Jay,

Hmmmmmm...... I have the most recent addtion of the Stine manual - 8th edition(?) (one of my BAR purchases)....I should've looked there! If rounded LEs only are worse than square then I have a Quest Astra (ready for fill and paint) and my Triskelion (in primer) and a FlisKits Whatchamacallit and dooHickey, Semroc Boid and an original (all in flying status) that are all set up "worst of the three". again.

scigs30,

I've started on a Star Stryker and noticed that the instructions say to round fin LEs but the pictures show square. Starting that build is what prompted me to finally "vocalize" this question after wondering about it for some weeks.

If it really says that then it is wrong.

Airfoiled, rounded, square.

I spent a year doing wind tunnel tests. Also I refer you to numerous reports published by the MIT Roc Soc.

Jerry

Anyplace those tunnel test results were published (kind of like Mike Selig's model airplane Reynolds number airfoil work in the Soartech journals, perhaps)?

I believe Jerry is correct.

In my copy of G.H. Stine's 7th edition it mentions that a square fin tip, not leading edge, will have a lower induced drag than a round one.

Fin optimization for a given airspeed would take a LOT of CPU time. There a lot of variables that come into play:

• fin geometry
• airframe OD
• airspeed

Greg

A rounded fin tip promotes vortex formation due to the ease of pressure differential equalization from surface to surface, and thus vortex drag which is induced drag since it is only attached to the body by a boundry layer and flow field.

A square edge reduces vortex drag, and an airfoiled tip makes the VD considerably lower and also lowers frontal area.

Fins like 2250 and Goblin with intentionally downward shaped, flat edges are designed to reduce vortex drag by sucking the vortex more into the wake of the body itself.

Airplanes use anti-vortex tips like little tails to break up most of the vortex power, but ask any pilot who has gotten too close behind a large plane, vortecies are still there and strong.

Just tech Jerry

Is that because square edges stand out better in pictures, thus highlighting the fin shape?

I had always thought it was easier to make round edges look perfect whereas slightly unsquare (slanted) edges are easily noticed.


Bill

I round all the edges but don't bother with tapering the trailing edges.

Save that effort for my Hand Launch and Catapult Launch Gliders. =)

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Jerry,

I noticed many of the early sounding rockets seemed to favor this design. Mark Mayhle used this in all of the Lasor series kits and the NCR Hypersonic kits I had were also very similar in design. USR Stinger Two also comes to mind as a real drag cheater.

I prefer to sand an airfoil on the root, leading and trailing edges with a belt sander leaving the tip squared with the 2250/2650 design. It just looks right .....

My best performing rockets use this proven design

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Actually, these days, unless an airfoil or rounded shape is specified, I leave my fins squared for this very reason. I want my rockets back! I'm usually trying to hold down the altitude. Lower altitude usually means less chance a RET will get my birds.

If I was entering a competition, I would definitely make an airfoil shape on my fins.

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I pretty much always round at least the leading and trailing edges of my fins... As a matter of fact, the ONLY ones I can remember leaving square are the ones on the "Pirates Booty" rocket I just built. The only reason I left them square was because the "blocky" look fit in with the scheme... I find it harder to do a nice, clean square edge than it is to round them