More sticky - butt or pseudo through wall

[mlrtime99]

So I'll be throwing together a minimum diameter rocket over the next few months and I'm wondering if anyone has any opinion on the below fin attachment.

The fin is a basic trapezoid -


A butt joint (ignoring the fillet) would wet about 645 mm SQ of surface area -


Routing a cutout for the fin (even though the fin would immediately bottom on the motor casing) would expose 3mm (thickness of airframe) of fiberglass to both sides of the fin bringing the wetted area up to 835 mm squared -


Adhesives are not my strong suit but does the latter option create some type of double sheering effect? Is there any reason NOT to do this?

 

[Steve Shannon]

So I'll be throwing together a minimum diameter rocket over the next few months and I'm wondering if anyone has any opinion on the below fin attachment.

The fin is a basic trapezoid -
View attachment 308121

A butt joint (ignoring the fillet) would wet about 645 mm SQ of surface area -
View attachment 308122

Routing a cutout for the fin (even though the fin would immediately bottom on the motor casing) would expose 3mm (thickness of airframe) of fiberglass to both sides of the fin bringing the wetted area up to 835 mm squared -
View attachment 308124

Adhesives are not my strong suit but does the latter option create some type of double sheering effect? Is there any reason NOT to do this?

No reason not to inset your fins. Keep in mind also that a good fillet can add a lot more bond area and create more strength than the actual butt joint.

 

[markkoelsch]

How fast will it go?

 

[JimJarvis50]

It would just be my opinion, not based on any specific testing, but ....

I've made fin cans in the past using a carbon tube and a couple of layers of tip to tip carbon that are pretty strong. I had a couple of 3" fin sections that I've handed to some pretty big guys and challenged them to break the fins. They can't (by hand anyway). I'm reasonably sure that if I made the same fin section on a cardboard tube that it would not be nearly as strong. I haven't actually done this with cardboard, but I'm pretty sure that the strength of the underlying tube is important for making that tube, the fins and the tip to tip into a single, strong structure. I think this is just taking advantage of the strength of the composite tube, relative to the lack of strength of the cardboard. So, if you accept that reasoning, then the next question is, why do you want to cut slots in that composite tube?

Jim

 

[Steve Shannon]

It would just be my opinion, not based on any specific testing, but ....

I've made fin cans in the past using a carbon tube and a couple of layers of tip to tip carbon that are pretty strong. I had a couple of 3" fin sections that I've handed to some pretty big guys and challenged them to break the fins. They can't (by hand anyway). I'm reasonably sure that if I made the same fin section on a cardboard tube that it would not be nearly as strong. I haven't actually done this with cardboard, but I'm pretty sure that the strength of the underlying tube is important for making that tube, the fins and the tip to tip into a single, strong structure. I think this is just taking advantage of the strength of the composite tube, relative to the lack of strength of the cardboard. So, if you accept that reasoning, then the next question is, why do you want to cut slots in that composite tube?

Jim

Good point. It would be interesting to see how much effect the slots have once the assembly is all cured. Certainly they would weaken the tube if simply left as slots, but is there any advantage to having the very short portion of the fin interlocking with the tube.


Steve Shannon

 

[JimJarvis50]

Good point. It would be interesting to see how much effect the slots have once the assembly is all cured. Certainly they would weaken the tube if simply left as slots, but is there any advantage to having the very short portion of the fin interlocking with the tube.


Steve Shannon

I think the trick to keeping the fins attached is to keep the underlying tube from deforming. If you asked me to deform a tube on purpose, I'd cut a slot in it, insert a lever (a fin would work) and then bend. I don't think a thin glue line gives all that much strength compared to what you're giving up.

Jim

 

[ThirstyBarbarian]

I read this thread title as having to do with "sticky-butt". Sounds like someone needs to tend to their personal hygiene!

 

[Steve Shannon]

I think the trick to keeping the fins attached is to keep the underlying tube from deforming. If you asked me to deform a tube on purpose, I'd cut a slot in it, insert a lever (a fin would work) and then bend. I don't think a thin glue line gives all that much strength compared to what you're giving up.

Jim

You may be right, Jim. You certainly have much more and better experience than me, but here's how I think.
I'm going to assume that fillets or tip to tip reinforcement would provide the exact same amount of strength in either case, so I'll disregard them and just discuss the glue joint itself.
I'll also assume both body tube and fin are 1/8 inch thick and I'll pick dimensions to simplify the math.
Assume the fin root is four inches long. The area of the butt joint then is 4/8 or 1/2 sq. inch.
If I inset the fin the glue joint consists of the area along both sides of the fin so the total area is just over 1 sq. inch.
You're right, that placing a fin into a slot allows the fin to exert more force on the tube, but that necessarily means the tube exerts more force on the fin as well.


Steve Shannon

 

[markkoelsch]

Jim, I tend to agree with you about not slotting the tube. It weakens the tube, and then you are sticking a fin into the slot. Let's think about the fin like a lever. Say the fin has a three inch span with about a 1/16" of material in the slot. That is a fairly large lever to be applied a weakened tube.

Surface mount has a different mechanism in that the tube is not weakened by slotting.

This is not condemning through the wall to the motor mount type of construction- minimum diameter is a different type of beast.

I remember many discussions on this topic. I remember several high end composite builders coming down on the surface mount side versus slotting the tube side. Look at some of the stuff Dave Trianno built/sold- surface mount. Look at the stuff Jim has built.

I think surface mount in minimum diameter is the way to go. You need to pick the correct materials, prepare the surfaces correctly, and execute well though.

One other thought I had at one time, which I discussed at length with Mick Kelly, was to bond radiused ribs to the root edge of the fin. The bottom radius would match the radius of the body, and the top radius would essentially be the top radius of the filet. This would add some structure, and increase bonding area. Another bonus if done accurately would be to ensure that the fin is straight. Just a thought- have not tried it yet.

 

[Binder Design]

I slot mine. More bonding area. Regarding weakening the tube where the motor casing goes? I've yet to see or hear of anyone causing an airframe to fail in that area. Ever.

 

[Steve Shannon]

Another way to think about it is this. Would you rather sit in a chair where the joints are butted together or mortised and tenons?


Steve Shannon

 

[JimJarvis50]

You may be right, Jim. You certainly have much more and better experience than me, but here's how I think.
I'm going to assume that fillets or tip to tip reinforcement would provide the exact same amount of strength in either case, so I'll disregard them and just discuss the glue joint itself.
I'll also assume both body tube and fin are 1/8 inch thick and I'll pick dimensions to simplify the math.
Assume the fin root is four inches long. The area of the butt joint then is 4/8 or 1/2 sq. inch.
If I inset the fin the glue joint consists of the area along both sides of the fin so the total area is just over 1 sq. inch.
You're right, that placing a fin into a slot allows the fin to exert more force on the tube, but that necessarily means the tube exerts more force on the fin as well.


Steve Shannon

I think if the tube (or the chair leg) can't be deformed, then your analysis is correct. Obviously, more surface contact would result in a better bond. But, keeping the tube itself from deforming is much more important. One way to think of this is that if you have tip to tip reinforcement, the first step to having that fail is for the tube to move in some manner under the reinforcement. You'll get much more strength from the fillet and tip to tip than the fin tab, so you want to protect those structures so they can do their job.

Jim

 

[JimJarvis50]

I slot mine. More bonding area. Regarding weakening the tube where the motor casing goes? I've yet to see or hear of anyone causing an airframe to fail in that area. Ever.

The air frame deforms allowing the fins to fail. This doesn't result in a broken air frame.

Jim

 

[dhbarr]

Has anyone done crenellated fins roots (crenroots?) for a true mortise-and-tenon approach?

Machined holes & tight tolerances should yield a proper bond line, but the rigidity of the attach point should go up substantially.

 

[Steve Shannon]

I think if the tube (or the chair leg) can't be deformed, then your analysis is correct. Obviously, more surface contact would result in a better bond. But, keeping the tube itself from deforming is much more important. One way to think of this is that if you have tip to tip reinforcement, the first step to having that fail is for the tube to move in some manner under the reinforcement. You'll get much more strength from the fillet and tip to tip than the fin tab, so you want to protect those structures so they can do their job.

Jim

One thing I think we can agree on, is that fillets and tip to tip reinforcement greatly exceeds the actual bond strength of the fin to the tube.


Steve Shannon

 

[mlrtime99]

I read this thread title as having to do with "sticky-butt". Sounds like someone needs to tend to their personal hygiene!

Hahah, as I saved the images the second was almost - wettedBOTTTOM

And thanks all for the insight, all valid points. Most of the adhesive failures I've seen (West System or better) the adhesive is completely intact and has pulled away material from itself that was crosshatched/scored.

Jim, I understand your point about the slot being a fulcrum point for a fin to torque about and destroy the airframe. If I think about it from a dampening and flexibility point of view, I can imagine a fin deflecting within some minimal limit that the slotted airframe and adhesive could cope with (1* or so) and then return to center. I wonder if the same adhesive on the airframe surface would do the same.

Sounds like I have a fun little experiment in the garage this winter.

Thank you all for entertaining me as I think out loud.

 

[JimJarvis50]

One thing I think we can agree on, is that fillets and tip to tip reinforcement greatly exceeds the actual bond strength of the fin to the tube.


Steve Shannon

Yep, we can agree on that. And I think we could agree that having the discussion is useful in any case. Two things come to mind for me. One is the issue of a loose motor. I'm going to give that a little thought, as it might be helpful to arrange for a nice snug fit - cheap insurance.

The second thing that comes to mind is that I was talking with a flyer about his design, which is a 5.5" fiberglass tube, likely to fly on 75mm motors. I suggested to him that he might consider surface mounting the fins instead of his normal design with through the wall fins. I also suggested that he could use removable motor mounts (i.e., a motor tube with centering rings, held in place via forward retention). I think such an approach would be fine if the centering rings were glued to the air frame, such that they actually supported the tube. But, if the adapter just slides in, the tube isn't really supported. I gave him bad advice, which I can fix.

Jim

 

[Steve Shannon]

Yep, we can agree on that. And I think we could agree that having the discussion is useful in any case. Two things come to mind for me. One is the issue of a loose motor. I'm going to give that a little thought, as it might be helpful to arrange for a nice snug fit - cheap insurance.

The second thing that comes to mind is that I was talking with a flyer about his design, which is a 5.5" fiberglass tube, likely to fly on 75mm motors. I suggested to him that he might consider surface mounting the fins instead of his normal design with through the wall fins. I also suggested that he could use removable motor mounts (i.e., a motor tube with centering rings, held in place via forward retention). I think such an approach would be fine if the centering rings were glued to the air frame, such that they actually supported the tube. But, if the adapter just slides in, the tube isn't really supported. I gave him bad advice, which I can fix.

Jim

At some point it's diminishing returns. If his 5.5 inch rocket is strong enough to be a minimum diameter, gluing the rings in, while that may strengthen the airframe slightly, doesn't serve as much purpose as being able to swap out motor mounts. As a friend of mine likes to say "Best is the enemy of good enough."

Edit: and gluing in a full length coupler might be even better than gluing in the centering rings.
Steve Shannon

 

[JimJarvis50]

At some point it's diminishing returns. If his 5.5 inch rocket is strong enough to be a minimum diameter, gluing the rings in, while that may strengthen the airframe slightly, doesn't serve as much purpose as being able to swap out motor mounts. As a friend of mine likes to say "Best is the enemy of good enough."

Edit: and gluing in a full length coupler might be even better than gluing in the centering rings.
Steve Shannon

Well, my feeling is that a 5.5" fiberglass air frame might be marginal without better support for the tube. A coupler in the fin area isn't a bad idea.

Jim

 

[Steve Shannon]

Well, my feeling is that a 5.5" fiberglass air frame might be marginal without better support for the tube. A coupler in the fin area isn't a bad idea.

Jim

I agree. If the airframe is internally unsupported then it's at its weakest. Tight fitting centering rings, even without glue would add structure. Glued in centering rings, or a glued in coupler would add even more rigidity.


Steve Shannon

 

[farsidius]

I read this thread title as having to do with "sticky-butt". Sounds like someone needs to tend to their personal hygiene!

Always wash your hands before using the bathroom if you've been eating cinnamon rolls.

 

[Binder Design]

The air frame deforms allowing the fins to fail. This doesn't result in a broken air frame.

Jim

Then all TTW fin joints should crack at the airframe junction from airframe deformation because we've weakened the tube by slotting it. There is essentially no difference other than the width of the tab.

 

[OverTheTop]

I built a 54mm Tomach and fully slotted the fins into the FG tube. For extra reinforcement I also put a layer of thin FG cloth on the inside of the airframe. Mach 1.8 on first flight. No cracked fillets.

Slotting the tube gives the fins something to bond to that is not in peel. Peel strength of epoxy is not that great, compared to both compression and tension (I have tried to locate the data quickly but it eludes me). I have researched this in the past and it is why I build with slotted tubes every time.

I reason that the tube is largely unaffected by the slot as the two sides are effectively tied together by the fin glued into the slot. The ends of the slot do get a bit more support (from each side) than the middle when the fin has moments applied to it during flight. A thinner fin slot would have less effect on the strength of the fincan than a thick one. As the fin torques one way or the other, part of the joint is in compression and part is in tension. No peel to be seen .

I did a mild crenellation on my Apache fins, but that was only to locate them to the correct depth when gluing. Placement was where I thought I would get best overall strength for the assembly.


BTW, these are CF skins with a G10 isogrid core. Overall thickness 4.2mm, for bonding to a 75mm airframe.