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Old Aug 06, 2004, 10:47 AM
Registered User
United States, VA, Warrenton
Joined Apr 2001
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Help With Knife Edge

I'm having trouble with knife edge on my 3DFoamy Katana (http://www.3dfoamy.com/katana_s.htm). The CG is set so that the plane flies hands off inverted and upright. It's powered with a Razor 400, 6:1 GWS gearbox, and 12x6 prop, about 2-2.5:1 thrust-to-weight ratio.

The problem is, it descends on it's side when I try to knife edge it. I can just barely get it to hold altitude with full rudder throw (~45 degrees) and full power. I've done quite a bit of searching on what makes a plane knife edge well and have this list:

1) CG set backward
2) Lots of fuselage side area
3) Lots of power
4) Big rudder

It seems to me that the Katana meets these requirements with the a catch on the fuselage side area - a lot of that area is behind the CG. I didn't find a consensus on this issue, but some have said that having more lifting area in front of the CG will make a plane knife edge better.

What are your thoughts? All are appreicated.

Thanks,
Dan
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Old Aug 06, 2004, 11:40 AM
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Ames, Iowa
Joined Oct 2002
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Quote:
Originally Posted by Ninjak2k
It seems to me that the Katana meets these requirements with the a catch on the fuselage side area - a lot of that area is behind the CG. I didn't find a consensus on this issue, but some have said that having more lifting area in front of the CG will make a plane knife edge better.
Well if you think about the CG as a pivot point. Imagine the plane hanging in knife edge position by a string attached to its CG. Now tilt the tail down so the plane looks like its in KE postion. Now its easy to see that all the air hitting in front of the CG will press the nose up (good) and all the air hitting behind the CG will press the tail up and nose down. (bad) But this does not mean you need to take away all the area behind the CG, you need just enough so that the rudder and area in front of the CG can hold the nose up enough.

I usually start with 2x as much area behind the CG as in front.
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Old Aug 06, 2004, 12:27 PM
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United States, TX, Rockwall
Joined Jun 2001
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I am certainlynot an expert, but I found that my Christian eagle from HL meets all your requirements as well. I found that the entry airspeed into the manuver was the key. Not enough airspeed and the rudder at full deflection would not hold. Too much airspeed and any amount of rudder would immediatly begin a knife loop.

I have a foam Yak that is similar to your Katana and it is the same thing. If it is decending in the knife adding power will help but it was because I did not carry enough airspeed into the manuver.

just a thought, but in fullscale aerobatics the entry airspeed is one of the most critical elements of a manuver.

Kirby
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Old Aug 06, 2004, 04:57 PM
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I tried adding more area to the top-front part of the plane to mirror the area on the bottom of my Katana with two pieces of fan fold foam on either side of the fuse. Ran it from the nose to the cockpit. Didn't seem to help, unfortunately.

If I give it enough power, it will pull up from its side to vertical, but it won't sustain knife edge without sinking. I was taking a gander at my rudder and wondering if it could go bigger.... May try that next.

~Dan
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Old Aug 06, 2004, 05:44 PM
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Ames, Iowa
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If it will pull to vertical then you have enough rudder and lateral are infront of the CG. Those two things just help keep the nose up. If you can get the nose to vertical then they are not your problem. Your problem is that you are simply not producing enough lift. There are two basic solutions to this:

A: like Kirby said, more speed. Start flying faster before you roll over to knife edge and while in KE keep speed up.

B: More fuse side area. Essentially, give yourself a bigger "wing" when flying KE. More area=lower wingloading=lower speed without sinking.

If your plane has better then 1:1 thrust to weight then techinically your plane can knife edge. You said you could pull it to vertical so just pull it very very close to vertical and feed in lots of power. This technically would be a knife edge harrier, but would look basically like a sliding hover.
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Old Aug 06, 2004, 06:36 PM
Scott Stoops
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United States, CO, Longmont
Joined Mar 2002
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Quote:
Originally Posted by Ninjak2k
What are your thoughts? All are appreicated.

Thanks,
Dan
Are you definately getting full rudder inflight ? (i.e. no control rod flexing?) From the looks of it, that model should knife-edge capably.

Scott
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Old Aug 06, 2004, 08:01 PM
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Pull-pull set-up fixed that problem with my Katana.

-Mo
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Old Aug 06, 2004, 08:42 PM
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I have a pull-pull setup on the rudder, but it seems soft. The strings are taut, but I can move the rudder by hand. Something is flexing and I'm not quite sure what.

~Dan

EDIT: I found the culprit. My control horn was loose in the rudder and was able to slide side-to-side. I've glued it in with a better glue and I'm really hopeful this will solve the problem for me.
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Old Aug 06, 2004, 08:59 PM
Team Broke
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Orlando/ Winter Garden, FL
Joined Feb 2003
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Quote:
Originally Posted by kepople

just a thought, but in fullscale aerobatics the entry airspeed is one of the most critical elements of a manuver.

Kirby
Speaking of full scale.....
Guys, check this guy out ! Must have Very large "marbles" !!!

No FAA guidelines at this airshow.

Jim
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Old Aug 07, 2004, 06:54 AM
veni,vidi, roto volubilis
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Aberdeen, Scotland
Joined Jul 2003
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isn't FAA a us organisation?, that vid is from china i beleive , and talking of large cahones

phil
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Old Aug 07, 2004, 07:18 AM
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Mostly in Norman, OK
Joined May 2002
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Quote:
Originally Posted by fishdeth
Speaking of full scale.....
Guys, check this guy out ! Must have Very large "marbles" !!!
fwiw more discussion here about Kayris (pilot in the clip):

http://www.rcgroups.com/forums/showt...ghlight=kayris

http://www.seqair.com/Hangar/Dovyden...e/CupHome.html
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Old Aug 09, 2004, 10:47 PM
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United States, VA, Warrenton
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Just wanted to report back here. The loose control horn was indeed the culprit! My Katana now knife edges! There is just a bit of coupling that I'm slowly getting out with some mixing. I think that's due mostly to my poor alignment when building.

~Dan
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