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Old 05-24-2019, 12:21 AM
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SEL SEL is offline
Officially Middle-Aged Rocketeer
Join Date: Mar 2005
Location: Portland, OR
Posts: 1,195

Originally Posted by teflonrocketry1
There is some new data about the aerodynamics of fins below a wider transition section. Check out the article titled "Increasing Rocket Aerodynamic Performance" by Thomas Salverson on page 42 in the May/June 2019 edition of Sport Rocketry. Essentially (on the right side of page 48) the portion of the fin inside the turbulent wake of the airflow behind a reducing transition doesn't do anything for the rockets stability. It is suggested that while a transition section below a wider payload section decreases the aerodynamic drag on a rocket, it induces a wake that interferes with and decreases the effectiveness of the rockets fins, and these fins should extend out beyond the rockets body until they are in the laminar airflow region beyond the diameter of the transition. I am working on a RockSim fix for this effect.

Also mentioned is that rail buttons create an asymmetric wake behind the rocket causing it to fly in an arc. Did you use launch lugs on your design that might have a similar effect? The original 1340/20 used a thin loops of wire for launch lugs visit:

Interesting - I'll pick up the magazine tomorrow. While the fins do extend somewhat beyond the payload diameter, it may not be enough if the wind is up. I used stand-off's for ( 2 ) lugs to fit a 1/4'' rod (see photo's below). I didn't know about the loop-lugs until long after I built mine (back in '96/'97) and rail buttons weren't common, so I just assumed the stock kit used the same style lug as the SR2250/2650.

I'd logged many flights on this model - I'm guessing upwards of 25 - in all kinds of weather, more than a few launches at the beach in similar winds. All were flown off a 4 foot 1/4'' stainless rod and as far as I can recall, all went where I aimed them. I had to go back to my original thread for this ( ) to remind myself what motor I used and it may just have been the combination of stiff winds and the (relatively) long burn E9, but I still would have expected it to weathercock into the wind rather than with.

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