knife edge

[loker]

does anyone know what is necessary in design to allow good knife edge with very little coupling? fuselage shape, center of gravity, distance from leading edge to nose, etc?

[tillmas]

Many things is the correct answer. Aerodynamically, the following are important:

1. Little Rudder to roll coupling (largely affected by rudder design, fin size and shape, and wing area)
2. a lifting fuselage side section
3. Little tendency to pitch off the thrust axis (affected by thrust angle, wing incidence, dihedral and decalage)
4. Cg does affect the knife edge (primarily in that the nose will want to drift up or down)

If I had to say that one of these mattered the most, I would say it was Cg (presuming that the plane could knife edge). As a pilot, you can control the others with input (aileron, elevator and rudder), but you can't overcome the Cg making the plane want to drift in flight. Remember that pattern folks used to fly knife edges long before there was mixing in the radio, you had to watch the plane and know how to respond to keep it flying the way you wanted.

That being said, it might be hard to balance the plane just for knife edge, as it might change its ability to do other things (like snap).

[loker]

I guess I should be more specific. my plane stays level in regular and inverted flight, and during knife edge pulls toward the belly. I guess I want to find out what causes this pitch toward the belly, and how in my next design I can make the plane do that less with(this is the most important part)as little coupling as possible.
I can maybe change the rudder shape as you suggested, maybe that would be easiest.

PS is this the type of stuff you can only figure out with computer generated wind tunnels or something?


PPS thrust line is perfectly straight back, there is no roll coupling, CG is good for all types of flight, the only problem I have is the belly pitch during knife edge.

[tillmas]

Does it pitch to the belly only when you apply rudder? In other words, if you perform a quarter rolll and don't apply any rudder, does the nose drift towards the ground and towards the belly, or does the drift to the belly just happen when you apply rudder?

The absolute answer to your second question is that you can design many good features in an airplane by refining an earlier design. CFD or wind tunnel testing do not provide fool proof methods of analysis either.

If the only problem you have with aerobatic stability is a little negative pitch on knife edge, I suggest you be very happy and just use that right stick a little more! Sounds like the kind of problem I would love!

[loker]

does the nose drift towards the ground and towards the belly, or does the drift to the belly just happen when you apply rudder?


good question, I don't know. I'll find out in the morning. I think only when I apply rudder. the only reason I complain about that is the belly pull is so strong.

anyway I'll find out about pitch tendency with no rudder in the morning. thanx.

[tillmas]

I was trying to find my old trim chart. I found the same thing on the NSRCA website (http://www.nsrca.org/trimA.htm). In your case, they suggest changing the wing incidence (makes sense) or shifting the Cg, or simply mixing in some elevator.

If the pitching is very strong, then likely the wing incidence is to blame. I wonder if you have some negative decalage in the plane?

[loker]

I probably should have mentioned this at the start, but this plane is a flat foam plane.

also, I checked the pitch tendency with no rudder, it isn't there, only when I apply rudder.

I know other foam planes have almost no pitch coupling, I just don't know what it is about their shape that makes it so.

[tillmas]

Check the plane for decalage and wing to thrust incidence. If it isn't one of those, then I suggest that you use a rudder that has a tapered trailing edge so that the rudder gets smaller as you get further from the h-stab.