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Fin Question

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Diamond Dog

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I put together this kit

http://www.rocketreviews.com/reviews/kits/qst_navaho.html

It has two stages. I put the booster stage fins on upside down and on the wrong end. So when I put on the booster stage the fins are upside down and higher up than they should be.

How will this affect the rocket?

I did an experiement and took a RTF rocket to launch. I took of the one piece fin assemble and turned it upside down so the top of the fins came off the rocket at a 90 degree angle instead of a slanted angle. I think the rocket actually flew straighter and higher.

I am concerned about the 2 stage rocket I have because the I dont know how aerodynamics affects 2 sets of fins when the second set is not going the same direction as the first set and when they are set on the rocket higher up than they should be. The fins line up pretty good with the 1st and 2nd stages though.
 

doxiedog315

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I have never had a problem,with reversed fins. i've built many that way,to keep them from breaking on hard landings.;)
 

jflis

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Diamond Dog,

:) Seems I saw this a moment ago in ROL... :)

I replied there, but will do so here as well.

More than anything, it is great that you realize you should be *asking* such a question and it's great that you have a place like TRF to do it in :)

In many cases, this error could be very serious, but in the case of the Navaho AGM, it doesn't look all that bad.

If the fins are swept back, then reversing them will cause the CP (Center of Pressure) to move forward a bit. Also, the having the fins further forward will move the CP forward also. Moving the CP foward decreases stability but doesn't necessarily mean it will become *unstable*.

"Stability" is loosely defined (in model rocketry) as having your CP *behind* your CG (Center of Gravity). CP needs to be calculated or measured using certain tools, but can also be estimated using simple methods. CG can be easily measured. Simply balance the model on your finger (NOTE: To locate the CG properly, you *must* have all motors installed and the recovery system installed in the rocket.)

Without the tools or knowlege to *calculate* the new CP to see if it is still stable, I would recommend (to be perfectly safe) that you add a *small* amount of nose weight to move the CG (Center of Gravity) forward about 1 body tube diameter. This will return the stability margin back to near where it should be if built properly.

Once that is done, everything should be fine as far as flight is concerned.

One final word of note: If the fins are mounted as you described, on the booster, the booster *may* not tumble properly when it is kicked off, but I doubt this will be any problem.

If you were a *purist*, you could score the fins on each side along the glue line, snap them off and put them back on correctly then touch up the damaged areas. I doubt that I would do this. I'd just add the nose weight... :)

Good luck and let us know how this works out!

Jim
 

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