Last week i tried to set up a 68" Extra300 to fly pattern.
The problem was the following:
the plane flies quite as expected, but in knife edge it pushes towards the belly quite hard.
I tried to find an explanation for this but don't have a clue.
All angles are set to zero except there is 2 degrees of right thrust. It does the same thing in both directions.

If anyone can shed some light on this...

regards,

Zaquron

This looks like pitch coupling. Does the same thing happen if you apply full rudder from upright, wings-level flight?

I have a 4*40, and I guess most 4*40 owners have seen this. One way to get rid of the problem (or at least minimize it) is to mix some up elevator to the rudder channel.

Hope this helps.

yes, it does the same in level flight with full rudder applied.
uh..
erm..
Pardon my ignorance.. but what is a 4*40?

The solution was quite obvious to me, but i still want to know the reason.
Any idea?

It's the abbreviation for the Sig Four Star 40 model. Pitch coupling ( there is also roll coupling, where the model wants to roll with rudder application) is the product of the relationships between the thrust line, center of gravity, wing and stab location, rudder area center, and dihedral when rudder is applied. Some pattern designs like the Eyeball and Banshee tried to eliminate it by putting thrust line, wing and stab location, and rudder area all on the same horizontal line. Nothing to these model airplanes, is there?>>>JIM

I don't know it this will help at all, but from experience in the full size extra 300 the plane actually pitches up toward the canopy during knige egde.... It almost sounds like downthrust is causing your pitch down. If it's not downthrust then it is aerodynamic, probably has to do with the flow over the fuselage at high angles of side slip, or beta. For example the bottom of the cowling is much more curved then the top so it stands to reason that the pressure would be lower on the bottom of the cowling then on top causing a downward pitching moment... same goes for the aft fuselage... why I don't see this in the full size extra, who knows.

Ty

Try raising the ailerons a couple of turns on the linkages. This will require retrimming a bit more up elevator to fly level which is what you want in knife edge if it's going to the belly.

Thanks 4 the replies!

I have an idea of what can cause this (special thnx to Acropilot_ty )
the fuselage has a curved shape on the top and an almost flat bottom aft the wings. So when its sideslipping it can generate some lift pulling the rear part towards the canopy. Something similar may happen at the cowling in the opposite direction.
Please tell me if you think this sounds believable.

regards,
Zaquron

HI ZAQURON! I hate to ask the obvious, but are you trimmed for hands off in level flight? Does your plane stay hands off when inverted? The bias of the ailerons can cancel out this trouble [as was stated before]. The majority of the drag is on the cockpit side, so all things being equal, I would expect a tendency to pull that way? You are flying a proven design, so it must be more to do with your set up, than it is the design of the plane.

Most model aircraft in my experiance pitch to the belly in knife edge. Once you are happy with the other trimming (hands off, CofG, thrust line etc) then put in a mix to keep the plane straight.

michpittsman,

You sound like you really know what you are talking about.

I posted a question at (http://www.rcuniverse.com/forum/Four...1431127/tm.htm) that I bet you could answer.

I do have a request for you in response to a comment you made:

>Pitch coupling ( there is also roll coupling, where the model wants to roll with
>rudder application) is the product of the relationships between the thrust line,
>center of gravity, wing and stab location,
>rudder area center, and dihedral when rudder is applied.

Could you elaborate, please? I am interested in knowing how pitch coupling is affected by these variables. For instance, given one of the pattern planes you mentioned ( "Some pattern designs like the Eyeball and Banshee tried to eliminate it by putting thrust line, wing and stab location, and rudder area all on the same horizontal line." ) what would happen to pitch coupling if each of the variables you mentions were changed. I am interested in a chart, kind of, like this:

Variable Change Affect on pitch coupling when rudder is applied
CG Moved Foward Pitch to Belly (or cockpit)

I am mostly interested in what happens when you move the horizontal stabilizer up or down.

Thanks

Chad H

HI CKH22! A few years ago I built a plane that was absolutely symetrical in every respect, you could not tell right side up from upside down. This was the most even handling plane I've ever flown. It was also the ugliest! Anyway, with an identical amount of rudder area above and below the centerline, and with the elevator centered on the thrust line, none of the tail surfaces put any counter productive torque on the plane in K.E. This plane is almost as easy to fly in K.E. as it is R.S.U. When the model design strays too far from this idealized[but ugly] example, that's why some planes behave poorly in K.E. Ample rudder area above, and below the fuselage centerline probably has more to do with great K.E. behavior, than stab placement.

I tested the knife edge in the full size Extra 300 today (midwing)... turns out I was wrong... it does pitch foreward towards the wheels not backward in knife edge... I guess the elevator control forces are so light that I never noticed it before. It also rolls in the same direction as the top rudder... if your holding left rudder it wants to roll left and visa versa. A decathlon definately pitches back during knife edge so if you analyse the aerodynamics on it vs. the extra you may find something.

Ty