Elevator and rudder balance horns

alcladrv · #1 03-13-2009, 05:01 AM

I've been flying my -7A for 4 years. My tail surface balance horns were built and finished "squared off" pretty much how they came. The distance between the control surface "horns" and the fixed surfaces on the stab and fin are minimal, about 3/8"-1/2".

I've notice that higher performance planes, King Airs and Citation jets come to mind, have a more "rounded" aerodynamic shape to the front of their tail control surface "horns". The other planes also have a much greater clearance between the fixed and movable tail surfaces, sometimes as much as an inch or so.

In level, cruise flight, my rudder horn pretty much stays "in trail", but I find I need to rest my left foot on the left rudder pedal to keep the ball centered. My elevator horns are raised above the stab surface by 3/8"-1/2"

Questions: Are there worthwhile reduced drag advantages to be gained by rounding the leading edge of the control "horns" ? When does the clearance between the horn and fixed surfaces get to be too much? How have other builders finished them off?

Thanks.

carguy614 · #2 03-13-2009, 05:20 AM

I would think that the clearance at 3/8 to 1/2 inch is enough. As for elevator trail, what I did was to fly the airplane straight and level at cruise speed, and trim the elevator perfectly. Land without adjusting the trim tab, and take a look at the tab deflection. You can add or subtract shims under the HS to slightly change the angle of attack of the leading edge of the HS until you get the elevators to trim perfectly in trail. Then you can start sticking on a trim tab to the rudder trailing edge to get the ball centered in cruise flight. This takes a little effort and time, but on my 9A, I was surprised at the speed it added. Make sure you have your wheel pants and fairings installed and tuned up before doing any of these adjustments. It takes more than a few flights to get them tuned up, but a perfectly rigged plane is truly a joy to fly.

All the best,
Chris

alcladrv · #3 03-13-2009, 05:54 AM

Chris,

About how much shimming beyond that specified in the plans for the stab did you have to add?

carguy614 · #4 03-13-2009, 06:10 AM

If I remember correctly, I ended up with just a .032 extra shim underneath the forward spar of the HS. I found that just a little shimming makes a big difference. I think it best to get the HS tweeked up first, then address the rudder trim. Seems that almost all RV's need a rudder trim tab. Go easy a little at a time, and you'll get it.

Regards,
Chris