Reinforcing fin fillet with CF

[Dane Ronnow]

@ThreeJsDad -- I've been thinking about your suggestion to cut the trailing edge of the fin in an ellipse, and, frankly, I know very little about the drag characteristics of a fin. I know the one on the left below has a low pitch rate, and although the stability is lower than I would like—.967 calibers at this point—OpenRocket indicates it will fly fine. And the flutter threshold is 453 mph. That too is fine with me. I could use more safety margin, but I'm not sweating it.

On the other hand, if the fin on the right below would have a significant difference in drag, could you explain it to me?. Thanks very much.

BTW, I'm not looking for performance from this rocket, just reliability.

 

[lakeroadster]

When he mentioned shaping the trailing edge into an ellipse, I think he was talking about the original fin shape. I changed the fin in OR from the minimum fin (center, in your illustration), to the ellipse (right), and I see no change between the two in stability or pitch rate. No change in apogee or VMAX either. [Edit] Aerodynamically, they strike me as essentially the same thing. I can't address the issue of drag. But I can say stability, pitch, apogee and VMAX are the same.[/Edit]

Personally, I like the hard angles of the center shape in your illustration, although the actual shape doesn't have as steep an angle on the outer end as yours does (see the minimum fin in post #58).

I agree, I like the chiseled look on the original overhanging fin better too.

 

[cls]

Rocket fins and airplane wings... Doing different things. Eliptical shapes are so pretty and aerodynamically functional in ways that rockets don't want. GH Stein wrote a chapter about this, take a look.

 

[Dane Ronnow]

Rocket fins and airplane wings... Doing different things. Eliptical shapes are so pretty and aerodynamically functional in ways that rockets don't want. GH Stein wrote a chapter about this, take a look.

I guess it's time to break down and buy the book, if for no other reason than to be able to engage in a conversation about drag with some degree of competence. I've just been lazy, hoping others would explain those things I know nothing about.

I do agree, though, those wings look pretty sharp on the Spitfire.

 

[ThreeJsDad]

Rocket fins and airplane wings... Doing different things. Eliptical shapes are so pretty and aerodynamically functional in ways that rockets don't want. GH Stein wrote a chapter about this, take a look.

I only have a couple of minutes as I am heading off to my Wife's class room in the hopes of creating a bunch of science nerds. Aero nerds actually.

I agree 100% with you. I have a couple great books on these subjects and one is even directed at Model aerodynamics. If we were discussing airfoils I would say there is a huge gap between the two efforts but when discussing purely planform some of the issues are similar. This is more the case for me due to my focus on Sailplanes. Drag is a big problem when there is no way to just add power any time one wants it.

Dane, how you built your fins is perfect and next time hit me up for some fiberglass or carbon scraps. I have a box in the corner of my shop that never seems to get empty.

The tip vortices that are such a problem with my planes is less of an issue with your fins because there is no airfoil. However we are still looking for laminar flow with a clean separation. When the separation isn't clean or happens at a weak spot in the fin we get flutter. The flutter comes from a twisting motion.

I once had flutter so bad on a large sci-fi rocket the .25" ply wood fins fluttered apart. They literally delaminated on the way up. They were hug fins on a J motor so Oh-Well.....It was awesome to watch though.

You can have corners on the fins and not have issues, the keys are to build the fins the way you do with a stressed skin system and to make sure the separation is clean. Even more small angles that approximating a radiused trailing edge would be an improvement and make them more durable during handling and landings.

Take a look at the Mosquito. That rocket has been upscaled quite a bit, there is no way those fins should survive but they do. That very boring rounded tip is there for a reason.

I will think about this some more and if I have an epiphany I will share it.

 

[ep29030]

On my Mega Mosquito, I did two layers of tip to tip from the top of the fin to the bottom of the airframe. Used 0.5oz/yd plain weave fiberglass and slow set epoxy. Very thin setup, but I never had a cracked fin as a result. Another setup I've seen is using epoxy and brown paper (grocery sack) to stiffen glider wings. Even typing paper and yellow glue would add strength to prevent cracked fillets.

 

[Dane Ronnow]

make sure the separation is clean

The more I read, the more I understand the behavior of air moving across a surface, and what happens when the boundary layer separates in a turbulent manner. What I don't understand (yet) is how to 'make sure the separation is clean.'

 

[Dane Ronnow]

Even typing paper and yellow glue would add strength to prevent cracked fillets.

That's where I was headed when I first thought about strips of CF or polystyrene laminated to a portion of the fin, and crossing the fillet. But I think I can get to a flutter-free fin by eliminating the overhang instead.

 

[Dane Ronnow]

@ThreeJsDad - I picked up a copy of Stine's Handbook of Model Rocketry and went straight to the chapters on stability and aerodynamics. I understand the boundary layer, and laminar and turbulent flow. I couldn't find a specific reference to 'clean separation', but I'm guessing it has to do with the turbulence leaving the fin and creating a wake. Still wondering how to make sure the separation is clean. Flat edges?

Anyway, I'm moving ahead with the modification to the fins. I'll post pics as I get into it. Actually, I'm probably going to move the modification posts over to the build thread on this rocket. Seems like it would be more at home there. If I do, I'll post a link to it here.

Meanwhile, thanks to everyone who offered advice on an issue that started out as reinforcing fillets to prevent ground-impact damage, but changed to preventing fin flutter. I appreciate the input.

 

[ThreeJsDad]

@ThreeJsDad - I picked up a copy of Stine's Handbook of Model Rocketry and went straight to the chapters on stability and aerodynamics. I understand the boundary layer, and laminar and turbulent flow. I couldn't find a specific reference to 'clean separation', but I'm guessing it has to do with the turbulence leaving the fin and creating a wake. Still wondering how to make sure the separation is clean. Flat edges?

Anyway, I'm moving ahead with the modification to the fins. I'll post pics as I get into it. Actually, I'm probably going to move the modification posts over to the build thread on this rocket. Seems like it would be more at home there. If I do, I'll post a link to it here.

Meanwhile, thanks to everyone who offered advice on an issue that started out as reinforcing fillets to prevent ground-impact damage, but changed to preventing fin flutter. I appreciate the input.

You got it !!! I use the term "clean separation" to express the idea of reducing vortices created by angle changes to the trailing edge, not beveled trailing edges, and anything that can create strong turbulence off the trailing edge of the fin or wing.
We want the air flow on both sides of the fin to come back together with as little separation from the fin as possible (laminar flow)

Think back end of a semi vs back end of a Porsche. That vortex on the back of a semi is no joke !!

Oh, round the leading edge and sharpen the trailing edge.

 

[Dane Ronnow]

You got it !!! I use the term "clean separation" to express the idea of reducing vortices created by angle changes to the trailing edge, not beveled trailing edges, and anything that can create strong turbulence off the trailing edge of the fin or wing.
We want the air flow on both sides of the fin to come back together with as little separation from the fin as possible (laminar flow)

Think back end of a semi vs back end of a Porsche. That vortex on the back of a semi is no joke !!

Oh, round the leading edge and sharpen the trailing edge.

Thanks! That answers my question. [Edit] Now I just have to figure out how to bevel the trailing edge of fins that are already attached to the rocket. Block sanding, I guess. I hope a 3/16" bevel will do. I changed my mind on this. Trying to do this with mounted fins, for me, anyway, is too hard to do and get the bevels even. And I don't want problems with one side of the fin having more lift than the other. I'll probably round the trailing edges and call it good. [/Edit]

 

[Dane Ronnow]

I've posted the fin modification over on the build thread:

https://www.rocketryforum.com/threads/2-6-inch-29mm-mpr-scratch-build.166792/page-3#post-2288428