Hello,I have a YT International ultimate biplane with a Laser 70. When i do knife edge the aircraft tries to roll out, to stop this i need a lot of aileron input which slows the aircraft down to much. I have been told there is too much dihedral,although both wings are dead straight.

I don't have a good answer for this question I was asked!

A airplane with a symmetrical fuse in the vertical plane and a mid mounted wing has no roll coupling. Any divergence from that configuration will cause roll in knife edge. It comes from the Clbeta (rolling moment due to side slip) aero term. The symmetrical airplane has a zero Clbeta.

The Ultimate biplane configuration, as good as it is otherwise, has a high wing that messed up the symmetry and it ends up having a value for Clbeta. Do a knife edge and it will roll. I remember seeing a design that had anhedral in the upper wing to help stop the effect. It looked pretty bad.

One trick to produce anhedral effect, which is what you want to correct roll in the direction of the rudder, is a downward onlt end plate. Make a couple of plates from lite ply or something similar that are flush at the top, but extend downward an inch or so. I would screw them on so they can be removed and trimmed.

Start with an inch or an inch and a half downward. The downward plate induces an anhedral effect, causing roll opposite to the rudder, countering the directional roll from the top wing. You'll have to "cut and try" to get the downward length correct. What you want at the start is a plate hanging down enough to cause opposite direction roll. Then you can trim a quarter inch and fly again. Once you get the size correct, you can paint or Monokote them to match and most people won't notice them.

I have used this trick on several planes to get good knife edge flight.

It's probably the sweep on the Ultimate that is doing you in. Sweep back, dihedral and a high wing location cause directional roll with rudder. Sweep forward, anhedral and low wing location cause roll opposite to the rudder. A designer has to "play" with these to get a neutral knife edge.

Ok guys. I have a plane you can figure out. I am at a lose so I need your help. I designed a profile, Everything is 0 and on center but, I have a full flying rudder. The plane has TONS of coupling in kife edge. Ail and ele. The only thing i can think of is the new rudder is messing it up. Any help would be great.

Thanks.

Heres another.

I think I am looking at two different airplanes, same design, different construction, however I think you answered your own question. The more symmetrical the airplane is top to bottom the less coupling there is. Assume the lateral area is the same top and bottom, the distribution of it is not even. The canopy is big and aft.

In the process of making the full flying rudder another thing happens. The top part is very effective in producing a couple about the airplane center line. There is nothing on the bottom to counter it.

Keep one thing in mind, for no coupling in knife edge you have to have everything symmetrical. If not then you need a bunch of effects that counter each other. The first is easier to obtain than the second (which is a wild guess in any science).

You would be better off with a total smaller area vertical tail, let some of it hang below the fuselage and just go with maybe a 50% or so movable rudder. Move the canopy futher forward and try to balance the upper and lower side areas.

It would look better too. (OK, just my opinion but in discussions with myself I find I value it highly, often to my detriment)

So it was the rudder! So my next qestion is, what effect does the counter balance on the top of the rudder have on knife edge? I would think it would make it roll againt the rudder input but what do i know.

You think you are an addict?? Welcome to the club. I have been doing it for 47 years, not all successfully, and have over 30 airplanes, about 2/3 of which are park/yard fliers, hanging in the garage and a handful in the workshop in the basement. When I moved into the present new house I have an open field across the street for 2 years. I took advantage of it. There are houses there now but the trees are all newly planted so the streets are wide and usuable for flying. I just make sure the motors are the GWS with rubber spinners just in case.

On the rudder. If you give left rudder with an airplane that has normal yaw roll coupling, similar to a shoulder wing airplane, it gives a left roll. The tip of the rudder gives a right aileron effect.

Your configuration is nearly neutral until you give rudder deflection. I would expect to not get any roll from the lower part of the rudder and right roll from the top of the rudder. Life is usually more complex than that so the above is just a educated (I hope) guess.

That is what i would think. Whe i put the plane in knife edge it will roll with rudder ( real bad too ) and also a ton of elevator is needed too.

Yes i am an accict, I wake up and think about planes and when i go to bed the last thing i think about is plane's. Man im sick! (grin) I am getting better though.