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UCD 60 knife edge advice please!

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UCD 60 knife edge advice please!

Old 04-19-2005, 08:29 AM
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RCUOrbiter
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Default UCD 60 knife edge advice please!

I know many very knowledgeable U-Can-Do 60 folk hang around here so I'm asking for a little advice. I've had a UCD 60 with a 91FS for a year or so and generally enjoyed the experience. So when I piled it in after a plug failure while hovering I bought another. It's built identically with the same servos & linkages, engine etc from the other plane. From what I can remember the CoG is in roughly the same place, just shy of 6inches from the LE. The big difference is that knife edge flight has a massive tug to the u/c. Previously 10% up elevator mix, and 5% opposite aileron was sufficient to fly KE straight as a die all day long.
This one I'm up to about 35% elevator mix but as I continue to increase rudder it's not enough with an exponentially increasing fierce tuck to the u/c. KE loops are impractical because tuck under becomes so fierce it's uncontrollable.

The behaviour is so different from the previous one that I'm looking for ideas from experienced owners as to the possible cause. Things like adjusting CoG, wing camber, incidence and so on. I'd just like to get it to the state I had with the old one. Anybody else had this problem and cured it?

PS. I have added right thrust to get to nearly 4degrees, just like the last one, and it seems about right. I haven't adjusted the down thrust from as supplied but didn't on the original one either.
--
S
Old 04-19-2005, 09:48 PM
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gjeffers
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Default RE: UCD 60 knife edge advice please!

my 2 ucd60s sucked at knife,same as yours,good luck
Old 04-20-2005, 09:36 AM
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seanychen
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Default RE: UCD 60 knife edge advice please!

KE tendency is much less when the CG is kept forward of 5.25" mark. I don't use any mixing, as the coupling is not linear with rudder input. I just use stick compensation. I have my UCD w/ YS 110. CG is at around 5.25" mark. I make up the "relatively" forward CG by maximizing elevator & rudder throws. I mounted the rudder servo inverted under the fuse just in front of the tail ramp, controlling rudder with pull-pull. This allows the elevators to go much lower without hitting rudder pushrod.
Old 04-20-2005, 11:50 AM
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Default RE: UCD 60 knife edge advice please!

When a model pulls to the belly or canopy in knife-edge flight, you can usually eliminate a portion of this by adjusting both ailerons up or down. Thrustline changes can help as well. It will be impossible to get almost any model trimmed so that you only have to roll to KE and apply rudder. You'll still have to add some aileron and elevator to hold a straight line. You can only adjust the model's geometry to get rid of the worst tendencies.

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Old 04-20-2005, 06:42 PM
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Default RE: UCD 60 knife edge advice please!

Bax has it right. I would suggest trying to mix in some down flaperonss & up elev for rudder deflection. Or just droop the ailerons down a little using the subtrims and try it out.

I've had my UCD for a long time now and I've fixed the coupling issue, but I don't know if you want to go this far.

After my last crash from a hover/deadstick, I lowered the stab by almost 2 inches, now there is just a little coupling that is easily corrected.

Good thing that each time I rebuild/repair, the plane gets lighter!! There's a lot of structure in the UCD that is not needed IMO.

Good luck.
Old 04-21-2005, 10:54 AM
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AcroJo
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Default RE: UCD 60 knife edge advice please!

Out of four UCD .60s I've found that on each one the wing incidence was off in relation to the previous one. All were on the positive side which I don't believe would contribute to diving to the wheels. Had more of a tendency to balloon up. I did a drastic mod for roll coupling and removed about 30 sq. in. off the rudder, top and back. Roll coupling solved as the rudder had too much mass above the longitudal center line. As Airbike-RCU mentioned, I too feel that the tendency to dive to the wheels is in the horizontal stab being placed too high. On the UCD .46 it is lower and does not suffer from this. My CG is @ 5-5/8 and require 35% on elv. to stop the dive. Joe
Old 04-21-2005, 03:35 PM
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RCUOrbiter
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Default RE: UCD 60 knife edge advice please!

Thanks for all the comments guys, I value your experience.

I take the point from those that say their UCD 60 was always difficult in KE, but that wasn't the case with my last one. Sure I know the elevator compensation is non-linear, but previously with 10% up elevator and 5% opposite aileron it was tamed enough for easy further control by hand (thumbs!). I could have emptied a tank flying KE circuits if I wished. This one is very, very fierce in the pull to the u/c and also has a pronounced roll tendency. The combination is such, and is so sensitive to the controls around that point that I can't maintain it for long. My point is that something is different about this one and I'm just digging around to get some ideas.

Since the last one is destroyed I can't check exactly where I previously ended up with the CoG or stabiliser incidence and I'm just going on memory. I do remember I needed some down elevator at one point during landing.

Although I do add some throttle during KE, I can't see any obvious down thrust which might contribute to the problem. Basically this model is built using the same parts from the old model, horns in the same place, same length linkages, same model and mixes selected in the Tx, balance the same as far as I can judge etc. I guess my memory on this last could be suspect. Usually I just adjust the CoG on any model to get a comfortable balance between agility and stability.

I think I'll start by nudging the CoG forward a little, and then experiment with aileron droop - both those are easy to try. It would be easy enough to drop the hor stab, but I'd be happy just with the manageable coupling I had before.

Thanks again
--
S
Old 04-21-2005, 11:08 PM
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seanychen
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Default RE: UCD 60 knife edge advice please!

Have you noticed that most low wing, high thrust-line aerobatic planes will tuck to the undercarriage during knife edge? Cap, Extra, UCD, Funtana,... I can't think of one that doesn't. The only exception that I've experienced is the Katana 40 profile, which tucks to the canopy during knife edge, but it's not really a low wing & high thrust-line.

When wing is low and thrust line is high, there is a natural tendency to pitch downward because the pull is above the resistance.

The Yak, on the other hand, has the least amount of KE coupling. It's mid thrust-line and almost mid-wing.
Old 04-22-2005, 09:43 AM
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Default RE: UCD 60 knife edge advice please!

I had 12% up elevator mixed with rudder.

Hated the plane. Only thing it did good was hover, and I had already mastered that long before I even got the U-Can't-Do 60.

I had my balance pretty far back. Saito 91- and then switched to a TT120. I had high torque digital coreless servos on it. So, I know the problems weren't just sloppy control surfaces from weak servos.

I tried engine thrust correction, wind incidence corrections, and balance corrections. Spent more time F***ing with it than actually flying it.

Ended up throwing it in the garbage can. POS I can't believe they are so popular. If you got one and like it-- then I'm glad for ya. Enjoy the hobby however you like. I should have smashed mine and threw it in the can months before I actually did. Nothing but a frustrating POS.

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