Rocket fin design question.....

In looking at a handful of rockets which use canard type fins one half to two thirds the rocket's length forward of the rearward fins, and in
the same line, I've noticed some behave nastily. Is there a formula for the air disruption a canard induces? Or a seperate design form for a canard? Is there a likewise formula for the cure? Let's say a .08" canard can be followed by no less than a .25" thick rearward fin.......or maybe a canard one half the size of the rearward fins must be twice the thickness......or maybe a canard must have a leading and trailing edge sanded down to one half the thickness of the center chord of the fin.
Chuck
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Chuck Rudy

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Area of the fin will factor more importantly than the thickness. In general, the total area of a canard fin *forward* of the CG must be offset by an equal amount of fin area *aft* of the CG, multiplied by the distance from the CG. i.e. 1 sq. inch of fin area, 8 units ahead of the CG, would need to be offset by 4 sq. inches of additional fin area 2 units aft of the CG.
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Grunt the Flat wrote:

Grunt
I'm not doubting your equation.....however it appears your formula allows for no disruption of air from the canard to the rearward fin, it's simply dealing with the 'shadow' of Cp.......just as a boat racer does not want to race behind another boat because the churned up water is less dense due to air bubbles, my feeling is the rearward fins are also in less dense air which lessens their effectiveness in 'pushing' off the air because of a loss of density.
Chuck
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wrote:

general,
equal
CG.
offset
The rear fins don't stay in the wake of the foward canard. Rockets occillate... especially those with CP out of whack... which translates to even less trailing in the wake. They could only experience perfect vortex conditions if the rocket flew true... no pitch, roll, or yaw... etc... but then, considering if exactly true and perfect flight were possible, you'd have the canard in the right place and rear fin size correct for CP, wouldn't you?
~Duane Phillips.
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Jerry Irvine wrote:

I was figuring an aero issue with disrupted air and the canards acting as vortex generators giving the fins less dense air to push off, but you believe it's Cp only?
Chuck
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Yes because al rockets fly at slight angles of attack as the fins correct stability. THis makes a wake around every fin. That wake damps out vs time what it experiences momentarily (ie a wave hitting the right side of the lower fin)
www.v-serv.com/vcp
I said it in laymans tgerms dont rag on me. I figured he would not know his rho from his reynolds.

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Two things (at a minimum) will strongly affect the flight path.
1) The Center of Pressure is moved forward by adding fins forward of the rear fins. The CP really needs to remain aft of the center of gravity.
2) The real world. There is very little chance of you aligning the forward fins exactly with the rear fins. Just like a two stage rocket with two sets of fins. If you try to line them up and one fin is just a bit misaligned with the other aft of it (or at a slight angle!), it will act like an aircraft control surface (a flap or vane) and generate a large side force causing the rocket to deviate. Two stagers are notorious for this as they can "cone" or "corkscrew" around or even go into "cruise missile mode" even when they should be highly stable. SOLUTION: Interdigitate the fins. Do not try to line them up. Perfect example: Astron Farside.
-Fred Shecter NAR 20117 http://cgi6.ebay.com/ws/eBayISAPI.dll?ViewSellersOtherItems&userid=shreadvector
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Fred Shecter wrote:

You doubt my abilities.........I'm hurt..... :-( ......I'm very hurt........I'm so hurt I'm going to shoot myself........NAH! ;-) Thanx for the help!

I'll catch you via email if I can cut through your addy.
Chuck

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For simple stability calculation, treat it as a 2 staged rocket. That will calculate the correct CP for the whole rocket. This works for any rocket with multiple or complex fin sets.
As to coupling, which other replies have mentioned, yes, it does seem to be a problem. Any allignment error in the canard fins can do VERY wierd things in flight. It's VERY common for Sidewinder style kits to do "interesting" aerobatics after launch.
BTW, for those doing Canard style BGs, my Neutral Point program handles it with no coding changes. You just have a negative number for the distance from the wing trailing edge to the stab leading edge, which not only includes the distance between the wing and the stab, but the wing and stab chord as well. The program uses wing TE and stab LE to define the moment arm, but internally calculates MAC to MAC offset.
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