If you know the outside diameter of a body tube, and you know you are going to use X number of fins, 3 or 4, usually, and you know the width/length of the fin, is there a way to determine what the fin span diameter will be?

For example I think it can be done for 4 fins but I can't figure out how to do it with 3 fins.

For example :

4 fins each 1" width a 90 degree angle to one another and a body tube OD of 1.0"..the fins span should be fin + OD + Fin or 3 " in this case.

So how would I do it for 3 fins?

TIA


Terry

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"Old Rocketeer's don't die; they just go OOP".....unless you 3D print them.

I could be mistaken here but it would seem it wouldn't matter how many fins you had. The diagram below is something I drew up to show why. My thinking says, from fin tip to center of the tube, back to another fin tip would determine fin span.

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The best rule of thumb is still twice the radius from center to tip.

If you think of the fin span as being used to evaluate a cross sectional area the air still has to deal with a cylinder whose radius is defined from the center of the model to the tip and has to deal with all the fins at once. The fins being evenly arranged around the airframe will create that cylinder regardless of three, four, or seven fins. Span and surface area are two different things. It is akin to the definition of wing area - that being the "shadow" the wing creates on a surface parallel to the chord line and the area defined by that shadow, not the total surface area of the wing upper and lower surface.

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Gravity is a harsh mistress
SAM 002
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"The complexity of living is eminently favored to the simplicity of not."

I'm actually trying to determine the diameter of the circle that the fin span develops.

Terry Dean

__________________
"Old Rocketeer's don't die; they just go OOP".....unless you 3D print them.

Then it can be no smaller than twice the radius from center to tip.

__________________
Gravity is a harsh mistress
SAM 002
NAR 91005
"The complexity of living is eminently favored to the simplicity of not."

In Autocad, if you offset the 1" tube drawn in the previous example, by 1" it will give you the circumference of a 3" fin span. A 1" circumference measures 3.1563" and the 3" span circle measures 9.4375". This is 3 times the inner tube size. This does not account for tube thickness though as my poly lines are set at zero thickness.