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Even...uneven fins...

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moocrew

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Alright So im working on a new scracth built. Had a few problems with CP but got them worked out a bit. Now im trying to decide which is better.

here a pic of the design.

Would the unaligned fins cause problems?

i tried to be as clear as possible but if you need more info or need something cleared up just ask.
 

moocrew

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im posting a second angle shot... hopefully you can see what im talking about...if not please ask.

-matt
 

jetra2

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Actually, the unaligned fins will likely make the rocket more stable, since there will be less turbulence. What happens with split-fin designs is that the air rushing off the back of the forward fin is a lot more turbulent than the rest, and when it hits that huge wall that is the 1/8" thick fin, it's like stopping a freight train on a dime. So, you will have a stable rocket. BUT, make sure you add enough noseweight to compensate for all those fins. Can you post a picture of the 2-D shot from Rocksim showing the CP and CG?

Thanks,
Jason
 

GL-P

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Can't wait to see that fly! Jetra2 from what I know is perfectly right.I think it is PML Endeavors with split fins have be known to have finflutter because the front fin creates turbulence which whacks into the rear fin and causeds fin flutter. My only concern is the long fins will flutter on their own w/o interference drag. How big is the rocket and what motors are you going to use in the cluster??? I hope nothing more than 3 Ds or Es:D
 

DPatell

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Wow, have extra fin stock?

Theoretically you should be fine unless you push it too hard. Those fins have large spans.
 

Elapid

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may be a problem...

you won't need any noseweight if you can keep it light since the fin area moves the cp way back. if you start glassing the fins and adding beefy fillets, you may have to add significant noseweight though...

depends on what motors you want to fly
 

Stymye

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looks cool can't judge the length but could be a real windcocker
 

Missileman

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Looks like a medievel weapon:D
Should be a spectacular flight.
 

moocrew

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well.....
If you want specs...
Then I'll give you more than you can shake a stick at...here goes nuthin! :D
------------------------------------------------------------------------
NOTE: These specs are AS listed in Space Cad....I budged a bit on weight just to see what my margin of weight was when building.
---------------------------------------------------------------------
Coefficient of Drag was set to .75 (Space Cad Default)

Lift-off weight 5.54 oz.
Empty weight 4.64 oz.

Length 3.28 ft.
(From Tip To end of body, not bottom tip of fins...the fins add another 2 inches.)

Center of gravity 2.53 ft
Center of pressure 2.84 ft.
(Im pretty sure this is from the tip of the nose.)

The top section is Bt 20 tubing and is 25.00 in. long (above coupler 11.02 in .... below coupler 1.51 ft.)

Transition is 1 inch long

Bottom section is exactly 6 inches long.
Bottom section is a bt 55 tube.
Bottom section is basicaly the motor mount with three 1/2A sized tubes the are recessed exactly .20 inches.

Nose Cone is 2.95 in. long (standard estes bt 20 nose)

Space Cad estimates the stability to be approx. 2.72 calibers
(its shaky!)
------------------------------------------

Now here are the flight predictions From Space Cad.
(I'll post Wrasp numbers after this)

Altitude 213.65 ft.
Speed 105.85 ft/s
Acceleration 774.61 ft/s 2 (2=squared)

Best Delay: 3.00 sec.

min launch rod length 1.81 ft.
chute diameter 1.90 ft.
------------------
WRASP sims.

max alt. 201.1 ft
time to max alt. 2.723 sec
delay time to max alt. 2.6 sec
launch rod velocity 59.9 ft/sec (3ft)
max vel 90.1 ft/sec Mach .08
Max accel'n 763.7 ftsec
23.74 G's
-------------------------------------------
The roc will launch on three estes A10T's
--------------------------------
When giving HxW of each fin Assume that there is a box around them and the measurements are from edge to edged
Width is the distance from the BT to the tip of each fin.
All measurements are in INCHES

All fins on Bt 55 are 2.5-3 mm thick and comprised of basswood

Bottom set 5 fins
W 3.20 H 3.10

Upper set on bt 55 tube. 4 fins
W 3.10 H 2.50
-----------------
All fins on bt 20 are 1 mm thick and are comprised of balsa

bt 20 fin specs

Bottom set 3 fins
W .50 H 2.75

Upper set 3 fins
W .35 H1.10
-----------------------


Well I can't think of anything else you could possibly need to know. :D

Here are the Blue Print pics from Space Cad showing CP and CG.

The rocket is named AVRiS
Anti
Vehicle
Rocket
System
I kinda took the name from a game i play (ut 2k4) and changed it a bit.

I do think that it will likely be a wind cocker because most of the weight is in the back..actually thats where more than half of the weight comes from.
Hopefully it still flys good.
Right Now Im Waitin of the glue to dry on the Ct rings for the upper BT sections Ill post pics soon on a different thread.

Thanks all of your advie and opinions guys!
:p
-Matt
 
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