Launch lug position-- Fin root or not?

[BABAR]

On my scratch builds I have always put the launch lug at the root of the fin/body tube joint.

I figured if the fin was straight, the lug would be straight.

If anything, it would strengthen the fin body tube joint.

Is this aerodynamically/performance-wise better or worse than just putting the lug on by itself independent of the fins?

Thanks,

Tom

 

[Gary Byrum]

I've never noticed any difference. Everything I build is scratch. My rule of thumb though it this. If I can put it in the fin joint, I will. I'm sure is provides a little more strength to the fin, but that's not my reasoning for doing it. It's mostly to hide it. Depending on what's in front of that lug, will determine whether it gets mounted on the side of the fin or centered between them.

 

[dhbarr]

That fin root is already a source of interference drag, so putting the lug in there isn't going to make it worse.

My hunch is about a half-lug better, but that's just a guess, no math. It'd be different if there were a significant fillet.

 

[The_Lone_Beagle]

There is actually a discussion about this in our "Bible," i.e., Harry Stine's Handbook of Model Rocketry...in my copy on the top of p. 152.

Bottom line, you are definitely cutting down on drag. His estimate was that a normally placed launch lug accounted for about 35% of the parasitic drag experienced by a rocket; by placing it at the fin root you cut it down to only about 20%.

 

[dhbarr]

Thanks for the reference, probably where I'm half remembering it from.

 

[jyawn]

Glad to know that the fin root location works OK for HP rockets too. When I do classes where kids scratch-build model rockets, I have them put the lug on a fin root. It's largely to make it easy for them to get it on straight, but also reinforces both the lug and that fin. I have them check to make sure the fin is straight first. It's a good exercise in eyeballing. : )

 

[qquake2k]

Honestly, I can't see where it would make a noticeable difference. If you're that concerned about drag, maybe for an altitude competition, just omit the launch lug altogether and use a tower.