1/16" Fiberglass Fin Flutter

[noahapiraino]

I recently got a Wildman Punisher Super Sport a 2.2" diameter kit with a 38mm motor mount, and was hoping to push it to supersonic for the first time on a J425. However the more I thought about it I started to get worried about potential fin flutter.
The fins are 1/16" fiberglass, so fairly thin. The root chord is 6.5", tip chord 1", and fin span 2.1". My fillets go about 0.25" up the fin span, and I read someone else say that the fillet could potentially be subtracted from the fin span since they'll increase fin stiffness/thickness at the root. There is also a shallow slope on the aft edge of the tip chord, and I was wondering if I could consider that part of the tip chord as well for the sake of the math sheets, increasing it's length to anywhere from 1.5-2".





I've plugged everything in to some fin flutter spreadsheets with a shear modulus of 850,000 psi which seemed to be a good estimate based on a couple other threads as well as a research paper I found. The sheets have been giving me a flutter boundary velocity of ~1200ft/s, but my OpenRocket simmed velocity is ~1350ft/s.



Does anyone have any experience flying supersonic on this kit or with fins this thin?

 

[mrwalsh85]

Meh.

I fly my 2.6" Madcow Screech (Thin Wall FG version) on K2050ST's regularly, and have flown it on a Loki L1400W. Always a good time.

Have fun!

 

[WILDMANRS]

I recently got a Wildman Punisher Super Sport a 2.2" diameter kit with a 38mm motor mount, and was hoping to push it to supersonic for the first time on a J425. However the more I thought about it I started to get worried about potential fin flutter.
The fins are 1/16" fiberglass, so fairly thin. The root chord is 6.5", tip chord 1", and fin span 2.1". My fillets go about 0.25" up the fin span, and I read someone else say that the fillet could potentially be subtracted from the fin span since they'll increase fin stiffness/thickness at the root. There is also a shallow slope on the aft edge of the tip chord, and I was wondering if I could consider that part of the tip chord as well for the sake of the math sheets, increasing it's length to anywhere from 1.5-2".

View attachment 630228

View attachment 630231

I've plugged everything in to some fin flutter spreadsheets with a shear modulus of 850,000 psi which seemed to be a good estimate based on a couple other threads as well as a research paper I found. The sheets have been giving me a flutter boundary velocity of ~1200ft/s, but my OpenRocket simmed velocity is ~1350ft/s.

View attachment 630229

Does anyone have any experience flying supersonic on this kit or with fins this thin?

There is no motor that will fit in that rocket that will hurt those fins ( I have tried )

 

[noahapiraino]

There is no motor that will fit in that rocket that will hurt those fins ( I have tried )

Thanks for the reassurance!
Sometimes it’s a little too easy to overthink things like this.
The kit parts were all very nice and I had a great time building it, very excited to fly soon. Thanks for making such great stuff!

 

[Donnager]

A J-425 will be just a blast of air to that rocket.

It should handle lots....

 

[lr64]

The shallow slope probably makes the fin more resistant to flutter, actually, by removing mass from a place where it would aggravate things.

Making larger fillets extending further up the fin would probably make them more flutter resistant without too much work. They could go up the fin in proportion to the height of the fin at that point. Whether it would help to make the fillets broader at the base probably depends on just how stiff the tube is in that area.

One could make lighter, stiffer, stronger fins by using a less dense material and making the fins somewhat thicker. Maybe airfoil shaped. The lighter fins would also reduce the need for weight in the nose, if any.

On the other hand, if it's good enough, why mess with it?

 

[Kane]

I recently got a Wildman Punisher Super Sport a 2.2" diameter kit with a 38mm motor mount, and was hoping to push it to supersonic for the first time on a J425. However the more I thought about it I started to get worried about potential fin flutter.
The fins are 1/16" fiberglass, so fairly thin. The root chord is 6.5", tip chord 1", and fin span 2.1". My fillets go about 0.25" up the fin span, and I read someone else say that the fillet could potentially be subtracted from the fin span since they'll increase fin stiffness/thickness at the root. There is also a shallow slope on the aft edge of the tip chord, and I was wondering if I could consider that part of the tip chord as well for the sake of the math sheets, increasing it's length to anywhere from 1.5-2".

View attachment 630228

View attachment 630231

I've plugged everything in to some fin flutter spreadsheets with a shear modulus of 850,000 psi which seemed to be a good estimate based on a couple other threads as well as a research paper I found. The sheets have been giving me a flutter boundary velocity of ~1200ft/s, but my OpenRocket simmed velocity is ~1350ft/s.

View attachment 630229

Does anyone have any experience flying supersonic on this kit or with fins this thin?

What altitude do you expect? The altitude at maximum speed seems low to me. What safety factor is being used in the boundary equation?