Rudder Trim

My prejudice is that fins and rudders are mostly an affectation in CL. That is,the plane is going to fly in a confined circle, and that other forces (leadout position, engine offset, CG)overpower the minor aerodynamic effect of the rudder and fin. I suspect that a Nobler built with no fin and rudder would fly about the same as a stock one.

Now that I have seen how minor changes in trim (wingtip weight, leadout position, flap / elevator ratio, etc) affect performance, is an "adjustable" rudder a trimming option? I hope your consesus is that it is not!

But you have to admit that it would look pretty slick to have an adjustable rudder with hinges on a CL plane, even if it made no difference in performance.

Larry F

My first stunt airplane was a Magician with stock leadouts. The front leadout was the down line and it would yaw in at outside square corners. After I removed the rudder several times hitting the ground on outside squares, I made a working rudder linked to the elevator so that it went out maybe 3/16 at full down. That cured the problem.

On an airplane with a high rudder, like the Twister, rudder offset tends to roll the airplane in, which is not good.

To me, the fin/rudder is an absolutely *vital* feature of C/L airplanes. Bear in mind that everything I'm saying refers to C/L stunt airplanes that you want to trim to fly cleanly. Having a large fin/rudder makes the airplane aerodynamically stable, and having the rudder adjustable makes it possible to trim the yaw angle.

I suggest never, ever, trying to adjust the static yaw angle with leadout position! Set the static yaw angle with the rudder to trim the roll response from level flight to maneuvering, and then set the leadouts to minimize any yaw acceleration. The ideal situation would be to set the leadouts so they never do anything to the airplane, and the wires just float in the middle of the holes. Of course that never happens, but its a good thing to try to aim for. If everything else is right, the rudder will wind up straight ahead, but if not (like you have too much or too little assymetry, too much or not enough flap differential, on and on), at least you can do something about it.

An example:
We had the original Medic (built by Bob Hazle, and presently owned by Paulette Erickson) and were trying to trim it. It was more-or-less terrible. It flew around level with a large inboard roll angle (outboard wing about 1-1.5" higher) both upright and inverted. Needs more tip weight, right? Well, good idea, except that in the maneuvers it hinged outboard very severely, while at the same time yawing outboard. The level flight yaw angle looked pretty decent, maybe about 0-1/8 inch of the outboard wheel could be seen behind the inboard wheel. The leadouts were very far forward, anomalously so, in an attempt to kill the outboard yaw in the maneuvers.

What is the problem? We had been moving the leadouts forward to minimize the yaw reaction, which gives a clue. I took a peice of cardboard from the back of a glow plug package, bent it in half, and taped it to the fixed fin/rudder with about 1/4" of offset. So, in the middle of this large fin/rudder was this teeny little rudder tab with a little bit of offset. Simultaneously, I took out about 1/2 oz of tip weight to try to kill the hinging. Flew it again, and suddenly the outboard wing was now hanging down all the time, the hinging was greatly reduced, and the model was now yawing inboard in the maneuvers. The static yaw angle was now displaying about 1/4-3/8" of the outboard wheel.

Why did this happen? Because altering the yaw angle made the inboard wing "lead" the outboard wing a little more, adding lift to the inboard in level flight, and causing the airplane to want to roll outboard. The opposite condition (outboard leading inboard) was leading Bob to pile on the tip weight, which only weakly effects the level flight, but drastically effects the manevuers. The torque from the CG offset scales with acceleration, but the differential lift stayed the same, do it hinged severly while giving indications in level flight of needing more tipweight.

After removing most of the removable tip weight, and having to add about 1/2 oz on the inboard wing, and moving the leadouts back to kill the inboard reaction yaw, the airplane now flies very cleanly through all the maneuvers, has the wing level in level flight and in the maneuvers, and next to no roll/yaw reaction in the maneuvers. In other words, hopeless dog to something that could win Advanced at the NATS. This in 3 flights, and courtesy of a tiny rudder adjustment.

This is just an illustration, but it is generally true. Ironically, you are incorrect that if you took the fin/rudder off a Nobler it wouldn't fly any differently. It would probably fly much better! Not because having a fin is a bad idea, but having a fin with a tremendous outboard offset (and airfoiled to boot) is absolutely fatal to the trim of the rest of the airplane. You wind up with a tiny amount of tip weight, but the wing still hanging down in level flight. The large yaw angle kills the overhead tension. Note that the leadouts have to be where the plans show, since any further forwards and the leadouts fight the rudder and create inboard yaw rates in the manevuers. The large aft fuselage would provide acceptable yaw stability and restoring force even without the fin.

Apocyphal stories about knocking it off and the airplane flying better are probably more due to it being grossly maladjusted, like shown on most kit plans.

It's no accident that my airplane looks like a Nobler from the side, with the one vital exception - the fin is on exactly straight ahead (measured with the same level of care as the wing and tail), symmetrically airfoiled, and has an adjustable rudder. It's very easy to see the effects of 1/2 a turn on a 2-56 kwik-link; in fact, if it's one turn off, I lose probably 50 points.

Bottom line - the fin/rudder is a critical factor in trim, whould be put on straight to begin with, and have very fine adjustment capability. Never, ever, try to set the static yaw angle with leadout position, and don't count on the lines to provide any more restoring force than absolutely necessary!

I followed most of this as well as some of Mr. Bucks trim explainations in recent threads on other Forums. But one concept I've heard in several places puzzles me.

Brett Buck indicated:

"The large yaw angle kills the
overhead tension."

I guess I believe that an outboard yaw angle will generate some tension simply because of the angled thrust vector. What is it about the overheads that makes this not so??

I've tried to visualize the relationship of the various centers and forces but havent found a reason for this overhead change.

Any hints.

Thanks

Also for what it is worth, I have a trio of reengineered Sky Rays, each with an adjustable rudder, that will be pressed into service once this strange white stuff disappears from the ground. The adjusting devices are quite simple: cut down 1/2A control horns, one on the fin and one on the rudder, connected by a short pushrod with a 2-56 clevis on one end, rudder hinged to the fin with two DuBro barrel type nylon hinges.
My thought in doing this was that,even if rudder offset proves unnecessary after I get the tip weight and lead-out rake properly set, it would give me something to play with to learn about trimming. Whether that proves to be the case remains to be seen.
Once flying weather returns to New England I will let you know whether the adjustable rudder seems to have been worth the effort.
Peter
<[Between the time I read the first two responses and when I posted my response, Brett Buck posted his typicaly illuminating response. Guess I must abandon any thought of having lead-out adjustment being a substitute for rudder adjustment.>]

Well, poop on all of you, especially Brett. I didn't want to do this, but I took a big slice down the fin / rudder of my Streak, and here we go with the adjustable rudder. I lost my lunch with that operation, but I did it.

Now what? A 1/2A control horn is a start, but how do you build the adjustments? I picture two rubber bands and some paper clips, and a loose screw to make the final adjustment. If you have done this, you probably have a better picture. I don't have a picture.

Sez you. (how 'bout that for witty riposte)

>Now what? A 1/2A control horn
>is a start, but how
>do you build the adjustments?

I use kwik-links to make the adjustments. Use nylon control horn, and a 2-56 Kwik-link on one end of a threaded rod, and a ball link at the other. Use a 2-56 bolt to hold the ball link to the fuselage (or fin in this case) put in 2" fo threaded rod, then the kwik-link going to the control horn.

This is the *only* place to trust kwik-links. They are dismally inadequate (as are any clevis) to the flap/elevator hookups.