Knife Edge coupling
 

Why do I need to feed in opposite aileron and up elevator to maintain knife edge flight. I have heard it can be taken care of using the radio, but I would like to try other means to fix it first. Basically I am having a hard time understanding why one plane differs from the other. The plane in question acts like it has negative main wing incidence, because it will try to roll out and tuck it's nose away from me. Please educate me here. What would you do?

I have thought of where to post this for some time until recently. It was after I aided as a caller for a local flyer who was practicing IMAC or tuning a new plane for IMAC. He was adjusting wing incidence which made me think if anyone knew how to fix such a thing it would be you guys in this forum.


Tim

RE: Knife Edge coupling
 

Hey Tim,

There are many variables envolved in answering your question. I'll see if I can list a few, and I'm sure others will list more:

Roll coupling:

- Wing dihedral is a biggy. The slip caused by the rudder brings the leading wing into a higher angle of attack, and the trailing wing into a lesser.......or negative AOA. The forward wing then produces more lift than the trailing and you get roll coupling in the direction of the rudder input.
- Wing placement in relation to the fuselage. High wing acts like dihedral, and low wing like anhedral.
- Rudder shape and location. If the rudder is not aerodynamically centered on the fuselage (and almost none are) its deflection will add some degree of roll.
- Horizontal stabilizer anhedral or dihedral. Same as the wing, but of course to a lesser extent.
- Fuselage shape. Since the top and bottom have different shapes, lift in a slip will not be the same around the plane's lateral CG, imposing a small roll tendency.

Pitch coupling:

- CG. A forward CG makes you add up trim, which is still there when you roll into a knife edge. Moving the CG back makes you trim down, increasing the tendency to pitch down in knife edge.
- Fuselage shape. The top may produce more lift in a slip than the bottom, pushing the nose to the wheels, or vise versa.
- Rudder shape and location. Again.
-Horizontal stabilizer position. It will be affected differently by the wing wash in level, inverted, or knife edge flight. In addition, deflection of the rudder may affect the lift to a larger degree on one side of the horizontal stab than the other.
- The affect of the spiral prop wash. May or may not be noticeable. Change the prop, and the affect may change.

Your probably getting the picture why each plane has different coupling characteristics. The easiest way to fix it is with a radio mix........as long as it's not too severe. Trying to mix out roll coupling in a high wing cessna model is an example of too severe. Aside from that, if you want the structural fixes, then:

Reduce dihedral to eliminate roll-out coupling, or lower the wing location.

Move the CG forward to eliminate pitch down coupling, or lower the location of the horizontal stab on the fuselage.

Good luck..........CJ

RE: Knife Edge coupling
 

CJ's correct... lots and lots of causes of coupling.

One thing to mention, though, is that roll coupling is typically linear with sideslip angle, where pitch coupling is typically nonlinear. This means that roll coupling is much more consistent to mix out with a linear mix on your transmitter. I flew Warren Thomas's Taylorcraft (high wing, similar to a J-3 Cub) and he had the roll coupling completely mixed out with a standard linear mix. The ailerons traveled about 30 degrees when you deflected the rudder, but it worked! Slow rolls, point rolls, rolling turns, flat turns were all a breeze. Now, you can also mix out a nonlinear couple with a multi-point mix, but this requires a lot of setup time to perfect, and is often not worth it.

RE: Knife Edge coupling
 

Alright I guess I'll try to mix it out and since I have never done it before what mix do you suggest.

Tim

RE: Knife Edge coupling
 

Just start with a linear mix (i.e. PGM. MIX or P-MIX) mixing rudder to elevator and rudder to aileron. depending on how severe the coupling is, start at maybe 5% mix, or maybe even 10%. Have a spotter with you to write down what changes it needs. Remember that the right rudder knife edge and left rudder knife edge may need different mix values, so check each direction. From there, it's just a matter of iterative tests and appropriate changes on the ground (or in the air if you feel like programming your radio in flight). Try to keep all tests at same speed (usually cruise speed) and at level altitude. I find it easier to mix roll out first, then deal with elevator, because when the plane is rolling out of knife edge it is difficult to see subtle pitching.

Now after all that, keep in mind that the coupling may be different on different lines other than knife edge (horizontal wings level, for one). Sometimes knife edge isn't the best neutral mix point, but it's an excellent starting point.

Hope this helps,
Baron

RE: Knife Edge coupling
 

Once set- do you leave the mixes on the entire flight?
Thanks
Eric

RE: Knife Edge coupling
 

I will start with a the roll coupling first as what you said makes sense to me, it would be much easier to stop the roll first. Do you recommend putting the mix on a switch, so that it is only used for knife edge, which is what I am working towards. Or just leave the mix on all the time so that I get opposite aileron everytime the rudder is input.

Tim

RE: Knife Edge coupling
 

I leave the mix on from take-off through landing.
I leave the mix on when the plane is parked in the pits too.......:D

RE: Knife Edge coupling
 

Thanks,
I'll give it a try this week.

I will however turn the mix off before heading home.

Tim

RE: Knife Edge coupling
 

There is no reason to ever turn rudder to aileron/elevator mixing off. Besides, who needs one more switch to remember/screw up? Leave it on and forget about it once it's set up. Unless you change the CG or prop, the mix will not change.

CJ

RE: Knife Edge coupling
 

Thanks all for the info.
Eric

RE: Knife Edge coupling
 

Leave the mix on all the time. The coupling arises not from the 'knife-edge' flight situation, but from ANY sideslip. Making a correction with rudder on a vertical upline will also induce a sideslip, therefore causing coupling.

RE: Knife Edge coupling
 

So you are saying it will make it a better plane all around once set properly? Or once set-up I will be able to slip into a landing without opposite aileron to hold the wings level.

Tim

RE: Knife Edge coupling
 

And maintain knife edge without the plane rolling out.

Tim

RE: Knife Edge coupling
 

Well, a sideslip to landing is such an extreme case of sideslip that any non-linearity in the coupling will show up. Typically, on aerobatic planes, the mix is used primarily to help maintain a given line (i.e. vertical, 45, horizontal) when making subtle corrections with the rudder (which induces sideslips).

If you set the mixing to hold solid knife edge at a given airspeed, then it will hold that knife edge at that airspeed. No guarantees on other lines or airspeeds... that will vary from plane to plane. Some planes like one particular mix value for all times, others require more or less correction in a different scenario. For instance, on a horizontal line with wings level, the roll couple could very well be different than it is in knife edge, because you've now introduced lift on the wings and placed each wing at a different sweep angle relative to the airflow.

The set-up is really the most critical step in making an excellent competition aircraft (aside from making sure it's built straight). The best plane on the market with a poor setup will fly poorly, where a lesser airframe with a good setup can be made to fly very well. Mixing out the coupling is, of course, just one of many steps in the complete setup process, but it is a critical one. Things vary so much with different planes, I've set up planes with two different rudder to aileron mix values (on a switch) for different particular maneuvers, and other planes seem to behave perfectly with a single mix value. Either way, once they're set up, they both fly good.

One last consideration...
How long are you at knife edge in any IMAC sequence? How much time is spent correcting horizontal, vertical, and 45 lines with rudder? With that in mind, knife edge is an excellent starting point for setting rudder to elevator/aileron mixing, but it may not be the final desired configuration.

Baron

RE: Knife Edge coupling
 

Hello
I went through that same problem and camer up with a perfect copuling rudder-elevator and used to use it for knife adge and 4 pint roll for instance
I quit using it when accidentally left the mix on and almost ruin the plane while giving rudder on a landing sequence

I recommend to do it manually (elevator input) it will give you higher skill level. and the possibility to use it in differente maneuvers not only knife edge.
regards
Omar

RE: Knife Edge coupling
 

I flew it last night and came up with about 12% roll coupling for a good solid knife edge. It still requires manual elevator input, but not as much since moving the CG forward a bit. I will still plan to move it farther forward because the tail still drops with power off.

Thanks for the input guys.

Tim

RE: Knife Edge coupling
 

If you adjust the balance and still need elevator input use anothe mix with elevator rudder. And fly on one stick. Most of the time it's only about 3 to 5%. Leave the mix on whhen your driving home and the trailer pull straighter.
Denis

RE: Knife Edge coupling
 

Simply amazing how fast things seem to change; back in the 70/80's pattern guru Don Lowe wrote a feature in one of his "Flying LOWE" columns for R.C.M. and discussed 'adverse' and pro-verse roll coupling and the simple yet scary fix of cutting partially through the (one piece wing's) center section and increasing or decreasing dihedral dependent on adverse or pro-verse yaw,expert advice for a time before computer mixing became available;what I question is "how much" do we fix in regard to airframe alignment opposed to a 'mix' for every trim problem.. then again in this case it doesn't translate as easily for a typical two piece wing, using a wing tube as is the norm now. AHHHHHHthe good ole days or were they?