What effect does the angle of the fin/rudder hingeline (w.r.t. the aircrafts datum line) have on knife edge flight in particular and anything else in general ? Does it change pitch coupling or roll coupling ? I remember reading a reference to it in Dave Patrick's book " Aerobatics" but he did not explain what exactly the effect would be as far as I can remember. I am in the process of scratch building a Cap 232 - the design has terrible pitch to the belly in KE if built to scale dimensions. A forward canted fin/rudder hingeline is what I have noticed on a couple of designs. I am aware that moving the tail plane position lower on the fuse will help in KE and I have found a way to do that without spoiling the looks. But will this help too - any adverse effects ?

Angling a hinge line will do a couple of things. If no change is made other than angling the hinge line, the effect of the surface will be lessened. Also, there will be a new direction of the force being created by the surface's deflection.

So you will need to increase the size of the surface if you wish to keep it's original effectiveness. And........

In the case of the rudder, you will need to angle the hinge line to give you a couple in the direction you wish. Since you're trying to counter a rudder couple that causes a nose down pitch, you'll need to have the hinge line angled to cause a nose up pitch. Rake the rudder hinge line to do that. Build the rudder hinge line to be farther aft at the top than it is at the bottom. When that rudder is deflected, it will generate a tail down force.

I would bet that none of the above is any revelation to you. I'm guessing that you were really hoping someone would pop in with a formula that would give you some numbers or an angle to use.

Ain't no free lunch. Truth is, the way most model designers work is from model to model. We build what we think will work and fly it to see. If it works, but works too well, we build another version with less change. If it works, but not enough, we build another version with more.

I would bet that Dick Hanson could give you advice whether or not raking the rudder hinge line is worth the effort. And I'd hazard a guess that he could give you a starting angle, if any were worthwhile.

BTW, the TAURUS model of years past was supposed to have had it's rudder hinge line raked to balance out any rudder couple. At least that's what I heard from a bunch of TAURUS bigots. They usually waxed eloquent and LOUD within earshot of the judges most often.

Funny thing is that some of these aero experts actually explained that the rake was to counter nose up pitch. Wonder if the judges they were trying to impress actually understood.....

The Cap design I am talking of has the hinge line raked forward .i.e. if we were to measure the distance from a line perpendicular to the datum line - the distance at the top of the rudder is shorter than the distance at the bottom which is opposite to the example you have given ( if I have understood it correctly). This can be seen on the Midwest Cap and World Models Cap. I am currently flying the WM version - it takes a 13% mix but interestingly enough, once put in, it does not show up at different flying speeds/defelctions in 3D or otherwise and I think the angle is no more than two to three degrees. Any thoughts ?