Rabe Rudder

Hi All....I'm looking for the article in Dec 2001 Stunt News about the Rabe rudder system..Anybody able to email it or post it??

Tnx, Mike, the "Old Harley Man" [email protected]

>Hi All....I'm looking for the article in Dec 2001 Stunt News
>about the Rabe rudder system..Anybody able to email it or
>post it??

Mike,

No offense, but unless you have a specific problem with a specific airplane, I think you're wasting time here.

The Rabe Rudder works, and can be made to be quite effective. I didn't think that a couple years ago, have since changed my mind.

However--and I am guessing a bit here--I do not believe any CL Stunt model you own or plan to build this winter is a likely candidate for a Rabe Rudder. For example, I have deactivated the unit on my Impact/Saito 56 combination.

Hi Dan....well, you are most likely correct in your assumptions. I am more curious than anything else. I just wanted to "mess" with it and when I asked about the mechanics of the system, I was "invited" to read the article I referred to. Tnx for the reply, Mike

Tnx, Mike, the "Old Harley Man" [email protected]

The Rabe rudder is designed to counter precession. I suppose if you are using a small prop and a lot of side area on your plane it may not make a lot of difference. But even Paul Walker was using one on his Mustang. Matt has used one on his last couple of Stuka designs. With the current plane he started out with the rudder fixed and near zero offset. He trimmed the plane to about as good as could by using all the adjustables on the plane without using the Rabe Rudder. His intent was to take it to its first contest that way thinking he would't have time to fiddle with the Rabe rudder. But, just for kicks, he hooked it up. There was an immediate impact on line tension in the usual problem areas, such that with no fiddling, adjusting, or anything, the trim had already improved. Since then he has done some trimming on the rudder to improve it even more so. But it never came back off.

I think the first question is going to be, "Is it needed" It will depend on your plane and power plant (including prop). Next question will be how you have it hooked up and trimmed. It is quite possible to hook up the Rabe rudder and mess things up. Matt has an adjustable pivot point that he designed on his elevator in place of the fixed horn. Funny thing was, when comparing notes with Paul, it was discovered that Paul had a very similar linkage point that he had put on his Mustang. By putting that adjustable pivot point (or linkage point) on the elevator you can adjust the ratio of the throw in addition to the amount of throw. You want more outside throw on the rudder on "down" elevator and very little throw (some even use slight inside throw) on "up" elevator.

Put it on, adjust it correctly, and it won't hurt. Use little throw if all you need is a little. Use more throw if you need more. If you have the adjustments there and set it up right, I think you will use it and keep it. It is a fixture in Matt's designs now.

Mike.
,

If nobody offers to help you with a reprint of the article, I'll xerox one for you. I which I was computer literate enough to copy it into a word document.

Al...Thanks for the offer.....Not too hard to put into a PDF file if you have a scanner....If you don't, you can take the document to a place with a scanner and have it put on a disk....How about this?? Mail me the article and I'll put it on a CD or floppy (or both) and send them to you....then you can copy the article to your PC from the disk.....

Mike

Tnx, Mike, the "Old Harley Man" [email protected]

I was about to offer the same thing. Once this is done, I'd love to have a copy myself. Is there a place to archive it, so anyone can snag it?

Kim Elmore

I'll send you a copy also if you want....

Tnx, Mike, the "Old Harley Man" [email protected]

Mike,

I've got the scanner but don't know how to create files. I guess I'll just have to copy it and snail mail the pages. Your address in the PAMPA reference manual?

Hi Al...Send me an email and I'll return it with my address...

Tnx, Mike, the "Old Harley Man" [email protected]

It certainly gives different results with differert planes.
I have had fits trying to make it work on some planes, and no problems on a few.
My biggest problem is cutting down the amount of movement and getting the right amount of assemetry.
I have the Rabe Mustunt article, If you Email me OLCEP@aol .com
with instructions on how to send it ; I will try to send you a
copy .
I have a good computer,and scanner/copier to go with a small brain

Here's a pic that might help. The rudder is ~1.25x3.5 in. cut into the back of the fin/rudder. The linkage uses Dubro ball socket ends, on a short piece of 2-56 rod. The horn on the left elevator is cut down to 3/16 in. high for the ball link which mounts about 1/4 in. back from the hinge line. The horn on the rudder is cut down and mounts a bit low, so the pushrod clears the elevator a full down. The picture is in neutral. The rudder, in this case, moves from about 1/8 in. right in neutral to 1/8 in. left at full up, and from 1/8 in. right to 5/8 in. right at full down. I lucked out. The travel, area etc. is just about perfect for this plane.

If you have a plane that turns quick and/or uses a large prop or one with a lot of inertia, Al's rudder will help a lot. The symptoms are that the plane can't be trimmed to fly smooth through the corner with just the fin offset and leadout position. If it still yaws and swings too much in sharp turns the Rabe Rudder will probably help. You probably don't want to use a full size rudder on most planes. It is likely to be way to much control. If you go with a smaller, trim tab style rudder you can use more movement and it will probably be easier to get it trimmed right.

The system actually feeds in left rudder at some point? If so, I can't wait to read the theory ... Maybe the left rudder input is just enough to offset the yaw but not enough to affect the direction of flight and thus line tension...Hmmmmm....

Tnx, Mike, the "Old Harley Man" [email protected]

>The system actually feeds in left rudder at some
>point? If so, I can't wait to read the theory ... Maybe
>the left rudder input is just enough to offset the yaw but
>not enough to affect the direction of flight and thus line
>tension...Hmmmmm....

Well, it does effect the line tension - in a good way, by preventing the outboard yaw that would otherwise occur on inside corners. "How much" is not necessarily the key point - it's more like "how constant". Yawing outboard in an inside corner will *temporarily* increase the line tension, but shortly later will reduce it, too, when the airplane swings back to the left. Rapidly varying line tension (in either direction) causes all sorts of problems with controlling the airplane. If you have some up elevator fed in, and the line tension varies around, it *changes* the amount of control deflection - more with more line tension, and less with less line tension. So when it pulls harder, it tightens up the corner, and if it pulls less, it opens up the corner. NOT what you want at all.

If it doesn't swing the nose outboard at first, it never swings back inboard later.

This is *really* important to understand.

The whole idea in trimming is almost never to "get more line tension". It's to make the control response more predictable and consistent. You have to have enough tension, but for the most part, centrifugal force is more than sufficient. Trying to get more can sometimes work, but usually, tricks to "get more" result in more tension and frequently, worse overall perfromance.

>The system actually feeds in left rudder at some
>point? If so, I can't wait to read the theory ... Maybe
>the left rudder input is just enough to offset the yaw but
>not enough to affect the direction of flight and thus line
>tension...Hmmmmm....

Suggest you go back and read Leonard's post that geometries can be chosen where no left rudder need to be used. And there are other ways to hook up a movable rudder to avoind any left rudder being used at any elevator position. I am preparing an article for Stunt News to show how I have been doing this since Al introduced his movable rudder concept in the early 1970's.

Meanwhile, Leonard explains one way do to so in his post #3 above.

Keith has a really cool rudder system. He makes Jacquard fabric with his Bearcat in the off season.

LAST EDITED ON Nov-04-04 AT 01:54 PM (CDT)
 
>Here's a pic that might help. The rudder is ~1.25x3.5 in.
>cut into the back of the fin/rudder. The linkage uses Dubro
>ball socket ends, on a short piece of 2-56 rod. The horn on
>the left elevator is cut down to 3/16 in. high for the ball
>link which mounts about 1/4 in. back from the hinge line.
>The horn on the rudder is cut down and mounts a bit low, so
>the pushrod clears the elevator a full down. The picture is
>in neutral. The rudder, in this case, moves from about 1/8
>in. right in neutral to 1/8 in. left at full up, and from
>1/8 in. right to 5/8 in. right at full down. I lucked out.
>The travel, area etc. is just about perfect for this plane.
>
>If you have a plane that turns quick and/or uses a large
>prop or one with a lot of inertia, Al's rudder will help a
>lot. The symptoms are that the plane can't be trimmed to
>fly smooth through the corner with just the fin offset and
>leadout position. If it still yaws and swings too much in
>sharp turns the Rabe Rudder will probably help. You
>probably don't want to use a full size rudder on most
>planes. It is likely to be way to much control. If you go
>with a smaller, trim tab style rudder you can use more
>movement and it will probably be easier to get it trimmed
>right.

Ron B.
F4Fguy

Therein lies the key to most of the problems people have trimming the Rabe rudder. The effect is subtle and the adjustment must be also. Most builders who've never used one before tend to crank in WAAAYYY too much movement.

Please note that by subtle I mean it's easy to go beyond the good effect and get some bad effect. Start with 0 movement and crank in no more than 1/32" at a time.Generally you don't need as much left as right throw, but you'll almost always get an improvement with some. Remember, the idea here is yaw control to obtain constant line tension,not to get more in some parts of the pattern than in others.

If you're adding a Rabe rudder to a design that didn't have one, it's a good idea to have a fully adjustable linkage to allow varying the left/right ratio. If you're building an existing Rabe rudder design use the designer's setup,he's already been there.

If I'd been the patient sort, I would have built the rudder adjustable first, trimmed the plane, and then hooked the rudder up to the elevator and re-trimmed the whole thing. Being impatient, and also noticing that the plane yawed too much in almost every sharp corner, I just hooked it up and used it. Like Matt's experience, the first try made such an improvement I'm going have to practice a couple hundred flights to get enough better that I'll be able to see if it needs more fine tuning.

As far as left and right, according to physics, Wild Bill's measurements, and Al's experiments, prop precession swings the nose in on outside turns, and out on inside turns, the same amount(if the plane is turning the same rate). On the practical side, we don't fly the insides and outsides symmetrically(gravity, wind, and hand movements), so less left rudder is needed. Like Brett said, it's really nice when the plane doesn't yaw 'cause then it is more predictable. I've found that judiciously whipping the plane in the various turns can eliminate most of the yawing, but it is really unpredicatable and hard to coordinate. The rudder does the job a lot easier.

Phil, you wrote:

>On the practical side, we don't fly the insides and outsides
>symmetrically(gravity, wind, and hand movements), so less
>left rudder is needed.

One would think that less left rudder is needed on insides. That is basically how Al sets up his system. With various geometries, (or control systems) it is possible to see what happens when no change in rudder position occurs or even slight more right rudder is added for the inside maneuvers. I have found that slightly more right rudder on insides (like an extra 1/32" throw) helps. I have discussed with several others who have experimented extensively with the Rabe rudder who have discovered the same thing.

>>>If you go with a smaller, trim tab style rudder you can use more movement and it will probably be easier to get it trimmed right.<<<

Yes, especially more movement allows easier linkage without free play - free play makes really ill effects on Rabe rudder. Look to pic it shows my model from WC - that small rudder allows 1:1 deflection to right.

Reason why it should go more right then left is little bit difficult to explain, but main reason is, that Rabe rudder is not perfect way to kill gyro yawing. Yaws come from pitching rate, not from elevator deflection and thus the Rabe rudder does not work for 100% (because of mass inertia). It means it cannot solve yaw for 100%. So if I set Rabe rudder to damp only 50% of yaw then I have some yawing and thus also variable line tension. And as Brett already wrote - line tension is damping the yaw also (variable) and thus for symetric response in positive ans negative corners, left rudder must go less then right.

I also did some tests with R/C gyro unit handling rudder on base of pitching rate (I wrote it in past here) that systen clearly needed more deflection and especially almost symmetric to both sides.

Igor, please tell us more about your yaw servo system.

Here is that old thread:

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=5716&forum=DCForumID1

Since that time I did not do further tests, but I have new quicker servos (they are now in 30$ range) and small LiPoly batteries.

I have new unit for electric stunter which has more aivailable inputs and especially 6V servo feeding - it makes it even quicker. I think I will use two gyro units with little more clever software. I plan to use one gyro measuring pitchig rate and another measuring and filtering only short yaw disturbances.

... may be next summer