Openrocket fin size

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boatpirate

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I have been designing and building this and for some reason I cannot make myself believe that the fins should be this small. Can someone look at the file and let me know if im in the ballpark? :y:

This is the first rocket I would have built completely from scratch. It's two wrapping paper tubes with a blue styrofoam nosecone.

View attachment Cat SCRATCH Fever.ork
 
I don't see anything wrong with your design. Length, especially extreme length, has a significant impact on stability. Even doing the math by hand (yuch!) will show this. If you want an example of smaller fins working on a very long rocket, look at any Super Roc contest model. One of the tricky things about length is what happens in wind. Too much stability and the bird will weathercock alarmingly. The bottom line is, a longer rocket needs less fin area to be stable but has other issues to deal with (airframe curvature developed while sitting on the pad waiting to launch due to uneven heating from the sun as an example.)
 
Thanks guys, i guess i mainly needed a little observation by someone other than me. I don't have anyone anywhere near me that's interested in model rockets. This is the first time I have talked with anyone who knows anything about it. I am almost completely self-educated in this hobby.
 
I will keep the airframe curvature issue in mind. That's interesting. I have had some what I thought were cool flights taking advantage of weathercocking. This IS the longest rocket I have.
 
I don't think the rocket is over stable. Long Tom rockets (L/D > 20) loose stability quickly in crosswinds as the non-zero angle of attack increases because the simple stability calculations do not consider the airframe cross-section changes with angle of attack. For this size rocket I like a 2" fin height, a 2" tip chord, a 4" root chord, with a 1.5" sweep back, and I'd set the trailing edge of the fins 1" from the rear of the BT, but that's my preference.
 
Hi,

I wouldn't make the fins any smaller, even though there appears to be a lot of stability margin. Like bobkrech said, long rockets loose a lot of stability even at low angles of attack.

If you sim the rocket with the C11 motor with 20 mph wind (the maximum allowed by the NAR code) and plot Stability margin and Angle of attack against time, you'll see that the rocket has a 23° AOA when it leaves the launch rod. At that AOA the rocket has only 0.5 calibers of stability. This is quite an extreme case, so I'd reckon the rocket to be stable, but I wouldn't make the fins any smaller.

Extremely skinny rockets can lose 10-20 calibers of stability in relatively small winds.


Cheers,
Sampo N.
 
bobkrech said:
I don't think the rocket is over stable. Long Tom rockets (L/D > 20) loose stability quickly in crosswinds as the non-zero angle of attack increases because the simple stability calculations do not consider the airframe cross-section changes with angle of attack. For this size rocket I like a 2" fin height, a 2" tip chord, a 4" root chord, with a 1.5" sweep back, and I'd set the trailing edge of the fins 1" from the rear of the BT, but that's my preference.
Click to expand...

Sampo said:
Hi,

I wouldn't make the fins any smaller, even though there appears to be a lot of stability margin. Like bobkrech said, long rockets loose a lot of stability even at low angles of attack.

If you sim the rocket with the C11 motor with 20 mph wind (the maximum allowed by the NAR code) and plot Stability margin and Angle of attack against time, you'll see that the rocket has a 23° AOA when it leaves the launch rod. At that AOA the rocket has only 0.5 calibers of stability. This is quite an extreme case, so I'd reckon the rocket to be stable, but I wouldn't make the fins any smaller.

Extremely skinny rockets can lose 10-20 calibers of stability in relatively small winds.


Cheers,
Sampo N.
Click to expand...

You two speak the truth, and the Long Tom is no exception. I had one do cartwheels 20 feet above the launch pad and it had the kit fins and recommended nose weight, and that happened when a gust blew across the launch pad. I ended up adding 20 grams more nose weight to make it a reliable flier.
 

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