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Secret to a clean knife edge

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Secret to a clean knife edge

Old 11-21-2007, 07:50 PM
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Default Secret to a clean knife edge

I can't seem to get my Ultimate to do a nice clean knife edge. It's the Seagull version, 54" wingspan. I can hover and harrier just fine. As a matter of fact, I can walk it around the yard on the tail wheel. I have a Ftuaba 3050 pull pull setup on the rudder. I can do a flat spin with no problem, but when it comes to a knife edge, the tail wants to drop no matter how much rudder I give it. I'm running an ASP 1.08 with a 16X4W prop. I even changed to a 15X4W and had the same results. I even thought maybe I didn't have enough speed, so I tried a 15X6 and 16X5 with no better results. I know it can be done, I've seen videos of Quique doing it.
Any suggestions?
Old 11-21-2007, 08:55 PM
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Default RE: Secret to a clean knife edge

Oops ... let me clarify. The whole plane drops. As I give it more rudder to keep it from falling, I find myself in a HAKE before the plane stops falling. That's better. It's almost like I'm not getting enough speed, but a 15X6 @11,600 RPMs ought to produce enough speed for a 10 pound airplane.
Old 11-21-2007, 10:10 PM
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Default RE: Secret to a clean knife edge

That sound strange to me. I have one with a Saito 1.25. It does a great knife edge anything above half throttle. I am running a 15x8 prop but you should still have more speed than I am getting. Two things come to mind. Your plane could be heavier than mine. You could also have your CG more to the nose.

David
Old 11-21-2007, 10:59 PM
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Default RE: Secret to a clean knife edge

9 pounds 12 ounces and the CG is right under the rear bolt of the top wing. (right at 5 7/8"). Just enough to have to use just a smidge of down elevator to keep it level. The only other bipe I had that could do it was a Pitts S. My Tecate wouldn't and neither would my SPAD bipe. it's gotta be something I'm doing wrong. It will do a KE loop in a heartbeat, and I found out this afternoon that it will even fly sideways. I'll try a 14X8 in the morning, but I can hear the pitch of the engine with the 15X6 and it doesn't bog down when I roll into a HAKE. The 15X6 even at 11,200RPM should still be close to 60 MpH.
Let me make sure my dihedral struts are straight. I just thought about that. If they're not straight, it could have that effect.
Old 11-22-2007, 12:41 AM
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Default RE: Secret to a clean knife edge

Maybe the rudder is experiencing blowback. What are you running for a rudder servo?
Old 11-22-2007, 07:20 PM
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Default RE: Secret to a clean knife edge


ORIGINAL: mrbigg

Maybe the rudder is experiencing blowback. What are you running for a rudder servo?
Futaba 3050
It will do a Knife Edge loop and HAKE but on a straight knife edge, the plane falls untill I get the tail low enough. Then if goes in to a High Alpha Knife Edge.
Old 11-23-2007, 10:35 AM
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Default RE: Secret to a clean knife edge

You got me on this one. I wish I could see it fly. I have flown 15-20 Ults over the years. I have never seen one do that. Normally a Ult will knife edge all day long.

David
Old 11-23-2007, 11:25 AM
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Default RE: Secret to a clean knife edge

You need speed when doing a Knife-edge and a 4 inch pitch prop might not generate that. My Venus with a 13x7 prop does nice KEs. My UCANDO.46 with 14x4 will do KE with the wind, but pushing into the wind it will drop.
Old 11-23-2007, 03:45 PM
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Default RE: Secret to a clean knife edge


ORIGINAL: GarySS

You need speed when doing a Knife-edge and a 4 inch pitch prop might not generate that. My Venus with a 13x7 prop does nice KEs. My UCANDO.46 with 14x4 will do KE with the wind, but pushing into the wind it will drop.
15x6 @11,600 produces more than enough speed.
I found the problem and have rectified it. Both wing struts were seemed to be towed out a couple degrees. I had to elongate the holes in the "L" brackets that hold the aft side of them in place. I finally got them square with the fuse. I didn't realize they were that far out. I couldn't really tell by looking, I was just grabbing straws. I must have done something right. After I retrimmed everything, it does a KE about as straight as my ultrasport. I no longer have to fight harriers and flat spins either. I didn't think it would make that much of a difference. The rudder seems FAR more effective than before, it's almost like adding another 4 inches to it!
What can I say ... it's only mt second bipe ARF. I'll just add it to my "ARF assembly checklist"
Thanks for all the tips and comments. I guess I was correct thinking it must be something I was doing wrong. I never even thought to check that when I was putting it together.
Old 11-23-2007, 10:39 PM
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Default RE: Secret to a clean knife edge

I'm glad you found it. I guess I should check mine. They must be close but maybe there is more there. I did have trouble on mine getting the top center rib to line up straight. I guess I should have checked the struts to. I have been thinking about putting an airfoil on them like the Goldberg uses. This would be a good time to do both.


David

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