Octoberfest Interests!!!!!

[Bruce R. Schaefer]

Tim,
I’m sure I’m not the only one thinking this, but if one cut the double-swept delta fincan fins into elliptical fins, and except for the disturbance in the laminar airflow around the minimum diameter airframe that the fincan adds, which might be negligible or at least counterbalanced with the tailcone GLR added to it, this model would be perfect, with a more elliptical nose cone depending on the simmed speed, for the Super Single Shot Sweepstakes. I just may have to buy another one from ya. 😀 Haven’t built mine yet, but it just keeps smiling and winking at me. 😉 Oktoberfest is just around the corner…

[denverdoc]

Bruce,

Hmmm. I recall a design from Estes that was BT-50 based, elliptical fins, and used an abrupt tailcone, sprint maybe? Claimed 1600′ on a C6. I built a couple. But never outperformed a BT20 based rocket of the same length. But more reliable for sure.

Its stretching a comparison, but I don’t think you can ever reclaim losses from drag by making the air move in two directions–first out, then in. Just making the air do a two step implies that extra energy is imparted to it, the source has to be rocket. Energy donation of course bad thing in this context.

The other notion i have is that while ellipses still rule the world of small reynolds number flying objects like hand launched gliders, it was curious to see that most of the really good kits/designs these days have the wings on backwards. In other words the LE taper is more severe. I was designing some BG’s recently for Naram, and kept looking at these record holding designs thinking that can’t be right.

Also, I seem to recall that Tim Van Milligan who is in the springs and is the force behind apogee (rock Sim etc) addresses this issue for rockets in one of his e-zine issues, and concludes that a rectangular fin is as good as any other. Sort of guessing, but the ellipse was based on RAF studies which showed induced drag was least with this planform. But planes always fly at a non zero angle of attack, rockets ideally do. So the ellipse may after all not be the optimal planform. Check out Apogee’s site, some fantastic and free info.
Cheers,
John

[Bruce R. Schaefer]

Actually, a clipped delta is fine, with elliptical fins only performing ~1% better. In a contest, 1% can mean a lot. It’s odd that they would, since the worst fin shape is the double swept delta, like the Event Horizon, for drag. Tip vortex and all. Don’t underestimate the boat-tail. Everything in life has a beginning, middle and end. It DOES reduce drag by quite a bit, at least in a wind tunnel. 😀 Sometimes it’s best to go with empirical results and not over-think things. Aerodynamics isn’t always (is it ever?) intuitive.

ADDED LATER: John, just don’t ask me where I read or saw this, but a boat-tail can take up to 0.2 off your drag coefficent, as I remember.

[Tim Thomas]

Bruce, I had the chance to fly my Thunderbolt over the weekend at Hellfire. I launched it with a Pro38 I285. It was the 2nd flight Sat. morning. It jumped from the pad, at about 2 seconds it went mach, and flat was gone. My Perfect flight read 6840ft. This weekend at Hartsel, she goes on a I300bt. It simms at 7560 @ 780 mph. The performance is there, now some DD and we should be good to go! With yours, Daves and mine in a 3way drag at Oct. we should put on a great show. The J350 sims at 47gs 1050 mph, 10,200 ft. and 20.5 sec to the top. Now thats a drag race! Not to mention the smoke and fire from 3 J350s’. Isn’t this why we got our Level2’s? 8)

[Bruce R. Schaefer]

Oh, yeah! 😀

[Conway StevensParticipant]

So Gang How about what everyone is pushing to get done and fly for Octoberfest!!! ( I mean it is the topic .LOL) I really really want to hear what you all want to do for this event. I just recently ordered the Motor and Case for my L3 project I am so dang excited!!! Man thats gonna rip! So its should go very nicley!!!! I also just met with my L3CC and I have the go ahead on the next step and await it and thats the flight sign off. WOO HOO!! come on October!!!

[Bruce R. Schaefer]

Well, that’s what we were talking about. 🙂 Best of luck, Conway, with your L3. If you need another ground crew member, just let me know. I’ll be turning to you for tips within a year’s time. I’m just going to fly one of Tim’s Rocky Mountain Thunderbolts in a drag race with him and Dave Way, and probably a K550 in GLR’s Vertical Assault. Other than that, I’ll be LCO or RSO or whatever is needed.

[Conway StevensParticipant]

Cool. Bruce!!! Drag race sounds fun. Can I join. My L3 may not be enough motor there to do much.You guys would prolly whip me!! Just kiddin. I must admit Im as excited as I could get. This has been a fun project. It will have Data aquisiton and on board Video as well as being a large motor with 50 lbs of rocket that should get about 13k at roughly Mach speed. I just wish it was like tomorow… LOL

[denverdoc]

Actually, a clipped delta is fine, with elliptical fins only performing ~1% better. In a contest, 1% can mean a lot. It’s odd that they would, since the worst fin shape is the double swept delta, like the Event Horizon, for drag. Tip vortex and all. Don’t underestimate the boat-tail. Everything in life has a beginning, middle and end. It DOES reduce drag by quite a bit, at least in a wind tunnel. 😀 Sometimes it’s best to go with empirical results and not over-think things. Aerodynamics isn’t always (is it ever?) intuitive.

ADDED LATER: John, just don’t ask me where I read or saw this, but a boat-tail can take up to 0.2 off your drag coefficent, as I remember.

Bruce,

Perhaps I wasn’t clear–my issue with the Giant leap design (which bears an uncanny resemblance to the H1000 rocket I lost at MHM) was with the notion of a fin can that adds diameter to the rocket. Sure a boattail might mitigate the damages, but in the context of ultra performance, my feeling is a single diameter tube with SM fins will do better vs a double taper to smooth a bulge.

I also don’t doubt that under right conditions a boat tail might reduce ones drag coefficient by .2 and you’ll find one on the booster of the L3 project I’m flying this saturday. So for the sake of argument, lets take a look at it. Nothing extraordinary, ogive nose, fineness ratio of near 20 (9.5’/6″), Nike trap fins. Say the CD w/o tail is .4. If i get the drop of .2 then I end up with .2. Meanwhile tho I have taken a potentially 4 inch min diameter and gone to 6, so the area has gone from x to 2.25X. The 50 percent reduction in Cd doesn’t reclaim the losss. I end up with a 12 percent gain in drag. Now if I had a really good reason to go from 4 to 6 inches inches as in using a bigger motor or nitrous tank, makes huge sense.

Hope thats a little clearer this go round,
J

[Bruce R. Schaefer]

Clear and agree. No arguments from me. 🙂 I also agree that a bulge aft isn’t the way to go. You have to make so many tradeoffs during the process of designing and building an altitude bird, you don’t want to start with something you can avoid in the first place.

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