Split Rudder?

[Soar_dude]

What are the benefits of converting a standard rudder to a split rudder? And what is the ratio of split provides the best results?
How come the glass slipper open class ships do not use it?

Soar Dude:)

[Quacker]

By "split rudder" I assume that you are refering to a v-tail. There are lots of opinions on this. This is my take:

V-tails have lower "wetted area"--are lighter-- stay clear of the ground on landing-- are less stuff to build.

That doesn't stop many find designs form having a conventional (cruciform) tail.

Q

[Ron Cichowski]

I'm assuming you are referring to a rudder like the Skeeter HLG where the shape is much like an inverted "L" with the leg pointing foreward over the vertical stabilizer.

On the "Glass Slipper" ships they can get enough rudder without adding area this way. They also have enough servo power that they don't need the aerodynamic balance that this configuration would provide.

Anyway that's my take on the whole thing. The balanced rudder does look attractive, but does add to the complexity of construction and probably does not provide any advantage in a competitive situation. So it would not be used on a strictly competitive aircraft.

Ron

[Quacker]

Well...today is "dumb day" for me. You're right Ron. I should read more carefully. Meanwhile, having flying surface ahead of the hinge helps or stops flutter altogether at high speeds. Especially so if the forward section is weighted to place the rudder center of mass ahead of the hinge. A strong servo helps to eliminate the need for this, but even a strong servo followed by any slop or flex in the linkage makes it worse.

Q

[John Gallagher]

I've heard that the split rudder is less effecient aerodynamically. Think of the fin-rudder as an airfoil on its side. A split rudder results in two different airfoils fighting each other. One at much higher angle of attack than the other.

[Soar_dude]

Yes I am reffering to the top portion of the rudder going over the vertical stabilizer. I am under the impression that the more rudder you have the better. I have a MM glidertech Grand Illusion that I have used the split rudder option and to me it seems to impart some roll when turning compared to my friends Spirit ARF.
I have a spirit 100" that I took the top 3" of the vertical stab and grafted it to the rudder. Is there any possiabilty that I could blow the rudder off during High speeds?

Soar Dude :)

[AndersHoberg]

I fly powered Giant Scale, and I always use what in that discipline is called "counterbalance" on rudders and elevators. Not necessarily trying to achieve a static balance, but more aerodynamic support. The trade-offs are: 1) more sensitive control surface, 2) more induced drag (controversial point) 3) risk of "stick-snatch" which is a full scale syndrome, if the counterbalance or spade is too large. I use around 1/3 - 1/4 of the total rudder surface as the counter balance.

The benefits are obviously lesser servo load, meaning a smaller servo can be used for the same size plane. In theory I get the same response/force on an aerodynamically balanced elevator with two JR 8411s (180 inch ounces of torque each) per elevator half (four in total) compared to a non balanced with two MPX Jumbo speed at 241 inch ounces each. My weight savings are 4 times 5+ ounces less 4 times 2 ounces or around 12+ ounces.

Some of the theory applies to your application.

[John Gallagher]

On full size aircraft and large scale RC the split rudder can reduce the control forces, but for a three meter or less span glider, it adds drag without much benefit. It's an old idea that was abandoned by most designers long ago.

[Soar_dude]

Why do say that a split rudder adds drag John? John where did you about differ angles of attack on a split rudder? I know you fly Giant scale with lotsa speed Anders but have you ever seen a Spilt rudder get blown off during high speed flying? I don't mean to sound trite but this is the second glider I have kit-bashed into a split rudder, and I do not want to lose it"Spirit 100 that is". the Gentle Lady I did seems ALOT more nimble then my standard tailed Easy Answer before I augred the Answer in.

Soar Dude

[AndersHoberg]

I think you will be safe adding a counter balance, as long as you ensure proper control surface hinging/integrity. Extending the theory to it's fullest, we are using flying stabs, which is another way to look at a counterbalance - in that case full counterbalance. Think of the elevator integrity when you design the hinge mechanism for the rudder. The rudder could be made flying too with the same phiysical proportions in mind. Then reap the benefits by using a micro or mini servo. Since you are adding control surface area, it will be more nimble.

[John Gallagher]

The fin/rudder is an airfoil. Looking down from the top, when you deflect the split rudder you have two different airfoils. One consists of the split top portion fin with the rudder and the other the not split bottom portion of the fin and rudder. The split portion winds up doing all the work because it's at a much higher angle of attack to the airstream. There's good reason why the Spirit and all the latest designs don't have the split rudder.
I've built several Skeeters and found that getting rid of the split improved the gliders ability to climb in thermals. It also makes it much easier to secure the rudder to the fin.