Level 2 HPR certification: Question about plywood shear modulus for fin flutter speed calculations

[kjhambrick]

Gotcha gotcha, thank you for the article on that. I'll look into it for a more accurate G value. My plywood thickness is 0.25 in, and the fins are trapezoidal.

According to my sim, my rocket weighs about 54 oz with motor, and I'm planning on launching with a J357 motor.

Adding TLAR to my analytical toolbox! Never knew such a handy tool had a name.

All --

What a great thread !

Thanks !!


@ParagonRocketry --

Thank you for the link to the Apogee "Peak of Flight" Article: How To Calculate Fin Flutter Speed By Zachary Howard.

I am going to try to follow the calculations with all the info provided here and maybe write a Q&D script wrapper around the calcs ...

I am also following @Adrian A's related TRF thread: Counter-intuitive AeroFinSim Lite result

Now that I know about Zachary's Article, I want to try the calcs for myself.

I was able to make a simplified OpenRocket SandHawk.ork File by starting with a RockSim file for an Estes Sandhawk, scaling and substituting LOC Parts for Estes Parts.

At least I didn't have to hand enter the fins ( as long as they scaled properly for the LOC Kit from the freehand fins for the Estes model )

The Dry mass of the scaled and edited Sandhawk is 0.87 Kg / 31 oz.

Sees to be close enough for this purpose since all I need is the fin geometry, as long as they scaled correctly ...

Q: Is a J357 Motor a Cessaroni Pro-38 5 Grain 658-J357 ?

If so, using my q&d Sandhawk.ork, OR came up with launch mass = 1.521 Kg / 51 oz ( a little less than your 54 oz launch mass but I didn't bother with laundry nor links, etc ).

And OR says the Apogee Altitude will be 4552 ft with a max Velocity of 1127 ft/sec ( Mach 0.98 at 95F Launch Site Temperature ).

But this should be close enough to try Zachary's Fin Flutter Speed Calcs.

Thanks again for this info !


Full Disclosure:

I just learned TLAR this week from @Rschub in his VERY interesting Level 2 Cert thread: Minnie-Magg drag mods for Level 2?

And now, @tsmith1315 shared BTDT here in this thread -- these are two very important techniques for HPR or for life in general

Good luck with your Level 2 Cert, @ParagonRocketry !

Please let us know how it goes !

-- kjh

@ParagonRocketry -- I just noticed that you edited your original post:

EDIT: Thank you guys so much for the information, I really appreciate all the good advice here and in my messages. I think the primary problem I had was that my fin thickness was off; for some reason, it was in my OpenRocket sim as 0.118 inches, while LOC and people who had experience with those fins said that it was 0.25 inches. I checked and they were right- which lead to a much more reasonable necessary G of 16,422 psi. I also thought I should post the article where I'm getting my calculations from, so here that is: https://apogeerockets.com/education/downloads/Newsletter291.pdf

Interesting, when I first converted the Estes .rkt to a LOC .ork my fins were also 0.118 inches thick which I changed to 0.25 after reading this thread ...

 

[Blast it Tom!]

I encourage you to build a separate rocket for L2.

Is he not allowed to use aomething like a Jolly Logic chute release to help him stay on the field? Or is there another reason? I'm a lowly L0...

 

[tsmith1315]

Is he not allowed to use aomething like a Jilly Lofic chute release to help him stay on the field? Or is there another reason? I'm a lowly L0...

No problem using JLCR or DD on L1 or L2, but it does add complexity that must work as planned.

I’d fly 3” to 4k’ with motor ejection on a good field with good visibility and low wind. Again, BTDT, but everyone has their own comfort zone.

 

[cls]

Is he not allowed to use aomething like a Jilly Lofic chute release to help him stay on the field? Or is there another reason? I'm a lowly L0...

of course that's allowed, and JLCR is great on your MD L0 rockets, too.

my point was, L2 is a journey, not a destination, and don't miss the learning opportunities along the way.

 

[Blast it Tom!]

of course that's allowed, and JLCR is great on your MD L0 rockets, too.

my point was, L2 is a journey, not a destination, and don't miss the learning opportunities along the way.

Thanks! That's a very good reason to build another rocket!

I think that right now, I only have one rocket that a JLCR would fit into (an Estes 24mm V-2).

 

[bjphoenix]

It doesn't need Young's modulus, and I don't know exactly why. Maybe fin flutter only is applied in shear?

I have not studied fin flutter so I don't know exactly what deflected shapes they are predicting. I always thought fin flutter had to do with bending of the fins in which case young's modulus would be important. If flutter is a torsional affect then a shear modulus would be important to the calculation and not young's modulus. (Torsion could be viewed as a specialized application of shear.) For normal elastic materials shear modulus is derived from young's modulus (and poisson's ratio) but this may not hold for wood since its properties are not the same in all directions. For instance the G for steel is about equal to E for steel divided by 2.6.
Doing calculations like this on plywood raises another question- what geometric properties are you using for the plywood itself? For instance in bending calculations you would use moment of inertia as a stiffness property and section modulus as a strength property, in torsion you would use the property J. All of these properties are different for plywood vs. a solid material since plywood has layers oriented in the strong direction and layers oriented in the weak direction. It is easy enough to find these properties published for normal plywoods that are used in construction but might be difficult to find these properties for aircraft plywood.

 

[kjhambrick]

I have not studied fin flutter so I don't know exactly what deflected shapes they are predicting. I always thought fin flutter had to do with bending of the fins in which case young's modulus would be important. If flutter is a torsional affect then a shear modulus would be important to the calculation and not young's modulus. (Torsion could be viewed as a specialized application of shear.) For normal elastic materials shear modulus is derived from young's modulus (and poisson's ratio) but this may not hold for wood since its properties are not the same in all directions. For instance the G for steel is about equal to E for steel divided by 2.6.
Doing calculations like this on plywood raises another question- what geometric properties are you using for the plywood itself? For instance in bending calculations you would use moment of inertia as a stiffness property and section modulus as a strength property, in torsion you would use the property J. All of these properties are different for plywood vs. a solid material since plywood has layers oriented in the strong direction and layers oriented in the weak direction. It is easy enough to find these properties published for normal plywoods that are used in construction but might be difficult to find these properties for aircraft plywood.

@bjphoenix --

Nor have I, but I intended to look into it years ago.

@ParagonRocketry referenced this article: Apogee - Peak of Flight - How to Calculate Fin Flutter Speed by Zachary Howard.

I would like to dive deeper into Fin Flutter some day when I've got more time (*) but the article above is only a few pages and I was thinking about taking a look this weekend.

I've not really studied the article but maybe the A's to your good Qs are in the article ?

-- kjh

(*) I bought a copy of the Dover reprint of Aeroelasticy by Berkhoff EDIT: oops, Bisplinghoff et al, years ago to play with flutter calcs but I never got to it ...

 

[user 30299]

I have not studied fin flutter so I don't know exactly what deflected shapes they are predicting. I always thought fin flutter had to do with bending of the fins in which case young's modulus would be important. If flutter is a torsional affect then a shear modulus would be important to the calculation and not young's modulus. (Torsion could be viewed as a specialized application of shear.) For normal elastic materials shear modulus is derived from young's modulus (and poisson's ratio) but this may not hold for wood since its properties are not the same in all directions. For instance the G for steel is about equal to E for steel divided by 2.6.
Doing calculations like this on plywood raises another question- what geometric properties are you using for the plywood itself? For instance in bending calculations you would use moment of inertia as a stiffness property and section modulus as a strength property, in torsion you would use the property J. All of these properties are different for plywood vs. a solid material since plywood has layers oriented in the strong direction and layers oriented in the weak direction. It is easy enough to find these properties published for normal plywoods that are used in construction but might be difficult to find these properties for aircraft plywood.

Here's your chance to study Fin Flutter. Attached is the original report that developed the Fin Flutter Equation (Eqn. 18 - Appendix).

The report will also show how the "J" (torsional modulus) is addressed.

There are many subsequent NACA and NASA reports on this topic - and many still continue to be generated.

But this is a good foundational report if you're going to weigh-in on this topic. It's an easy read.

 

[Blast it Tom!]

@bjphoenix:
You got me to thinking about the Young's modulus not appearing, and wondering what deflected shapes they were considering. In that light, I wonder if vortex shedding is a primary mechanism, where the vortices alternate off of each side and tend to twist the fins, rather than, say, bending them from the root like a cantilever.

But I suppose rather than wondering I ought to just read the posted references!

 

[user 30299]

@bjphoenix:
You got me to thinking about the Young's modulus not appearing, and wondering what deflected shapes they were considering. In that light, I wonder if vortex shedding is a primary mechanism, where the vortices alternate off of each side and tend to twist the fins, rather than, say, bending them from the root like a cantilever.

But I suppose rather than wondering I ought to just read the posted references!

Test on Monday . . . .

Bring your No. 2 pencil . . . .

Completely fill in the ovals . . . .

 

[tsmith1315]

Here's your chance to study Fin Flutter. Attached is the original report that developed the Fin Flutter Equation (Eqn. 18 - Appendix).

Thanks for that!

 

[Blast it Tom!]

Test on Monday . . . .

Bring your No. 2 pencil . . . .

Completely fill in the ovals . . . .

So yeah...

When I was young I had the cockeyed notion to build a 3 engine cluster "D" motor (Estes 24 mm) version of their old "Streak", except this would recover by parachute. Calculated required fin area by the "cardboard cutout" method. I'd read Estes TR-1 or whatever it was and was sure I could help Werner with the Saturns... Ridiculous fins, too long and too thin. I'm pretty sure cantilever flutter broke them (dramatic failure!), but of course I have only my 50+ yr old memories. I recall my dad saying he wondered if that would happen... thanks for the warning, Pops!

So here I am now, with all the analytical ability, and nowhere near enough time... but a quick scan showed that the report indeed consideres a torsional mode along with the cantilever mode. I say mode because it's a vibration; I'm kind of assuming it involves what we call a "critical speed", or "unbalance resonance" in rotating machinery, except this is an aeroelastic system or some exotic thing like that!

 

[user 30299]

So here I am now, with all the analytical ability, and nowhere near enough time... but a quick scan showed that the report indeed consideres a torsional mode along with the cantilever mode. I say mode because it's a vibration; I'm kind of assuming it involves what we call a "critical speed", or "unbalance resonance" in rotating machinery, except this is an aeroelastic system or some exotic thing like that!

All things rocketry are "exotic". That's why we do this stuff - right?

 

[Blast it Tom!]

All things rocketry are "exotic". That's why we do this stuff - right?

Abso-positivimitly-lutely!

 

[Bill Hanson]

Lots of great info in this discussion. That said, let me ask whether we might be barking up the wrong tree.
Fin flutter helps us understand when the fin might break due to bending or torsion, but most of the time (admittedly small sample size) fins don’t break in the middle. Instead, fins normally break at the attachment joints.
Now, for something like a thru-the-wall fin, my intuition is that flutter might give us a good indication of when the fin would break, but for surface-mount fins, the limiting factor is probably the strength of that joint/fillet.
I don’t know enough to make a call one way or the other, but so far nobody seems to have brought this into the cross-check, so I thought I’d bring it up for discussion.
- Bill H

 

[user 30299]

Lots of great info in this discussion. That said, let me ask whether we might be barking up the wrong tree.
Fin flutter helps us understand when the fin might break due to bending or torsion, but most of the time (admittedly small sample size) fins don’t break in the middle. Instead, fins normally break at the attachment joints.
Now, for something like a thru-the-wall fin, my intuition is that flutter might give us a good indication of when the fin would break, but for surface-mount fins, the limiting factor is probably the strength of that joint/fillet.
I don’t know enough to make a call one way or the other, but so far nobody seems to have brought this into the cross-check, so I thought I’d bring it up for discussion.
- Bill H

Most definitely Bill ! Joint/Connection failure is the one item that plagues all things structural, and
connections tend to be underdesigned or overdesigned, and the least understood element.

I bet you've seen plenty of interesting fin failures at the Spaceport Cup.

 

[Buster]

L2 Madcow 4.0 Patriot 1/4 plywood fins J350. No problem here Commandante!

 

[kjhambrick]

L2 Madcow 4.0 Patriot 1/4 plywood fins J350. No problem here Commandante!

Congratulations on your Level 2 flight, @Buster !

Does your Patriot have the 38mm or 54mm motor mount ?

I imagine the narrow span, TTW Patriot fins will handle any existing 38mm motor without any issues at all.

And depending on your filets the joints will probably do just fine with any 54mm motor too

As for flutter, those 1/4 inch narrow span Patriot fins shouldn't flutter at all on any motor that fits in the hole ...

Congrats again !

-- kjh

EDIT: When and where in TX did you fly your Patriot ?

 

[Buster]

Congratulations on your Level 2 flight, @Buster !

Does your Patriot have the 38mm or 54mm motor mount ?

I imagine the narrow span, TTW Patriot fins will handle any existing 38mm motor without any issues at all.

And depending on your filets the joints will probably do just fine with any 54mm motor too

As for flutter, those 1/4 inch narrow span Patriot fins shouldn't flutter at all on any motor that fits in the hole ...

Congrats again !

-- kjh

EDIT: When and where in TX did you fly your Patriot ?

Thanks 38mm and I was in Memphis at the time. Mid-South Rocket Society.

 

[86mustang408w]

I encourage you to build a separate rocket for L2.

I would have to agree. I was thinking about it using one, but will build all new for the L2 specifically for the cert flight. Some good info though in the first few posts. Thank you guys.