How Do You Do A Knife Edge

On Mon, 15 Oct 2007 09:09:12 -0700, Ted wrote in :

You have to "take the roll out" by returning the aileron to neutral.

Leave the rudder alone for a little while. Just practice

1/4 rolls: roll to knife edge, pause, roll inverted, pause, roll to the other knife edge, pause, roll upright.

Working on getting the roll stopped at the right points. The pause doesn't have to be long--just long enough for you to see what it takes to stop the roll.

If you roll from upright to knife edge using right aileron, left rudder will be on top.

If you roll from upright knife edge using left aileron, right rudder will be on top.

OH! It's possible that you have some "coupling" with the rudder, so that when you input it, it causes the plane to roll as well as to yaw.

I'm not very good at mixing out rudder coupling myself. It can involve a small percentage of aileron and/or elevator mixed with the rudder as the master channel. I've heard and read a lot about it, but haven't had a lot of success taming my bad knife-edge planes (especially a Hangar 9 Ultra Stick). I tend to just fish around with the sticks until I reach some equilibrium. It's pretty ugly. :o(

At any rate, you can test the response to rudder from upright. Put in right or left rudder and see how the plane reacts to just that input. It may roll as well as yaw and it may dive or climb, too. You may need to go past neutral on the aileron stick to hold the plane in knife edge.

You can also roll the plane to knife edge, wait a half-second to make sure that it has stopped rolling, then input the correct top rudder. If it starts rolling at that point, then you know you have coupling issues.

Marty

:

The rudder to roll coupling is pretty much the same in straight and level and knife-edge (nit-pickers will note that all the interactions are not linear, and the above is approximate ... hence the "pretty much")

So if you mix it out in level flight, you're 95% of the way there to mixing it out of knife edge. Mixing it out in straight and level is much easier to do pilot workload wise though :)

So, fly your plane straight and level, at a medium or medium/fast speed. Take a couple runs across the field without touching the controls to make sure the trims are perfect, since the accuracy of the rest depends on a good baseline.

Then, just input pure rudder stick (full or medium deflection), don't pull or push the elevator, and especially don't touch the ailerons.

The plane will make a skidding turn. It will also likely bank into or out of the turn (most likely into it)

The bank is what you are trying to eliminate. You do that by using a Rudder to aileron mix. It will likely be a small negative number, but it may vary. On the ground check that moving your rudder makes your ailerons move SLIGHTLY to oppose the rolling motion that you were getting.

Fly it again and repeat the rudder stick activation. Hopefully it will bank less. adjust the amount of the mix until moving the rudder stick gives you a pure skidding turn, with no change in bank angle.

It is wise to put this mix on a switch, so that you can enable it during aerobatics, but turn it off for landing.

The other Marty

and all this time I thought that was why God gave us thumbs.

:

Thanks Marty, I do have rudder coupling built in and that is causing the plane to dive if I try the aileron roll. I will inhibit the mix and try your apporach. Thanks agin.

Ted