New Fin Idea - Hole in Fin

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edwardw

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So I'm building a 38mm rocket to be powered by a G64 and F12 AT reloads. I've got done designing it and find it....well...blah. It's a simple 3FNC rocket. So to spice things up I decided I would find the center of each fin (they are trapezoids) and then drill a 3/4" hole right through the fin. I thought it would be interesting and give it a new look. Anyone know how this will effect stability? I'm thinking I will have to find the square inches of then fin before hole, then the square inches of the hole. I just add the two together and arrive at a fin shape that has the square inches of the fin plus the square inches of the hole, so when I drill the hole out I'm left with the original square inches.

Any ideas?

Edward
 

wwattles

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While the quest for meeting the exact same amount of surface area with the hole as without may be an interesting problem in mathematics, unfortunately it would not equate directly to rocket performance. By cutting a hole you also increase the eddies created by the fin (you've created an additional leading and trailing edge), as well as increased the frontal surface area on the forward edge of the fin and the hole. Those will affect your drag calculations, which will throw some moderately more complex variables into your CP calculations.

Generally speaking, by adding the hole but increasing the overall size of the fin to compensate, you will be pulling the CG backwards (both the eddies and the frontal surface will do this) while the righting forces will also increase because the distance from the CG on the moment arm will increase (longer fin - further from center).

I don't have any empirical data to back this up - it's just from what I remember from my classes on that sort of stuff a while back. So my 2 cents has depreciated to about 1 1/2 cents worth.

WW
 

Elapid

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seems like it would just increase drag, making the model slower and not fly as high...

(see signature)
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edwardw

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Lower is okay on this model. It weighs about 20 ounces and sims to 4K. It is only 38mm body tube and 24" long overall. Gonna be hard to find if I don't do something to keep it lower and slower.


I think I'm just going to oversize the fin and cut the hole. And if I don't like it, I'm going to cut from the hole to the back edge in a straight line :)


Edward
 

aerojacket

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As noted above, the hole is going to increase drag with the small additional frontal area and resulting vortices and eddys. (Eddies?) The removal of material with shift the c.g. forward a slight (might be barely measurable) amount. IMHO, the loss of material will be offset by the extra drag which will be stabilizing.

Drill the holes, go fly as a heads-up. Let us know how it goes.
 

heada

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As noted above, the hole is going to increase drag with the small additional frontal area and resulting vortices and eddys. (Eddies?) The removal of material with shift the c.g. forward a slight (might be barely measurable) amount. IMHO, the loss of material will be offset by the extra drag which will be stabilizing.

Drill the holes, go fly as a heads-up. Let us know how it goes.
psst....this thread is 14 years old. I'm hoping they've flown it at least once since then.
 

K'Tesh

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psst....this thread is 14 years old. I'm hoping they've flown it at least once since then.
And the OP's last activity on the forum was 9.5 years ago.
 

AlphaHybrids

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Rocket has flown a couple times and as an upscale. Lots of fun. Whistles a bit. One iteration I used a router to route a bevel on the bottom of the circle and on the opposite side bevel the top side. Had a slow spin to it.

Edward
 
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