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

[ParagonRocketry]

Hi all!

I was wondering if I could get some help considering shear modulus G for my Level 2 HPR certification. I'm planning on using the same rocket that I used to get my Level 1 certification (3.1" LOC Sandhawk), and currently with my OpenRocket sim and possible motor configuration I'm looking at running at about Mach 0.9. Because of this, I'm a bit worried about fin flutter during my flight. I've found the equation regarding calculating fin flutter velocity online, but I'm running into some problems with the value of G.

After consulting Matweb, the G value for plywood is 20,000-30,000 psi (0.138-0.207 GPa), and by my calculations it needs to be over 160,000 psi (1.103 GPa). I know that going tip-to-tip fiberglassing would work, but I don't have enough time to do that for my best-case timeframe (and I would absolutely hate re-stripping the bottom third of my rocket). If I stay subsonic with my motor, and my fins can handle it, I think it would work without needing to go tip-to-tip.

So, my question for you all is: what's an accurate shear modulus value for aircraft-grade plywood?

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

Thanks again so much!

 

[kjhambrick]

@ParagonRocketry --

I've not read this article: In-plane mechanical properties of birch plywood but it might tell you what you want to know.

Just curious, how thick is the plywood and what type of airfoil did you build into your fins ?

How much does your rocket weigh and what motor are your flying for your L2 cert ?

Maybe the TLAR(*) analytical method will be sufficient ?

-- kjh

(*) - TLAR - That Looks About Right -- thanks to @Rschub: TRA: Minnie-Magg drag mods for Level 2

That Looks About Right, an elegant weapon for a more civilized age.”

 

[tsmith1315]

So, my question for you all is: what's an accurate shear modulus value for aircraft-grade plywood?

All LOC page says is 1/4” Baltic birch plywood for that kit. With no specification or grade, can a value be that accurate? Maybe a call to LOC could point you to some data.

Maybe the TLAR(*) analytical method will be sufficient ?

A very useful method, that is.

Using BTDT* analysis, many of us can verify that a 3” rocket with 1/8” LOC plywood fins of unremarkable size/shape can easily handle a moderate thrusting J, such as a J275 or J350.

Perhaps someone has done BTDT analysis on this particular kit.


*Been There Done That

 

[user 30299]

Hi all!

I was wondering if I could get some help considering shear modulus G for my Level 2 HPR certification. I'm planning on using the same rocket that I used to get my Level 1 certification (3.1" LOC Sandhawk), and currently with my OpenRocket sim and possible motor configuration I'm looking at running at about Mach 0.9. Because of this, I'm a bit worried about fin flutter during my flight. I've found the equation regarding calculating fin flutter velocity online, but I'm running into some problems with the value of G.

After consulting Matweb, the G value for plywood is 20,000-30,000 psi (0.138-0.207 GPa), and by my calculations it needs to be over 160,000 psi (1.103 GPa). I know that going tip-to-tip fiberglassing would work, but I don't have enough time to do that for my best-case timeframe (and I would absolutely hate re-stripping the bottom third of my rocket). If I stay subsonic with my motor, and my fins can handle it, I think it would work without needing to go tip-to-tip.

So, my question for you all is: what's an accurate shear modulus value for aircraft-grade plywood?

The shear value you're looking for is known as "Modulus of Rigidity". The standard value is 89,900 psi.
This is for Baltic birch, 5-ply, 6mm thickness (1/4").

That shear value is shown for 4mm through 21mm thicknesses, 3-ply through 15-ply. But it all has that
same 5-ply progression - 6mm thickness.

There is 11 and 12-ply Baltic birch (6mm thickness) but I have not found any official shear values for it.
It does have a higher standard in its manufacturing, and additional adhesive layers usually improve the
mechanical properties of plywood products.

 

[Blast it Tom!]

G... there's some good thoughts with regards to the TLAR and BTDT methods. However, if you really want to make a calculation, I'd look at the "standard" value given above, and then turn it around and ask, "what value of G gets me in trouble?" If it's 87,000 psi, you may want to rethink your design. If it's 20,000, you're probably good to go. That sort of thing.

I'm surprised the calc doesn't also need E, the modulus of elasticity, or Young's modulus. Or do you already have that? I'd take the same approach with that value as well.

Wood by its nature is all over the place with these values. We use selection and cross-laying, and binders to try to make something uniform enough that we can count on a narrow enough range of values to be able to make the sorts of determinations that you want to make (and I do as well, I like calcs...).

 

[user 30299]

@ParagonRocketry --

I've not read this article: In-plane mechanical properties of birch plywood but it might tell you what you want to know.

For future reference the article you mention does not cover Modulus of Rigidity, aka Panel Shear.
It only looks at standard shear across the thickness. The authors do take the time to point out that
they opted not to do Rigidity testing; Four-Point Bending Test. It's laborious testing - been there,
done that.

 

[kjhambrick]

Thanks @QFactor.

Shucks, I thought I might have found something useful there

This is an interesting subject to me -- something I once studied long ago in Materials Science but never really had an opportunity to apply.

Also see a very recent, related post: @Adrian A's topic under TRF Rocketry Electronics & Software > Counter-intuitive AeroFinSim Lite result

Thanks again !

-- kjh

 

[kjhambrick]

All LOC page says is 1/4” Baltic birch plywood for that kit. With no specification or grade, can a value be that accurate? Maybe a call to LOC could point you to some data.



A very useful method, that is.

Using BTDT* analysis, many of us can verify that a 3” rocket with 1/8” LOC plywood fins of unremarkable size/shape can easily handle a moderate thrusting J, such as a J275 or J350.

Perhaps someone has done BTDT analysis on this particular kit.


*Been There Done That

Thanks Tim, BTDT is another Keeper

-- kjh

So Tim, you're not a shrubber but are you a pythonite ?

 

[tsmith1315]

So Tim, you're not a shrubber but are you a pythonite ?

Quite.


In spirit, anyway…

 

[user 30299]

Thanks @QFactor.

Shucks, I thought I might have found something useful there

This is an interesting subject to me -- something I once studied long ago in Materials Science but never really had an opportunity to apply.

Also see a very recent, related post: @Adrian A's topic under TRF Rocketry Electronics & Software > Counter-intuitive AeroFinSim Lite result

Thanks again !

-- kjh

What you referenced is a good article, and gives a good overview of testing and strength properties for Birch plywood.
It only lacks the one design value that's critical to fin flutter.

All is good.

 

[Rschub]

I feel summoned.

This is the reference you need.

Good luck in the rabbit hole, but beware the rabbit, he has sharp pointy.....look at the bones!

I will say that the stiffness of the supporting structure is probably just as large a factor in the flutter speed as the fin flutter properties taken alone.

BTDT is an excellent method and follows the prime directive if used correctly, "A wise man learns from the mistakes of others."

 

[T-Rex]

If you shred, you don't get your cert and you lose your Sandhawk. If you take the time (and effort) to do it right, in theory, you get your cert and keep your Sandhawk, your choice young Jedi.
Also, a 3" rocket to ~4000ft, I hope you are using a chute release/dual deploy, and a tracker.

ETA: removed a good bit of curmudgeon-ness.

 

[Blast it Tom!]

I feel summoned.

This is the reference you need.

Good luck in the rabbit hole, but beware the rabbit, he has sharp pointy.....look at the bones!

I will say that the stiffness of the supporting structure is probably just as large a factor in the flutter speed as the fin flutter properties taken alone.

BTDT is an excellent method and follows the prime directive if used correctly, "A wise man learns from the mistakes of others."

Nice "rabbit" hole paper! I note that it is also before NASA, so they would have had this for building plywood rockets; apparently this was deemed a "no-go."

Unfortunately, I cannot D.L as I'm not on Scribd - all these blasted subscriptions are like death from 1000 small cuts. DoD should have it though.
I've done a lot with elasticity, but not with composites (which is basically what plywood is). It'd be interesting to go through a good formulation of the approach.

If you shred, you don't get your cert and you lose your Sandhawk. If you take the time (and effort) to do it right, in theory, you get your cert and keep your Sandhawk, your choice young Jedi.

ETA: removed a good bit of curmudgeon-ness.

I was quoting the curmudgeonly version and you fixed it! I thought that was some pretty good advice - In my 30+ years of engineering, I've found that there's often some dang good reasons for experienced-based rules-of-thumb.

 

[T-Rex]

I was quoting the curmudgeonly version and you fixed it! I thought that was some pretty good advice - In my 30+ years of engineering, I've found that there's often some dang good reasons for experienced-based rules-of-thumb.

I thought about it after I hit enter. Some folks tune out us curmudgeons, and some get offended. Either way it doesn't help, so I fixed it.

 

[SolarYellow]

I feel summoned.

This is the reference you need.

Good luck in the rabbit hole, but beware the rabbit, he has sharp pointy.....look at the bones!

I will say that the stiffness of the supporting structure is probably just as large a factor in the flutter speed as the fin flutter properties taken alone.

BTDT is an excellent method and follows the prime directive if used correctly, "A wise man learns from the mistakes of others."

There are versions of that report available from many, many locations on the web. There seem to be two main ones, a color scan that has all the yellowing of old paper, and a B&W that is pretty rough. The best seems to be the one from Google books:

https://books.google.com/books/about/ANC_Bulletin_18_Design_of_Wood_Aircraft.html?id=81mrHAAACAAJ

 

[Blast it Tom!]

There are versions of that report available from many, many locations on the web. There seem to be two main ones, a color scan that has all the yellowing of old paper, and a B&W that is pretty rough. The best seems to be the one from Google books:

https://books.google.com/books/about/ANC_Bulletin_18_Design_of_Wood_Aircraft.html?id=81mrHAAACAAJ

I figured. Thanks! I'm just too busy right this moment to go after it.

I thought about it after I hit enter. Some folks tune out us curmudgeons, and some get offended. Either way it doesn't help, so I fixed it.

Tune us out at your peril! Like I said, when I hear of these BTDT rules of thumb, I tend to say, "There was probably an experimental investigation or a calculation to support that at one point - however approximate". Of course, the known roughness of the transition zone, vortex shedding, and all those crazy fluid-flow issues should be enough to give one pause. I, for one, have stored your little rule in the "someday I might need that" section of the rocketry lobe of my brain.

 

[Rschub]

I'm not on Scribd

Sorry, I just put the first link I found, I have time, but not all day.

 

[Blast it Tom!]

Sorry, I just put the first link I found, I have time, but not all day.

Oh, didn't mean to imply anything... I found a copy on everyspec.com. Thanks indeed for posting it!

 

[SolarYellow]

The everyspec copy is an extra-crappy version of the crappiest of the three I found. You'll like the one on the Google link a lot better.

 

[Blast it Tom!]

The everyspec copy is an extra-crappy version of the crappiest of the three I found. You'll like the one on the Google link a lot better.

I agree. It's very nice that they have it hyperlinked, etc. I just like having a local copy. It bugs me that this stuff is being swept up and put behind paywalls when it's supposed to be free...

 

[SolarYellow]

If you mouse over the "READ EBOOK" button, there is a drop-down menu that includes the option to download a PDF.

 

[Blast it Tom!]

If you mouse over the "READ EBOOK" button, there is a drop-down menu that includes the option to download a PDF.

YOU DA MAN! Thanks!

 

[ParagonRocketry]

@ParagonRocketry --

I've not read this article: In-plane mechanical properties of birch plywood but it might tell you what you want to know.

Just curious, how thick is the plywood and what type of airfoil did you build into your fins ?

How much does your rocket weigh and what motor are your flying for your L2 cert ?

Maybe the TLAR(*) analytical method will be sufficient ?

-- kjh

(*) - TLAR - That Looks About Right -- thanks to @Rschub: TRA: Minnie-Magg drag mods for Level 2

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.

 

[ParagonRocketry]

All LOC page says is 1/4” Baltic birch plywood for that kit. With no specification or grade, can a value be that accurate? Maybe a call to LOC could point you to some data.



A very useful method, that is.

Using BTDT* analysis, many of us can verify that a 3” rocket with 1/8” LOC plywood fins of unremarkable size/shape can easily handle a moderate thrusting J, such as a J275 or J350.

Perhaps someone has done BTDT analysis on this particular kit.


*Been There Done That

I think that 1/4" is going to be a lot better measurement for my fin thickness. I just checked, and for some reason my calculations had the thickness at 0.118in when it should have been that quarter-inch value. Maybe my OpenRocket sim just assumed they were 1/8" instead of 1/4".

With the 1/4" fin thickness, my calculations work out really well for supporting Mach 0.9. Thank you!

 

[ParagonRocketry]

The shear value you're looking for is known as "Modulus of Rigidity". The standard value is 89,900 psi.
This is for Baltic birch, 5-ply, 6mm thickness (1/4").

That shear value is shown for 4mm through 21mm thicknesses, 3-ply through 15-ply. But it all has that
same 5-ply progression - 6mm thickness.

There is 11 and 12-ply Baltic birch (6mm thickness) but I have not found any official shear values for it.
It does have a higher standard in its manufacturing, and additional adhesive layers usually improve the
mechanical properties of plywood products.

Fantastic, I really appreciate that value. With that and my updated fin thickness, I think I should be in the clear for my flutter speed. Thanks so much!

 

[ParagonRocketry]

G... there's some good thoughts with regards to the TLAR and BTDT methods. However, if you really want to make a calculation, I'd look at the "standard" value given above, and then turn it around and ask, "what value of G gets me in trouble?" If it's 87,000 psi, you may want to rethink your design. If it's 20,000, you're probably good to go. That sort of thing.

I'm surprised the calc doesn't also need E, the modulus of elasticity, or Young's modulus. Or do you already have that? I'd take the same approach with that value as well.

Wood by its nature is all over the place with these values. We use selection and cross-laying, and binders to try to make something uniform enough that we can count on a narrow enough range of values to be able to make the sorts of determinations that you want to make (and I do as well, I like calcs...).

Very true on all counts. Here's the article that I'm working out of to calculate flutter speeds: https://apogeerockets.com/education/downloads/Newsletter291.pdf

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

Either way, you were also right about wood being inconsistent; the author of the article actually points that out on Page 4. I'm going to try to stay 20% below my max allowed speed because of that.

 

[ParagonRocketry]

If you shred, you don't get your cert and you lose your Sandhawk. If you take the time (and effort) to do it right, in theory, you get your cert and keep your Sandhawk, your choice young Jedi.
Also, a 3" rocket to ~4000ft, I hope you are using a chute release/dual deploy, and a tracker.

ETA: removed a good bit of curmudgeon-ness.

Exactly true; wise words. That's why I was prepared to use tip-to-tip, it would have just set me back a month to the next launch, which would have been problematic. Shredding would hurt my soul.

I'll be using a GPS, but no chute release or dual deploy. That was going to be for my next rocket- so it looks like I'll have little chance of losing it, but it's going to be a lot of fun searching for a while. That, and the fact that it's twice as high (so possibly twice as far) is daunting too.

Either way, fun memories to look back on when putting dual deploy in the next one!

 

[thzero]

Build a specific rocket for the l2!

 

[David_Stack]

I'll be using a GPS, but no chute release or dual deploy. That was going to be for my next rocket- so it looks like I'll have little chance of losing it, but it's going to be a lot of fun searching for a while. That, and the fact that it's twice as high (so possibly twice as far) is daunting too.

Just remember that if you leave the approved range area, that will be viewed as a failed cert flight. Per Tripoli (and likely NAR has similar wording): "Rocket drifting outside the specified launch range" is one of the grounds for a non-cert flight.

 

[cls]

'm planning on using the same rocket that I used to get my Level 1 certification (3.1" LOC Sandhawk)

I encourage you to build a separate rocket for L2.