Tube fin diameter relative to body tube diameter?

[boatgeek]

I am advising students on a project that will likely use 3” tube fins on a 5.5” body tube. A question nagging at the back of my mind is whether there’s a rule of thumb for minimum tube fin diameter like there is for flat fins. Normally you want the span of a flat fin to be at least the tube diameter to make sure it’s clear of the boundary layer. I’ve seen the problem in action on a silly rocket that had 18mm tube fins all the way around a BT-80 body tube.

Anyone have experience with tube fins about half the diameter of the body tube?

 

[cls]

Hi, make the tubes as long as their diameter. 1 caliber. So, 3" long, 3" diameter. Ok to have 4, 5, 6, 7, or 8; how many fit around your 5.5" airframe?

 

[BABAR]

. I’ve seen the problem in action on a silly rocket that had 18mm tube fins all the way around a BT-80 body tube.

hey! I resemble that remark!



https://www.rocketryforum.com/threa...r-wide-body-bt-80-horizontal-recovery.166124/

See post 14

 

[BABAR]

double post

 

[boatgeek]

Hi, make the tubes as long as their diameter. 1 caliber. So, 3" long, 3" diameter. Ok to have 4, 5, 6, 7, or 8; how many fit around your 5.5" airframe?

Looks like 8 will fit, with a little extra space between. Maybe the students will throw 4 flat fins in the gaps. It's a silly rocket anyway.