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Rudder Coupling Causes & Fixes

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Rudder Coupling Causes & Fixes

Old 04-29-2004, 10:05 AM
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HenRam
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Default Rudder Coupling Causes & Fixes

Hi All,

Can anyone explain the aerodynamics involved in a rudder and elevator design that causes rudder coupling while on knife edge. What would be the best way to fix a design to prevent this without having to get into heavy radio mixing.
Old 04-29-2004, 11:07 AM
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vinnie
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Default RE: Rudder Coupling Causes & Fixes

Do you mean Yaw coupling? Gravity maybe? Which way does it go?
Old 04-29-2004, 11:41 AM
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rmh
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Default RE: Rudder Coupling Causes & Fixes

In most cases -simply changing CG will fix or change this
However - this is only for a model moving at a good speed and where you are simply trying to hold altitude.
There are lots of reasons other than CG- among them high weight ,needeing more "fuselage lift, plus lousy lateral area distribution - for a couple .
Old 04-29-2004, 11:44 AM
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Default RE: Rudder Coupling Causes & Fixes

As I understand it, the coupling of rudder is due to mis-matched areas above and below the stabilizer. Due to separation, you build up more pressure on the "upwind side" of a surface than you lose on the "downwind" side. If you have more rudder above the stabilizer than below, this should cause some "up" elevator effect with extreme rudder deflection in either direction.

Tapering the rudder so that the areas are relatively equal above and below the stab should cure the problem.
Old 04-29-2004, 03:41 PM
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HenRam
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Default RE: Rudder Coupling Causes & Fixes

While on knife edge, the model tends to tuck towards it's belly. (Up elevator is required to keep the model moving straight) I have a UCD3D that does pretty much the same thing and could only be fixed by mixing it out. Obviously, this tail design is not very effect for knife edges. Based on some of the responses, it would appear that extending the rudder futher below the stablizer would help alleviate this problem....

The model in question is the Mad 3D Taco which you can see on our website at [link=http://www.flyingthingz.com]www.flyingthingz.com[/link] I have attached the picture here for your reference as well.
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Old 04-29-2004, 03:47 PM
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Default RE: Rudder Coupling Causes & Fixes

It's hard to see the whole rudder in the picture, but the bottom of the rudder extends to where the bottom of the fuselage is. This bird really 3Ds very well, but does not knife edge. I would like the design to be fully 3D capable in every aspect out of the box. Maybe it could be done, maybe not.

Right now with the current setup, we have been able to land this bird as it's flat spinning due to all the exagerated control surfaces and it's floating capablilites. It's pretty impressive to fly. Knife edges on the other hand are difficult without having to go through a lengthy mixing process. We want to market this plane as a 3D trainer and wouldn't want a newbie to have to go through a mixing out process to get it to fly right.

I'm hoping the right approach would be to redesign the tail components to make it work as an all around performer...
Old 04-29-2004, 05:12 PM
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Default RE: Rudder Coupling Causes & Fixes

CG is still principle cause -
rudder taper has little or nothing to do with it
you can slant hinge line and it helps -
I have setup and trimmed about a jillion patter setups and when all ist goot -typically the CG will get most of it out
On a 3D stup -the aft cg will make this impossible without some upmix.
Old 04-29-2004, 05:20 PM
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Default RE: Rudder Coupling Causes & Fixes

Try a little more of the fuselage area below the wing. Kind of looks like there is more area above than below.

Fairly easy idea to test... tape on a piece to the belly of that profile plane. You'll know if its the correct idea in just a few minutes.

I am assuming you already have the CG back as far as you dare put it...
Old 04-29-2004, 05:26 PM
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Default RE: Rudder Coupling Causes & Fixes

Do some experiments by moving the ailerons up and down (both). If it pitches towards the belly on the knife edge (both sides) you should propably rise both ailerons. You should also take a look of f3a trimming charts such as this: http://www.mat.uc.pt/~pedro/aeromode...s/trimming.txt
Old 04-29-2004, 05:29 PM
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HenRam
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Default RE: Rudder Coupling Causes & Fixes

You'd be surpised, but no, it's not as far back is it could be... Normally this would aid in 3D manouevers, but this bird just does it somewhat naturally... Flying at half throttle she floats nice and straight with the elevator perfectly centered and very little trim...

I'm going to attempt to make the fuse equal on both the bottom and top doing what you recommended. I should have the results by the weekend... Do you know what is happening to the air as it passes the control surfaces to cause the tucking????

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