Rabe Rudder Question

LAST EDITED ON Jul-18-03 AT 08:12 AM (CDT)
 
Having never used a Rabe rudder, but contemplating its addition on next years model, I have a question. Being that I fly clockwise, how do you set up a rudder to travel more on insides and less on outsides the opposite way the rudder operates on a counterclockwise flying model. Will the horn on the elevator have to be on the top side? Help Al!
Alan Resinger

You shouldn't have to put it on top, just put the linkage on the other side of the airplane. The differential through is determined by how far aft of the hingeline the horn pivots.

The way I understand the function of the Rabe rudder when you are Flying clockwise the control input will have to be just the reverse of counterclockwise flying. The normal set up is for more rudder with "down" input and less with "up".

you get nose out rudder with down elevator. you adjust the amount of throw based on the outside square loop of the horizontal square 8 to get the cleanest turning without yawing.
Andrew R

>The way I understand the function of the Rabe rudder when
>you are Flying clockwise the control input will have to be
>just the reverse of counterclockwise flying. The normal set
>up is for more rudder with "down" input and less with "up".

Just a note to explain what "phantomflier" just said.

The Rabe rudder is used to offset the undesirable gyroscopic precession of the rotating propeller. For left handed propeller rotation ("normal" engines) gyroscopic precession will swing the nose to the left on outside maneuvers and will swing the nose to the right on inside maneuvers. So the linkages for the rudder travel must be changed when flying clockwise, or as "phatomflier" just said, you will want the rudder to definitely deflect to tend to yaw the nose left on inside maneuvers, and somewhat less or even right rudder for the outside maneuvers, just the opposite as for counterclock wise flying.

Keith,
That is what I meant in my first post. I knew that on normal counter clockwise ships the rudder deflected more on outsides turns and less on inside turns. I was sure it had to be the opposite on clockwise flying models. How do you hook up the controls to get that kind of geometry? Does anyone remember what issue of Stunt News had an atricle on the Rabe rudder?
Alan

Alan,

If you are using the same kind of linkage between the rudder and the elevator that Al advocates, then instead of having the linkage from the bottom of the right elevator to a horn on the right side of the rudder, do it all on the left side of the elevator/rudder.

The Rabe rudder article was in the Nov/Dec 01 issue of Stunt News.

>Alan,
>
>If you are using the same kind of linkage between the rudder
>and the elevator that Al advocates, then instead of having
>the linkage from the bottom of the right elevator to a horn
>on the right side of the rudder, do it all on the left side
>of the elevator/rudder.
>
>The Rabe rudder article was in the Nov/Dec 01 issue of Stunt
>News.

Now I have never thought about this, but the engine is running the same direction (counter clock wise) regardless of which way the airplane is pointed. The Rabe rudder gives more rudder on outside turns and less on inside turns when flying counter clockwise. It would seem that you would want more rudder on insides when flying clockwise. I would say just to leave it alone and you would have that, except the concept exaggerates the rudder offset with outside and gives little in the opposite direction. So, basically, you would want to put your linkage on the top of the outside of the elevator to keep the effect the same.

The Rabe rudder gives
>more rudder on outside turns and less on inside turns when
>flying counter clockwise.

No. It actually gives left rudder on insides and right rudder on outsides.

???

If you pitch up, the gyroscopic moment yaws right. I think it has nothing to do with direction of flight in circle. Clockwise or counterclockwise. Am I missing something ???

However – Len wrote it well, the asymmetric ratio can be a problem, it should be designed opposite, but can still be on the same side. Just arms have to be little different. It enough to put the rudder horn little aft of its original position. That is all.

LAST EDITED ON Jul-18-03 AT 03:12 PM (CDT)
 
Alan,
I think Keith, Ukee Jim and phantom flyer all have it correct. The airplane will always yaw to the right on insides and to the left on outsides because the propeller rotation is the same regardless of which way you are flying. With normal clockwise flying the outsides are critical because the airplane is trying to yaw inward on nose down pitching maneuvers. when flying clockwise, the inside maneuvers become the critical maneuvers where the airplane tries to yaw inward.

The installation of the movable rudder controls is the same, with a difference. For clockwise flying the rudder should be adjusted to be slightly left of neutral when viewed from behind, much the same as setting the rudder a little right for counter-clockwise flying. We find the best adjustment if the rudder moves asymmetrically more to compensate for the pitching direction which causes the more critical yaw. For counter-clockwise flying, we begin with the rudder offset slightly to the right with neutral elevator and move it asymmetrically farther to the right than to the left with down and up elevator deflections. We obtain this asymmetry by using a rudder pickup located aft of the elevator hinge line. For clockwise flying, the movable rudder pickup, pushrod and rudder horn are still on the outside (right) side of the airplane. Now, however, the inside maneuvers will be more critical so the difference in installation is to change only the asymmetry. The elevator pickup for the rudder pushrod must now be ahead of the elevator hinge line to make the rudder travel more to the left on up elevator than it travels to the right on down elevator. I use about 3-1 but it isn't very critical unless you are trimming for ultimate performance. If you locate the rudder pickup on a line extending down and about 45 degrees ahead of the elevator hinge line you will be in the ballpark and should be able to reach final trim by adjusting the pushrod length for offset and selecting rudder horn holes for sensitivity.

Generally, not much movement is needed unless you are swinging a really big prop. Unfortunately, it is much easier to build counter-clockwise asymmetry into the elevator than clockwise asymmetry where the pickup will tend to interfere with the bottom of the stab. You will perhaps need a linger pickup to clear the stab and a porportionally longer rudder horn to bring sensitivity back to normal.

I think all of the above is correct, but I would stand corrected if someone like Keith or F4F Guy (ron Burn) points out mistakes. It just isn't one of those things I spend much time thinking about, in fact, you are the first person in 35 years to ask this question.

As I read this it slowly dawns on me that putting the movable rudder mechanism in the left side of the airplane will still give left rudder on up and right travel on down but the asymmetry problem practical installation problem is cured. Installing the elevator pickup behind the elevator hinge line on the left elevator will give asymmetrically increased rudder travel to the left, which should be correct for clockwise flying.

Keith had it correct on his post No 6, where he suggested moving the mechanism to the left side of the airplane. I just had to spend some time reasoning through it to reach the same conclusion Keith had already posted

Thanks Al.
I figured that my inside turns would require the increase in rudder offset and the outsides would require less. Brads post was a little confusing when he was talking about his travel. I get the impression that he means that the rudder travel does not pass the center line of the model but does move slightly to the inside of a pre-set amount of outboard rudder offset.
I'm in the process of building another Beringer Sportster but intend to utilize the larger Sukhoi wing and moments. Power to be either a 56 or 72 with the resultant large prop.

Hey Alan, wouldn't it be easier if you just learned to fly in the "right" direction? (Just kidding.)

Leonard,
I know it's hard to believe but it isn't the first time I've heard that.
Alan

Alan,

The properly adjusted rudder may very well pass the centerline of the airplane. Several of mine do. The theoretical optimum would be for the rudder to move equal distances from its elevator neutral offset position. This will almost always move the rudder across the airplane's centerline. The practical optimum lies in less than a full correction on outward yawing pitching maneuvers and overcorrecting on inward yawing pitching maneuvers to force the nose slightly outward instead of simply correcting/balancing the inward yaw from the propeller.

The only satisfactory answer lies in experimentation/trimming. There is good information in the Stunt News article on how to trim.

>Alan,
>
>The properly adjusted rudder may very well pass the
>centerline of the airplane. Several of mine do. The
>theoretical optimum would be for the rudder to move equal
>distances from its elevator neutral offset position. This
>will almost always move the rudder across the airplane's
>centerline. The practical optimum lies in less than a full
>correction on outward yawing pitching maneuvers and
>overcorrecting on inward yawing pitching maneuvers to force
>the nose slightly outward instead of simply
>correcting/balancing the inward yaw from the propeller.
>

Of course, the assymetry is allowable because nose-out yawing moment increases the line tension, and thus the lines provide more resistance to yawing too far. Nose-in reduces the line tension, and thus a slightly excess movement reduces the tension and results in a lot more angle.

In other words, 1/2 a degree too much inboard rudder results in a lot larger yaw angle than 1/2 a degree too much outboard rudder.

It's just as true of many other adjustments. A tiny bit too little tipweight has a lot more drastic effects than a tiny bit too much, for instance, for similar reasons.

Alan,

The more I think about this, the less sure I am that I have given correct information. Now, I think possibly installing the elevator pickup on the opposite elevator will only give the same results, that is, the asymmetry will be the same with the pickup installed on the bottom of either elevator.

We are back to my original post which stated the pickup would have to be ahead of the elevator hinge line, or behind the hinge line only if installed on the top of either elevator.

Before installing one of these on your airplane, it only makes sense to make a mock-up to determine exactly how to get the optimum asymmetrical travel. It is your airplane. Ultimately, you will have to take the necessary steps required to get it right. The goal is 3-1 travel with the rudder moving more on "up".

Most classic stunters get along just fine without a movable rudder, particularly those with engines smaller than .60. I think it only necessary to use a movable rudder with larger engines, particularly 4 strokes with large propellers.

The movable rudder is usually misunderstood and frequently misapplied. Its purpose is to remove gyroscopic wobble from corners, not to create wobble in the effort to get more line tension. the trim process is one of smoothing the corners, which, in of itself will increase tension.

Al,
A mock-up is what I was planning. I'm beginning to think that geting the attachment ahead of the hinge-line is a necessity. I'll let you know when I get the mock-up built.
Alan

LAST EDITED ON Jul-20-03 AT 08:22 PM (CDT)
 
installing
>the elevator pickup on the opposite elevator will only give
>the same results, that is, the asymmetry will be the same
>with the pickup installed on the bottom of either elevator.

Correct,but with the rudder horn on the other side the assymetry is reversed,normally not a good thing at all.
>

>Before installing one of these on your airplane, it only
>makes sense to make a mock-up to determine exactly how to
>get the optimum asymmetrical travel. It is your airplane.
>
AMEN.

I
>think it only necessary to use a movable rudder with larger
>engines, particularly 4 strokes with large propellers.

AMEN again.
>
>The movable rudder is usually misunderstood and frequently
>misapplied. Its purpose is to remove gyroscopic wobble from
>corners, not to create wobble in the effort to get more line
>tension. the trim process is one of smoothing the corners,
>which, in of itself will increase tension.

DOUBLE AMEN.
>
The reason you don't need as much rudder correction on up,flying counter-clockwise is,as has been said because the combination of forces is already producing a nose out couple.Flying clockwise reverses the situation,and,probably,the trim requirement.See my earlier post re an easy demo.If flying inverted produces the desired result,simply moving the linkage to the opposite corner should do the trick.Leave the linkage geometry as is.In any case your'e going to have to play with it to get it right.And if your'e not running large or heavy props,don't do it at all.It's only one more thing to misadjust.

LAST EDITED ON Jul-20-03 AT 05:20 PM (CDT)
 
As Al says, if you have the correct amount of rudder, the airplane won't yaw at all. Therefore, assuming constant line tension and RPM everywhere in the pattern, the layman would think that the rudder should move the same in both directions.

It makes sense that overdoing the correction would be more onerous on inside maneuvers, where the yaw lightens line tension. Having a Rabe rudder would be interesting, but I have more movable surfaces now than I can handle.

Ron B.
F4Fguy

Easy demo:
Invert the airplane,reverse the handle,your'e flying upright and clockwise.The rudder still works.

Ron,
I agree that inverting the plane gives a demo but now the unsightly horn is on the top rather than the bottom of the surface. Don't think that I want the horn and connecting pushrod out in open view when the plane is sitting on the tarmac.

Alan

Alan,

I agree with you that an installation on top of the tail surfaces is unsightly. It is possible to install it below but you may very well have to do considerable hollowing on the bottom of the stab to clear to forward projecting elevator pickup on down elevator

Ron B.
F4Fguy

See my post #20? above.Just move it to the opposite side on the bottom.

Ron:

Hate to disagree with you but moving the control to the opposite side on the bottom will only make the problem of precession worse. When you move it to the opposite side it will have to go on top.

As Keith points in #4 and Al in #10 The plane will yaw to the right on insides and the left on outsides. Flying clockwise this means the plane will yaw to the inside of the circle on inside maneuvers and the outside of the circle on outsides maneuvers. This means we will want more left rudder on insides and less on outsides. Placing the controls on the bottom would result in less rudder on insides and more on outsides. Placing the controls on the top will give the desired input. The other solution is to leave the controls where they have traditionally been located, provided the geometry can be set up to give the proper amount of left instead of right rudder.

Ron B.
F4Fguy

Exactly,and moving the linkage to the other side on the bottom will reverse the differential.This will give more throw left on up elevator,and less right on down.You'll still have play with the throws,since the level flight precession couple is reversed flying clockwise.

Al.
I've thought of that considering the large amounts of elevator throw that the Baringer designs use with their control system geometry. I sure would like to get the problem solved as the large props are surely affecting the quality of the turns.
Alan

>Al.
>I've thought of that considering the large amounts of
>elevator throw that the Baringer designs use with their
>control system geometry. I sure would like to get the
>problem solved as the large props are surely affecting the
>quality of the turns.
>Alan

There are always smaller props. I am headed there myself.

Also, my rudder goes left on insides. It has too, to diminish the right yaw induced in every inside corner.