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CG Change?

Old 11-10-2005, 04:37 PM
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CJKling
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Default CG Change?

Hey guys....
I have a Mid-Star 40 from Sig, and I added a different sized rudder to it. I understand that this will cause a slight change in center of pressure and center of gravity. I was wondering if anyone knew how adversely this will affect it or not. Thanks
Old 11-10-2005, 05:26 PM
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Dr1Driver
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Default RE: CG Change?

Simply rebalance it where it was originally. Unless the new rudder is HUGE, don't worry about the center of pressure.

Dr.1
Old 11-10-2005, 06:13 PM
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CJKling
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Default RE: CG Change?

Its at least 1.5-2 times larger than the old one. Thanks.
Old 11-10-2005, 07:30 PM
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britbrat
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Default RE: CG Change?

You're weren't planning on knife-edge were you?

That big rudder could be susceptible to flutter if you fly too fast.
Old 11-10-2005, 11:09 PM
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Default RE: CG Change?

Rudder won't affect the CG for pitch but you may notice that in turns you need to use a bit of top aileron to keep it from winding in. By adding that much "fin" area you've changed the spiral stability value and an overly large fin area tends to make a design want to tighten the turn. Like most things in life fins have a happy medium. How your model flies will be determined by where on the scale the original design was.
Old 11-11-2005, 07:51 AM
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rmh
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Default RE: CG Change?

The funny part is that adding a bigger fin and rudder can worsen the knife edge flight!
Adding some area to the front of the fuselage is far more effective in getting better rudder effect.
The "why" is that the area is more balanced to the CG - On serious aerobatic stuff we did for pattern - the models were deliberately setup such that a slight tail wag may occur at lower speeds. (not enough stability in yaw).
The models would then roll very easily with minimum pitching corrections.
Your Mid Star is not a prime candidate for doing a lot of this kind of corrective surgery. It is tho a great model for all around sport flying . If you swap off the fin to rudder size ratio and then add more rudder throw - the model will push around hammerheads more easily/quickly (using slight power to start the rotation)
Old 11-11-2005, 09:26 AM
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britbrat
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Default RE: CG Change?

Adding more fin area can certainly worsen knife edge, as you describe, but adding more rudder should assist it it some cases -- those cases where there is inadequate rudder authority (ignoring pitching moments).
Old 11-11-2005, 07:06 PM
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Default RE: CG Change?

and I added a different sized rudder to it.

Its at least 1.5-2 times larger than the old one.
With all due respect why did you do that?

Bill

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