what?

I haven't yet tried 'papering' any of my fins. I understand the reasons for doing balsa this way, but is it worth it to paper basswood? It seems like a tight enough grain already...

Sorry, I'm rambling...:caffeine:

Ok, one more (for now)... Are polycarbonate fins and/or BT's considered a 'polished' finish in Sims (OpenRocket)?

Thanks in advance,

~Jeff

Much of my decision-making process on fin thickness (and probably true for many/most others) is just experience with what works and what does. For very small rockets (13mm motors or lighter-weight rockets on 18mm), 1/16" (0.0625") balsa is usually fine as long as the fins are of a fairly normal size. Moving up into C motors or low-power rockets with larger fins or more highly swept fins and 3/32" (0.09375") balsa starts becoming a safer bet for added strength with minimal addition of drag. In small rockets with sturdy fin shapes (clipped delta/elliptical/trapezodial), 3/32" balsa might do just fine even with D and E motors, but for heavier rockets 1/8" balsa would, again, give a bit more insurance.

Balsa also comes in different densities, which correlates well with its strength. Soft, contest-grade balsa is extremely light, but also relatively weak, and can be easily dented with a fingernail, or sometimes even just squeezing it hard between fingers. 3/32" contest grade balsa might be equivalent in strength to 1/16" medium-density wood.

Adding paper will help to add stiffness and strength, and is probably the equivalent of another 1/32"-1/16" of wood.

For plywood fins, usually about half the thickness of an equivalent balsa fin will do, i.e. 1/8" balsa --> 1/16" plywood, 1/16" balsa --> 1/32" plywood, and so on. On small competition models you can go even farther. I flew a payload altitude model on a C motor that used minimum-size rectangular fins made from 1/64" plywood that worked just fine. Thinner woods become much more apt to fluttering, though, so when using extremely thin materials it's best to avoid swept fins entirely (specifically trailing edge sweep; swept leading edges are not really a problem).

For basswood, a thickness somewhere between balsa and plywood would be good. 1/8" balsa --> 3/32" basswood sounds about right. Papering should add some strength to basswood as well, its effect is just more pronounced with balsa fins because the wood is so soft initially.

Fiberglass is a bit stronger than plywood, and carbon fiber is a bit stronger and stiffer than fiberglass. I think something like 1/8" balsa --> 3/32" basswood --> 1/16" plywood --> ~0.050" fiberglass --> ~0.040" carbon fiber. I'm not sure where Lexan would fall, but I'd be tempted to treat it like basswood.

Someone who has actually run tests on the tensile strengths and flexibilities of all these materials can chime in and correct my assumptions and guesstimates.

Do you think .080 lexan is overkill for LPR? Would .040 or .060 be better? I found my local Ace Hardware carries all 3 sizes in ~8x11" sheets

I don't have enough experience yet of what works and what doesn't I'd rather not find out the hard way :duck:

Mckailas Dad said:

Ok, one more (for now)... Are polycarbonate fins and/or BT's considered a 'polished' finish in Sims (OpenRocket)?

Click to expand...

Thanks again

McKailas Dad said:

Do you think .080 lexan is overkill for LPR? Would .040 or .060 be better? I found my local Ace Hardware carries all 3 sizes in ~8x11" sheets

I don't have enough experience yet of what works and what doesn't I'd rather not find out the hard way :duck:



Thanks again

Click to expand...

Ditto what Mirospeed posted. couldn't have been more concise if I'd tried.

YES .080" Lexan is overkill for LPRs up to a pound. Just about all my MPR birds up to 3.3lbs with lexan fins are .060". I've used .080" occasionally but only on really big, WIDE Span fins.
working with Polycarbonate one really has to pay attention to fin spans when picking a proper thickness. I've used as thin as .010" on some small 13mm models but the span has to be pretty short. If you have a source for .040 and .060 I'd get some of each but save the .080" for really big stuff..it really gets heavy.

Lots of Estes Kits with bodies under BT-80 use 3/32" balsa, many smaller 18mm motor models 1/16"(.063"). Balsa is some amazing stuff, very stiff and strong for it's weight. Rarely do I use 1/8" or thicker balsa unless I'm trying to match a fin thickness on upscales.

I try to use materials that will withstand the landing I expect the rocket to have. Therefore, a rocket that often uses streamer recovery and a rapid descent will have more robust fins than a rocket that will always use a big parachute and a slow descent.

McKailas Dad said:

Ok, one more (for now)... Are polycarbonate fins and/or BT's considered a 'polished' finish in Sims (OpenRocket)?

Click to expand...

As for this, polished is probably as close as anything. I wouldn't worry about it too much; the finish is probably used to some very small degree in calculating the drag coefficient, but really, the difference in frontal area between having fins that are 0.080" thick and 0.085" thick would probably affect that calculation to a greater degree, and all things considered, the simulation probably won't be that precise in the first place. Polished or gloss should both be close if the fins are smooth and shiny.