fin strength/length

[moocrew]

Hi all!
yet again I'm back to good old space cad designing my 20th scratch built roc....so i figured for this one i want something slightly different.
I came up with this bird. I named it shockwave because of the wide fins span and short body length almost reminds me of some sort of shockwave. anyways....

the fins dimensions are... 1 1/4" x 9 1/2" ......refer to pic posted....

The design is stable and launchable, but im not sure if the fins will be strong enough at the root. I plan on using basswood that is about 3/32" thick. I'm pretty sure this will be strong enough in that the fins wont split in half, because the roc doesn't sim to weigh more than 2.5 oz....so its a pretty light bird

My question is...
with the root edge only being about 1" long..can i attach them with CA and make elmers fillets and still be safe in saying that the fins wont rip of?
The bird will launch on a max engine size of A10T so not a whole lot of force.

any help is much appreciated.
thanks.
-matt

 

[powderburner]

The following comments are based purely on intuition, and NOT on analysis----
I think that, while it is theoretically possible for you to run into a flutter problem, your rocket design will probably be drag-limited to velocities low enough that you might remain below the flutter-critical speeds.
For low-power models, I think you could get by with balsa construction for these main fins. After all, they are not that much different than boost-glider wings . . .


If you are really worried, put the fin roots through the BT down to the MMT. (Now that's really something, TTW construction for an A10 motor!)

 

[moocrew]

Im willing to agree with you about the flutter the roc's simmed top speed will be about 60ft/s. Which i would think is pretty well under flutter speed...
I actaully considered TTW but i think that may be just a tad much for this. I think that the elmers fillets should be sufficient.
I'll stick with basswood thou i think...i just have this thing against balsa fins...i don't like em. besides I'm going for low flight height on this one.
Thanks for your input. Im probably going to start construction sometime later today.

thanks again.
-matt

 

[powderburner]

If you use balsa, it can warp after a time and curl up, especially with long thin fins like your design. Once that starts your rocket will be pretty much un-fly-able.
Don't have as much experience with basswood so I don't know whether it has the same tendencies to warp.

 

[moocrew]

hmm...even if its sealed?... As are you, I'm not real sure either... I usualy seal all my fins with CA glue. I always thought that warping had something to do with moisture.... :: shrugs :: I guess you never know till you try....

 

[moocrew]

oh for anyone who cares...heres the spacecad file for this bird. i don't care if anyone builds it, but if you do please send me back some info I'd like to know how it will peform for other people.

 

[Elapid]

let us know how it fares!

 

[moocrew]

I'm close to finishing the transition I made from paper. That part always take the longest. I've finished and mounted the engine tube. all thats left is to mount and fillet the fins and add a cool paint design..
-matt

 

[Stymye]

interesting design!,,

I have a feeling that the flightpath may be very wiggly, I recommend flying it at a personel launch first if possible.
than looking the fins over for cracks..they will probably see alot of flex... I like the design.

 

[bobkrech]

An interesting design indeed.

Kind of like a Fliskits Praetor gone wild.

https://www.fliskits.com/products/01prod_fs.htm

There's some stuff here about the big fins.

https://www.rocketreviews.com/reviews/kits/flis_praetor.html

Build it, fly it and post a picture.

Bob Krech

 

[moocrew]

I have several preator style designs that i have made on space cad. I've always liked the BIG fins designs. Im more of a show type guy i don't care how they fly to much. I guess if this one goes well, you may be seeing more large fins designs from my drawing board

 

[OARJeepr]

wood warping is caused by uneven forces in the wood not by moisture. Moisture can create these forces (one side getting wet/uneven drying) but it is not necessary. I've ripped perfectly flat dry boards and had them twist up on me.

Generally even the best sealants can only slow, not stop, vapor transfer into and out of wood. That's why finished doors that work great in the winter will stick in the summer. CA may work better, I don't know because its not something I would ever use in woodworking.

 

[moocrew]

I think im going to use a thin CA. Its not like its really going to hurt at all.