Forward swept fins on a L3 rocket

I've got the beginnings of a design in mind that calls for forward swept fins... I'm figuring something in a 75mm M powered design. I'm thinking that I'd use some 5mm birch plywood for the ins, and tip-to-tip FG to make it work... Or am I begging for fin flutter and a shred?

Thanks!
Jim

Depends. What motor are you looking at and what weight are you thinking?

Tonimus said:

Depends. What motor are you looking at and what weight are you thinking?

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Not really sure... I'm open to suggestions. Not too big and heavy though, and maybe a baby M.

I'm thinking just to get the cert, but mostly spend my time in the lower engine ranges.

At 35 pounds and 4 inches in diameter, according to ThrustCurve, most Ms will be well in the safe zone as far as speed is concerned. If that is in line with what you are looking at (or anything larger) then you should be fine as long as your fins aren't too extreme. I think a couple layers of glass on top of aviation ply would handle most situations.

What kind of fin profile are you thinking?

Tonimus said:

At 35 pounds and 4 inches in diameter, according to ThrustCurve, most Ms will be well in the safe zone as far as speed is concerned. If that is in line with what you are looking at (or anything larger) then you should be fine as long as your fins aren't too extreme. I think a couple layers of glass on top of aviation ply would handle most situations.

What kind of fin profile are you thinking?

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I'm not sure what you mean by profile. I was thinking something reminiscent of the Estes Javelin (2005), with a slightly forward sweep to the fins. Possibly 6" OD on the airframe though.

I mean a profile view of the fin. If you're not talking a ton of forward sweep, I don't see an issue. Perhaps radius any inside corners instead of making them sharp, just to avoid stress concentration.

Thanks for the input!

Pointy Side Up!

Any time. Now if you want to talk about overbuilding, THAT is my specialty.

You could also use some carbon fiber in the fin construction for strength.
There's no good reason that the fins cannot simply be made strong enough to handle the airflow.

Tominus sounds like he has a firm knowledge of these matters when it comes to the aerodynamic stresses induced on the fins, so I'de want more of his input if it were me.

I watched an L3 cert attempt at Southern Thunder 2015. It was CF body tube with approximately 3/16" G10 fins that were forward swept. At about 350' the fins shredded, the coupler buckled, the payload and nose cone came down. The sustainer was somehow able to restabilize and continue to thrust. Needless to say, it failed to pass.

The fins failed due to design issues. The coupler failed due to insufficient insertable length. I wasn't even L1 yet and I knew it was a bad design. Scratch builds for cert flights are not something I would personally recommend. But that is up to the flyer. And greatly depends on their full understanding of rocketry dynamics. Obviously this guy needed a lot more knowledge before he was ready to scratch build an L3 rocket.

Um, don't you have to get your Level 2 first? :confused2:

Bat-mite said:

Um, don't you have to get your Level 2 first? :confused2:

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It's called planning ahead...

I'm pretty sure the probability of a shred is associated more with size and materials than it is to the general shape, whether swept forward or backward.

K'Tesh said:

It's called planning ahead...

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Just make sure that you engage the TAPs you plan to use for the L3 on this and they are comfortable with the plan and sign off on everything.

Also, if you post a rocksim/openrocket file for the rocket up here we'll be better able to help you analyze the fin design an whether or not it's sufficient.

K'Tesh said:

It's called planning ahead...

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Oh, I forgot that some people are capable of that.

NateLowrie said:

Just make sure that you engage the TAPs you plan to use for the L3 on this and they are comfortable with the plan and sign off on everything.

Also, if you post a rocksim/openrocket file for the rocket up here we'll be better able to help you analyze the fin design an whether or not it's sufficient.

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Thanks Nate... I'm still a ways off on going for it. It'll be awhile before I can really put some serious effort into it.

Oh, and Thanks to all for the input!

K'Tesh said:

It's called planning ahead...

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From the sounds of it, being stuck in China, planning is all he is capable of doing...

however I would think you would have nearly unlimited access to g10

Mendal said:

From the sounds of it, being stuck in China, planning is all he is capable of doing...

however I would think you would have nearly unlimited access to g10

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Well, I can do some designing work too...



As to g10 (which I expect is gunpowder)... Nope... I need money for that, and that's something I don't have either.

G10 is fiberglass.

rstaff3 said:

I'm pretty sure the probability of a shred is associated more with size and materials than it is to the general shape, whether swept forward or backward.

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The X-29 used a specially constructed wing to control the problems with a forward swept wing.

rstaff3 said:

I'm pretty sure the probability of a shred is associated more with size and materials than it is to the general shape, whether swept forward or backward.

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I think both matter a good deal.

Shape and size does matter. Long swept fins with some span to them with small fin roots are significantly more prone to flutter and shred than something like a clipped delta of a smallish span with a generous root cord.

Almost any fin shape can be used if the structure and composition are well thought out and suited to the shape and size of the fin.

Somewhat bigger than you plan it, but the Rebel Space Raptor 6.0 has flown on a N2500

[video=youtube;PULuDUBhG6Y]https://www.youtube.com/watch?v=PULuDUBhG6Y[/video]


The kit and its construction materials are described on their homepage, but it also advises to reinforce the kit, so it's hard to estimate how strong this particular rocket was.

Reinhard