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Carden 40% Edge coupling problem

Old 11-10-2004, 10:09 PM
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jack01
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Default Carden 40% Edge coupling problem

I've got a Carden 40% Edge that's got some knife edge coupling issues. According to Dennis, the plane should have little to no knife edge coupling in it when balanced between 6 1/4 and 7 inches from the leading edge. Currently, I'm balanced at 6 1/4 and the plane has a considerable amount of up elevator mixed in to keep it from going towards the belly in a knife edge. It's around 5-7 degrees. I've got some up trim in it for straight and level flight. The coupling also gets ALOT worse when the plane is rolled level and rudder is applied. What is causing this? I'd like to know any and all things I need to check. Also, the plane has an unusual tendency to try and snap if I pull up too hard or too little. It's not a violent snap. It's easily corrected by a little aileron and rudder. I understand that pulling up too hard causes an accelerated stall, but then why would pulling up harder fix the problem? Adding power doesn't seem to affect it.


Thanks in advance!!
Old 11-11-2004, 12:21 AM
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GRH
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Default RE: Carden 40% Edge coupling problem

Jac,
This is the exact same thing Bill asked the other day.

I don't remember what amount of pitch mix I had with the Edge but I know I used a nonlinear mix and that it was asymmetric left to right. If I recall I used more up elevator mix with left rudder knife edge than with right rudder....P-factor causes this asymmetry. I think I only used a couple of degrees of elevator to mix out this airplane. If he's looking at percentages and doesn't have much total throw then I can see where the percentage could be significant. The problem with the Edge is that the mix is nonlinear and I'm not sure if anyone really knows why.

Pitch mix is almost always a function of angle of attack and typically becomes more nose down with increasing angle of attack. Often times this is just the natural nose-down pitch stability that you get with increases in angle of attack.

The real thing is to get the CG far enough forward to get the airplane to lock in after the snaps and mix the rest. I had to relearn how to fly to compete with that airplane. The roll mix is much more than it should be because of the 0.5 degree dihedral in the wing...this led to having a noticeable amount of opposite roll mix. I recall this being worse than the
pitch mix but linear enough to mix out. That airplane's weird in another aspect because it seemed like when you're transitioning from inverted to a 45 deg bank angle (like in the second half of an 8-point roll) you have to hold up elevator to make it stay on line...it didn't do it every time which made it hard to learn.

7 degrees seems like it's way too much...how much rudder deflection do you use in knife edge? Set your low rate up just so you have enough rudder to hold knife-edge and see how much the elevator deflects from it's trim deflection. I used a nonlinear mix that had the elevator level off after 70% of total rudder deflection. The up trim won't matter...the tail is just along for the ride. You basically change its incidence every time you command a different elevator deflection.

Pulling up to slowly will typically reveal any asymmetry you have between wing panels. One will almost always let go (stall) before the other. The best way to get past this is with an abrupt elevator input so the airplane will pass quickly through that angle of attack regime where wing CLmax occurs.

If you continue to move the CG more forward the amount of up elevator mix should start to reduce. What prop/motor are you running? What's the weight of this airplane? What servos are you using on the elevator and what's the gearing (servo arm length to control horn height)? When you roll inverted and try to do a rudder-only flat turn what is the pitch coupling like compared to knife-edge (zero alpha) and upright (positive alpha)? What speed are you performing these passes at? 75% throttle?

George

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